Research

INTRODUCTION

Marine ecosystems are changing rapidly, given strong increases in anthropogenic pressures, such as climate change and fishing (Halpern et al. 2015, Jouffray et al. 2019, Gissi et al. 2021). Excessive exploitation of living marine resources, in particular, has caused and still causes abrupt collapses of fish stocks while their recovery potential is low (Sguotti et al. 2019, Steneck and Pauly 2019). Abrupt changes from an abundant resource toward overexploitation can be analyzed with regime shift theory, which describes the abrupt change of a system state due to natural, social, or economic stressors (Scheffer et al. 2001). The old and new system states are both stable in themselves and are characterized by an instable transformation period (Petraitis and Dudgeon 2015). These shifts can be perceived either as a surprise by resource users or as a logical consequence of an overuse of living marine resources (Möllmann et al. 2015).

Because of its explanatory potential, the regime shift concept gained attention in the scientific discourse over the last decade and today represents an applied scientific concept (Scheffer et al. 2001, Beisner et al. 2003, Beaugrand 2004, deYoung et al. 2008, Conversi et al. 2015). Historically seen, the concept was developed in the 1970s and originates from two distinct scientific contexts: first, mathematical modelling was the basis for quantifying abrupt shifts given gradual transitions in terrestrial community components (May 1977). Second, observations and statistical analyses detected biological and physical differences between regimes in lakes (Scheffer et al. 2001) and the North Pacific (Steele 2004, Wooster and Zhang 2004).

Given its origins and widespread application within different disciplinary contexts (Scheffer et al. 2001), there is to date no common agreement on a clear or mandatory definition of the regime shift concept within scientific communities (Steele 2004, Conversi et al. 2015, Möllmann et al. 2015). Existing definitions semantically range from “dramatic, abrupt changes in the community structure that are persistent in time, encompass multiple variables, and include key structural species” (Conversi et al. 2015) to “low frequency, high-amplitude changes in oceanic conditions that may propagate through several trophic levels and be especially pronounced in biological variables” (Collie et al. 2004). Other explanations also include changes on several system parts, levels, and geographical scales, such as “persistent radical shift[s] in typical levels of abundance or productivity of multiple important components of marine biological community structure, occurring at multiple trophic levels and on a geographical scale that is at least regional in extent” (Bakun 2005). Hence, the regime shift concept appears to be of a certain explanatory and practical use, as it is similarly defined among scientists shaping the concept to their specific contextual needs. However, the regime shift’s precise conceptual content is still not fixed scientifically, i.e., it is debated whether the switch between true alternate stable states truly exists (Conversi et al. 2015, Sguotti et al. 2022).

The concept of regime shifts seems to hold an analytical and practical value as it is widely applied in the context of marine research in, e.g., the Baltic Sea (Möllmann et al. 2008, 2009), the Black Sea (Daskalov et al. 2007), or the North Sea. The latter represents a well-investigated area where regime shifts were identified (Weijerman et al. 2005, Kenny et al. 2009, Sguotti et al. 2022) for the 1980s because of sudden temperature increases, leading to shifts in phyto- and zooplankton (Beaugrand 2004, Capuzzo et al. 2018, Edwards et al. 2020). The North Sea is among the most rapidly warming world oceans (Intergovernmental Panel on Climate Change [IPCC] 2022), and therefore considered to be a climate change hot spot. Hence, changes at multiple trophic levels are becoming more frequent and likely (Sguotti et al. 2022).

Atlantic cod (Gadus morhua) in the North Sea (hereafter: North Sea cod) represents a paradigmatic example for investigating regime shifts as it has experienced a collapse in the 1990s (Cook et al. 1997) because of overfishing and climatic changes (Rose 2019, Sguotti et al. 2019). Since the 1980s, the North Sea cod stock remains in a low stock state (ICES 2022) because of temperature increase limiting young cod survival (Beaugrand et al. 2002, 2003, Blöcker et al. 2023a) and reducing the adult cod’s thermal habitat (Blanchard et al. 2005, Rindorf and Andersen 2008). In some scientific literature, this collapse was framed as a regime shift, which caused the stock to switch to an alternative system state, where recovery is hindered by external drivers (Cook et al. 1997, Fauchald 2010, Lynam et al. 2017, Sguotti et al. 2019, Blöcker et al. 2023a). Efforts to enhance recovery, such as the implementation of a cod recovery plan in 2004 (EC 2004), were unsuccessful as the stock remained below sustainable reference levels (ICES 2022). Management of North Sea cod takes place in a rather linear manner, meaning that regime shifts in fisheries management are not sufficiently considered yet (King et al. 2015, Levin and Möllmann 2015). Ecosystem-based management, which is the underlying approach in North Sea fisheries management, allows for the inclusion of stressor interactions (humans included) and their implications for whole ecosystems (Levin and Möllmann 2015). However, North Sea cod is managed by using a single-species approach, focusing strongly on the management of fishing mortality and less on environmental dynamics strengthening and driving possible regime shifts (Sguotti et al. 2019, Blöcker et al. 2023b).

Hence, the example of North Sea cod not only highlights the practical and urgent need for the detection of regime shifts in general, but it also calls for a more accurate explanation of what a regime shift actually is. A common understanding among various stakeholders, such as decision-makers, environmentalists, and fishers, in this context appears to be of vital importance to inform and determine the policies of recovery for fish stocks. Therefore, if the concept itself is weakly defined and poorly understood, it holds the danger of leading management efforts and measures into a dead end (Möllmann and Diekmann 2012, Sguotti et al. 2019).

Against this background and with the aim to tackle the understanding of the regime shift concept, we studied its framing among stakeholders, in the German context, involved in science, nature protection, management, and North Sea cod fishing. We conducted interviews and identified convergences and divergences in the theoretical framing of the concept to address the above-mentioned problem setting. Hence, the analysis performed here aims at clarifying the meaning of regime shift among stakeholders and explores its relevance for explaining the changes taking place within North Sea cod, as well as for implications for fisheries management.

Ecological regime shifts and tipping points in marine science

The regime shift and tipping point concepts used in marine systems are closely related and one seldom appears without the other (Möllmann et al. 2015, Milkoreit et al. 2018, Sguotti and Cormon 2018). However, their terminology stems from various disciplinary contexts (May 1977, Wooster and Zhang 2004), and both are not clearly defined among scientists (Möllmann et al. 2015, Milkoreit et al. 2018, Mathias et al. 2020).

Terminology-wise, regime shifts are linguistically described by using synonyms such as “phase transitions” or “alternative stable states,” which conceptually highlight the transitional process from one state to another (Möllmann et al. 2015). Besides these contemporary uses, the term regime shift holds a theoretical history which goes back to the 1960s (Thom 1972). In marine research it was first used in the late 1980s, analyzing shifts in fish populations dominance between anchovies (Engraulis encrasicolus) and sardines (Sardina pilchardus) (Lluch-Belda et al. 1989). From then on, the term continuously gained popularity in the 1990s (Milkoreit et al. 2018).

At the turn of the century, “tipping points” were conveyed linguistically by using synonymous phrases such as “critical transitions,” “critical point,” “threshold,” “abrupt change,” and “punctuated equilibrium” (Dakos et al. 2015, Milkoreit et al. 2018). Nowadays, the notion of tipping points is an analytical notion in itself, but also conceived to be an addition to the already existing terminology while semantically embedded in the overarching regime shift concept (Milkoreit et al. 2018).

Among the synonymous ways of describing regime shifts and tipping points, differences in the conceptual structures can be found. “Skeptics” and “believers” divide the marine community (Scheffer and Carpenter 2003, Conversi et al. 2015). At first it was assumed that systems respond gradually to drivers in regime shifts, whereas abrupt changes toward another system state were only theoretically described by using conceptual models (Holling 1973). Nowadays, three conceptual types of a system response to drivers are accepted: (1) linear and continuous, (2) non-linear and continuous, and (3) non-linear and discontinuous (Scheffer et al. 2001, Möllmann and Diekmann 2012). The latter originates from “catastrophe theory,” where a system reaches a certain point, the tipping point, at which it can no longer withstand stressors because of decreased resilience and transitions into a new, alternative stable system state (Scheffer et al. 2001, Scheffer and Carpenter 2003). Alternative states are indicated by three regime shift indicators, which are (1) abrupt changes in time series, (2) multimodality in the state variable (e.g., fish stock size), and (3) a bi-fold relationship with the driver that determines alternative stable states (Scheffer and Carpenter 2003). In contrast to theory, states in reality are, however, not strictly stable and systems appear to be rather dynamic and fluctuating (Möllmann et al. 2015). Foundations for catastrophe theory, and therefore regime shifts, were already laid in 1885 by the French mathematician Henri Poincaré (Barrow-Green 2005). But it was only in the 1960s and 1970s that these mathematics were introduced to physical and ecological applications, when discussing ecosystem stability became popular among scientists (Lorenz 1963, Holling 1973). In the case of non-linear and discontinuous systems, the return to the original state might be hindered by emerging functions and feedback mechanisms stabilizing the new system: this phenomenon is called hysteresis (Scheffer et al. 2001, Möllmann and Diekmann 2012). However, there is scientific uncertainty about whether all three responses fall under the conceptual definition of regime shifts or only the latter one (Steele 2004). For all types, a significant driver is required to shift the system across a (tipping) point toward an alternative state. Hence, a regime shift implies the whole process of a system’s transition from one state to another state, whereas a tipping point is one of the regime shift’s characteristics and determines the inflection point at which the system begins to transfer (Steele and Henderson 1984, Scheffer et al. 2001, Möllmann and Diekmann 2012, Möllmann et al. 2015, Sguotti et al. 2019).

Drivers play an important role in the regime shift concept and are conceived as the underlying forces. Based on differences in their predominance and their effect on a state variable (Möllmann et al. 2015), shifts in climate and ecosystems are distinguished (Möllmann and Diekmann 2012): climate shifts imply abrupt changes in diverse climatic characteristics that induce bottom-up changes, and are therefore conceptualized as drivers of an overarching ecosystem shift (Hare and Mantua 2000, Bakun 2005, Dakos et al. 2015). These ecosystem shifts are driven by an interaction between external drivers as well as internal system processes, e.g. trophic control (Scheffer et al. 2001, Scheffer and Carpenter 2003). They consider changes in the abundance of marine community components and can occur at large space- and time-scales (Bakun 2005).

Furthermore, there is no common ground for the temporal and ecosystem organizational scale of regime shifts. Time-wise, variables in time series can be distinguished as either slow or fast (Milkoreit et al. 2018). Fast variables, e.g., a certain fish species, are of primary interest and experience quick dynamics that are influenced by slow-changing variables, such as temperature increases over time (Walker et al. 2012). Also, the time-related identification of a regime shift differs per definition. The scientifically shortest period defined for an alternative new state is at least five consecutive years before another change occurs (Norström et al. 2009). At ecosystem organizational levels, regime shifts have no limits in either way: they can be defined as shifts on population level (Sguotti et al. 2019), on a system’s internal feedback like trophic functioning (Alheit et al. 2005), or on community level, which for instance include a northward shift of cold water–preferring species (Beare et al. 2004, Petitgas et al. 2012, Baudron et al. 2020).

Diversity in the regime shift and tipping point concepts can, furthermore, be found on the methodological level. Starting off with easier models that consider time series of one ecosystem component (Ludwig et al. 1978, Steele and Henderson 1981, 1984, Scheffer et al. 2001, Steele 2004), models were continuously broadened in complexity to assess alternative system states and ecological shifts driven by a multiplicity of drivers. These analyses were performed on a retrospective level to assess whether a regime shift had taken place already (Collie et al. 2004, Möllmann and Diekmann 2012, Sguotti et al. 2020). However, given steady increases in anthropogenic pressures, models that predict the likelihood of a regime shift to occur were developed by using early warning signals (Scheffer et al. 2012). Their predictive ability, however, remains limited because of environmental stochasticity (Dakos et al. 2015). These varieties in methods, data availability, and selection bear an impact on the detection of regime shifts. Hence, statistical modelling supports empirical evidence of regime shifts through detecting the underlying drivers and processes (Möllmann and Diekmann 2012).

Finally, the detection and knowledge of regime shifts and tipping points possess an important impact on management measures. Alternative stable states, involving hysteresis, ask for significant and drastic actions to reduce the intensity of drivers for achieving an initial system state (Möllmann and Diekmann 2012). Within this context, the application of the terms regime shift or tipping point to describe various phenomena can ignore significant differences and lead to or even create the belief of conceptual similarity (Milkoreit et al. 2018). Using both concepts and considering their great semantic differences might express the notion of a deep conceptual understanding by scientists, but it could also indicate the loss of focus and the lack of a commonly shared understanding (Milkoreit et al. 2018).

Despite the apparent conceptual imprecisions, the notion of regime shifts seems to hold a certain analytical value for studying changes in marine ecosystems shown by its worldwide application. It could therefore be conceived as a boundary object as it holds enough immutable content, while at the same time flexible interpretations and applications across scientific disciplines, methods, theories, contexts, and research objects are possible (Star and Griesemer 1989).

Epistemology of concepts: black boxes and boundary objects as analytical tools

The notion of regime shift exhibits a long conceptual history and has generally been used to understand and scientifically study the causes of abrupt changes in an ecological system driven by environmental and/or anthropogenic impacts. The notion of regime shift has once been borrowed “from describing phenomena such as lacustrine ecology or fire regimes and [has now been] applied to complex socio-ecological phenomena” (Kull et al. 2017).

For this to be done, applying a translational approach (Schlesinger 2010) opens up conceptual gaps with the aim to exhibit and analyze the various framings of the concepts among stakeholders. It takes a meta-perspective on the concepts of regime shifts, abrupt changes, and tipping points while also systematizing and analyzing their framings. Such a perspective is informed by research undertaken in the area of science and technology studies and on the sociology of scientific knowledge that investigate the development, understanding, and application of scientific technologies, infrastructures, practices, methods, or theories. In this context, theories and concepts can be “black-boxed” (Latour 1987). The analytical notion of black-boxing (Johnson and Lidström 2018) refers to the fact that a concept can be used in various and sometimes considerably differing contexts without being semantically or pragmatically explained, adapted, or fined-tuned to it (Jasanoff 2006).

To analyze this semantic heterogeneity of the regime shift concept, we suggest using a critical realist approach (Sayer 1999) that avoids the theoretical traps of positivist and relativist rationales (Stone-Jovicich 2015). Such a perspective holds the important advantage that we can move back and forth between “empirical realities and the social processes that produce the [...] understandings of those realities” (Kull et al. 2017) and their meanings. This enables us to explore the middle ground and the social contingencies between these two conceptual ends. Consequently, the regime shift concept and its connected notions could be understood as socially produced and semantically multifaceted entities. Such a framing opens up a perspective that analytically engages with a “cohesive heterogeneity” as outlined in the analytical concept of a boundary object (Star and Griesemer 1989). Boundary objects represent entities that are “plastic enough to adapt to local needs and the constraints of the several parties employing them, yet robust enough to maintain a common [and socially shared] identity across sites” (Star and Griesemer 1989). Hence, they practically hold the potential to cover social and semantic distinctions while at the same time bridging differences or worldviews between scientific disciplines and/or social actors (Kull et al. 2015). Hence, boundary objects could be conceived as concrete objects, abstract notions, or concepts that are shared by and are accessible for different social groups who do not or only hold partly overlapping knowledges or epistemologies.

To summarize, one can say that the theory of black-boxing and the notion of boundary objects share the conceptual aspects of specific non-specificities. Thus, the social aspect of the black-boxed boundary objects surpasses the actual and precise semantics of what the notion regime shift actually means. In doing so, it opens up a perspective on the social dimensions and the social sense of meaning and the interesting fact that sociality gathered around a theory like regime shifts is possible and does not depend on an exact definition every member in a social group shares. On the contrary, black boxing and boundary objects appear to populate a socio-scientific world in which the regime shift concept represents a communicative tool to express the perceptions and assessments related to it and assess them.

METHODS

To investigate the conceptual framing and assessment of the boundary object regime shift in the context of abrupt changes in North Sea cod stock, we conducted semi-structured interviews with relevant stakeholders and performed an inductive data analysis (Fig. 1) (Dawson 2009, Gläser and Laudel 2010). The concepts and their surroundings are here analyzed and discussed within the German context, given that most interviewees are German and work in Germany.

Data collection

First, we started with an in-depth reading of the relevant scientific literature about the current regime shift of North Sea cod and analyzed the thematic structure of this scientific discourse (Fig. 1, step 1). Scientific literature was searched by using the Web of Science platform (Clarivate) and Google Scholar (https://scholar.google.com/), applying the following search words without specification for searches in title or abstract: “Regime Shift*” OR “Tipping point*” OR “Abrupt change*” AND “North Sea” OR “Atlantic cod” OR “Gadus morhua.” Only literature focusing on these topics was considered relevant, leading to a total of 13 articles. Based on this content-oriented contextualization, a thematic interview guide was developed, addressing major issues revolving around the subjects of North Sea cod, regime shifts, tipping points, and abrupt changes (Fig. 1, step 2). Major topics of the interview guide thematically considered the regime shift and tipping point concepts, regime shifts in the North Sea, the framing of North Sea cod, its management and policies, and future aspects related to this stock. These topics were chosen to conceptually tackle the complexity of regime shifts (from linear to discontinuous, from fast to slow) as they developed in the scientific literature (Beaugrand et al. 2003, Kenny et al. 2009, Sguotti et al. 2019), while the issue of regime shifts in North Sea cod was used to contextualize and reify the concept (Sguotti et al. 2019).

To be more precise, the interview guide included questions touching upon what the regime shifts, tipping points, and abrupt changes concepts imply (see Appendix Table SI1). The North Sea, as territorial entity, was used to spatially situate the experience of such changes and assisted in reflecting on its causes and effects of the current state of North Sea cod and how it has changed over time. Finally, the questions dealing with the management of North Sea cod stock and its development aimed at developing a probable assessment of estimated futures mirrored against the content discussed during the interview. Each interview ended with the opportunity for interviewees to address further topics not raised during the interview (see Appendix Table SI1).

To thematically explore the field and the people associated with the topic of regime shifts in the North Sea cod stock, a screening of newspaper articles was performed (Fig. 1, step 3). The search for articles was performed in newspaper archives by using the word “Atlantic cod” (in German: “Kabeljau”), which was conceived as a central semantic node for the news coverage. The publishing outlets considered were weeklies and daily-appearing German newspapers with a national and regional North Sea focus. The archives dated back to the mid-1940s, which ensured historical consistency while also covering the increase of cod in the 1960s, as well as the collapse taking place from thereon. In total, 20 newspaper articles were included. From these articles four stakeholder types related to North Sea cod were identified: people involved in fisheries, decision-makers and managers on various institutional levels, as well as scientists and environmentalist.

All interviewees were chosen according to a quota sample, which represents a basic ingredient in the context of the purposive sampling method, and which implies that selected subgroups were chosen on the basis of certain features determined by the first author. Thus, interviewee requirements were defined beforehand and against the background of the scientific literature analyzed to assure a balanced representation of all relevant stakeholder groups (Dawson 2009). Relevant interviewees had to fulfill the following requirements: (1) currently working or have been working on the topic of North Sea cod or is/was associated with North Sea fisheries, (2) associated with North Sea cod and North Sea fisheries for at least three years to ensure that they were familiar with the topic, and (3) belonging to one of the four relevant stakeholder groups identified in the media analysis. Even though the experts interviewed were involved in North Sea topics, they also often had knowledge regarding other areas, such as the Baltic Sea, because of their educational backgrounds, e.g., working on Baltic Sea projects as well. Final interviewees were selected by (1) the first author’s knowledge regarding the interviewees, (2) the third author’s knowledge regarding the interviewees, and (3) the four major stakeholder types.

In total, we interviewed 18 stakeholders with a strong focus on Germany (Fig. 1, step 4, Fig. 2), whereas all management, fisheries, and environmental non-governmental organizations (eNGOs) representatives were German, and two scientists who were from abroad (Table 1). Because of the COVID-19 pandemic, interviews were not conducted on site and therefore held online with the help of Zoom software. Also, interviewing active fishers turned out to be difficult because of the pandemic, as on-site visits were impossible; only one former fisher could be interviewed. On request, major topics to be addressed in the interview were provided prior to the interview for preparation purposes. All interviews were performed by the first author from January to March 2021, voice recorded (lasting between 25 and 60 minutes), and transcribed verbatim.

Data coding and analysis

To understand semantic structure of the regime shift concept, we applied a qualitative approach by screening and coding interview data, and by performing a sequential content and thematic analysis (Dawson 2009, Bhattacherjee 2012). This consisted, first, of an iterative and separate reading of four chosen interviews (each from one stakeholder group) by three authors (AMB, HS, MD) to cooperatively develop preliminary categories for the analysis (Fig. 1, step 5; Appendix Table SI2). Detailed descriptions of each category were carried out upon compliance between the authors (AMB, HS, MD) to create an empirically informed basis for analysis and to secure intercoder reliability (Saldaña 2015). Then, these categories were critically assessed by expert knowledge of the authors (AB, CM). This iterative process resulted in a total of 12 analytical categories, which were divided into four main topics: (1) regime shift, (2) North Sea and related impacts, (3) North Sea cod, (4) North Sea cod - collapse and management (Table 2; Appendix Table SI3). Based on these categories, an analytical guide was prepared for further analysis.

In a second step, the 14 remaining interviews were evenly distributed among three authors (AMB, HS, and MD) implying that each author analyzed at least one interview from one stakeholder group (Fig. 1, step 6). These interviews were coded accordingly by using an analytical guide as a coding scheme, assigning the categories within the text (Dawson 2009, Bhattacherjee 2012). Regular feedback rounds were held between the authors to ensure mutual coding reliability. During a final discussion of this step, the authors decided to focus on the main categories of regime shift, North Sea and all impacts, and North Sea cod, as they consistently contributed to the research question (Fig. 1, step 7). The last topic (North Sea cod - collapse and management) was removed as it appeared to be semantically out of focus for this study. The topic entailed information whether the stock was seen as collapsed or not and how the management reacted to transitions in the stock. Based on the questions asked, answers within this topic did not contain relations to regime shifts, tipping points, or abrupt changes and solely focused on management implications (Appendix Table SI3).

In a third step, each author was assigned one main topic, each with three categories, for an in-depth analysis (Fig. 1, step 8). By reusing the inductive rationale for the detailed analysis, the major categories containing all material were divided into further sub-categories (Dawson 2009, Gläser and Laudel 2010). General groups were created at a first reading and then sub-categories were developed by re-reading them. These sub-categories represent the detailed knowledge of each interviewee and exhibit their understanding of the topics of “regime shift,” “North Sea and related impacts,” and “North Sea cod” (Fig. 1, step 9). At the end, the three sub-categories regime shift, tipping points, and abrupt change (Table 2), all stemming from the “regime shift” topic, were determined to provide a semantically structured and content-related overview of the regime shift conceptualization and reveal convergences and divergences concerning the depletion of the North Sea cod.

RESULTS

The analysis of the interviews revealed many topics revolving around (1) regime shifts, (2) tipping points, and (3) abrupt changes. Because of this first result, the three main concepts form the center of our study in which we analyze their conceptual structure. Emphasis is put on theoretical convergences and divergences explained in terms of consequences for the North Sea cod as a paradigmatic real-world example.

Regime shift concept in perspective

Our analyses show that a discourse exists around the regime shift concept within a wide range of stakeholders. Interviewees have different perceptions about the definition of the regime shift concept, what it entails, when exactly a regime shift can be determined as such, which timing is needed, and what consequences result from it. Interviewees find it difficult to use the concept (Interviewee code I14, I12), since it is widely applied as a buzzword and not so much because of its semantic or theoretical content (I12). Hence, a big concern raised by the interviewees lies within the definition of the concept. Interviewees stress that a precise definition is difficult to grasp as the word “regime shift” is often used without an adequate explanation of underlying causes (I12) or causal relationships (I9, 12). Such a lack of a clear definition hinders the determination of what event qualifies as a regime shift (I3, I15).

Regime shift knowledges

The regime shift concept is widely applied in science nowadays, and still, knowledge about it is diversely dispersed across different interview partners. From the interview analyses, three types of knowledges came forward: (1) non-knowledge, (2) general knowledge, and (3) detailed knowledge (Fig. 3):

1. Non-knowledge (Fig. 3a) is expressed as simply not knowing anything about the concept, never having heard about it before, or not knowing details (Fig. 3a, I1, I3, I7). Non-knowledge was represented by three interviewees (management: 2, eNGO: 1). These interviewees were given a regime shift example by the interviewer (i.e., the change from an oligotrophic to eutrophic lake) as an input. Based on this example one interviewee revised his/her former non-knowledge by comparing the newly gained knowledge about a social regime shift to the abrupt occurrence of the COVID-19 pandemic and the rapidly occurring home office situation (I7).
2. General knowledge was expressed by seven interviewees (management: 2, eNGO: 2, science: 2, fisheries: 1; Fig. 3b). Here, the regime shift concept is known and often described in terms of an abrupt change (I2, I4, I5, I9, I13, I14, I16). The main drivers associated with causing a regime shift are the warming temperature effect induced by climate change and fishing pressure. These drivers can cause shifts on various system levels, causing an impact on ecological processes. Transitions in species composition (change in dominance of species) were conceived as the major effect of a regime shift (I2, I5, I9, I13), followed by changes on the whole ecosystem level (I2, I9), the fish population level (I4, I14), or in terms of a shift northwards in the North Sea (I5, I16). For fish populations, both an increase and a decrease in the stock size were considered as a regime shift (I5, I14). On an economic level, the COVID-19 pandemic is mentioned as a regime shift, having an effect on the fishery companies (I13).
3. Detailed knowledge was represented by four interviewees (science: 3, eNGO: 1; Fig. 3c). Regime shifts are here associated with a shift of the system from one state toward another indicating that the existence of multiple stable states is known (I10, I11, I12, I15). Tipping points are here associated with the point at which a system actually undergoes a change (I11). Moreover, steadiness of the different states is described by one interviewee, highlighting that a system transforms from a steady state through a non-steady state to a new steady state (I10). More knowledge is expressed by describing these transformations between system states with the use of words such as “discontinuity” and “catastrophic type” (I12). Two other interviewees raise the aspect of “irreversibility” (I11, I15), which describes the limited possibility for a system in a new state to return to its former system state.

These changes between states can take place across multiple system levels. Interviewees mentioned the ecological process of a regime shift across multiple trophic levels the most (Fig. 3c). Thus, a shift from low to high abundances of phytoplankton causes alterations in the food web, and therefore leads in the end to changes in the abundance of predator fish (I11, I12). Also, changes on population level and in species composition are often articulated (I15). In the context of a cross-disciplinary framing, an ecological regime shift can influence socio-cultural and economic systems, e.g., fishers need to adapt to losses in catches or changes in prices regarding the market supply (I11).

The two main drivers mentioned causing these regime shifts are, as for general knowledge, climate change–induced warming and fishing pressure (I10, I11, I12; Fig. 3c). Interviewees explained that in early stages of the development of the concept, only systemic dynamics were considered to cause a regime shift. The influences and connectedness of external drivers were only included at a later stage of the concept’s development (I12, I15).

Furthermore, regime shifts are distinguished temporality-wise into the past and future accordingly to the drivers associated. In the context of the past, regime shifts are in most cases related to anthropological drivers. The exploitation pressure on the system (here the North Sea) has increased greatly since the Second World War in the past 70 to 80 years (I5). In terms of the future, the effect of climate change is becoming stronger in the past 20 years, bearing increasing pressure on the North Sea (I9). Given increasing impacts, the questions raised lie within possible usage options within the next 10 years, the spatial distribution of species, and how many of them might possibly be caught (I13). Furthermore, if management continues with business as usual at maximum sustainable yield–level (MSY-level), regime shifts will appear more frequently and become more important in defining boundaries for fish stocks than management (I13). The temporal question, however, remains, “whether we as humans will ever experience this state [former North Sea state] again” (I10)?

Interviewees also considered the time span of a regime shift. On the one hand, an event “must happen pretty suddenly [...] to be called a regime shift” (I15), whereas on the other hand a system needs to level off in the new state (I10). It is not clear which time span is exactly defined as a norm for a regime shift. Changes can take “a long time” (I15) or up to “about a million years” like the ice tides (I15). These long-term changes take place “without any particular difference in the dynamics” (I12) and are, despite their rather long time span, still called regime shifts. Hence, what a regime shift represents in terms of its time span appears to be difficult to define (I12).

Regime shifts in situ: North Sea cod and neighboring examples

Interviewees used, in their assessment of the regime shift concept, examples from the North Sea and the Baltic Sea. As indicated in scientific literature (Beaugrand 2004, Capuzzo et al. 2018, Edwards et al. 2020), they tend to refer to a North Sea regime shift in the 1980s with transitions on all trophic levels as a paradigmatic example (I11, I12). Even though this shift is negatively related to young cod survival (Beaugrand et al. 2002, 2003), one interviewee highlights that the shift took place on the plankton level, which does not necessarily imply a regime shift on the cod level in itself (I12).

Related to North Sea cod, the collapse is determined as a regime shift in scientific literature (Sguotti et al. 2019, Blöcker et al. 2023a, 2023b). Here, interviewees refer to the gadoid outburst in the 1960s, the suddenly strong increase of gadoid species, as a regime shift (I2, I14): “the cod stock has never been as high as during the time of the gadoid outburst” (I2). In contrast to the scientific literature, two interviewees do not agree in defining the subsequent decrease of North Sea cod as a regime shift (I2, I12), and another interviewee stated that overfished fish stocks have never been considered as regime shifts in his work (scientist I13). Those who frame the increase through the gadoid outburst as a regime shift conceptualize the decrease as a decline to former normal levels (I2, I12).

In contrast, some interviewees see the decline of the cod stock as a regime shift, but divide it into internal and external factors and dynamics (I5, I15, I16). Internal factors relate to changes on population levels, where the interplay of herring and cod stocks is mentioned by one interviewee (I15). because herring preys on cod eggs and larvae, the strong herring fishery after the Second World War led to an increase in cod. Subsequently, fishing on the depleted herring stock was reduced, inducing an increase in herring and, consequently, a cod decrease (I15). External dynamics causing the regime shift are related to overfishing (I5, I15, I16, I17, I18), whereupon the cod stock “may never come back to the same extent” (I15). Next to the shift on population level, a climate-induced spatial shift northwards takes place by increasing water temperatures of the southern North Sea (I16). Climate change shifts cold water–preferring species, like cod, northwards and enhances the introduction of new species from the southern North Sea (I2, I5, I14).

Even though interviewees expressed themselves regarding a regime shift related to North Sea cod, most interviewees only disclosed knowledge after they were asked by the interviewer. Examples from the neighboring Baltic Sea appeared to be more illustrative and functioned as a conceptual background (I2, I4, I9, I13). Here, the predominant example concerns the cod-sprat relationship, where a regime shift occurred from a “cod dominated system to a sprat dominated system” (I13). As cod preys on sprat egg, a high fishing mortality on cod reduced the pressure on sprat, which in return increased (I13). Another example of a regime shift is related to the Baltic herring stock, which experiences a prey mismatch because of increases in sea temperature and is therefore not performing well (I9).

Consequences and adaptations

Based on the mentioned examples of regime shifts, interviewees highlighted consequences from and adaptations to shifts. A major consequence of ecological regime shifts are alterations in the social-economic system (I2, I13, I14, I16, I17, I18). The 1960s gadoid outburst led also to a strong increase of haddock, which was therefore heavily fished by Scottish fishers. Because of contracts between fish factories and Norway, who also caught haddock, Scottish fishers could not land their fish and “discarded, an estimated 70,000 tones” (I14). Moreover, increases in one species can result in switches of the target species, and therefore in the technical and economic orientation of a fleet. Irish and English fishers started, for example, fishing for edible crab in the German Bight using traps, because of a strong crab increase (I14).

The reduction of the high-quality food source North Sea cod stock and its spatial drift northwards led to further severe changes in fishing fleets (I16, I18). From 1993 to 1996, the German fishing fleet underwent a switch from several small vessels, fishing at Heligoland, to fewer large vessels, which were capable of reaching the more northern fishing grounds (I16). Several fishing companies were no longer able to generate sufficient revenue because of fleet downsizing and the spatial shift of cod, finally forcing them to go out of business (I16, I18). Not only changes in the fish population level but also regime shifts in the North Sea species composition (because of climate change) appear as short-term threats to fishing companies “because it is unclear what will happen afterwards” (I13).

Generally seen, regime shifts raise adaptation questions for management. Questions such as “how should we deal with new species? How shall they be managed in the long-term?” are now stressed (I13). Hence, adaptation in management is needed (I4, I9, I5, I13) because a regime shift can cause irreversibility to the former state. The fisheries management system needs to adapt with new sustainable measures, adjusting fishing quotas and fishing reference levels to the new situation, which could mean lower catch levels (I9). In the case of North Sea cod, fishers have to prolong their fishing trips to reach new fishing grounds to fish their full quota, and if they do not manage to do so they lose the quota in the next year (I13), reducing their planning capabilities and therefore their livelihood security (I4).

To summarize, one can say that the concept of regime shifts is permeated by quite a variety of subordinate aspects that exhibit the heterogeneous semantics and meanings of what the concept represents, stands for, and/or is supposed to be. The concept is perceived differently among the stakeholder groups, with non-knowledge and general knowledge being distributed evenly among the stakeholder groups. Detailed knowledge, however, was mainly existent among scientists. This is no surprise, as scientists were selected on the basis of their North Sea cod work, which is scientifically framed as a prestige example that experienced consequences of a regime shift (Beaugrand et al. 2002, 2003), or a regime shift itself (Sguotti et al. 2019, Blöcker et al. 2023b). This shows that the regime shift concept, a scientifically based concept, appears to be communicated only within a scientific bubble with nearly all of its characteristics, whereas outside this bubble, the concept is rather limited to the basic understanding of a system changing because of external factors.

Conceptual dimensions of tipping points

The analysis regarding the conceptual structuring of tipping points clearly exhibits that it is not widely known among stakeholders. Only four interviewees (management: 2, eNGO: 1, science:1) referred to the definition of the concept and applied it to examples while also criticizing its application on various levels. They (I2, I4, I10, I12) converge in a generic definition of the concept by depicting a tipping point simply as the point where a system changes to another system state. However, differences can be located in detail: two interviewees underlined that tipping points are catastrophic (I12) and determine the threshold until which a system’s resilience is still sufficient to buffer existing disturbances (I10). To explain these aspects, interviewees refer to examples in abiotic and biological phenomena (I2, I10). Abiotically seen, temperature can be understood as an important tipping point (I10) because a system remains in a supposedly stable state until temperature rises to a certain threshold (the tipping point), where “cascades are initiated which cause an abrupt change in the regime” (I10). Biologically seen, the idea of a tipping point is the point at which pelagic fish species are more dominant than benthic species in the system (I2).

Furthermore, the use of the tipping point concept is critically assessed (I12) by various interviewees. Not only the concept in principle, but also underlying aspects, such as its temporality, inherent dynamics, and relevance for management, are debated (I4, I10, I12). As in the case of the regime shift concept, the notion of a tipping point has a high attractiveness to being used because of its conceptual and semantic specific non-specificities. It appears to be a fancy word and “people like the sound of it” (I12) while the notion is used to indicate the point or threshold of an abrupt change (I2, I10, I12) in which the temporal scale is difficult to justify (I10, I12). Hence, “what does abrupt [in an ecological system] mean?” (I10). Ecological changes take place at a considerably slower temporal scale than all of a sudden (I10, I12), and rather represent gradual dynamics in reality than rapid changes (I12). Because of these temporal and causal inconsistencies, some interviewees conclude that the underlying dynamics triggering a tipping point are hardly understood (I4, I12). This can, for example, be seen in the fact that scientists, generally speaking, relate climate warming to tipping points, but do not clarify whether it directly or indirectly affects the system under scrutiny (I4), even if it is known that warming, and aspects such as invasive species, are involved in causing change. However, a detailed picture of the effects appears to be lacking (I4) that might have contributed to the fact that the concept of tipping points is not high on political agendas (I4).

To summarize, the tipping point concept is not strongly defined within the stakeholder groups, but apparent in all but fisheries. Its application is difficult because of its elusive character, such as differences in temporality, which hinders the ability to precisely define this concept.

Abrupt changes concept across space and time: temporality–cause–effect–response

From the interviews with different stakeholders, it becomes clear that abrupt changes appear to be the most straightforward concept. They are described in various ways by interviewees (management: 4, eNGO: 5, science: 1). Still, one should keep in mind that this does not reflect how well the concept itself is known or defined among interviewees because no interviewee question explicitly aimed at this concept’s definition or knowledge.

One aspect clearly came forward from the interview analyses: abrupt changes appear to be differently framed in their temporality, causes, and effects, and the response(s) that follow them. Moreover, the concept is considered at different governance levels (e.g., supranational, regional) and can be explicitly applied to different marine ecosystems while it can also be expanded to disciplines and fields, such as ecology, economy, and social issues (“an abrupt change for all of us was of course COVID,” I3).

Regarding the concept’s definition, an abrupt change is conceptualized by interviewees as a process that happens not in a short- but in a medium- to long-term period (temporalities) (I1, I5). The speed at which an abrupt change takes place depends significantly on the strength of its causes, with climate change and fisheries cited as the main drivers (I9). They lead to changes “that nature, and even less fisheries, can handle” (I5). This includes impacts on plankton in general (I11); the size of fish stocks, such as total biomass (I5); and the interactions within the whole food web (I9). Considering the concept of abrupt change in general, response is related exclusively to EU fisheries policy and management pictured here as the adjusting screw of the whole (I4), with a temporal distinction being made between short-term (“how to respond politically”) and long-term measures (I4).

The four dimensions identified (temporality, cause, effect, response) are applied to examples from (1) the Baltic Sea and (2) the North Sea (regional scales):

1. In the Baltic Sea, a short-term change in drastic quota reductions for cod and herring is described (temporality), without clarifying what causes this change. It only explains that this abrupt change can be equated with drastically reduced fishing opportunities for cod and herring (effect), which has led to a “dilemma” in the Baltic Sea fishery (I3). Hence, it remains open to discussion how fisheries can or should be helped to deal with this dramatic situation (response).
2. In the North Sea, abrupt changes entail North Sea fish stocks (strong collapses in the biomass and productivity) (I7, I6, I11) and plankton in general (I11): they are in themselves depicted as changes, as well as their interaction within the entire food web (I9). Two abrupt changes took place in (temporality) the late 1980s to early 1990s (I7, I6, I11) and the late 1990s to early 2000s (I2, I11). Contrary to the statement that abrupt changes happen in the medium to long term, one interviewee described this process with a special short-term focus on the decline of North Sea fish stocks (“within a short time,” I7, I6). However, the causes “are partly understood” (I11), with overfishing (“due to overfishing the stocks have completely collapsed”) being identified as one of the most important drivers (I7, I6). The response to abrupt changes is viewed in relation to the social-ecological system. The abrupt changes caused, on the one hand, a surprise among fishers, but did not lead to an adjustment of their fishing behavior (socioeconomic response, I7, I6), while on the other hand a change in the entire food web (ecological response) is mentioned (I9).

Apart from a regional scale, the analysis of the abrupt change concept also reveals a focus on the supranational level, i.e., the EU Common Fisheries Policy. Here, both the discard ban and the sharp reduction in fishing opportunities (also known as total allowable catch, TAC) for North Sea cod are understood as an abrupt change by the interviewees (I10, I4) (effect). An interesting aspect to note is that there is a legitimization of knowledge. Thus, either the field of work in general (“at least for the field I work in,” I10) or in particular (“from a fisheries policy perspective,” I4) are used to legitimize the respective statement. In EU administration, temporality of the abrupt changes is limited to the “recent past” (I10), e.g., to a period of the last 10 years (“I can’t say how things have gone in the last ten years,” I4). Nevertheless, the cause of these changes is not delineated further and consequently remains open. It is only mentioned that an abrupt change in the management system was preceded by “years of arguments and discussions until changes in the regime were decided” (I1). In this context, those responsible for EU fisheries management are sharply criticized (I1), as action was only taken (abrupt changes in the EU fisheries system) when it was already almost too late in terms of time (I1).

To conclude: the concept of abrupt changes is considered and structured in different dimensions (environmental, economic, social), which are not clearly related to specific interviewee groups. Particularly when the concept is depicted against a regional background, there exists a comprehensive knowledge about the time frame, inherent causes, and reactions to the changes among all stakeholders. In this view, the concept is perceived in positive (e.g., introduction of the discard ban) or negative ways (e.g., reduction of fishing opportunities), whereas abrupt changes on other scales (e.g., shift of species as in the North Sea mackerel and herring stock) are linguistically presented in more neutral terms nevertheless leading to responses often depicted in negative terms (e.g., conflict between fishing nations). Finally, a remarkable distinction exists in framing of ecological and management related temporalities: the latter is temporarily relegated to the recent past whereas the former is often depicted as lying further in the past.

Our results show that the three concepts are perceived and framed differently among stakeholders (Table 3). Whereas the regime shift concept’s general knowledge is represented by all stakeholder groups, detailed knowledge is clearly present within the science group. The tipping concept, in contrast, is defined by only four interviewees. The abrupt changes concept appears to be the most straightforward and even though no interview question clearly asked for a definition, all stakeholder groups except fisheries used this concept in their answers. Similarities among the concepts were found regarding factors such as temporality or the contexts in which they are applied. The temporal scale on which these concepts happen are not clearly defined as they are considered to be processes in, for example, ecology over time. But the concepts are applied in not only ecological but also social, economic, and governance contexts (social-ecological system).

DISCUSSION

For the first time, we performed a socio-conceptual analysis of the various understandings of regime shifts, tipping points, and abrupt changes. Our analysis revealed that all three concepts are diversely framed and depicted by the groups involved, such as scientists, environmentalists, decision-makers, and those directly involved in fisheries. Whereas the knowledge types for the regime shift concept vary among the stakeholder groups (detailed knowledge existed mainly among scientists, and general knowledge is evenly spread among stakeholder groups), tipping points are seen rather as elusive, including criticism in their application. Abrupt changes, in contrast, appeared to be more straightforward, being applicable to several real-life examples. Outside the scientific domain, the regime shift concept is known for its basic processes (e.g., abrupt changes), which is also reflected by the strong representation of the abrupt changes concept among interviewees. However, whether a change is functionally assigned to one of the concepts or not depends on how temporality, drivers, and consequences are perceived.

The regime shift, tipping point, and abrupt changes concepts were identified as black-boxed and as boundary objects at the same time. The black box of the regime shift notion appears to be closed, with its semantic, analytical, and pragmatic content largely established and supposedly agreed upon. However, this is not the case, as our analysis indicates, even though some of our interview partners assumed that it can be used and understood without further elaboration. This clearly indicates that these concepts are not used because of their semantic validity or analytical accuracy, but because of the support of scientists, stakeholders, decision-makers, and managers involved in and devoted to it. Hence, the social use of the notion stands above its actual meaning or epistemological value (Latour 1987). All three concepts are used within various contexts (e.g., social, economic, management, ecology) without applying one single clear explanation (black-boxed) (Latour 1987, Jasanoff 2006), and still, these concepts function as translators, social bridges, or boundary objects among those involved in the framing of the concept, such as different scientific disciplines and stakeholders holding various epistemological backgrounds (Star and Griesemer 1989, Kull et al. 2017). Communities around a certain issue are created while supporting the development of more or less clear analytical concepts (Bowker and Star 1999). This is only possible because of the weak structure of the concepts when used on the one hand, but their strong structure when used by those individually involved with boundary objects on the other hand (Star and Griesemer 1989). However, boundary objects are temporary entities. In terms of stable meanings, managing their content is key for the development and maintenance of semantic consistency, as well as for communicative coherence across those involved in questions revolving around regime shifts, tipping points, and abrupt changes.

We have, moreover, empirically shown that three dimensions of knowledge concerning the detail of the regime shift concept exist among the interviewees, ranging from non-knowledge to general knowledge to detailed knowledge (Fig. 4a, knowledge).

Whereas the former implies no knowledge about the regime shift concept at all, the second and third include more and more detail by the inclusion of abrupt changes (abrupt changes are here seen as being an entity included in the regime shift concept; the differentiation to the abrupt change concept itself is not made) and multiple system states, respectively. Similar differences in knowledges emerge for the tipping point and abrupt changes concepts, where a generic definition is agreed upon, but discrepancies in detail were revealed by our analysis. Differences in these knowledges evolve from cultural beliefs, social values, norms, and from human experiences generated by the interaction with the environment in which our interview partners engage with them. Humans frame and perceive the environment in various ways and consequently frame changes in them differently (Sterling et al. 2017, Schwermer et al. 2021a). Thus, the concepts used to capture and explain change semantically vary or differ.

Another commonality is the discourse revolving around temporalities within the concepts’ understanding (Fig. 4a, temporality). Temporality and temporal dimensions are defined by each person individually, and imply how time is individually perceived, causally assigned and conceptually expressed, such as the past, present, and future. Time itself is then practically applied to something or used for framing something (Caldas and Berterö 2012) and our analyses exhibited that no agreed-upon time-concepts for the processes of abrupt changes, tipping points, and regime shifts exist among our interview partners. Whereas regime shifts are understood as processes in time, abrupt changes and tipping points rather imply particular moments in time. One might need to ask here: what is abrupt? What is a tipping point? Abrupt can be defined as “sudden and unexpected” (Oxford Learner’s Dictionaries 2022a) and point as “a particular time or stage of development” (Oxford Learner’s Dictionaries 2022b). The interviews revealed that abrupt changes at or within certain points in time do not simply exist, while some interviewees state that in theory abrupt changes and tipping points shall take place rather fast and suddenly. However, they also highlight that, in reality, ecosystems underlie certain drivers and processes that can undergo short- to long-term changes. Hence, from a socio-conceptual point of view, there are no such things as abrupt changes and tipping points at a particular point in time, referring to the difficulties in apprehending and applying the concepts.

Seeing abrupt changes and tipping points as processes in reality partly challenges the use of quantitative methods in science, where tipping points and regime shifts are properties of models (Fig. 4a, scientific methods). Detection methods such as change point analyses, for instance, reveal abrupt changes in a time series, but set the points of change to particular points (e.g., certain years) in the time series (Erdman and Emerson 2007, Killick and Eckley 2014), which entails that these temporal processes elaborated by interviewees cannot be directly found. The question hence remains: which of these abrupt changes detected is then a tipping point or abrupt change in the regime shift context? It remains with the scientists’ tentative judgement to decide what it actually is. Hence, from a quantitative method point of view, abrupt changes and tipping points theoretically align with their definitions and occur suddenly at a certain time (Erdman and Emerson 2007, Killick and Eckley 2014). The comparison of the stakeholder framing with the scientific methodological approaches, furthermore, shows that there is a discrepancy in the framing of abrupt changes, tipping points, and the scientific possibility of detecting them. Therefore, conceptual reflection is needed to assess tipping points and abrupt changes in terms of their temporality. The awareness that abrupt changes and tipping points are not certain points in time, but rather time spans, needs to be considered to enhance compliance about certain sudden events. Both, quantitative (e.g., change point analyses) and qualitative data (e.g., interviews with stakeholders) as well as their analyses, could be included if a method standardization for regime shift detection is developed. The focus is then put on how to align different framings cooperatively (Star and Griesemer 1989). In addition, it is crucial that the terminology of concepts like tipping points, abrupt changes and regime shifts is applied consistently. Definitions for their specific use in certain contexts (e.g., species, organizational level, drivers, ecosystem, time scale) shall be provided and used persistently throughout the study cases where they are applied. In this way, knowledge about the boundary object in question could become more consistent and could assist in translating and coordinating the meaning of important concepts and their implications between science, policy, administration, and stakeholders (Star and Griesemer 1989). Through their involvement in scientific projects, stakeholders can for example make valid contributions to increase the relevance and robustness, as well as the understanding and acceptance, of scientific results outside the academic world (Figure 4b, stakeholder engagement) (Köpsel et al. 2021).

As we have seen, the elusiveness of the concepts investigated here makes their complexity difficult to grasp, and their application to case studies challenging. We showed that changes in the North Sea, like the decline of North Sea cod, are considered as a regime shift in scientific literature (Sguotti et al. 2019, 2020), but not among all interviewees (Figure 4a, examples used). Hence, differences in perception and assessment of these concepts exist as based on interviewees’ backgrounds and knowledge, even though the example presented (North Sea cod collapse) was the same. These differences hold implications for fisheries management. Fisheries management needs to react to changes in the marine system to provide a governance approach that sustains the social-ecological system (Ostrom 2009). A common understanding, if changes in the marine realm are considered as regime shifts, is crucial to adapt measures in cases of hysteresis and irreversibility (Figure 4b, common understanding) (Scheffer et al. 2001). Decision-makers need to weigh whether management measures will support a new regime, or force, if possible, the transition back to a former state. If the northward shift and the introduction of southern species in the North Sea (Beare et al. 2004, Petitgas et al. 2012, Baudron et al. 2020), for instance, are considered an irreversible regime shift, conflicts among EU fishing nations may arise and a new quota policy and new policy regimes may be required. In addition, another challenge consists in the fact that systems per se constantly fluctuate and are not fully stable (Möllmann et al. 2015). The collapsed North Sea cod, shown here as a case study, is currently in such an unstable, low state. Recovery might be possible if fishing is reduced to sustainable levels and environmental conditions change for the better (Blöcker et al. 2023a). However, management is currently not incorporating regime shift dynamics sufficiently to react in time or even proactively to such abrupt changes leading to possible irreversibility (King et al. 2015, Levin and Möllmann 2015, Möllmann et al. 2021). As for North Sea cod, strong management regulations were implemented reactively, after the stock had already declined (Blöcker et al. 2023a, 2023b). As shown in our results, this reaction is not necessarily due to non-knowledge of the possibility of regime shifts, as decision-makers indeed know about their existence. But it appears rather to be difficult to integrate abrupt dynamics into fisheries management processes, even if these abrupt dynamics are taking place over several years and can be seen, rather, as short- to long-term changes: hence, temporal processes as identified by the interviewees. Here, a high flexibility in management is necessary (King et al. 2015, Levin and Möllmann 2015, Blöcker et al. 2023b). The total allowable catch (TAC) is based on stock assessment, where environmental variability of predicable, frequently occurring events is included. Less frequent effects are included to a lesser extent (King et al. 2015). Not only the scarcity of long time series, but also the lack of understanding of linkages of underlying processes, make the detection and inclusion in management difficult (King et al. 2015, Levin and Möllmann 2015). This way, flexible management measures to prevent fish stocks, like North Sea cod, from a collapse and therefore from a possible irreversible regime shift are difficult and simply applied too late (reactively). Also, multi-annual plans, such as the North Sea cod recovery plan, limit a flexible adjustment to abruptly changing stock dynamics due to being in force long-term (EC 2004). However, reducing fishing mortalities to sustainable advised reference levels would be a first attempt for a foundation for North Sea cod recovery (Blöcker et al. 2023a, 2023b). It is useful to quickly determine whether perceived changes in systems are indeed regime shifts to develop appropriate measures quickly. Together with the inexplicit time spans defined within a regime shift, and its related concepts, this clearly calls for a more flexible management approach (Schwermer et al. 2021b).

Furthermore, our study was conducted in a German context, with most interviewees being German. This means that the results outlined here are, until cross-cultural research is undertaken, culturally and topically bound. One has to bear in mind that besides the fact that scientific concepts are conceived as internationally shared, they are in many cases littered with contextual implications and disciplinary rationales because science does not take place in a decontextualized vacuum. Aspects such as the references to the situation in the Baltic Sea as a comparison indicate this while further research might be needed to check whether different concepts or understandings of regime shifts, abrupt changes, and tipping points exist in the same or in other systems. There are, hence, nationally bound limits implicated in the scientific validity of our paper because of the data gathered which call for further research. The method and analysis applied, however, build on and represent a solidified social study of science approach that displays enough systematic rigor and conceptual precision to be applied to comparable or even other scientific disciplines, systems, or management contexts.

Regime shift dynamics appear in diverse marine ecosystems worldwide and need to be incorporated into ecosystem-based management to deal with probable changes on time and to determine if measures shall be implemented before, during, or after a regime shift (Levin and Möllmann 2015). Fisheries management needs to consider the entire social-ecological system (Ostrom 2009) and take the various temporalities into consideration. This can be done with ecosystem-based fisheries management (Westley et al. 2011, Long et al. 2017), namely the sustainable exploitation of fish stocks according to sustainable reference levels and thus the preservation of the fishers' livelihoods as well as the fish markets (Fig. 4b, sustainable fisheries management).

CONCLUSION

We have explored the conceptual framing of the regime shift concept and the related concepts of tipping points and abrupt changes by highlighting that these concepts are semantically framed in various and different ways among stakeholders. Our analyses showed that differences in the detail of knowledge, the examples used to explain the concept and in the perception of time and temporality hold the potential to induce considerable misunderstandings for sustainable fisheries management, such as for Atlantic cod in the North Sea. Hence, the increased use of the concepts in marine science (Sguotti et al. 2019, Möllmann et al. 2021, Arif et al. 2022, Stockholm Resilience Centre 2022) demands for an improved and conceptual clarity to enhance a sustainable fisheries management. Furthermore, the frequent use of these concepts in media in the past decade, mainly due to the anticipated effects of climate change on the planet (van der Hel et al. 2018), promotes the use of these concepts among various stakeholders and is an urgent pointer to create a more commonly shared understanding. The frequent use of these concepts in media in the past decade, mainly due to the anticipated effects of climate change on the planet (van der Hel et al. 2018), promotes the use of these concepts among various stakeholders and is an urgent pointer to create a more commonly shared understanding. Such an endeavor holds the potential to provide a basis for an improved and above all shared ecosystem dynamics understanding in science and society, and pave the way toward acceptable management measures, which could support the sustainable use of marine resources (Schwermer et al. 2021a).

In contrast, the perspective among stakeholders is of having a common understanding about and similarity of the regime shift and its related concepts, whereas in reality they do not, can conceal differences in patterns and beliefs, as well as hide the diversity of thoughts around non-linear dynamics (Goguen 2005, Milkoreit et al. 2018). Hence, knowing that different knowledges about these concepts exist is crucial to think about how a commonly shared understanding about how and why regime shifts happen should or can be developed. Localized respectful and trustful communication among stakeholders can be enhanced, if the awareness of different conceptual framings (as based on varying framings around these concepts) is raised and if these differences are taken seriously into consideration and are discussed (Sterling et al. 2017).

The regime shift concept and its related notions of tipping points and abrupt changes remain without one clear definition but with different framings depending on the context they are used in. We have shown that these concepts are not yet standardized and might require more time to develop into improved definitions, with consistent methods by also including stakeholder knowledges for the support of stakeholder empowerment in local level institutions (Schwermer et al. 2021a). Hence, the heuristic or shared thinking around the regime shift, tipping point and abrupt changes concepts and their application has to be unraveled into, e.g., underlying meanings, causalities, drivers, and consequences to develop acceptable fisheries management measures for the sustainable use of marine resources such as fish stocks.

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