Decreasing body size has been proposed as a universal response to increasing temperatures. The physiology behind the response is well established for ectotherms inhabiting aquatic environments: as ...higher temperatures decrease the aerobic capacity, individuals with smaller body sizes have a reduced risk of oxygen deprivation. However, empirical evidence of this response at the scale of communities and ecosystems is lacking for marine fish species. Here, we show that over a 40‐year period six of eight commercial fish species in the North Sea examined underwent concomitant reductions in asymptotic body size with the synchronous component of the total variability coinciding with a 1–2 °C increase in water temperature. Smaller body sizes decreased the yield‐per‐recruit of these stocks by an average of 23%. Although it is not possible to ascribe these phenotypic changes unequivocally to temperature, four aspects support this interpretation: (i) the synchronous trend was detected across species varying in their life history and life style; (ii) the decrease coincided with the period of increasing temperature; (iii) the direction of the phenotypic change is consistent with physiological knowledge; and (iv) no cross‐species synchrony was detected in other species‐specific factors potentially impacting growth. Our findings support a recent model‐derived prediction that fish size will shrink in response to climate‐induced changes in temperature and oxygen. The smaller body sizes being projected for the future are already detectable in the North Sea.
Aim
The negative correlation between temperature and body size of ectothermic animals (broadly known as the temperature‐size rule or TSR) is a widely observed pattern, especially in aquatic ...organisms. Studies have claimed that the TSR arises due to decreased oxygen solubility and increasing metabolic costs at warmer temperatures, whereby oxygen supply to a large body becomes increasingly difficult. However, mixed empirical evidence has led to a controversy about the mechanisms affecting species’ size and performance under different temperatures. We review the main competing genetic, physiological and ecological explanations for the TSR and suggest a roadmap to move the field forward.
Location
Global.
Taxa
Aquatic ectotherms.
Time period
1980–present.
Results
We show that current studies cannot discriminate among alternative hypotheses and none of the hypotheses can explain all TSR‐related observations. To resolve this impasse, we need experiments and field‐sampling programmes that specifically compare alternative mechanisms and formally consider energetics related to growth costs, oxygen supply and behaviour. We highlight the distinction between evolutionary and plastic mechanisms, and suggest that the oxygen limitation debate should separate processes operating on short, decadal and millennial time‐scales.
Conclusions
Despite decades of research, we remain uncertain whether the TSR is an adaptive response to temperature‐related physiological (enzyme activity) or ecological changes (food, predation and other mortality), or a response to constraints operating at a cellular level (oxygen supply and associated costs). To make progress, ecologists, physiologists, modellers and geneticists should work together to develop a cross‐disciplinary research programme that integrates theory and data, explores time‐scales over which the TSR operates, and assesses limits to adaptation or plasticity. We identify four questions for such a programme. Answering these questions is crucial given the widespread impacts of climate change and reliance of management on models that are highly dependent on accurate representation of ecological and physiological responses to temperature.
Increasing sea temperatures are predicted to decrease body size of marine ectotherms based on the temperature size rule. This will impact fisheries yields, but empirical evidence of the process is ...still limited.
We used fishery‐independent bottom trawl survey data from 1970 to 2017 to examine the trends of length‐at‐age of four commercially important demersal fish species (cod, haddock, whiting and saithe) in two study areas facing increasing sea temperatures: the West of Scotland and the North Sea. We then compared the trends of length‐at‐age with annual bottom sea temperatures.
The mean length‐at‐age of adults declined over the study period, in all species and in both areas, except for cod in the West of Scotland. A common trend of decline in adult length was inversely correlated with bottom sea temperatures. Correlations with temperature at seven yearly time‐lags were significant and negative in the North Sea. Correlations were only significant at lags of 1 and 2 years in the west of Scotland, where sea temperature warming was twice as slow.
The mean length‐at‐age of juveniles concurrently increased, which has not hitherto been reported. This trend, shared by all species and both regions, correlated positively with rising temperature, suggesting that our study species have a faster growth rate due to increased temperatures.
Synthesis and applications. We examined the body size of a range of commercially exploited fish species, at different age groups, from two management regions. We found that juvenile fish have been getting bigger and adults smaller in both regions. These changes were correlated with rising sea temperatures, providing empirical evidence that global warming is affecting the size of commercial fish species. The effects of these changes on productivity of fish populations and fisheries yield now require investigation. Temperature changes should, therefore, be included into forecasts used in fisheries science in order to mitigate the impact of global warming and maximise sustainable yields.
We examined the body size of a range of commercially exploited fish species, at different age groups, from two management regions. We found that juvenile fish have been getting bigger and adults smaller in both regions. These changes were correlated with rising sea temperatures, providing empirical evidence that global warming is affecting the size of commercial fish species. The effects of these changes on productivity of fish populations and fisheries yield now require investigation. Temperature changes should, therefore, be included into forecasts used in fisheries science in order to mitigate the impact of global warming and maximise sustainable yields.
Many commercial fish stocks are beginning to recover under more sustainable exploitation regimes. In this study, we document the temporal and spatial changes in one remarkable example of stock ...recovery: northern European hake (Merluccius merluccius). Analysing data from several scientific surveys, we document a dramatic increase in estimates of biomass between 2004 and 2011 throughout the larger area now occupied by the stock. The largest increase occurred in the North Sea, where hake have been largely absent for over 50 years. Spatio‐temporally resolved commercial landings show that high densities occur in the North Sea only between April and September, suggesting a density‐dependent seasonal habitat expansion to suitable temperature and depth conditions. These changes have implications for the management of the stock which are discussed. Notably, if discards are banned as part of management revisions, the relatively low quota for hake in the North Sea will be a limiting factor (the so‐called ‘choke’ species) which may result in a premature closure of the entire demersal mixed fishery in the North Sea, jeopardizing many commercial fisheries in the region. This example of the unforeseen consequences of improved stewardship highlight the need for a more holistic, regional and responsive approach to managing our marine ecosystems.
Climate change is having a significant impact on the biology and ecology of fish stocks and aquaculture species and will affect the productivity within seafood supply chains in the future. The ...challenges are further amplified when actors within the fisheries and aquaculture sectors have very different ideas and assumptions about climate change and what risks and opportunities they entail. In order to address the challenges of climate change, several countries have developed national adaptation plans. However, fisheries and aquaculture are rarely included in these plans, resulting in a general lack of documented adaptation strategies within these sectors in most countries. This paper introduces guidelines for the development of climate adaptation plans (CAPs) within fisheries and aquaculture, applying a co-creation approach that requires the participation of scientists, industry representatives, policymakers, and other relevant stakeholders. The objective is to provide a stepwise approach to facilitate and enable stakeholders to plan strategies toward climate adaptation. The guidelines are based on practical experience and include a three-step process: (1) assessment of risks and opportunities; (2) identification of adaptation measures, and (3) operationalization of CAPs. The three-step process is also part of a larger cycle, including implementation, monitoring, and evaluation, again generating iterative feedback loops over time. Lessons learned are discussed, and we highlight the advantages and challenges of developing CAPs. While the guidelines are designed for and tested within fisheries and aquaculture systems, the CAP approach is also employable for other natural resource-based systems.
•Importance of considering prey-predator interactions when managing fish stocks.•Traditional single stock approach would recover VIa cod but not VIa whiting.•Reduction of juvenile bycatch necessary ...to recover the VIa whiting stock.•Little overall impact of grey seals predation on cod and whiting.
The latest reform of the Common Fisheries Policy (CFP) which regulates the exploitation of fish stocks in European waters entails a move from the traditional single stock management towards Ecosystem Based Fisheries Management (EBFM). Meanwhile the Marine Strategy Framework Directive dictates that Good Environmental Status (GES) should be achieved in European waters by 2020. Here we apply an EBFM approach to the west of Scotland demersal fisheries which are currently facing several management issues: depleted stocks of cod (Gadus morhua) and whiting (Merlangius merlangus), increased predation from grey seals (Halichoerus grypus), and large bycatch of juvenile whiting by crustacean fisheries. A food web ecosystem model was employed to simulate the outcomes of applying the traditional single stock fishing mortalities (F), and management scenarios which explored F ranges in accordance with the CFP. Ecosystem indicators were calculated to assess the performance of these scenarios towards achieving GES. Our results highlight the importance of considering prey-predator interactions, in particular the impact of the top predators, cod and saithe (Pollachius virens), on juvenile cod and whiting. The traditional single stock approach would likely recover cod, but not whiting. Exploring the F ranges revealed that a drastic reduction of juvenile whiting bycatch is necessary for the whiting stock to recover. Predation from grey seals had little impact overall, but did affect the timing of cod and whiting recovery. With the exception of whiting, little difference was observed between the single stock scenario, and the best scenario identified towards achieving GES. The findings advocate for the use of ecosystem modelling alongside the traditional single stock assessment models used for tactical decision making in order to better inform fisheries management.
Mixed fisheries and the marine ecosystems that sustain them are complex entities and involve multiple and potentially conflicting management objectives and stakeholder interests. The presence of ...multiple trade-offs complicates the identification of strategies that satisfy various policy requirements while being acceptable to affected stakeholder groups. This creates a demand for tools and processes that support learning, cooperation and planning. We report on the application of decision support methodology used in combination with a co-creation approach to scenario based planning for the demersal fisheries of the West coast of Scotland. These fisheries face significant challenges, such as the depletion of key stocks and increased predation by seals. In collaboration with stakeholders we identified generic management alternatives and indicators to evaluate their performance in a structured evaluation using Multi Criteria Analysis. We identify the potential and limitations of this approach and suggest how it can contribute to Ecosystem Based Fisheries Management (EBFM). This approach does not provide tactical management advice, but stimulates learning and creates an opportunity for stakeholders to search for strategic and policy relevant solutions in an EBFM context.
Changes in fish distribution are being observed across the globe. In Europe's Common Fisheries Policy, the share of the catch of each fish stock is split among management areas using a fixed ...allocation key known as ‘Relative Stability’: in each management area, member states get the same proportion of the total catch each year. That proportion is largely based on catches made by those member states in the 1970s. Changes in distribution can, therefore, result in a mismatch between quota shares and regional abundances within management areas, with potential repercussions for the status of fish stocks and the fisheries that depend on them. Assessing distribution changes is crucial to ensure adequate management and sustainable exploitation of our fish resources. We analysed scientific survey data using a three‐tiered analytical approach to provide, for the first time, an overview of changes in distribution for 19 northeast Atlantic fish species encompassing 73 commercial stocks over 30 yr. All species have experienced changes in distribution, five of which did so across management areas. A cross‐species analysis suggested that shifts in areas of suitable thermal habitat, and density‐dependent use of these areas, are at least partly responsible for the observed changes. These findings challenge the current use of relative stability to allocate quotas.
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