Warming temperatures and smaller body sizes : synchronous changes in growth of North Sea fishes

Funded by Marine Scotland SciencePeer reviewedPostprin

Comment on "Fisheries Management"

The recent article by O’Leary et al. (2011) raises an important question about the relationship between science and those who manage fisheries. They contend that fishery managers do not give due cognisance to scientific advice and consistently set Total Allowable Catches (TACs) above values advised by scientists (which they define as ‘‘political adjustment’’). The authors claim that the consequence of this is that there is a high probability of stock collapse in the next 40 years. They use a simulation model to argue that this probability may exceed 80% at the mean level of political adjustment adopted by managers, depending on the degree of environmental variability and life history strategy of the fish

Systematic bias in estimates of reproductive potential of cod stocks: implications for stock/recruit theory and management

Stock/recruit relationships, describing the relationship between the parental population and the number of offspring produced, are a central tool in population ecology. For fish populations the stock/recruit relationship uses spawning stock biomass (SSB) to represent reproductive potential of the parental population. This assumes that the proportion of SSB comprised of females and the relative fecundity (number of eggs produced per unit mass) are both constant over time. To test these two constancy assumptions female-only spawner biomass (FSB) and total egg production (TEP) were estimated for the Northeast Arctic cod stock over a 56-year time period (1946-2001). During that time period the proportion of females (FSB/SSB) varied between 24 and 68% and the variation was systematic with length such that SSB became more female-biased as the mean length of spawners increased and more male-biased as mean length decreased. Over the same time period, relative fecundity of the stock (TEP/SSB) varied between 115 and 355 eggs g-1 and, like FSB/SSB, was significantly, positively correlated mean length of spawners. Because both FSB/SSB and TEP/SSB covaried with length composition, SSB is systematically biased estimate of reproductive potential. FSB and TEP were evaluated as possible replacements for SSB in stock/recruit relationship. Both indices gave a different interpretation of the recruitment response to reductions in stock size (over-compensatory) compared to that obtained using SSB (either compensatory or depensatory). The threshold level of stock size below which recruitment becomes impaired was estimated for each of the different stock/recruit relationships using piecewise linear regression. There was no difference between SSB and FSB in the assessment of stock status, however, in recent years (1980-2001) TEP fell below the threshold level more frequently than SSB fell below. This suggests that using SSB as a measure of stock reproductive potential may lead to overly optimistic assessments of stock status

Fishing for MSY: using “pretty good yield” ranges without impairing recruitment

Pretty good yield (PGY) is a sustainable fish yield corresponding to obtaining no less than a specified large percentage of the maximum sustainable yield (MSY). We investigated 19 European fish stocks to test the hypothesis that the 95% PGY yield range is inherently precautionary with respect to impairing recruitment. An FMSY range was calculated for each stock as the range of fishing mortalities (F) that lead to an average catch of at least 95% of MSY in long-term simulations. Further, a precautionary reference point for each stock (FP.05) was defined as the F resulting in a 5% probability of the spawning-stock biomass falling below an agreed biomass limit below which recruitment is impaired (Blim) in long-term simulations. For the majority of the stocks analysed, the upper bound of the FMSY range exceeded the estimated FP.05. However, larger fish species had higher precautionary limits to fishing mortality, and species with larger asymptotic length were less likely to have FMSY ranges impairing recruitment. Our study shows that fishing at FMSY generally is precautionary with respect to impairing recruitment for highly exploited teleost species in northern European waters, whereas the upper part of the range providing 95% of MSY is not necessarily precautionary for small- and medium-sized teleosts.</jats:p

Limiting inter-annual variation in total allowable catch strategies. An application to ICES roundfish stocks

This study evaluated through simulation management strategy that stabilise catch levels by setting bounds on the inter-annual variability in Total Allowable Catches (TACs). An integrated modelling approach was used, which modelled both the ‘real’ and observed systems and the interactions between all system components. The modelling framework therefore allowed evaluation of the robustness of candidate management strategies to both the intrinsic properties of the systems, and the ability to observe, monitor, assess and control them. Strategies were evaluated in terms of level of risk (measured as the probability of spawning stock biomass falling below the biomass limit reference level for the stock) and cumulative yield. The simulation approach used provides a powerful tool for the examination of the performance of candidate management strategies. It has shown that better management is not necessarily going to be achieved by improving the assessement, since even with a perfect assessment (where the simulated working group knew stock status perfectly) stocks may crash at fishing levels that standard stochastic projections would suggest were safe. Also explicitly modelling the assessment process can result in quite different outcomes than those predicted by the simple projection traditionally used by stock assessment working groups. This is because the simple projection assumes that the status of the stock in the current year is known without error and that the target fishing mortality can be achieved without error. However, in practice the assessment is based on last years data and the effect of any management measure on SSB is only manifest, following the implementation of the quota, at the end of the following year. The choice of target and fishing mortality levels and minimum stock levels results from ICES interpretation of the precautionary approach. This lead to the definition of fishing mortality and biomass reference points that are intended to prevent over-fishing and to trigger recovery plans when a stock is overfished respectively. Although, fishing mortality and biomass reference points were originally intended to be independent, a fishing mortality level implies a corresponding biomass level. In the case of saithe a fishing mortality of 0.40 (i.e. the FPA level) would drive the stock to Blim, suggesting that the choice of biomass and target reference points are not consistent for this stock

Limiting inter-annual variation in total allowable catch strategies. An application to ICES roundfish stocks

This study evaluated through simulation management strategy that stabilise catch levels by setting bounds on the inter-annual variability in Total Allowable Catches (TACs). An integrated modelling approach was used, which modelled both the ‘real’ and observed systems and the interactions between all system components. The modelling framework therefore allowed evaluation of the robustness of candidate management strategies to both the intrinsic properties of the systems, and the ability to observe, monitor, assess and control them. Strategies were evaluated in terms of level of risk (measured as the probability of spawning stock biomass falling below the biomass limit reference level for the stock) and cumulative yield. The simulation approach used provides a powerful tool for the examination of the performance of candidate management strategies. It has shown that better management is not necessarily going to be achieved by improving the assessement, since even with a perfect assessment (where the simulated working group knew stock status perfectly) stocks may crash at fishing levels that standard stochastic projections would suggest were safe. Also explicitly modelling the assessment process can result in quite different outcomes than those predicted by the simple projection traditionally used by stock assessment working groups. This is because the simple projection assumes that the status of the stock in the current year is known without error and that the target fishing mortality can be achieved without error. However, in practice the assessment is based on last years data and the effect of any management measure on SSB is only manifest, following the implementation of the quota, at the end of the following year. The choice of target and fishing mortality levels and minimum stock levels results from ICES interpretation of the precautionary approach. This lead to the definition of fishing mortality and biomass reference points that are intended to prevent over-fishing and to trigger recovery plans when a stock is overfished respectively. Although, fishing mortality and biomass reference points were originally intended to be independent, a fishing mortality level implies a corresponding biomass level. In the case of saithe a fishing mortality of 0.40 (i.e. the FPA level) would drive the stock to Blim, suggesting that the choice of biomass and target reference points are not consistent for this stock

Fleet dynamics in fisheries management strategy evaluations

Fisheries managers use scientific evaluations of management plans to determine whether such plans will be sustainable. Most extant evaluations do not account for changes in fleet dynamics in response to management measures, and are likely to be flawed as a result. In this thesis, I develop a new simulation model to address this issue. I present motivating case studies of management strategy evaluations for haddock, survey-based management approaches, and multi-species catch quotas, in order to highlight the need for an improved spatio-temporal fishery modelling framework. I characterise the response of the Scottish whitefish fleet to short-term real-time area closures, as an example of the type of fleet dynamics that a new model would need to be able to simulate for cod in the North Sea. I demonstrate using two complementary methods that such closures are unlikely to have directly encouraged skippers to avoid cod-important areas, and are therefore unlikely to have reduced cod mortality. I develop and implement a new spatio-temporal fishery simulation model which is flexible and powerful enough to account for fleet responses and thereby enable insightful quantitative analysis and evaluation of the wide range of management approaches. Finally, I report initial tests of the model, which demonstrate that a vessel seeking to maximise weekly profit will act differently (and with different fish stock implications) to one that is allowed a maximum weekly catch. With this model and the further future developments of it that I outline, scientists will be in a much better position to advise fisheries managers on stock sustainability over long-term time scales.Fisheries managers use scientific evaluations of management plans to determine whether such plans will be sustainable. Most extant evaluations do not account for changes in fleet dynamics in response to management measures, and are likely to be flawed as a result. In this thesis, I develop a new simulation model to address this issue. I present motivating case studies of management strategy evaluations for haddock, survey-based management approaches, and multi-species catch quotas, in order to highlight the need for an improved spatio-temporal fishery modelling framework. I characterise the response of the Scottish whitefish fleet to short-term real-time area closures, as an example of the type of fleet dynamics that a new model would need to be able to simulate for cod in the North Sea. I demonstrate using two complementary methods that such closures are unlikely to have directly encouraged skippers to avoid cod-important areas, and are therefore unlikely to have reduced cod mortality. I develop and implement a new spatio-temporal fishery simulation model which is flexible and powerful enough to account for fleet responses and thereby enable insightful quantitative analysis and evaluation of the wide range of management approaches. Finally, I report initial tests of the model, which demonstrate that a vessel seeking to maximise weekly profit will act differently (and with different fish stock implications) to one that is allowed a maximum weekly catch. With this model and the further future developments of it that I outline, scientists will be in a much better position to advise fisheries managers on stock sustainability over long-term time scales

A review of fishery-independent assessment models, and initial evaluation based on simulated data

Large uncertainties in catch data (officially-reported landings and discards) are undermining the ability of scientific organisations to provide valid management advice based on the conventional approach of analytical stock assessments. There is thus an urgent need to consider alternative tools that do not depend on long series of precise age-structured catch data. This paper presents four fishery-independent assessment models developed under the EU project FISBOAT (Fishery Independent Survey Based Operational Assessment Tools). It also reports on rudimentary tests based on simulated data, using the same data sets and protocol as an evaluation study conducted by the US National Research Council in 1997. The survey-based assessment models at hand are able to reliably capture the major signal in biomass and recruitment, although they smooth out transient changes. However, they cannot provide absolute abundance estimates, only relative values on an arbitrary scale. The survey-based approaches could provide more rapid updates of the state of stocks than catch-based methods

Deriving condition indices from standard fisheries databases and evaluating their sensitivity to variation in stored energy reserves

To evaluate interstock differences in condition, it would be advantageous to develop stock-level condition indices from standardized databases on weight and length. This study describes a method for estimating stock-level condition when individual-level observations on length and weight are not easily accessible. For each year in a 56-year time series (1946-2001) for Northeast Arctic cod (Gadus morhua), pseudo-observations of weight and length were generated by pairing the Norwegian and Russian values for weight-at-age provided annually to the assessment working group with estimates of length-at-age derived from the same databases. A weight-length relationship fit to each year was then used to predict weight-at-length, i.e., girth, for a range of standard lengths (30-120 cm). This index was uncorrelated with both the liver condition index and the abundance of Barents Sea capelin (Mallotus villosus), suggesting that at the stock level, the girth of cod is not necessarily indicative of the magnitude of stored energy reserves. Partitioning body size into length-at-age and girth revealed long-term trends in body size. In particular, large/old cod showed substantially higher values of both length-at-age and girth that could be the result of long-term increases in fishing mortality