Assessments of many West African fish stocks rely on fishery dependent catch and effort data. Typically, these treat the catch data as error free and some assume that fishing power does not change over time. To address these issues we develop a fleet based surplus production model that accounts for increases in fishing power. It allows errors both in effort and catch data so avoiding the assumption that catch data are exact. Mean annual fleet fishing power increase can be estimated when data from multiple fleets are available provided it can be specified for at least one fleet. The model is tested using simulated data and then applied to western stocks of anchovy (Engraulis encrasicolus) and bonga shad (Ethmalosa fimbriata) in the Fishery Committee for the Eastern Central Atlantic (CECAF) area. Both stocks appear to be over-exploited and near to collapse. Corrections for fishing power are important in the anchovy assessment and help to explain conflicting trends in the data. Uncertainty in the assessments is explored with a range of sensitivity tests.

a) Five stocks of small pelagic fish important to Ghana are assessed for the period 1990-2017. These are based on the stock area as used by FAO that also includes catches from Côte d’Ivoire, Togo and Benin. The stocks assessed include anchovy (Engraulis encrasicolus), round sardinella (Sardinella aurita), flat sardinella (Sardinella maderensis), bonga shad (Ethmalosa fimbriata) and Cunene horse mackerel (Trachurus trecae). b) Ghana accounts for the largest proportion of the total catches for all stocks from the four countries in the assessment unit. Total catches are variable and show some long term decline for four of the five stocks. Catches of horse mackerel have increased. c) Fishing effort data are available for 6 fleets. Effort trends are highly variable but Ghanaian fleets, that account for most of the catch show a marked increase. d) Catch per unit effort data (cpue) suggest that all stocks, except horse mackerel have declined from the 1990 level but may have stabilised or increased slightly in recent years. However, the trends may be over-optimistic as the effort data are not corrected for increases in fishing power. Horse mackerel cpue shows no long term trend. e) A surplus production model is used to assess the stocks using catch and effort data by fleet. The model also accounts for increasing fishing power over time, missing catch data as well as observation errors in the catch and effort data. f) Four of the five stocks show similar downward trends in stock biomass and increasing fishing mortality. They appear to be fished above FMSY and the current biomass is below BMSY. The estimates of FMSY are uncertain but the estimated stock status relative to MSY is robust to different modelling assumptions. The horse mackerel stock shows no clear trend in biomass and while is appears to be fished above FMSY, the stock status is highly uncertain. g) In all five stocks Ghanaian fleets dominate the fishing mortality with the artisanal fleet having a large impact. Côte d’Ivoire makes a large contribution to fishing mortality on sardinella and horse mackerel. Togo makes a significant contribution to anchovy mortality h) Estimates of the rate of increase in fishing power vary between fleets and species. Most notable is the above average increase in fishing power by the Togo artisanal fleet for anchovy and round sardinella, and the Benin artisanal fleet for most stocks. i) Equilibrium analyses suggest that all five stocks are at high risk of stock collapse at current (2017) rates of fishing, though there is large uncertainty in the case of horse mackerel. Much of the catch appears to be dependent on in year production (recruitment and growth). j) Despite uncertainty in the estimates of BMSY and FMSY, the model described in the analysis provides the basis for a multi-fleet, fishery model that can be used to investigate management scenarios and economic impacts.

A large portion of the catch in many stocks may comprise discards which need to be accounted for in assessments in order to avoid bias in estimates of fishing mortality, stock biomass and reference points. In age-structured assessment models, discards are sometimes treated as a separate fleet or are added to the landings before fitting so that information about discard behaviour and sampling error is lost. In this paper, an assessment model is developed to describe the discard process with size as a covariate while retaining age-structured population dynamics. Discard size selection, high grading and bulk dumping of fish at sea are modelled so that the temporal dynamics of the process can be quantified within the assessment. The model is used to show that discarding practices have changed over time in a range of Northeast Atlantic demersal fish. In some stocks, there is a substantial increase in high grading and evidence for bulk discarding which can be related to regulatory measures. The model offers a means of identifying transient effects in the discard process that should be removed from both short-term forecasts and equilibrium reference point calculations.

Fish and FisheriesVolume 20, Issue 3 p. 596-596 BOOK REVIEW Fish Population Dynamics, Monitoring and Management Sustainable Fisheries in the Eternal Ocean By I. Aoki, T. Yamakawa & A. Takasuka Springer, Japan, 2018. ISBN: 9784431566199. Robin Cook, Corresponding Author Robin Cook robin.cook@strath.ac.uk orcid.org/0000-0002-9604-0204 Mathematics and Statistics, University of Strathclyde, Glasgow, UKSearch for more papers by this author Robin Cook, Corresponding Author Robin Cook robin.cook@strath.ac.uk orcid.org/0000-0002-9604-0204 Mathematics and Statistics, University of Strathclyde, Glasgow, UKSearch for more papers by this author First published: 15 January 2019 https://doi.org/10.1111/faf.12352Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue3May 2019Pages 596-596 RelatedInformation

ICES assessments of cod (Gadus morhua) in the west of Scotland (ICES Division 6a) suggest the biomass has collapsed and that fishing mortality rate (F) has remained high. In contrast, other stocks in the same fishery, and adjacent cod stocks all show marked declines in fishing mortality and some recovery of the biomass. The perception of the status of 6a cod appears to be dependent on the assumption that the fishery exploitation pattern is flat topped. An assessment that allows the exploitation to take a domed shape produces results that suggest a marked decline in fishing mortality rate and that the spawning stock biomass has recovered to the minimum biomass reference point, B-lim. The reduction in F is consistent with substantial reductions in fishing effort and shows a similar pattern to stocks taken within the same fishery. The management implications arising from the two assessments differ substantially. The analysis indicates that benchmark assessments need to test assessment model conditioning assumptions more widely and that management advice needs to consider a more comprehensive range of information about the stock and fishery.

The decrease in groundfish stocks in the North Atlantic since the mid-1900s coupled with increases in grey seal (Halichoerus grypus) populations is responsible for an enduring controversy between fishers and conservationists regarding the role seals have played in stock declines. We used a Bayesian state-space model to investigate stock trends in the presence of grey seals and associated maximum sustainable yield (MSY) reference points in the West of Scotland. This study provides new estimates of seal predation mortality on haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) and updates the estimates for cod (Gadus morhua), which together form the traditional main components of the mixed demersal fishery in this area. Grey seal predation mortality is greatest on cod, resulting in estimates of total natural mortality higher than those used in the current ICES assessments. Seal predation mortality is low for haddock and whiting. Considering seal predation in stock assessments changes the scale of biomass and fishing mortality estimates for the three stocks. The estimates of F 0.1 and F MSY are sensitive to seal predation for cod and whiting but not for haddock. In all cases, MSY decreases with increased seal predation.

A scavenger is an animal that feeds on dead animals (carrion) that it has not killed itself. Fisheries discards are often seen as an important food source for marine scavengers so the reduction of discards due to the Landing Obligation may affect their populations. The literature on scavenging in marine ecosystems is considerable, due to its importance in the trophic ecology of many species. Although discards undoubtedly contribute to these species' food sources, few can be seen to be solely dependent on carrion (including discards). Ecosystem models predicted that discards contributed very little to the diet of scavengers at a regional scale. A reduction in discards through the Landing Obligation may therefore affect populations for a few species in some areas, but generally this is unlikely to be the case. But it is challenging to identify how important discards might be to scavengers, as they are taxonomically diverse and vary in the role they play in scavenging interactions.

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Synthesis studies of fish stocks worldwide suggest improving status of mainly target species that are fully assessed. Other analyses, primarily based on catch data alone, but which include a wider range of species as well as bycatch, present a different view. Catch-only analyses could be more robust if fishery-independent data were used and discards accounted for. We develop a model that uses only survey biomass at length and landings data to estimate fishing mortality, spawning stock biomass (SSB) and discards. An analysis of species from the North Sea shows the model results compare well with most fully assessed stocks. When applied to bycatch species with limited data, trends in fishing mortality and SSB typically reflect those of the target species. In the last decade, mean fishing mortality rates have tended to decline, while mean SSB has increased. Despite increasing SSB, recent mean recruitment appears to have been lower than previously which may limit future biomass recovery. Species usually associated with more northerly distributions appear to show the greatest effect of weaker recruitment, which may be linked to climate. Estimated discards have tended to decline in magnitude as a result of reduced fishing mortality and associated lower total catches. The model offers a simple way to use both landings and survey data to obtain more detailed population trends for data limited species.

Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.

Nature Ecology & Evolution 1, 0170 (2017); published 26 May 2017; corrected 12 June 2017. In the original version of this Article, the European Commission was mistakenly included as an affiliation for Christos D. Maravelias. His contribution to this work was exclusively completed while at the Hellenic Centre for Marine Research.

The role grey seals have played in the performance of fisheries is controversial and a cause of much debate between fishers and conservationists. Most studies focus on the effects of seal damage to gears or fish and on prey population abundance but little attention is given to the consequences of the latter for the fisheries. We develop a model that quantifies the economic impact of grey seal predation on the West of Scotland demersal fisheries that traditionally targeted cod, haddock and whiting. Three contrasting fishing strategy scenarios are examined to assess impacts on equilibrium fleet revenues under different levels of seal predation. These include status quo fishing mortality (SQF, steady state with constant fishing mortality), open access fishing (bioeconomic equilibrium,BE) and the maximum economic yield (MEY). In all scenarios, cod emerges as the key stock. Large whitefish trawlers are most sensitive to seal predation due to their higher cod revenues but seal impacts are minor at the aggregate fishery level. Scenarios that consider dynamic fleet behaviour also show the greatest effects of seal predation. Results are sensitive to the choice of seal foraging model where a type II functional response increases sensitivity to seal predation. The cost to the fishery for each seal is estimated.

Cod (Gadus morhua) are preyed upon by grey seals (Halichoerus grypus), and there is debate over the impact this has had on the decline of stocks and their prospects for recovery. We analysed a depleted stock to the West of Scotland and show that seal predation rate is consistent with a type II functional response. Forward projections of a model including the functional response under varying levels of fishing and seal population size suggest that stock recovery is possible under current conditions, but there is a modest probability that the stock will decline further in both the short and long term. The potential recovery is fragile and sensitive to relatively small increases in either fishing or seal predation. Forward projection models that exclude the functional response estimate a lower probability of stock decline and may underestimate the risk to the stock. At low stock sizes and high fishing mortality rates, functional response models project slower recovery but the opposite is true at low fishing mortality.

Many commercial fisheries seek to maximise the economic value of the catch that they bring ashore and market for human consumption by discarding undersize or low value fish. Information on the quantity, size and species composition of discarded fish is vital for stock assessments and for devising legislation to minimise the practice. However, except for a few major species, data are usually extremely sparse and reliant on observers aboard a small sample of fishing vessels. Expanding these data to estimate total regional discards is highly problematic. Here, we develop a method for utilising additional information from scientific trawl surveys to model the quantities of fish discarded by the commercial fisheries. As a case-study, we apply the model to the North Sea over the period 1978-2011, and show a long-term decline in the overall quantity of fish discarded, but an increase in the proportion of catch which is thrown away. The composition of discarded catch has shifted from predominantly (∼80%) roundfish, to >50% flatfish. Undersized plaice constitute the largest single fraction of discards, unchanged from the beginning of the 20th century. Overall, around 60% of discarded fish are rejected on the basis of size rather than for reasons of species value or quota restrictions. The analysis shows that much more information can be gained on discarding by utilising additional sources of data rather than relying solely on information gathered by observers. In addition, it is clear that reducing fishing intensity and rebuilding stocks is likely to be more effective at reducing discards in the long term, than any technical legislation to outlaw the practice in the short term.

Summary Grey seal predation has been blamed by fishers for the decline of Atlantic cod stocks and has led to calls for seal culls. In the West of Scotland, estimates of cod consumption by seals have exceeded reported catches and spawning biomass, focussing attention on the interaction between fishers and seals. Bayesian models making different assumptions about seal predation were used to estimate the size of the West of Scotland cod stock between 1985 and 2005 and the mortalities due to fishing and seal foraging. A simple population model was used to identify the likely direction of cod population change at recent mortality rates. All model configurations suggest that the total mortality of cod has remained fairly stable and high for many years regardless of the assumptions on seal predation. The high mortality explains the long‐term decline of the stock. The best‐fitting model suggests that mortality due to fishing reduced substantially in the decade up to 2005, but has been replaced by increased seal predation mortality on a smaller cod stock. Given total mortality estimates, the stock is unlikely to recover even at present reduced levels of fishing. Synthesis and applications. Our model offers a method of estimating seal predation mortality as part of routine stock assessments that inform fishery management. The analysis shows that predation by seals can be an important component of the total stock mortality. It also shows that assuming invariant natural mortality, as adopted in many standard fish stock assessments, may lead to incorrect perceptions of fishing mortality, over‐estimating the benefits of reducing fishing mortality when there is density‐dependent predation. It is essential to consider predation by top predators when formulating appropriate advice for managing the fishery.

The European Red List is a review of the conservation status of European species according to IUCN regional Red Listing guidelines. It identifies those species that are threatened with extinction at the regional level, so that appropriate conservation action can be taken to improve their status. This Red List publication summarises results for all described native European marine fishes.

Discarding by fisheries is perceived as contrary to responsible harvesting. Legislation seeking to end the practice is being introduced in many jurisdictions. However, discarded fish are food for a range of scavenging species; so, ending discarding may have ecological consequences. Here we investigate the sensitivity of ecological effects to discarding policies using an ecosystem model of the North Sea—a region where 30–40% of trawled fish catch is currently discarded. We show that landing the entire catch while fishing as usual has conservation penalties for seabirds, marine mammals and seabed fauna, and no benefit to fish stocks. However, combining landing obligations with changes in fishing practices to limit the capture of unwanted fish results in trophic cascades that can benefit birds, mammals and most fish stocks. Our results highlight the importance of considering the broader ecosystem consequences of fishery management policy, since species interactions may dissipate or negate intended benefits.

Methods of assessment that depend upon commercial catch data can be undermined by misreporting or where parts of the catch, such as discards, are not accounted for. An age-structured model that makes use of survey data alone, and avoids this problem, is developed within a Bayesian framework so that routine stock summary statistics such as fishing mortality, recruitment and spawning stock biomass can be estimated with associated levels of uncertainty. It is also possible to estimate catch on a relative scale which can be compared to reported catches. The model is applied to West of Scotland haddock (Melanogrammus aeglefinus), a stock with suspected high catch misreporting. Stock trends derived from the model are consistent with conventional assessments that use catch data during periods of low misreporting. Estimated proportions of fish at each age in the catch correspond closely with observed values. Model estimates of total catches suggest substantial misreporting in some years, though the precision of the estimates is very low. Revised estimates of natural mortality are obtained from the model that are higher than conventional values used for this stock. These new values are generally consistent with those obtained from multispecies predation modelling for the adjacent North Sea stock. The model provides many of the basic quantities used for management advice. It should not be regarded as a replacement for more comprehensive analyses, but an additional tool to explore available data when catch information is unreliable.

There is considerable interest in the state of the world’s natural fishery resources. The paper by Froese et al. (2012) is a recent example of applying a set of ad hoc decision rules to a time series of catch data in order to assign the world’s fisheries to categories of exploitation and hence make generalisations about their current status. They conclude that the percentage of stocks that are over-exploited is worse than previously reported in FAO (2010). The approach used by Froese et al. is based on an algorithm proposed by Froese and Kesner-Reyes (2002) which has been heavily criticised both on theoretical grounds and from simulation studies (Branch et al. 2011; Daan et al. 2011; Wilberg and Miller 2007). In their recent paper, Froese et al. (2012) produce additional analyses to support their method which assumes that maximum sustainable yield (MSY) lies in the interval (0.5Cmax, Cmax), where Cmax is the maximum observed catch in the time series. Unfortunately, these analyses do not support their contention that MSY for a particular stock is related to maximum catch in a predictable way and renders their conclusions unsafe