scholarly journals Challenges to fisheries advice and management due to stock recovery

2018 ◽  
Vol 75 (6) ◽  
pp. 1864-1870 ◽  
Author(s):  
Rob van Gemert ◽  
Ken H Andersen
Keyword(s):  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Forage Fish ◽  
Fish Stocks ◽  
Density Dependent ◽  
Piscivorous Fish ◽  
Predation Mortality ◽  
Management Procedures ◽  
Dependent Growth ◽  
New Situation

Abstract During the 20th century, many large-bodied fish stocks suffered from unsustainable fishing pressure. Now, signs of recovery are appearing among previously overfished large-bodied fish stocks. This new situation raises the question of whether current fisheries advice and management procedures, which were devised and optimized for depleted stocks, are well-suited for the management of recovered stocks. We highlight two challenges for fisheries advice and management: First, recovered stocks are more likely to show density-dependent growth. We show how the appearance of density-dependent growth will make reference points calculated with current procedures inaccurate. Optimal exploitation of recovered large-bodied fish stocks will therefore require accounting for density-dependent growth. Second, we show how a biomass increase of large-bodied piscivorous fish will lead to a reverse trophic cascade, where their increased predation mortality on forage fish reduces forage fish productivity and abundance. The resulting decrease in maximum sustainable yield of forage fish stocks could lead to conflicts between forage and large-piscivore fisheries. Avoiding such conflicts requires that choices are made between the exploitation of interacting fish stocks. Failure to account for the changed ecological state of recovered stocks risks creating new obstacles to sustainable fisheries management.

scholarly journals Implications of late-in-life density-dependent growth for fishery size-at-entry leading to maximum sustainable yield

2018 ◽  
Vol 75 (4) ◽  
pp. 1296-1305 ◽  
Author(s):  
Rob van Gemert ◽  
Ken H Andersen
Keyword(s):  
Density Dependence ◽  
Maximum Sustainable Yield ◽  
Sustainable Yield ◽  
Fish Stocks ◽  
Density Dependent ◽  
Large Size ◽  
Adult Growth ◽  
Stock Assessments ◽  
Stock Recruitment ◽  
Dependent Growth

Abstract Currently applied fisheries models and stock assessments rely on the assumption that density-dependent regulation only affects processes early in life, as described by stock–recruitment relationships. However, many fish stocks also experience density-dependent processes late in life, such as density-dependent adult growth. Theoretical studies have found that, for stocks which experience strong late-in-life density dependence, maximum sustainable yield (MSY) is obtained with a small fishery size-at-entry that also targets juveniles. This goes against common fisheries advice, which dictates that primarily adults should be fished. This study aims to examine whether the strength of density-dependent growth in actual fish stocks is sufficiently strong to reduce optimal fishery size-at-entry to below size-at-maturity. A size-structured model is fitted to three stocks that have shown indications of late-in-life density-dependent growth: North Sea plaice (Pleuronectes platessa), Northeast Atlantic (NEA) mackerel (Scomber scombrus), and Baltic sprat (Sprattus sprattus balticus). For all stocks, the model predicts exploitation at MSY with a large size-at-entry into the fishery, indicating that late-in-life density dependence in fish stocks is generally not strong enough to warrant the targeting of juveniles. This result lends credibility to the practise of predominantly targeting adults in spite of the presence of late-in-life density-dependent growth.

scholarly journals Effects of multispecies and density-dependent factors on MSY reference points: example of the Baltic Sea sprat

2017 ◽  
Vol 74 (6) ◽  
pp. 864-870 ◽  
Author(s):  
Jan Horbowy ◽  
Anna Luzeńczyk
Keyword(s):  
Baltic Sea ◽  
Gadus Morhua ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Stochastic Simulations ◽  
Natural Mortality ◽  
The Baltic Sea ◽  
Density Dependent ◽  
Dependent Growth ◽  
The Baltic

In this paper, maximum sustainable yield (MSY) parameters for the Baltic Sea sprat (Sprattus sprattus) are estimated in relation to pressure from cod (Gadus morhua) predation and the influence of density dependence on sprat growth. This study is based on long-term deterministic and stochastic simulations in which sprat density-dependent growth and predation mortality are considered. The resultant model is a relatively simple tool that allows for streamlined analyses of problems typically approached using complex multispecies models. The analysis indicates that estimates of the MSY parameters (i.e., MSY and FMSY) and equilibrium biomass differ significantly between approaches that hold growth and natural mortality constant and those that allow for density-dependent growth and natural mortality. Based on the cod biomass observed in the 1980s, the MSY parameters estimated by a model that accounts for density-dependent growth and by a model assuming constant growth may differ by a factor of 2. As such, the MSY parameters decline (approximately linearly) with the size of the cod stock.

The art of ecosystem-based fishery management

2014 ◽  
Vol 71 (3) ◽  
pp. 479-490 ◽  
Author(s):  
Michael J. Fogarty
Keyword(s):  
Fishery Management ◽  
Decision Rules ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Individual Species ◽  
Emergent Properties ◽  
Nontarget Organisms ◽  
Modeling And Analysis ◽  
Adoption And Implementation ◽  
Management Procedures

The perception that ecosystem-based fishery management is too complex and poorly defined remains a primary impediment to its broadscale adoption and implementation. Here, I attempt to offer potential solutions to these concerns. Specifically, I focus on pathways that can contribute to overall simplification by moving toward integrated place-based management plans and away from large numbers of species-based plans; by using multispecies or ecosystem models and indicators that permit the simultaneous and consistent assessment of ecosystem components while also incorporating broader environmental factors; and by consolidating individual administrative and regulatory functions now mostly dealt with on a species-by-species basis into a more integrated framework for system-wide decision-making. The approach focuses on emergent properties at the community and ecosystem levels and seeks to identify simpler modeling and analysis tools for evaluation. Adoption of ecosystem-based management procedures relying on simple decision rules and metrics is advocated. It is recommended that we replace static concepts for individual species focusing on maximum sustainable yield with a dynamic ecosystem yield framework that involves setting system-wide reference points along with constraints to protect individual species, habitats, and nontarget organisms in a dynamic environmental setting.

Estimating Fmsy from an ensemble of data sources to account for density dependence in Northeast Atlantic fish stocks

2020 ◽  
Author(s):  
Henrik Sparholt ◽  
Bjarte Bogstad ◽  
Villy Christensen ◽  
Jeremy Collie ◽  
Rob van Gemert ◽  
...  
Keyword(s):  
Maximum Sustainable Yield ◽  
Data Sources ◽  
Fishing Pressure ◽  
Fish Stocks ◽  
Northeast Atlantic ◽  
Density Dependent ◽  
Production Models ◽  
Pool Model ◽  
Age Structured ◽  
The 1960S

Abstract A new approach for estimating the fishing mortality benchmark Fmsy (fishing pressure that corresponds to maximum sustainable yield) is proposed. The approach includes density-dependent factors. The analysis considers 53 data-rich fish stocks in the Northeast Atlantic. The new Fmsy values are estimated from an ensemble of data sources: (i) applying traditional surplus production models on time-series of historic stock sizes, fishing mortalities, and catches from the current annual assessments; (ii) dynamic pool model (e.g. age-structured models) estimation for stocks where data on density-dependent growth, maturity, and mortality are available; (iii) extracts from multispecies and ecosystem literature for stocks where well-tested estimates are available; (iv) the “Great Experiment” where fishing pressure on the demersal stocks in the Northeast Atlantic slowly increased for half a century; and (v) linking Fmsy to life history parameters. The new Fmsy values are substantially higher (average equal to 0.38 year−1) than the current Fmsy values (average equal to 0.26 year−1) estimated in stock assessments and used by management, similar to the fishing pressure in the 1960s, and about 30% lower than the fishing pressure in 1970–2000.

scholarly journals Impact of time-varying productivity on estimated stock–recruitment parameters and biological reference points

2020 ◽  
Vol 77 (5) ◽  
pp. 836-847
Author(s):  
Carrie A. Holt ◽  
Catherine G.J. Michielsens
Keyword(s):  
Sockeye Salmon ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Time Varying ◽  
Simulation Framework ◽  
Fish Stocks ◽  
Time Varying Parameters ◽  
Stock Recruitment ◽  
Or Benchmarks ◽  
Time Varying Models

Models with time-varying parameters are increasingly being considered in the assessment of fish stocks, but their reliability when used to derive biological reference points or benchmarks has not been thoroughly evaluated. Here, we evaluated stock–recruitment models with and without time-varying productivity in a simulation framework for sockeye salmon (Oncorhynchus nerka) under different scenarios of productivity and exploitation. Ignoring trends in productivity led to overestimates of productivity and underestimates of capacity when both exploitation rates and productivity declined over time, resulting in an underestimation on average of benchmarks of biological status. Despite being less biased, time-varying models had relatively poor fit based on AICc and BIC model selection criteria. Our simulation results were compared with empirical analyses of 12 Fraser River sockeye salmon stocks in British Columbia, Canada. Although benchmarks were less biased when based on time-varying models, underlying true benchmarks based on spawner abundances at maximum sustainable yield, SMSY, trend downwards when productivity declines, which may not be aligned with conservation objectives. We conclude with best practices when adapting biological benchmarks to time-varying productivity.

scholarly journals Status and targets for rebuilding the three major fish stocks in Lake Victoria

2020 ◽  
Author(s):  
Laban Musinguzi ◽  
Mark Olokotum ◽  
Vianny Natugonza
Keyword(s):  
Fisheries Management ◽  
Nile Tilapia ◽  
Lake Victoria ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Nile Perch ◽  
Fish Stocks ◽  
Illegal Fishing ◽  
Stock Status ◽  
Low Levels

AbstractWe determined fisheries management reference points for three major fish stocks in Lake Victoria (Nile tilapia, Nile perch and Dagaa) for Uganda and the whole lake. The aim was to ascertain stock status and define reasonable objectives and targets for rebuilding to sustainable levels. Dagaa was found to be healthy in Uganda and the whole lake but tending to overfished status. In Uganda, the stock status of Nile tilapia and Nile perch was recruitment impaired but tending more towards collapsed and overfished status respectively. In the whole lake, the stock status of Nile tilapia and Nile perch was collapsed and overfished respectively with the latter tending more towards recruitment impaired. Estimates of maximum sustainable yield (MSY) showed that catches could be increased under good management. Rebuilding the Nile tilapia and Nile perch stock biomasses to MSY level (Bmsy) could respectively increase the catches above the current level by 9.2% and 29.5% in Uganda and by 72.8% and 15.1% in the whole lake. The immediate objective for fisheries management should be to rebuild biomass for the Nile tilapia and Nile perch stocks to Bmsy. Elimination of illegal fishing practices has proved to be effective. In addition, management needs to keep catches at low levels until biomass for the stocks is ≥Bmsy for at least three consecutive years.

scholarly journals Grey seal predation mortality on three depleted stocks in the West of Scotland: What are the implications for stock assessments?

2018 ◽  
Vol 75 (5) ◽  
pp. 723-732 ◽  
Author(s):  
Vanessa Trijoulet ◽  
Steven J. Holmes ◽  
Robin M. Cook
Keyword(s):  
Gadus Morhua ◽  
State Space Model ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Melanogrammus Aeglefinus ◽  
Grey Seal ◽  
The West ◽  
The North ◽  
Predation Mortality ◽  
Stock Assessments

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 F0.1 and FMSY are sensitive to seal predation for cod and whiting but not for haddock. In all cases, MSY decreases with increased seal predation.

scholarly journals Maturation reaction norm evolution under varying conditions of eco-evolutionary change

2021 ◽  
Author(s):  
Jenilee Gobin ◽  
Michael G Fox ◽  
Erin S Dunlop
Keyword(s):  
Genetic Model ◽  
Reaction Norm ◽  
Key Factors ◽  
Phenotypic Data ◽  
Fish Stocks ◽  
Density Dependent ◽  
Rates Of Evolution ◽  
Dependent Growth ◽  
Norm Evolution ◽  
Maturation Age

Probabilistic maturation reaction norms (PMRNs) are commonly used to infer evolution of maturation age and size in wild fish stocks, but how well estimates from phenotypic data actually reflect underlying genotypes is debated. We used an eco-genetic model of a commercially harvested freshwater fish to simulate populations undergoing various levels of fisheries-induced evolution and density-dependent feedback and evaluated effects on the estimation of PMRNs. We estimated PMRNs from phenotypic data sampled from simulated populations (age, length, and maturation status of individuals), as is done on wild stocks, and compared estimates with the known maturation genotypes of individuals in the simulated population. PMRN estimates were robust to changes in the strength of density-dependent growth and high levels of fisheries-induced evolution. However, our ability to detect slower rates of evolution was limited, especially when individuals matured within a narrow range of ages. This study suggests that the widely applied method of estimating PMRNs from readily available phenotypic data to detect underlying evolution of maturation schedule is robust to some key factors that vary in wild populations.

scholarly journals Quirky patterns in time-series of estimates of recruitment could be artefacts

2014 ◽  
Vol 72 (1) ◽  
pp. 111-116 ◽  
Author(s):  
M. Dickey-Collas ◽  
N. T. Hintzen ◽  
R. D. M. Nash ◽  
P-J. Schön ◽  
M. R. Payne
Keyword(s):  
Time Series ◽  
Stock Assessment ◽  
Meta Analysis ◽  
Reference Points ◽  
Fish Stocks ◽  
Stock Recruitment ◽  
Modelling Assumptions ◽  
Meta Analyses ◽  
Recruitment Model ◽  
Recruitment Time

Abstract The accessibility of databases of global or regional stock assessment outputs is leading to an increase in meta-analysis of the dynamics of fish stocks. In most of these analyses, each of the time-series is generally assumed to be directly comparable. However, the approach to stock assessment employed, and the associated modelling assumptions, can have an important influence on the characteristics of each time-series. We explore this idea by investigating recruitment time-series with three different recruitment parameterizations: a stock–recruitment model, a random-walk time-series model, and non-parametric “free” estimation of recruitment. We show that the recruitment time-series is sensitive to model assumptions and this can impact reference points in management, the perception of variability in recruitment and thus undermine meta-analyses. The assumption of the direct comparability of recruitment time-series in databases is therefore not consistent across or within species and stocks. Caution is therefore required as perhaps the characteristics of the time-series of stock dynamics may be determined by the model used to generate them, rather than underlying ecological phenomena. This is especially true when information about cohort abundance is noisy or lacking.

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