Maximum Sustainable Yield Reference Points Research Articles

Enhancing the utility of known-biomass production models: a case study of the Bay of Biscay and Iberian Coast ecoregion

Our general purpose is to support the use of known-biomass production models (KBPMs), illustrating their usefulness by addressing the evolution of surplus production (SP) over time and the factors affecting it (e.g. environment). We also demonstrate the utility of KBPMs for multispecies management objectives or for estimating maximum sustainable yield reference points without a stock recruitment function, among other worthwhile applications. To do so, we present different uses of KBPMs, illustrating their application on demersal species in the International Council for the Exploration of the Sea (ICES) area, specifically for megrim, white anglerfish and European hake stocks. The proposed analytical approach involved fitting single-species and multispecies KBPMs, conducting retrospective analyses and assessing the effects of environmental variability on SP. The findings show that, in general, stock SP increased after a decline in biomass and SP, except for white anglerfish in the southern area. Megrim stocks are the least productive, while hake and northern anglerfish are the most productive. Retrospective analysis revealed SP shifts in northern hake stock for reasons other than biomass variability. Hence, the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation (AMO), two key climate variability modes in the North Atlantic, were tested for their links to SP, revealing a positive connection between SP and AMO, although further research is necessary. Beyond the specific results of our particular KBPM application, our main conclusion is that KBPMs can serve as a tool complementary to more complex assessment models for resolving unaddressed issues and crosschecking available assessment results.

Turning reference points inside out: comparing MSY reference points estimated inside and outside the assessment model

Abstract Reference points are used in fisheries management to infer stock status and inform future fishing opportunities. They can be estimated externally to the assessment model assuming attributes such as stock size to be known without error, or internally where the uncertainty can be accounted for in the estimation. Little is known about the effect of choosing one approach over another on reference point estimates. We compare maximum sustainable yield (MSY) reference points obtained internally in the assessment model (SAM) with those obtained by external estimation (EqSim) via simulations using 11 stocks with known and unknown stock–recruitment relationships (SRRs). Internal estimation of reference points does better on average than external estimation in terms of bias, variance, and SRR selection. Coverage probability of the reference point confidence intervals is good for the internal approach. Stochasticity considerations via re-sampling in the external approach can lead to bias. Bias in reference points have consequences on future yield, stock development, and status. However, it is difficult to predict them by solely looking at bias because they depend on current stock status, recruitment, and extent of bias, but also bias direction since a harvest control rule might compensate for the bias in the future.

Understanding fishery interactions and stock trajectory of yellowfin tuna exploited by Iranian fisheries in the Sea of Oman

Abstract The predominant policy for remedying the world fishing crisis aims at maximum sustainable yield (MSY) by adjusting gear selectivity and fishing effort to maintain sustainable stock levels. The yellowfin tuna (Thunnus albacares) fishery in the Sea of Oman has experienced intense increases in removals since 1980, with particularly high levels since the 1990s. Here, we apply a statistical catch-at-age model to time-series of catches and fishery-dependent length composition data to obtain a preliminary and general understanding of the population dynamics of this stock since the start of the fishery in 1950–2019. Despite limited data, population models consistently indicate a sharp decline in population status since the beginning of the time-series across a variety of assumptions on stock productivity and life history. The gillnet fishery takes almost exclusively immature individuals, with high fishing intensity and removal rates. Both reference models indicate the population is essentially at the same relative stock status in 2019 (10% of unfished), but with very different future projections and higher absolute stock size when recruitment is estimated. The yellowfin tuna population in 2019 is below estimated MSY reference points (based either on unfished size or spawning output at MSY) for current relative stock size, and over the fishing intensity at MSY, indicating current overfishing. Adjusting the interactions of that fishery with the population, while continuing to collected biological composition data representative of each fleet in the fishery, will help mitigate current stock decline and provide the ability to refine future population status determination and forecasts through more informed stock assessments.

The Risky Decrease of Fishing Reference Points Under Climate Change

In Europe, implementation of sustainable fisheries management has been reinforced in the latest common fisheries policy, and presently marine fish stocks are mostly managed through assessment of their exploitation and ecological status compared to reference points such as Maximum Sustainable Yield (MSY). However, MSY and its associated fishing mortality rate F-MSY are sensitive to both stock characteristics and environment conditions. In parallel, climate change impacts are increasingly affecting fish stocks directly and indirectly but might also change the exploitation reference points and the associated level of catch. Here we explored the variability of MSY reference points under climate change by using a multi-species model applied to the Eastern English Channel, a highly exploited semi-continental sea. The spatial individual-based OSMOSE explicitly represents the entire fish life cycle of 14 species interacting through size-based opportunistic predation. The model was first parameterized and run to fit the historical situation (2000-2009) and then used to assess the ecosystem state for the 2050-2059 period, using two contrasting climate change scenarios (RCP 4.5 and RCP 8.5). For each condition, a monospecific MSY estimation routine was performed by varying species fishing mortality independently and allowed estimation of reference points for each species. The F-MSY estimated with OSMOSE were mostly in accordance with available values derived from stock assessment and used for fishing advice. Evolution of reference points with climate change was compared across species and highlighted that overexploited cold-water species are likely to have both MSY and F-MSY declining with climate warming. Considering all species together, MSY under RCP scenarios was expected to be higher than historical MSY for half of them, with no clear link with species temperature preferences, exploitation status or trophic level, but in relation with expected change of species biomass under climate change. By contrast, for 80% of cases F-MSY projections showed consistent decreasing pattern as climate conditions changed from historical to RCP scenarios in the Eastern English Channel. This result constitutes a risk for fisheries management, and anticipation of climate change impacts on fish community would require targeting a smaller fishing mortality than F-MSY to ensure sustainable exploitation of marine stocks.

Local sensitivity equations for maximum sustainable yield reference points

Quantitative fishery harvest strategies in many countries throughout the world are based on maximum sustainable yield (MSY) reference points (RPs); in particular, the fishing mortality rate that produces MSY (i.e. Fmsy) and the population biomass (Bmsy) that results from fishing at Fmsy. There is often great interest in the sensitivity of MSY RPs to their inputs and underlying assumptions. We perturb (i.e. ω) inputs to generate influence curves which we denote as Fmsy(ω) and Bmsy(ω) as functions of ω. We derive diagnostic equations for the slopes of Fmsy(ω) and Bmsy(ω) at the origin (i.e. no perturbation). These equations provide a conceptual understanding of how Fmsy and Bmsy will change in response to changes in inputs, which one does not get from a simple computational sensitivity analysis. We show that a leading factor determining the sensitivity of Fmsy is the amount of curvature in the equilibrium yield curve near Fmsy. Another leading factor affecting the sensitivity of Bmsy is if the stock–recruitment function indicates low density-dependence in recruitment productivity. With some exceptions, we find that Fmsy will increase if the body growth rate or asymptotic size increases, 2) proportions mature increase at younger ages, and 3) natural mortality rates decrease and the Ricker stock–recruitment function is used. If the fishery selectivity function of age is monotone increasing then Fmsy will also usually increase if peak selectivity is shifted to an older age. Usually the signs of the slopes for Bmsy and Fmsy are the same.

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

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.

Bioeconomic multistock reference points as a tool for overcoming the drawbacks of the landing obligation

The landing obligation policy was one of the major innovations introduced in the last Common Fisheries Policy reform in Europe. It is foreseen that the policy will affect the use of fishing opportunities and hence the economic performance of the fleets. The problem with fishing opportunities could be solved if single-stock total allowable catches (TACs) could be achieved simultaneously for all the stocks. In this study, we evaluate the economic impact of the landing obligation policy on the Spanish demersal fleet operating in the Iberian Sea region. To generate TAC advice, we used two sets of maximum sustainable yield (MSY) reference points, the single-stock MSY reference points defined by ICES and a set of multistock reference points calculated simultaneously using a bioeconomic optimization model. We found that the impact of the landing obligation is time and fleet dependent and highly influenced by assumptions about fleet dynamics. At fishery level, multistock reference points mitigate the decrease in the net present value generated by the implementation of the landing obligation. However at fleet level, the effect depends on the fleet itself and the period. To ensure the optimum use of fishing opportunities, the landing obligation should be accompanied by a management system that guarantees consistency between single-stock TACs. In this regard, multistock reference points represent an improvement over those currently in use. However, further investigation is necessary to enhance performance both at fleet level and in the long term.

Modelling age-dependent movement: an application to red and gag groupers in the Gulf of Mexico

We develop and test spatial population dynamics models that estimate age-dependent offshore movement of fish populations from spatial fishery data. Spatially aggregated population dynamics models produced biased estimates of maximum sustainable yield (MSY) reference points when spatial dynamics were simulated. Spatial population dynamics models provided relatively unbiased estimates of MSY reference points regardless of whether spatial dynamics were simulated. We demonstrate that by using conventional fishery data that are disaggregated spatially, it is possible to estimate movement with age and obtain more accurate estimates of management reference points. The new spatial models were fitted to data for Gulf of Mexico red grouper (Epinephelus morio) and gag grouper (Mycteroperca microlepis) to estimate spatial distribution and offshore movement with age. Offshore ontogeny was estimated to be stronger for gag grouper than red grouper and predicted a larger fraction of older gag grouper in deeper waters.

Impact of maximum sustainable yield-based fisheries management frameworks on rebuilding North Atlantic cod stocks

Fisheries management frameworks based on maximum sustainable yield reference points have been put forward by all agencies and organizations responsible for providing scientific advice for the management of 22 major North Atlantic cod stocks. These frameworks provide a structure for rebuilding depleted cod stocks and for future sustainable fishing. Unfortunately, in most cases these frameworks have not been fully implemented. Although information is lacking for some stocks, it would appear that a large proportion of North Atlantic cod stocks are either experiencing a fishing mortality rate that exceeds that required to achieve maximum sustainable yield, are at a biomass level far below that which provides maximum sustainable yield, or have both occurring. Despite this general failure, there are some notable successes, which have led to stock rebuilding and substantial fisheries, providing an incentive for striving to achieve rebuilding and sustainable management objectives for other Atlantic cod stocks. Management frameworks based on harvest control rules that result in fishing mortality rates below those associated with maximum sustainable yield are essential when stock size is low in order to rebuild to levels capable of producing maximum sustainable yield.

Can stock–recruitment points determine which spawning potential ratio is the best proxy for maximum sustainable yield reference points?

Abstract Legault, C. M., and Brooks, E. N. 2013. Can stock–recruitment points determine which spawning potential ratio is the best proxy for maximum sustainable yield reference points? – ICES Journal of Marine Science, 70: 1075–1080. The approach of examining scatter plots of stock–recruitment (S–R) estimates to determine appropriate spawning potential ratio (SPR)-based proxies for FMSY was investigated through simulation. As originally proposed, the approach assumed that points above a replacement line indicate year classes that produced a surplus of spawners, while points below that line failed to achieve replacement. In practice, this has been implemented by determining Fmed, the fishing mortality rate that produces a replacement line with 50% of the points above and 50% below the line. A new variation on this approach suggests FMSY proxies can be determined by examining the distribution of S–R points that are above or below replacement lines associated with specific SPRs. Through both analytical calculations and stochastic results, we demonstrate that this approach is fundamentally flawed and that in some cases the inference is diametrically opposed to the method's intended purpose. We reject this approach as a tool for determining FMSY proxies. We recommend that the current proxy of F40% be maintained as appropriate for a typical groundfish life history.

Fitting a non-parametric stock–recruitment model in R that is useful for deriving MSY reference points and accounting for model uncertainty

Abstract Cadigan, N. G. 2013. Fitting a non-parametric stock–recruitment model in R that is useful for deriving MSY reference points and accounting for model uncertainty. – ICES Journal of Marine Science, 70:56–67. Modelling the relationship between parental stock size and subsequent recruitment of fish to a fishery is often required when deriving reference points, which are a fundamental component of fishery management. A non-parametric approach to estimate stock–recruitment relationships is illustrated using a simulated example and nine case studies. The approach preserves compensatory density dependence in which the recruitment rate monotonically decreases as stock size increases, which is a basic assumption of commonly used parametric stock–recruitment models. The implications of the non-parametric estimates on maximum sustainable yield (MSY) reference points are illustrated. The approach is used to provide non-parametric bootstrapped confidence intervals for reference points. The efficacy of the approach is investigated using simulations. The results demonstrate that the non-parametric approach can provide a more realistic estimation of the stock–recruitment relationship when informative data are available compared with common parametric models. Also, bootstrap confidence intervals for MSY reference points based on different parametric stock–recruitment models often do not overlap. The confidence intervals based on the non-parametric approach tend to be much wider, and reflect better uncertainty due to stock–recruit model choice.

Impact of egg production and stock structure on MSY reference points and its management implications for southern hake (Merluccius merluccius)

Spawning stock biomass (SSB) is one of the most common measures of stock status. However recent research on reproductive potential has shown that alternative variables may be an improvement over SSB. In the case of European hake (Merluccius merluccius), it is known that large individuals produce more eggs by unit of body weight, and their quality is better than those of small individuals. Under these circumstances, application of reproductive potential may be pertinent in the implementation of the Johannesburg agreement, since it is not only the spawning biomass but also its age or length structure that defines stock productivity and its ability to achieve maximum sustainable yield (MSY). In this contribution we used an age-length structured population model to assess the impact of different reproductive indices (total spawning biomass, female spawning biomass and egg production) on MSY reference points. First, we analyzed how these different indices alter our perception about per recruit productivity. Second, we analyzed the quality of these alternative reproductive indices to explain and predict recruitment using different model structures (Ricker and Beverton–Holt) and Bayesian inference. Third, we combined per recruit models and stochastic stock–recruitment relationships to estimate the probability distributions of MSY biological reference points (MSY, Fmsy, Stockmsy, and Fcrash). Our results show that, for hake stocks where larger fish have a strong contribution to stock reproductive potential, use of alternative reproductive indices will affect estimations of stock sustainability. The change in perception of relative contribution of each length class is exhibited in two opposite ways: reproductive potential per recruit and the shape of the stock–recruitment relationship defined with steepness. In our case, the change from SSB to egg production, resulted in an increase in steepness which counteracted the depletion per recruit resulting in a larger Fmsy.

Evaluating the stock–recruitment relationship and management reference points: Application to summer flounder (Paralichthys dentatus) in the U.S. mid-Atlantic

The stock–recruitment relationship is one of the most uncertain processes of fish population dynamics, and is highly influential with respect to fisheries management advice. The stock recruitment relationship has a direct impact on reference points commonly used in contemporary fisheries management. Simulation analysis has shown that the steepness of the Beverton–Holt stock–recruitment relationship is difficult to estimate for most fish stocks, which has led to the use of proxy reference points. Proxy maximum sustainable yield reference points based on spawning biomass-per-recruit, which are commonly used when the stock–recruitment relationship is uncertain, are a linear function of steepness. Risk in terms of lost yield is generally lower when steepness is underestimated compared to when steepness is overestimated because the yield curve is flat when steepness is high (close to one: recruitment is independent of stock size), indicating that using a lower value of steepness might be appropriate. Simulation analysis based on data for summer flounder in the US mid-Atlantic indicates that steepness can be estimated from the data. Steepness is estimated to be close to one and a high steepness is supported by estimates for related species and from life history theory. Current target (F35%) and threshold (F40%) spawning biomass-per-recruit reference points used for summer flounder imply steepness values of 0.73 and 0.66, respectively, for the Beverton–Holt stock–recruitment relationship.

A continuous hockey stick stock–recruit model for estimating MSY reference points

Abstract Mesnil, B., and Rochet, M-J. 2010. A continuous hockey stick stock–recruit model for estimating MSY reference points. – ICES Journal of Marine Science, 67: 1780–1784. With political commitment to restore stocks to levels where they can produce maximum sustainable yield (MSY), fisheries managers request evaluation of management plans that include options for an FMSY policy. The procedure to estimate FMSY with dynamic-pool, stock assessment models is well established for common stock–recruitment relationships (S–RR), and this capacity is extended to another S–RR, a piecewise function known as the hockey stick (HS), which is frequently assumed when the data do not support more elaborate functions. However, the HS is not continuous, which makes it problematic for this application, where differentiable functions are required. The bent-hyperbola model proves to be an adequate continuous equivalent to the HS for estimating FMSY.

Eco-certification of sustainably managed fisheries—Redundancy or synergy?

There is an increasing global awareness of the consequences of overfishing and the need to implement sustainable fisheries management practices. The traditional custodians, governments and regional fisheries management organizations, have been slow to react creating a niche for environmental and non-profit groups to become increasingly active through eco-certification and sustainability ranking programs. While both the traditional and the new players tend to share a similar approach based on maximum sustainable yield reference points, they differ in scope and delivery mechanisms. The potential exists for confusion with regard to who should be determining whether a fishery is sustainable with the cost of possible redundancy being ultimately borne by the public. However, there is also the potential for synergy and, given the limited progress thus far with respect to achieving sustainable fisheries by traditional means, perhaps both are needed for the time being—a “belt and braces” approach.