Bioeconomic modelling of seal impacts on West of Scotland fisheries

It has been several decades that groundfish stocks have decreased around the UK. Meanwhile, grey seal population has increased. This has created a controversy between fishers and conservationists as regards to the role grey seals have played in the stock depletion. Currently, opinions are still divided, and further studies need to be done to mitigate these conflicts.A bioeconomic model able to quantify the economic impact of grey seal predation on West of Scotland demersal fisheries for cod, haddock and whiting was developed. The biological part of the model accounts for seal predation and fishing catches and is linked to an economic model accounting for fleet revenues and costs.Three scenarios are tested. The "status quo F" model assesses seal predation impacts on fleet revenues at the biological equilibrium. Two dynamic models are also studied to determine seal impacts when fleet behaviour is considered: the maximum economic yield scenario (MEY) where the fishery net profit is maximised and the bioeconomic equilibrium (BE) model where the profits are dissipated in the long-run.Cod is the fish the most impacted by grey seal predation so is the key stock in evaluating fishery effects. While the biological impacts can be important, seal predation is not economically important at the fishery level but some fleets are more sensitive than others. The large whitefish trawlers are likely to be the only fleet that could benefit from a reduction in grey seal predation. The following increase in its revenues would be certainly improved by fishery regulations.It has been several decades that groundfish stocks have decreased around the UK. Meanwhile, grey seal population has increased. This has created a controversy between fishers and conservationists as regards to the role grey seals have played in the stock depletion. Currently, opinions are still divided, and further studies need to be done to mitigate these conflicts.A bioeconomic model able to quantify the economic impact of grey seal predation on West of Scotland demersal fisheries for cod, haddock and whiting was developed. The biological part of the model accounts for seal predation and fishing catches and is linked to an economic model accounting for fleet revenues and costs.Three scenarios are tested. The "status quo F" model assesses seal predation impacts on fleet revenues at the biological equilibrium. Two dynamic models are also studied to determine seal impacts when fleet behaviour is considered: the maximum economic yield scenario (MEY) where the fishery net profit is maximised and the bioeconomic equilibrium (BE) model where the profits are dissipated in the long-run.Cod is the fish the most impacted by grey seal predation so is the key stock in evaluating fishery effects. While the biological impacts can be important, seal predation is not economically important at the fishery level but some fleets are more sensitive than others. The large whitefish trawlers are likely to be the only fleet that could benefit from a reduction in grey seal predation. The following increase in its revenues would be certainly improved by fishery regulations

The effects of grey seal predation and commercial fishing on the recovery of a depleted cod stock

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

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

The decrease in groundfish stocks in the North Atlantic since the mid-1900s coupled with increases in grey seal 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 MSY reference points in the West of Scotland. This study provides new estimates of seal predation mortality on haddock and whiting and updates the estimates for cod, 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

Skill assessment of models relevant for the implementation of ecosystem-based fisheries management

The advance of ecosystem-based fisheries management worldwide has made scientific advice on fisheries related questions more complex. However, despite the need to take interactions between fish stocks, and between stocks and their environment into account, multispecies and ecosystem models are still hardly used as a basis for fishery advice. Although reasons are numerous, the lack of high-level guidance for target-oriented skill assessments of such models contributes to the mistrust to use such models for advice. In this study, we propose a framework of guiding questions for a pragmatic and target-oriented skill assessment. The framework is relevant for all models irrespective of their complexity and approach. It starts with general questions on the advice purpose itself, the type of model(s) and data available for performance testing. After this, the credibility of the hindcasts are evaluated. A special emphasis is finally put on testing predictive skills. The skill assessment framework proposed provides a tool to evaluate a model's suitability for the purpose of providing specific advice and aims to avoid the bad practice of incomplete skill assessments. In the case of multiple models available, it can facilitate the evaluation or choosing of the best model(s) for a given advice product and intends to ensure a level playing field between models of different complexities. The suite of questions proposed is an important step to improve the quality of advice products for a successful implementation of ecosystem-based fisheries management

Bioeconomic modelling of grey seal predation impacts on the West of Scotland demersal fisheries

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

Mixed-stock analysis of Atlantic herring (Clupea harengus): a tool for identifying management units and complex migration dynamics

We developed and validated a mixed-stock analysis (MSA) method with 59 single-nucleotide polymorphisms selected from genome-wide data to assign individuals to populations in mixed-stock samples of Atlantic herring from the North and Baltic seas. We analysed 3734 herring from spawning locations and scientific catches of mixed feeding stocks to demonstrate a "one-fits-all" tool with unprecedented accuracy for monitoring spatio-temporal dynamics throughout a large geographical range with complex stock mixing. We re-analysed time-series data (2002-2021) and compared inferences about stock composition with estimates from morphological data. We show that contributions from the western Baltic spring-spawning stock complex, which is under management concern, have likely been overestimated. We also show that a genetically distinctive population of western Baltic autumn spawners, ascribed low fisheries importance, contributes non-negligible and potentially temporally increasing proportions to mixed-stock aggregations, calling for a re-evaluation of stock definitions. MSA data can be implemented in stock assessment and in a variety of applications, including marine ecosystem description, impact assessment of specific fleets, and stock-rebuilding plans