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New Research Highlights the Importance of Density-Dependent Factors in Fisheries Management: MIDs analysis

2025.1.15

Fisheries management relies heavily on the concept of maximum sustainable yield (MSY), a principle that has been at the forefront of global and European Union policies for decades. Under the EU’s Common Fisheries Policy, maintaining fishing catches at or below MSY levels has been a cornerstone of sustainable fisheries management. A critical component of this approach is FMSY—the level of fishing mortality that achieves the maximum sustainable yield over the long term.

Recent studies, however, highlight the need for a paradigm shift in how FMSY is calculated and implemented. Traditionally, FMSY estimates assume that fish growth and natural mortality remain constant, with density-dependent effects confined to the early stages of fish life (e.g., recruitment). But this simplification excludes the variability in growth, natural mortality (M), and maturity that occurs as fish populations respond to environmental conditions and food availability which can also be linked to density. New research, published in the ICES Journal of Marine Science, challenges this conventional approach and underscores the significant impact of density dependence on FMSY and MSY estimates.

You can find the recent paper here 

Key Findings from the Research

The research explored how accounting for density dependence in fish growth and natural mortality can affect fishing mortality reference points. It evaluated the effects of density-dependent growth and natural mortality on FMSY using age-structured models, estimated FMSY with simpler surplus-production models that incorporate density-dependent dynamics, and compared these results with those from traditional models assuming constant growth and M.

The findings show that incorporating density-dependent factors leads to higher FMSY estimates compared to conventional models. In many cases, MSY values also increased when density dependence was included, though the changes were less pronounced than for FMSY. Surplus-production models that account for density dependence produced estimates of FMSY closer to the “true” value than age-based predictions that assumed constant growth and M. These results suggest that overlooking density-dependent processes in fisheries models may result in overly conservative quotas, potentially limiting sustainable fishing opportunities.

Implications for Fisheries Management

The implications of these findings are significant for fisheries management. By including density-dependent growth and natural mortality, managers can align quotas more closely with biological and environmental realities, adopt adaptable strategies that reflect changes in fish growth and survival, and address systematic biases in current FMSY calculations. This could lead to healthier fish stocks and ecosystems over time. However, adopting these advanced methods is not without challenges. Estimating density-dependent effects introduces more complexity and uncertainty into stock assessments. Higher FMSY values could increase fishing pressure, risking overfishing if models are not carefully calibrated. Additionally, significant changes in data collection, modeling, and policy frameworks would be required to implement these methods effectively. The authors of the research recommend a cautious approach, supported by robust data collection and management strategy evaluations. These evaluations are essential to test the robustness of new methods under various scenarios and mitigate risks of unsustainable fishing practices.