Romagnoni, G. | Search bioRxiv

bioRxiv2023

Evidence of spatial regime shifts: contrasting spatial dynamics of three Atlantic cod (Gadus morhua) populations in the North Sea.

Recent scientific assessments have started to consider the presence of multiple populations of Atlantic cod (Gadus morhua) in the North Sea due to genetic evidence. The Atlantic cod stock as a whole has experienced a regime shift in the North Sea and is now considered to be in a persistent depleted state. Here we investigated if the regime shift detected for North Sea cod manifested differently across areas (South, North-West and Viking (North-East)), revealing different dynamics and recovery potential for the corresponding populations. Methods from change point analyses to stochastic cusp models were applied to detect the regime shifts. We found that cod in the Southern North Sea has undergone a regime shift and is now in a persistent depleted state. The Viking population is in a low state but with high resilience, making its dynamics more predictable and thus easier to manage. Finally, the north-western population did not show signs of a regime shift and is in a high biomass level. Several abiotic and biotic drivers (e.g. fishing, warming) contribute to these differences. Our study provides useful information for the restructuring of management units of cod in the North Sea and highlights the importance of looking at regime shifts spatially.

ecology↗

bioRxiv2018

When is a fish stock collapsed?

Marine fish stock collapses are a major concern for scientists and society due to the potentially severe impacts on ecosystem resilience, food security and livelihoods. Yet the general state of harvested fish populations has proven difficult to summarize, and the actual occurrence rate of stock collapses remains unclear. We have carried out a literature review and multi-stock analysis to show that numerous definitions exist for classifying stocks as collapsed, and that the classification of a stocks status is sensitive to changes in the collapse definitions formulation. We suggest that the lack of a unified definition has contributed to contrasting perceptions on the state of fish stocks. Therefore, we comprehensively define what constitutes a fish stock collapse and provide a time-series based method for collapse detection. Unlike existing definitions, our definition is process-based, because it links together three important phases of collapse events: the abrupt decline, an ensuing period of prolonged depletion, and potential recovery. Furthermore, these phases are specified in terms of population turnover. Through systematic evaluation, our definition can accurately distinguish collapses from less severe depletions or natural fluctuations for stocks with diverse life histories, helping identify the stocks in greatest need of reparatory measures. Our study advocates the consistent use of definitions to limit both alarmist and conservative narratives on the state of fish stocks, and to promote cooperation between conservation and fisheries scientists. This will facilitate clear and accurate communication of science to both the public and to policy-makers to ensure healthy fish stocks in the future.

ecology↗