Overfishing or environmental change: Establishing the frequency of changes in productivity of marine fish stocks

The relative importance of environmental conditions and stock abundance in determining the productivity of fish stocks has been a subject of an on-going debate. The controversy can be formulated as four competing hypotheses: 1) productivity is driven by fishing pressure, which affects abundance, subsequent recruitment; 2) productivity is regime-driven, with periods of good and bad productivity unrelated to abundance; 3) productivity is random from year to year and unrelated to abundance and is temporally uncorrelated; and 4) both stock abundance and regimes of good and bad conditions interact to affect productivity. The goals of this study are (1) to evaluate the support for each of these hypotheses by examining the productivity of marine species using a large number of stocks, and (2) to evaluate the same hypotheses with respect to recruitment. This project uses historic data from about 230 assessments from the RAM Legacy Database. Each of the four hypotheses will be formulated as alternative models, and the support for the hypotheses evaluated using model selection via AICc and AICc weights. The specific models are (1) a biomass-dynamic model relating surplus production to stock size, (2) a regime shift model accounting for temporal shifts in productivity; (3) a model that assumes productivity to be random and (4) a biomass-dynamics model that has regime changes in productivity parameters. Then a similar analysis was performed on recruitment. I found that when considering production the Abundance Hypothesis best explains 18.3% of stocks, the Regimes Hypothesis 38.6%, the Mixed Hypothesis 30.5%, and the Random Hypothesis 12.6%. When considering recruitment, the stock-recruitment Hypothesis best explains 17% of stocks, the Regimes Hypothesis 45%, the Mixed Hypothesis 21% and the Random Hypothesis 18%. If the production of a stock is determined by periodic regimes and the assessment of the stock does not recognize the shift in regimes, then the management system with respect to sustainable yield is incorrect. I do not suggest that we should abandon the goal of maintaining fish stocks at high abundance. Rather I simply show that it is unlikely that such policies will assure high and sustained recruitment. Thus, future work should identify and evaluate management strategies that would be robust to irregular jumps in average productivity.