The Synergistic Effect of Climate Change and Fishing on Multiple Populations of Atlantic Cod (Gadus morhua).

The world´s oceans are impacted by multiple, cumulative stressors of which overfishing and climate change are believed to be the most disruptive for marine populations and ecosystems. While the consequences of these stressors on marine systems have been extensively studied, there is still much to explore on their mechanisms of interaction i.e. whether they act in an additive or synergistic way. Moreover, the variable responses of populations from one species to climate change have been often overlooked. Here, we apply an analytical technique based on catastrophe theory, the stochastic cusp model, to unravel the effect of temperature changes and fishing pressure on 20 cod (Gadus morhua) stocks distributed across the North Atlantic. We find that fishing pressure and temperature act in a synergistic way, inducing non-linear discontinuous dynamics in stock biomass. In particular, while fishing pressure controls the biomass of the stock, temperature modifies their relationship from linear to discontinuous. The effect of fishing on the North Atlantic cod stocks and their response to warming varies both along a north-south and an east-west gradient. Our study highlights the significance of understanding the interaction between climate change and other stressors which can trigger non-linear dynamics difficult to foresee and manage. Moreover, the study points towards the great importance of understanding the variable dynamics of multiple populations within a species in order to apply differentiated and effective management approaches under climate change.