Synergistic effects of fishing-induced demographic changes and climate variation on fish population dynamics

Abstract

The synergistic effects of fishing, climate\nand internal dynamics on population fluctuations are\npoorly understood due to the complexity of these interactions.\nIn this paper, we combine time series analysis and\nsimulations to investigate the long-term dynamics of\nan overexploited population in the Mediterranean Sea,\nand its link with both fishing-induced demographic\nchanges and hydroclimatic variability. We show that the\ncyclicity of the catch per unit of effort (CPUE) of European\nhake Merluccius merluccius (EH) vanished in the\n1980s, while the correlation between the CPUE and a\nlocal environmental index increased. Using simulations,\nwe then show that the cyclicity observed in the EH biomass\nbefore the 1980s can have an internal origin, while\nthat its disappearance could be due to the fishinginduced\nerosion of the age structure. Our results suggest\nthat fishing can trigger a switch from internally generated\nto externally forced population fluctuations, the\nlatter being characterised by an increasing dependency\nof the population on recruitment and ultimately on environmental\nvariability. Hydroclimatic modifications occurring\nin the Mediterranean in the early 1980s could\nhave enhanced these changes by leading to a mismatch\nbetween early life stages of EH and favorable environmental\nconditions. Our conclusions underline the key\neffect of the interaction between exploitation and climate\non the dynamics of EH and its important consequences\nfor management and conservation.