Nonlinear response of Black Sea pelagic fish stocks to over-exploitation

A 3-trophic level prey-predator model investigated the underlying nonlinear dynamics governing long-term (1960 to 1999) changes in pelagic fish stocks in the Black Sea. The model first explored the steady state dynamics under various combinations of consumption, harvesting and mortality rate values, and identified the critical parameters and their ranges that control equilibrium characteristics of fish stocks in the pristine state of the ecosystem. This knowledge was then used to describe progression of the stocks under temporally varying harvesting regimes. Although idealized within the structure of the model, the simulations reproduced empirical observations reasonably well. The model possesses different single equilibrium solutions during different phases of the system and associated regime shift dynamics. The late 1960s represented the disruption period of heavilyexploited top predator stock and the successive proliferation of weakly-exploited small and medium pelagic stocks. The high stock regime of small pelagics persisted for a decade and then rebounded back and forth between the low and high abundance regimes. The 1990s ecosystem represented a gradual switch of small pelagics to a high stock regime, whereas other groups preserved their low stock regimes. Simulations further explored optimum harvesting conditions for balanced stocks of small and medium pelagics by the end of next decade. The present study highlights how a simple model, when carefully tuned, may provide detailed information on fish stock dynamics and realistically reproduce the empirical observations. The model also illustrates the value of a minimalist deterministic approach for multi-species fishery management strategy development.