Stock dynamics and predator-prey effects of bonito and bluefish as top predators in the Black Sea

Abstract

This study uses surplus production model based methods for data-poor stocks (BSM and CMSY) to assess the stocks and estimate key reference points of bonito and bluefish in the Black Sea. Our results demonstrate that the CMSY method, using catch data only, yields similar results to the more accurate BSM method, fitted with CPUE data, and therefore is suitable in assessing of data-poor stocks. We explore the ecological impacts of the two stocks on other commercial species and compare impacts of predation and fishing. Prior to 1995, the consumption of bonito and bluefish on anchovy, horse mackerel, and sprat, exceeded the removal of those prey species by the fisheries. Later on, the trends reversed, with catches of prey species becoming more than three times higher than their predation by bonito and bluefish. Horse mackerel, the main prey of bluefish has declined to critical levels since 1995, which is likely contributing to the general decline of bluefish, along with overfishing. Our results demonstrate, that heavy fishing of bonito and bluefish, have contributed to their current depleted states. The biomass trajectories of the two stocks display regime shift-likedynamics. Due to the present steady positioning of low stock regimes, the recovery of the two stocks, would need decisive and possibly prolonged measures, including reduction of fishing pressure, efficient control of under-sized catch, as well as insuring sufficient prey biomass availability, by keeping small pelagic prey stocks at safe levels.