Stock assessment of the Black Sea anchovy
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The Black Sea anchovy (Engraulis encrasicolus) is economically the most important fish species in Turkey since it provides 60% of the total fish catch. The Turkish fleet catches 90% of the anchovy stock in the Black Sea due to the anchovy winter migration to the Turkish coast. Over the last 50 years, the Black Sea basin has been subjected to pollution due to human sourced nutrient accumulation in the NW basin via the rivers Danube, Dniester and Dnieper and transportation. The ecosystem has been destroyed by the invasion of alien species and overfishing. Moreover, top predators have been overfished and could not recover completely, including marine mammals and large pelagic fish species. Small pelagic stocks benefited from this situation and increased in abundance, however, the invasion of the ctenophore Mnemiopsis leidyi caused large damage to stocks especially the anchovy stocks. Stock assessment studies have been undertaken for different reasons, and main reasons initiated from the extreme decrease in anchovy stocks and especially in the last decade driven by accession negotiations with the European Union. Even though the first stock assessment study of anchovy stock was done in 1988 based on the surplus production model, it could not be carried out comprehensively again until the last decade. The Turkish fishery statistics based on commercial catch data and this data prevents using the analytical model which is necessary for an appropriate stock management. The aim of the study is to explain the fluctuation of the anchovy stock as a function of fishing effort, using surplus production models. The models were applied, assuming both equilibrium and non-equilibrium conditions. Catch data for 44 years, between 1968 and 2011, were obtained from TurkStat. Fishing effort data were determined according to four scenarios: the number of boats larger than 10 m, total engine power of the boats larger than 10 m, the number of purse seiners and multi purpose trawlers, and sonar effect added form of the third scenario. Examination of the scenarios was achieved using Schaefer and Fox models and additionally Prager’s non-equilibrium based production model namely ASPIC. The non-equilibrium approach models provided more reliable results and closer estimations to the real data, when compared to equilibrium approach models. Among ASPIC simulations best fit was obtained from the Gengrid model. None of the model simulations could explain the sharp decrease observed in 1990 and 2005.