Estimation of a stock–recruitment relationship for Black Sea anchovy ( Engraulis encrasicolus) under the influence of nutrient enrichment and the invasive comb-jelly, Mnemiopsis leidyi

Abstract

The Black Sea is typical of a highly degraded marine ecosystem. Nutrient loads increased dramatically in the 1970s, which appeared to benefit the Black Sea anchovy ( Engraulis encrasicolus). However, the accidental introduction of the comb-jelly Mnemiopsis leidyi, which preys on young anchovy and competes with adults, contributed to a collapse in the anchovy fishery in the late 1980s. Recent efforts to model Black Sea anchovy population dynamics emphasize general environmental influences and do not take these ecosystem shocks explicitly into account. In this paper, I estimate anchovy recruitment using the traditional Ricker model and under rapid nutrient enrichment. Mnemiopsis’ influence on anchovy is captured as a shift in the parameters of the recruitment function. By incorporating these environmental disturbances, the seemingly monotonic relationship between parent stock and recruitment for anchovy disappears and is replaced by a dome-shaped stock–recruitment relationship subject to regime shifts.