Trends of anchovy (Engraulis encrasilocus, L.) biomass in the northern and central Adriatic Sea
Keywords:anchovy, Adriatic Sea, Virtual Population Analysis, Laurec-Shepherd tuning, DeLury model with recruitment index
Anchovy (Engraulis encrasicolus, L.) is one of the most important commercial species of the northern and central Adriatic Sea. The mean annual catch of anchovy estimated by IRPEM for these areas, in the time interval 1975-1996, is equal to 25,000 tonnes. Estimates of anchovy stock biomass at sea in the time interval 1975-1996 were obtained using two population dynamics methods based on different data inputs: Virtual Population Analysis (VPA) and the DeLury model with recruitment index. VPA was carried out tuning the estimated fishing mortality rate at age by fitting on corresponding Catch Per Unit of fishing Effort (CPUE). Both VPA and the DeLury model yielded sensible results. The effect on the assessments due to the use of a different birth date and thus of split-year data was investigated. Biomass values as well as patterns over time so estimated were similar on the basis of both assessment methods and calendar year versus split-year data. In particular, the biomass in more recent years (around 100,000 tonnes) was lower than in the second half of the 1970s and first half of the 1980s (over 200,000 tonnes). The minimum value (lower than 50,000 tonnes) was always estimated in 1987, when a strong drop in the catch and crisis of the anchovy fishery took place. Though high values of both fishing effort and fishing mortality/exploitation rate were obtained for some years before 1987, very low levels of recruitment in 1986 and 1987 seem to be mainly responsible for the collapse of the stock.
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