Effect of anthropogenic and environmental factors on the blue whiting Micromesistius potassou off the Catalonian coast, 1950-1982

Abstract

Measurements were analysed from a total of 144 781 individuals of blue whiting Micromesistius poutassou Risso sampled from commercial landings at Blanes (1950 to 1964) and Barcelona (1961 to 1982). Spain. Total length ranged from 60 to 410 mm. The left-skewed length frequency distribution shows heavy fishing mortality. Length groups 190-260 mm (1 to 3 yr old) support the fishery. Overall mean length for the period was 204 +/- 0.12 mm. Annual mean length varied as a function of the relative frequency of the smaller lengths and of strong recruitment to the fishing grounds, attaining highest values every 10 to 11 yr (1955/56, 1967, 1977). Optimal recruitment seems to be correlated with high primary production levels and with a delay in thermocline formation. The latter may be related to this species' change from a pelagic to a benthic habitat. A heavy increasing fishing effort since 1967 unbalanced environmental interactions with the population and lowered the annual mean length. Analysis of historical series and thelr relationships with environmental factors may be helpful in predicting yield levels.