The rise and fall of the Barents Sea capelin (Mallotus villosus): a multivariate scenario

Abstract

The Barents Sea capelin is a major forage species for many apex predators, for example, fish, marine mammals, sea birds, and man. The fishery for Barents Sea capelin increased exponentially in the early 1970s with the demise of the Atlanto-Scandian herring fishery along the Norwegian coast. The collapse of the herring spawning-stock resulted in the disappearance of young herring from the southern Barents Sea and the diet of the larger (ca. 3+ ages), piscivorous cod. Since then the only major dietary alternative for these larger cod has been capelin. The over-all trend in the biomass of the Barents Sea cod. however, has been one of decline since the late 1940s. As there was a marked decrease in the biomass of the 3+ ages of cod from ca. 3 • 106 tonnes to ca. 1 • 106 tonnes from 1974 to 1983, this would have simultaneously caused a great reduction in the predation pressure from cod on the capelin. The Barents Sea capelin is an r-strategist. Its weight-at-age is inversely related to population size, and thus the number of cohorts and subsequently age-at-maturity are positively related to population size. Principal components analysis (PCA) indicates that the biomass of 2+ ages of capelin and population characteristics already exhibited a declining trend from about 1975, despite a declining cod stock. Coupling of independent Russian and Norwegian data clearly shows that the percentage occurrence of prawns in the stomach of 3+ cod increased disproportionately from the mid-1970s despite a decrease in the cod biomass, probably indicating a lack of capelin as primary-choice prey. The nominal capelin catch between 1978 and 1983 is probably an unreliable estimate of fishing mortality. A drastic decline in the capelin stock occurred after 1983/1984, with marked predation from the potentially strong 1983 cod year-class. The capelin, as an r-strategist, is very difficult to manage in the fluctuating biological and climatic environment of the Barents Sea. Fisheries for capelin should be limited within the framework of multispecies perspectives, with much of its production being primarily used to sustain an elevated long-term yield of cod.