Historical trends in abundance and current condition of walleye pollock in the eastern Bering Sea

Abstract

Walleye pollock ( Theragra chalcogramma) is an important commercial species throughout much of its range around the rim of the North Pacific Ocean. Along the North American coast, the largest concentrations occur in the eastern Bering Sea, where catches of pollock in 1981–1985 averaged ∼1.0 million metric tons (t). Cohort analysis indicates that the biomass of pollock may have increased substantially in the eastern Bering Sea, from < 5 million t in the early 1960s to a peak of > 13 million t in the early 1970s. This increase appears to be the result of the recruitment of a series of stronger than average year classes originating in 1965–1969. The increase in abundance coincided in timing with the development and peak years of the pollock fishery, with catches reaching a maximum of 1.9 million t in 1972. Abundance then declined and remained relatively stable at a moderately high level of 7.6–9.0 million t in 1976–1980, before increasing to ∼10.0 million t in 1981 and 1982, as a result of the recruitment of the strong 1978 year class. Abundance in more recent years has again declined to ∼8 million t in 1985, but moderately good recruitment is anticipated to arrest this decline. Hypotheses to explain the observed fluctuations in abundance of pollock, including cannibalism, replacement by pollock of environmental niches occupied by other species and temperature anomalies were discussed. Environmental conditions would appear to be the most likely cause of variation in year-class strength and population abundance.