Interannual changes of biological properties of walleye pollock Theragra chalcogramma in the central Bering Sea

Walleye pollock, Theragra chalcogramma, is a key species in the Aleutian Basin ecosystem. The horizontal distribution and biological characteristics of the pelagic walleye pollock were examined in the Aleutian Basin in order to elucidate any interannual changes that may have occurred during the last decade. Historical survey data obtained between 1989 and 1999 from Echo Integration Mid-water Trawl Surveys (EIMWT) were used to analyze distributions, length-age relationships, spawning activities, and parasite infection. Commercial fisheries in the Aleutian Basin showed a peak in catches in the late 1980s, and these rapidly declined in the early 1990s. Acoustic surveys indicated a geographical contraction of summer distribution in the early 1990s, in accord with the decrease in catches by the commercial fishery. Winter survey results revealed that pollock became concentrated in the Bogoslof spawning area, as the spawning biomass decreased in the 1990s. Age compositions showed that the 1978-year class was dominant in the early 1990s, and was replaced by the 1989-year class in the mid- to late-1990s. At the same time, length at age increased, and the modal length of spawning pollock increased from 480 mm in 1989 to 560 mm (male) and 580 mm (female) in 1999. Over the last decade, the historical strong 1978-year class disappeared, and significant changes were observed in the biomass, horizontal distribution, size composition, growth, and spawning period in Aleutian Basin pollock stock. Environmental change associated with the decadal-scale climate variability was thought to be responsible for the interannual changes in some of the biological properties. Under this environmental regime, there were biomass fluctuations and biological properties, such as growth and fecundity, may have been influenced by density-dependent effects.