Echo integration-trawl survey of walleye pollock (Theragra chalcogramma) in the Southeastern Aleutian Basin during February and March 1995

Abstract

Results from the winter 1995 echo integration-trawl survey of spawning walleye pollock (Theragra chalcogramma) in the southeastern Aleutian Basin near Bogoslof Island are presented. The survey, conducted in two passes between 26 February and 9 March, covered an area between 165°51'W and 170°27'W long., from the Aleutian chain north to between 53° 45' N and 54° 40' N lat. Isolated pollock aggregations were encountered off the edge of the shelf north of Akutan Island and over deep water northeast of Bogoslof Island. Extremely dense pollock aggregations were observed along the north side of Umnak Island from 168° W to 169° 30' W long. Pollock vertical distribution ranged from 250 m to 750 m below the surface; their average depth was higher in the water column during pass 2 than during pass 1. Sex composition in hauls ranged from 8% to 94% female, averaging around 60% female. Evidence of vertical stratification by sex showed males inhabiting lower depth layers than females. Little evidence of a "non-spawner" component of the pollock biomass (as had been observed in 1993 and 1994) was observed, and most female pollock were in a pre-spawning reproductive state. The spawning biomass estimate, 1.10 million metric tons (I), was more than twice the March 1994 estimate of 0.49 million t. This large increase in the Bogoslof spawning population can be partially explained by strong recruitment of pollock from the 1989 year class. However, population and biomass increases also occurred across most of the age groups. Potential reasons for the population increase are discussed.


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