Origins of Chum Salmon Caught Incidentally in the Eastern Bering Sea Walleye Pollock Trawl Fishery as Estimated from Scale Pattern Analysis

Abstract

Approximately 74,500 chum salmon Oncorhynchus keta were intercepted in the 1994 U.S. walleye pollock Theragra chalcogramma B-season fishery in the eastern Bering Sea and Aleutian Islands. Using scale pattern analysis, we estimated the stock composition of age-0.3 chum salmon (fish that had spent three winters in the ocean) from this incidental catch. A conditional maximum-likelihood discrimination model, assessed through a series of simulation runs using hypothetical stock mixtures, was 83.3–92.3% accurate. Our fleetwide, unstratified proportion estimates closely resembled results of a concurrent stock composition study based on allelic frequencies of the 1994 chum salmon bycatch. Interception estimates weighted by time, which depend on the accuracy of National Marine Fisheries Service week-stratified bycatch estimates, indicated that about 50% of the incidentally caught chum salmon originated from Asia (Russia and Japan), 18% from western and central Alaska, and 32% from southeastern Alaska, British Columbia, and Washington. The western and central Alaskan proportion increased over the course of theB-season fishery, although the numbers intercepted remained stable. A comparison of our regional interception estimates with estimated run sizes indicates that bycatch in the 1994 B-season walleye pollock fishery did not greatly affect returns to western Alaskan chum salmon fisheries.