Assessment of adult salmon in near-surface waters of Cook Inlet, Alaska

Important commercial fisheries occur in the marine and estuarine waters of Upper Cook Inlet in south-central Alaska as adult Pacific salmon return to spawn in their natal rivers. The most valuable harvests occur on sockeye salmon (Oncorhynchus nerka (Walbaum)) runs to the Kenai, Kasilof, and other regional rivers. Fisheries management has been based on commercial catches, acoustic counts of the fish in the rivers, and run-timing models. However, when run sizes are too low for the fishery to operate, managers require alternative information. Acoustic techniques potentially provide a fishery-independent measure of run size. However, the near-surface orientation of these fish precludes a conventional down-looking acoustic approach. Side-looking transducer orientation achieved sufficient sample coverage to provide a viable assessment. Comparisons were made between side-looking assessments and ground truth information. The detection efficiency of the side-looking system was investigated as a function of sea state and bottom depth, and three surveys were conducted for comparison with abundance data from the fisheries. The results of the three surveys indicated that the acoustic techniques are a viable alternative to the traditional fisheries-based management approach.