Midwater Trawling for Sockeye Salmon Underyearlings (Oncorhynchus nerka) at Babine Lake, 1967

This report describes the method and results of midwater trawling for underyearling sockeye salmon (Oncorhynchus nerka) at Babine Lake, British Columbia, in 1967. The work was part of a joint Fisheries Research Board - Fisheries Service program to assess a large-scale sockeye development project (Canada Department of Fisheries, MS, 1965). One of the requirements of assessment was to measure changes in the density and distribution of young sockeye in the lake nursery area as the size of the population was increased. Estimates of density had been made for sockeye in Babine Lake by Johnson (1956, 1958) using catch per unit effort by surface townetting and later by McDonald (1969), using purse seines. However, the catch by both types of gear was extremely variable and the resulting estimates of density did not provide the desired precision. The limitation of these techniques appeared to stem mainly from rapid changes in the vertical distribution of the fish and, thus, their availability to the gear. Townets were effective only for a period of an hour or less at evening twilight. Seines caught sockeye throughout the night but not by day. Apparently at dawn the young sockeye descended somewhere below the depth of the seine (about 16 m).