Acoustic and Trawl Surveys to Locate Eulachon Aggregations in the Lower Fraser River, British Columbia

Abstract

AbstractCoordinated acoustic and trawl surveys were performed to locate aggregations of adult eulachon Thaleichthys pacificus in the lower Fraser River, British Columbia, during spring 2001 and 2002. Aggregations were encountered in one portion of the study area in both years. They contained mature, spawning, and spent eulachon of both sexes, of which over 80% were spent males. These groups were 880–2,000 m long and averaged 117–140 m wide. Within them, mean fish density was 0.27–0.29 fish/m3 and species composition was 87–99% eulachon. They occurred in 6–8.5 m of water over substrate that was mainly sand with a small fraction of pea gravel. We did not determine whether these eulachon spawned in that area or had congregated there for another purpose, such as holding or migration. Other species occasionally encountered in appreciable numbers were juvenile pink salmon Oncorhynchus gorbuscha and chum salmon O. keta. Mean in situ target strengths of eulachon were −53.3 dB at 123 kHz and −50.3 dB at 208 kHz. Coordinated acoustics and trawling were effective for locating eulachon aggregations at low to moderate flows (river discharge of 3,200 m3/s or less), but were hampered by entrained debris during higher flows of spring runoff. Under suitable river conditions, the high resolution and rapid spatial coverage of this technique are apt for habitat preference studies and point source environmental assessments in large, turbid rivers.