Skeena & Nass Sockeye Lakes Hydroacoustic Surveys Report 2007

Executive Summary

The Skeena Fisheries Commission 2007 sockeye (Oncorhynchus nerka) fry hydroacoustic survey program consisted of a survey of Damdochax, Wiiminosik, Lakelse, Kitsumkalum and Kitwanga lakes. Hydroacoustic data was collected using a Biosonics DT-X split beam echo sounder with a 200 kHz transducer. The Kitsumkalum and Kitwanga lake surveys replicated previous DFO surveys while the other lakes’ survey designs were modified or developed for the first time in 2007.

Limnetic fish were sampled using two different methodologies. The primary catch method was with a 2 x 2 m mid-water trawl. The second method was with two 12 m floating Swedish gillnets which had variable mesh size panels. Temperature and dissolved oxygen measurements were taken using a YSI meter. Bathymetric maps of Damdochax and Wiiminosik lakes were produced from GPS georeferenced depth data collected from the Biosonics DT-X echo sounder.

Wiiminosik Lake was the only lake where no bathymetric map was available prior to the survey. The maximum depth (25 m) of the lake was surprising since it was deeper than the much larger Damdochax Lake. The Damdochax Lake bathymetry agreed closely with the results from a previous survey.

Trawl catches in Damdochax, Wiiminosik, and Kitsumkalum lakes exceeded 45 young-of-the-year O. nerka in each lake. The “small” size class hydroacoustic estimates were apportioned 100%, 90% and 98% to young-of-the-year O. nerka for each of the former lakes. Trawl catches were poor in Lakelse and Kitwanga lakes, which make it impossible to apportion the “small” size class hydroacoustic estimates with any degree of certainty.

Of all the 8 lakes/lake basins, the highest “small” size class densities were found in the east basin of Wiiminosik Lake followed by the west basin. Damdochax Lake had the next highest density estimates followed by the Kitwanga Lake south basin. The Lakelse Lake north basin and Kitsumkalum Lake had similar, but lower, densities. When the “small” size class densities were expanded for the size of the lake to get population estimates, the trend reverses with the lakes with the highest densities having the smallest population estimates. Despite having low densities, the large size of Kitsumkalum and Lakelse lakes result in higher population estimates than Damdochax and Wiiminosik lakes.