Fisheries Survey of the Limnetic Zone of Sullivan Lake, Washington, Using Hydroacoustics and Gill Nets, September 2003

We conducted a fisheries survey of the limnetic zone of Sullivan Lake, Pend Oreille County, Washington during September of 2004. The study represented one of several efforts by WDFW, the Kalispel Tribe, the U.S. Forest Service, and Eastern Washington University (EWU) to better understand the physical and biological processes affecting the status, life history, and ecological interactions of various aquatic species in Sullivan Lake.

The objectives of this study were to evaluate the species composition, depth distribution, density, and abundance of fishes in the limnetic zone.

We conducted a gill net survey between September 23-26, 2003, comprised of 51 overnight gill net sets and a hydroacoustic survey that included 27 transects on September 23. Kokanee and cutthroat trout were the dominant fish species captured in limnetic gill nets, comprising 77% and 12% of the relative abundance, respectively. Vertical distribution of acoustically detected fish was highest between 12 and 20 m depth, with relatively few fish detections below 30 m. Mean density of all acoustically detected fish (30-800 mm) was 7 fish per 10,000 m³, or 409 fish per hectare. The abundance of kokanee age-1 to 3 was 67,000 (120 kokanee per hectare), with approximately 10,000 age-3 kokanee. Length-at-age and relative weight for all kokanee, and relative weight for cutthroat trout over 300 mm were below the national average, indicating that food resources may be limiting fish production. This result was consistent with the EWU analysis that showed oligotrophic conditions and a zooplankton community that had the characteristics of size selectivity and heavy predation by zooplanktivorous fishes.

This study should provide fishery managers the necessary information to make informed decisions, especially when combined with the results of other ongoing research on Sullivan Lake.