Factors Limiting Net Pen Rainbow Trout Production at Banks Lake, WA

The Washington Department of Fish and Wildlife implemented the Banks Lake Fishery Evaluation Project (BLFEP) in September 2001 with funds from the Bonneville Power Administration.  Despite annual stocking events, rainbow trout represent a small proportion of fish caught in the annual creel survey.  The area of study was restricted to 3.4 km radiating north from Dry Falls Dam to monitor net pen releases occurring only on the south end of Banks Lake.  The objective of this project was to evaluate the abiotic and biotic variables limiting net pen rainbow trout production in Banks Lake including water temperature and dissolved oxygen, size of prey and zooplankton densities, entrainment, and acute predation by piscivores.  Water temperature and dissolved oxygen were within the physiological limits for rainbow trout survival.   Daphnia spp. (>1.0 mm) suggests that the prey base is sufficient for planktivores.  Entrainment netting and hydroacoustic monitoring was conducted in 2009 and 2010.  No rainbow trout were captured in 2009 and net modifications were made to increase the probability of capturing rainbow trout the following year.  In 2010, hydroacoustic estimates of entrainment and netting for species verification both showed increased entrainment rates in the four weeks following a release of 100,000 rainbow trout suggesting that entrainment limits rainbow trout production.  Gillnetting surveys were conducted to collect diets from smallmouth bass and walleye before and after net pen releases to quantify species-specific predation rates on rainbow trout released from net pens.  Smallmouth bass were the most abundant predatory fish in Banks Lake, but their diets consisted of primarily crayfish before and after the trout release.  Walleye are the second most abundant predator in Banks Lake and their diets shifted from primarily yellow perch to rainbow trout following the release.  This data suggests that walleye are the main predatory species in Banks Lake and acute predation may play a significant role in the survival and overall sustainability of rainbow trout.