Salmonid Consumption in the Sanpoil River Arm of Lake Roosevelt by Walleye Using Bioenergetic Modeling

Rainbow trout Oncorhynchus mykiss and kokanee salmon O. nerka, make adfluvial migrations out of the Sanpoil River into Lake Roosevelt. Despite enhancing natural reproduction for rainbow trout and stocking hatchery-raised kokanee, poor numbers of both species have returned to the Sanpoil River in recent years. Predators staging at the river/lake interface during the juvenile migration season are thought to have consumed large numbers of these migrants. To determine what percent of kokanee salmon and rainbow trout were consumed by non-native walleye Sander vitreus, we performed mark/recapture studies, utilized gastric lavage and computed salmonid consumption with age-specific bioenergetic modeling. The Spokane Tribal Hatchery released 10,080 kokanee yearling and 589,580 kokanee fry into the river. A mark/recapture study utilizing a rotary screw trap estimated a population (± 95% CI) of 14,578 (5,117 - 24,038) redband rainbow trout yearlings and 23,738 (8,332 - 39,143) 2 and 3-yr old spring-run redband rainbow trout migrated out of the river into Lake Roosevelt. Walleye population estimates were made over 26 occasions from 24 March to 14 July, 2010, which recaptured 101 of 1,456 double and triple marked walleye. Program MARK software was used to estimate their population (± 95% CI) at 12,257 (7,223 - 17,292), which was further stratified into age classes using an age/frequency table. Gastric lavage data from 321 walleye (130 - 540 mm total length) showed that kokanee salmon and rainbow trout made up 61.8 % and 12.3 % of the diet by weight, respectively. Input variables for the Wisconsin Fish Bioenergetics 3.0 software included: temperature, age-specific growth, dietary weight percentages per sampling week, and prey caloric density. Walleye consumed (± 95 % CI) 469,991 (276,551 - 662,084) (79.7 %) of the 589,580 released kokanee fry, 3,767 (2,216 - 5,306) (37.4 %) of the 10,080 released kokanee yearlings, 2,688 (1,606 - 3,845) (18.4%) of the 14,578 estimated rainbow trout yearlings and 6,394 (3,763 - 9,009) (26.9%) of the 23,738 estimated rainbow trout 2 and 3 year olds.