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r, , '~--1#--Jv~j- Hcd/~Grjswold . I q C( 2. 680 Transactions of the American Fisheries Society 121 :68~85. 1992 NOTES ~ (j "') (,LBG AK ~ lt~f'2-y'l -- Predation by Northern Squawfish on Live and Dead Juvenile Chinook Salmon DENA M. GADOMSKI AND JUDY A. HALL-GRISWOLD. U.S. Fish and Wildlife Service National Fisheries Research Center-Seaule Columbia River Field Station Cook. Washington 98605. USA Abstract.-Nonhern squawfish Ptychocheilus oregon- ensis is a major predator of juvenile salmon ids Onco- rhynchus spp. migrating downstream through the Co- lumbia River. High predation rates occur just below dams. Ifnonhern squawfish selectively consume salmo- nids killed or injured during dam passage, previous es- timates of predation monality may be too high. We con- ducted laboratory experiments that indicate nonhern squawfish prefer dead juvenile chinook salmon O. tshawytscha over live individuals. When equal numbers of dead and live chinook salmon were offered to nonh- ern squawfish maintained on a natural photoperiod (15 h light: 9 h darkness), significantly more (P < 0.05) dead than live fish were consumed, both in 1,400-L circular tanks and in an 11,300-L raceway (62% and 79% of prey consumed were dead, respectively). When dead and live juvenile chinook salmon were provided in proponions more similar to those below dams (20% dead, 80% live), nonhern squawfish still selected for dead prey (36% of fish consumed were dead). In additional experiments, nonhern squawfish were offered a proponion of 20% dead juvenile chinook salmon during 4-h periodS of ei- ther light or darkness. The predators were much more selective for dead chinook salmon during bright light (88% of fish consumed were dead) than during darkness (31 % were dead). The development of hydroelectric power has caused populations ofanadromous salmonids On- corhynchus spp. to decline in the Columbia River for a variety of reasons (Raymond 1968, 1988). Juvenile mortality during downstream migration has increased due to injuries from passage through dams. Schoeneman et aI. (1961) reported mortal- ities of 11 % for juvenile salmon ids passing through turbines and 2% for those passing through spill- ways at McNary Dam on the Columbia River. Predation is more intense near dams due to higher I Present address: HC-64 Box 9965, Stanley, Idaho 83278, USA. densities of both predators and juvenile salmonids (Sims et aI. 1981; Vigg 1988; Beamesderfer and Rieman 1991). A dominant predator in the Co- lumbia system is the northern squawfish Ptycho- cheilus oregonensis. which was estimated by Rie- man et aI. (1991) to have ingested 11 % of all juvenile salmonids entering John Day Reservoir via McNary Dam (immediately upstream) during 1983-1986. Twenty-six percent of this loss was in a 0.6-km2 area (0.3% of the total reservoir area) immediately below McNary Dam (Rieman et aI. 1991). Ifnorthern squawfish feed on dead or moribund juvenile salmonids, mortality due to predation may be less than previously estimated. The propor- tions oflive, injured, and dead juvenile salmonids consumed by northern squawfish in John Day Reservoir are unknown because the condition of prey at the time of capture could not be deter- mined from gut samples (Poe et aI. 1991). The high density of northern squawfish below McNary Dam (Beamesderfer and Rieman 1991) could be due in part to a preference for dead or injured prey. Predators may capture higher-than-expected proportions of weaker individuals from prey pop- ulations (Coutant et al. 1979; Temple 1987), al- though such differential predation may be more a result of differences in prey vulnerability than predator selection. The objectives of our study were to determine whether northern squawfish consume dead juve- nile salmonids, and, if so, in what proportions when both live and dead prey are available. Ex- periments were conducted with two proportions of available dead juvenile salmon, 50% and 20%. The 50%-dead experiments were designed to test general northern squawfish selection when live and dead salmon were equally available; the 20%-dead experiments more closely approximated propor- tions of dead salmonids available below dams. Additionally, we compared predation on live and dead salmon during light and dark periods, be- cause this information would aid in understanding diel feeding patterns observed in the field and in evaluating the best times to release juvenile C:;'