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S. B. Mathews 



School of Fisheries 

 University of Washington 

 Seattle, WA 98195 



School of Fisheries 



University of Washington 



Seattle, WA 98195 



Present address: 



NorthemSoutheastem Regional Aquaculture 



Association 

 103 Monastery, Sitka, AK 99835 



M. LaRiviere 



DIGESTION RATES AND GASTRIC 



EVACUATION TIMES IN RELATION TO 



TEMPERATURE OF THE SACRAMENTO 



SQUAWFISH, PTYCHOCHEILUS GRANDIS 



Squawfish, Ptychochilus sp., are large piscivorous 

 cyprinids which have a reputation of being major 

 predators on salmon and trout, although documen- 

 tation for this is poor. Brown and Moyle's (1982) 

 review on squawfish concluded that squawfish are 

 not likely to affect salmonid populations in free flow- 

 ing streams (Falter 1969; Ebel 1970; Buchanan et 

 al. 1980, 1981), but that significant predation could 

 occur in areas where streams are altered (dams, 

 diversions) and in relation to fish releases. 



Sacramento squawfish, Ptychocheilus grandis, 

 have been reported to prey heavily on juvenile 

 salmonids in the Sacramento River, CA, especially 

 below Red Bluff Diversion Dam (RBDD) (Hall 1977), 

 and have been implicated in the continuous decline 

 of Chinook salmon, Oncorhynchus tshawytscha, in re- 

 cent decades (U.S. Bureau of Reclamation 1983, 

 1985). Currently governmental agencies charged 

 with the management of anadromous fishes in 

 California are attempting to decrease the number 

 of squawfish in the Sacramento River, especially 

 near RBDD. The justification for Sacramento 

 squawfish removal is based on a report by Hall 

 (1977). Unfortunately, the estimate of squawfish 

 predation rates by Hall (1977) were made without 

 knowledge of the digestion rates or gastric evacu- 

 ation times of Sacramento squawfish in relation to 

 temperature and is likely an overestimate. 



Bentley and Dawley (1981) found that northern 

 squawfish, P. oregonensis, consumed 14.3 g offish 

 per day at 10°C. Based on this estimate Sacramento 

 squawfish below RBDD would consume only 3 or 

 4 salmon/day (mean size of hatchery salmon released 

 into the Sacramento River is 4.0 to 5.0 g). This esti- 

 mate is lower than the 20 salmon/day calculated by 

 Hall (1977) for Sacramento squawfish below RBDD. 

 The ability of predatory fish to consume prey is 

 mediated, at least in part, by the digestion rate and 

 the extent of gastric evacuation (Grove and Craw- 

 ford 1980; Jobling and Wandsvik 1983). Several 

 workers (Falter 1969; Steigenberger and Larkin 

 1974; Persson 1979, 1981, 1982; Jobling 1980; Smith 

 1980; Hofer et al. 1982) have shown that digestion 

 rates in fishes increases with increasing tempera- 

 ture. 



The purpose of this study was to determine diges- 

 tive rates and time for gastric evacuation of the 

 Sacramento squawfish in relation to temperature. 

 Sacramento squawfish digestion rates increased 

 with increasing temperature, while evacuation times 

 decreased with increasing temperature. 



Methods 



Sacramento squawfish (x = 370 mm standard 

 length [SL], range = 300-456 mm SL) were cap- 

 tured, using hook and line or a boat electrofisher, 

 immediately below Red Bluff Diversion Dam 

 (RBDD). The length-weight relationship for Sacra- 

 mento squawfish was Y = 4.03 -t- 2.66X Fish were 

 transported to University of California, Davis, and 

 treated immediately with nitrofurazone or potas- 

 sium permagnate. The fish were held for several 

 days at their capture temperature before the tem- 



FISHERY BULLETIN: VOL. 85. NO. 1, 1987. 



159 



