and Steigenberger and Larkin (1974) (Table 1); how- 

 ever, the times noted in this study are bracketed by 

 the other studies. Falter's equations predict that 

 gastric evacuation would be complete in about 29 

 h at 6° and 10°C and about 10 h at 16.5° and 20°C. 

 In contrast Steigenberger and Larkin's data predict 

 that northern squawfish would complete gastric 

 evacuation in 84, 51, 23, and 13 h at 6°, 10°, 15°, 

 and 20°C, respectively. 



Table 1 .—Estimated time (hours) for total evacuation of stomach 

 contents of Sacramento squawfish held at selected temperatures. 



There were several differences in the protocol be- 

 tween the studies of Falter (1969), Steigenberger 

 and Larkin (1974), and the present investigation 

 which may account for the differences noted in 

 digestion and gastric evacuation. Differences in pro- 

 tocol included length of acclimation period, meal 

 size, predator size, and number of prey per meal. 

 Falter apparently acclimated the northern squaw- 

 fish in his experiments for variable time period, but 

 not <48 h. Falter fed only one prey item at a time 

 then sacrificed the squawfish in his studies and 

 dissected the alimentary tract. The squawfish in 

 Falter's study ranged from 100 to 550 mm total 

 length. Steigenberger and Larkin (1974) used north- 

 ern squawfish between 150 and 400 mm fork length, 

 only allowed an overnight acclimation period to each 

 temperature and also sacrificed the squawfish. In 

 this study Sacramento squawfish (size range 300-456 

 mm standard length) were acclimated for at least 

 14 d to each temperature, fed four prey items, and 

 then had their alimentary tracts pumped. Jobling 

 (1981) pointed out that gastric evacuation is volume- 

 dependent and that as the number of prey per meal 

 increases gastric evacuation time increases. 



In the present experiment digestion was linear 

 during the second phase of the digestive process. 

 Jobling (1981) stated that the difference between 

 the surface-to-volume ratio between large and small 

 food particles is important in determining the 

 pattern of digestion. Jobling further suggested 

 that an exponential function describes the digestive 



process of small, easily digested prey items, but a 

 linear expression gave the best fit for large prey 

 items. 



The digestive rates and gastric evacuation times 

 of the Sacramento squawfish in relation to temper- 

 atiire suggest that the consumption estimate of Hall 

 (1977) of 20 juvenile salmon/day per squawfish was 

 likely an overestimate. Water temperatures at 

 RBDD are typically near 4.5°C in January and may 

 increase to 10 °C by late March (U.S. Bureau of 

 Reclamation unpubl. data). Bentley and Dawley 

 (1981) found that northern squawfish only consumed 

 14.3 g of fish per day at 10°C which would equal 

 <10 fish per day even if the average size was 1.5 

 g. I suggest that squawfish would not consume 20 

 salmon/day between January and March during this 

 period when approximately 80% of the juvenile 

 salmon migrate downstream. As water tempera- 

 tures continue to increase, predation by squawfish 

 would likely also increase. However, juvenile salmon 

 migrate primarily at night in the Sacramento River 

 (Hallock and Van Woert 1954; Vondracek et al. fn. 

 1) which limits the predation by the visually oriented 

 squawfish. 



I suggest that the predation rate of Sacramento 

 squawfish on juvenile salmon at RBDD is lower than 

 previously believed because of the physiological ef- 

 fects of temperature on squawfish digestion during 

 January to March. 



Acknowledgments 



I am grateful to Larry Brown, Irene Chew, Steve 

 Hanson, Ned and Elaine Knight, Susan Loeb, Stuart 

 Moock, and Nick Villa for helping to collect squaw- 

 fish. Irene Chew, Bruce Herbold, and Eric Wikra- 

 manayake assisted during the digestive trials. 

 Juvenile chinook salmon were provided by the Cole- 

 man National Hatchery through Roger Guinee and 

 Kathy Clemens. I thank Joseph J. Cech, Jr., Bruce 

 Herbold, Peter B. Moyle, and two anonymous re- 

 viewers for their critical comments on the early 

 drafts of the manuscript. This work was supported 

 by a grant from the California Department of Water 

 Resources. 



Literature Cited 



Bentley, W. W., and E. M. Dawley. 



1981. Effects of supersaturated dissolved atmospheric gases 

 on northern squawfish, Ptychocheilus oregonensis. North- 

 west Sci. 55:50-61. 



Brown, L. R., and P. B. Moyle. 



1982. The impact of squawfish on salmonid populations: A 

 review. N. Am. J. Fish. Manage. 1:104-111. 



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