perature was adjusted to the experimental tempera- 

 tures. The temperature was adjusted upward 1.0°C 

 or downward 0.5°C per day until the experimental 

 temperature was reached. Temperatures were main- 

 tained using an immersion heater and thermo- 

 regulator. Fish were then held for 14 d at the 

 experimental temperature. Sacramento squawfish 

 were fed mosquitofish, Gambusia ajfinis; golden 

 shiner, Notemigonus crysoleucas; or threespine 

 stickleback, Gasterosteus aculeatus, in excess dur- 

 ing the holding period. 



Sacramento squawfish were starved 72 h at 5° 

 and 10°C and 48 h at 15° and 20°C prior to the 

 digestive trials. Digestive trials were 4, 16, 32, 

 and 48 h at 5°C; 2, 4, 8, 16, and 32 h at 10°C; 1, 

 2, 4, 8, and 16 h at 15°C; and 2, 6, and 10 h at 

 20°C. 



Sacramento squawfish were force-fed juvenile 

 Chinook salmon obtained from the Coleman National 

 Fish Hatchery (mean wet weight = 3.7 g). Each 

 squawfish was fed four salmon because squawfish 

 captured below RBDD in 1982 averaged approx- 

 imately four salmon (x = 3.9, Vondracek et al.^) in 

 their foreguts. The weight of each squawfish was 

 estimated before a digestive trial. Each juvenile 

 Chinook salmon was weighed before the feeding 

 trials. The salmon were selected by size to insure 

 that each squawfish received an equivalent size ad- 

 justed ration. I attempted to feed a ration of about 

 2.0% of the squawfish wet weight. The mean ration 

 actually fed was 1.8%. Squawfish were selected by 

 size for each digestive time period to ensure an even 

 distribution of sizes. 



During force feeding Sacramento squawfish were 

 placed into a V-shaped trough lined with polyethy- 

 lene foam. Once in the trough another piece of foam 

 was placed over the fish to restrain it. No anesthetic 

 was used. The chinook salmon were introduced into 

 the anterior portion of the alimentary tract of the 

 squawfish using a large syringe (Falter 1969). The 

 syringe (18 mm diameter) was inserted into the 

 esophagus and past the pharyngeal teeth with the 

 plunger removed. Once in place the salmon were 

 introduced into the syringe. The plunger was then 

 replaced and depressed. Groups of three to five 

 squawfish were placed into small circular tanks 

 maintained at the desired temperature immediate- 

 ly after feeding. Individual fish were identified by 



>Vondracek, G., S. R. Hanson, and P. B. Moyle. Sacramento 

 squawfish, Ptychocheilus grandis, predation on juvenile chinook 

 salmon, Oncorhynchus tshawytscha , below the Red Bluff Diver- 

 sion Dam in the Sacramento River, California. Manuscr. in prep. 

 Wildlife and Fisheries Biology, University of California, Davis, CA 

 95616. 



placing a numbered Floy^ anchor tag between the 

 rays of the dorsal fin. 



After the prescribed digestion period Sacramento 

 squawfish were netted from the small experimen- 

 tal tanks and placed into the foam-lined trough. A 

 catheter connected to a small water pump was in- 

 serted into the anus. Digestion tract contents were 

 flushed through the mouth and collected in a fine 

 mesh net. The digestive tract contents were weighed 

 (salmon were weighed individually if digested <30% 

 and en masse if >30%) and placed in a drying oven 

 in 60°C for 24 h. Dry weights did not change after 

 24 h. An initial dry weight of each ration was deter- 

 mined by sacrificing 5 to 10 salmon prior to the 

 digestive trials. Mean percent dry weight of the 

 salmon was 20.8 ± 2.0% for the 10° and 15°C trials 

 and 21.9 ± 2.1% for the 5° and 20°C trials. If the 

 dry weight of the digested ration exceeded the esti- 

 mated initial dry weight, the percent of the ration 

 was set to 100%. The dry weight of the digested 

 ration exceeded the estimated initial dry weight dur- 

 ing 17 trials with a mean of about 104%. 



Digestive rates at each temperature were deter- 

 mined by linear regression of the percent of the ra- 

 tion remaining in the alimentary tract versus time 

 after force feeding. The initial wet weights of the 

 salmon fed to the squawfish were not used in the 

 regression analysis. Time for alimentary tract evac- 

 uation for each temperature was assumed to be the 

 point where the extrapolated regression for diges- 

 tion intersected the x axis (time after force feeding). 



Results 



The digestive rates of Sacramento squawfish were 

 directly related to temperature, while the gastric 

 evacuation times were inversely related to temper- 

 ature (Fig. 1). The digestive rates were 1.8%/h at 

 5°C, 2.6%/h at 10°C, 6.3%/h at 15°C, and 8.2%/h 

 at 20°C. Gastric evacuation times were 61 h at 5°C, 

 38 h at 10°C, 17 h at 15°C, and 14 h at 20°C. 



The digestive process appeared to involve at least 

 two phases. During the initial phase, the wet weight 

 of the ingested salmon increased. The duration of 

 the initial phase was inversely related to the ex- 

 perimental temperature. At 5°C the initial phase 

 was at least 16 h, 4 h in duration at 10°C, 2 h at 

 15°C, and approximately 2 h at 20 °C. During the 

 second phase the percent dry weight of salmon re- 

 maining in the digestive tract decreased linearly 

 with time. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries, Service, NOAA. 



160 



