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s 156 The Southwestern Naturalist vol. 30, no. 1 March 1985 Notes 157 TABLE 1.-Results of feeding trials. Squawfish (SQF) were first fed channel catfish with spines removed (CCSPL); if they ate any of the CCSPL they were then fed channel catfish with spines intact (CCSP). RBT = rainbow trout; CTT = cutthroat trout. Standard deviation in parentheses. TABLE 2.-Summary of feeding trials in Table 1. SQF = Colorado squawfish; CCSPL = channel catfish with spines removed; CCSP = channel catfish with spines intact. Mean total length and standard deviation in millimeters. Trial Species Number of fish Length (mm) Range Mean Days starved Days exposed to prey No, eaten Mean length eaten (mm) 1 SQF' 3 453.470 459(9.3) 5 4 - - CCSPL 40 59.105 76(10.7) 0 - 8 74(10.6) IA CCSP 40 61.63 75(7.9) 0 - 0 - 2 SQF 3 428.440 435(6.1) 5 4 - - CCSPL 40 68-98 77(7.6) 0 - 2 71, 75 2A CCSP 40 51.108 75(10.6) 0 - 0 - 3 SQF 3 422.510 4fi(1(95.1) 5 4 - - CCSPL 40 59-126 79(12.3) 0 - 2 66, 73 3A CCSP 40 58.106 80(12.0) 0 - 0 - 4 SQF 3 401-486 438(43.6) 5 4 - - CCSPL 40 65.97 77(7.6) 0 - 0 - 5 SQF 3 406.496 448(45.2) 5 4 - - CCSPL 40 64.97 76(7.6) 0 - 1 77 5A CUSP 40 62.111 76(10.4) 0 - 0 - 6 SQF 3 410-457 433(23.5) 5 4 - - CCSPL 40 59-125 78(12.6) 0 - 1 80 6A CCSP 40 58-102 72(10.8) 0 - 0 - 7 SQF 3 420.496 447(42.5) 5 4 - - CCSPL 40 65.98 76(7.5) 0 - 1 71 7A CCSP 40 62.111 76(10.4) 0 - 1 83 8 SQF 3 407.463 438(28.6) 5 4 - - CCSPL 40 59.125 79(12.6) 0 - 4 80(16.8) 8A CCSP 40 58.99 77(10.6) 0 - 1 77 9 SQF 3 398-477 435(39.7) 5 4 - - RBT 40 62.104 84(10.8) 0 - 7 76(10.6) 10 SQF 3 388.507 961(64.1) 5 4 - - CTT 40 63-102 90(9.1) 0 - 13 90(10.3) `Flww 3 squawfish were the only ones used in a preliminary vial. They were given a 2"ay rest period before testing sumed more frequently than were catfish with spines intact. However, catfish with spines removed were still consumed far less frequently than were trout (2.4 vs. 4.0 per trial). There could be several reasons for trout preference. First, the stumps remaining after catfish spines were clipped might still feel harsh to the squawfish's mouth. However, in one preliminary trial three Colorado squawfish consumed 20 CCSPL in 5 days, indicating that the stumps provide no real hinderance to consumption. Behavior and distribution differences could explain the low channel catfish utili- zation. Catfish in our trials formed dense schools that shifted continuously across the bottom of the aquarium. The trout, in contrast, formed very loose schools at the surface that were easily dispersed when a predator approached. These differences in behavior between the two species could be one of the reasons why even catfish without spines were taken less frequently than trout. A final possibility relates to the second component of Ivlev's (Experimental ecology of the feed. ing of fishes, Yale Univ. Press, 1961) definition of prey vulnerability-predator preference. Colo- rado squawfish may innately prefer other species over channel catfish, or they might have been conditioned to trout through years of feeding on them. There have been no studies on condition- ing with Colorado squawfish; however, Mauck and Coble (J. Fish. Res. Board Can., 28:957.969, 1971) found prior conditioning had no effect on northern pike preference. In predator preference studies that involved ictalurids (Beyerle and Williams, Trans. Amer. Fish. Soc., 97: 28.31, 1968; Mauck and Coble, J. Fish. Res. Board Can., 28:957-969, 1971), they were always the least preferred species but were still consumed to some degree. In our laboratory study, it appeared that Colorado squawfish were not interested in catfish as prey. Squawfish could be fed trout daily and would consume them immediately, yet they had to be starved for 5 days before they would attempt to eat catfish. Nothing is known about how squawfish seek their prey, but using Greenway's (Experi- SQF Salmo sp. CCSPL CCSP N 30 80 320 280 R TL 446 87 77 76 SD 34 10.3 10.1 11.1 No. eaten - 20 19 2 R TL - 84 80 80 SD - 12.0 10.2 - Range - 63-102 58-100 77-83 No. not eaten - 60 301 278 z TL. - 88 78 76 SD - 9.7 10.1 11.0 Range - 69-104 59.126 55.111 % consumed - 25 6 1 mentia, 21:489.498, 1965) method of classifying predators by body type and mouth structure, Moyle (Inland fishes of California, Univ. Calif. Press, 1976) classified Colorado squawfish as rov- ing predators with lie-in-wait affinities. Thus, a single fish close to the surface might seem more attractive ui it Colorado squawfish than it tightly schooled group of fish on the bottom. The six squawfish that were observed attempting to eat and the few, although unobserved which ate CCSP, were not injured and the catfish did not lodge in the throat. Reist (Can. J. Zool., 58;1245.1252, 1980) showed that the size ratio between predator and prey is important in determin- ing the effectiveness of spines in preventing predation. The Colorado squawfish reported by McAda (Southwestern Nat., 28:119-120, 1983) was 550 mm long and the catfish was 120 mm, a predator-prey length ratio of 4.6:1. Fish and Wildlife Service data for the three Colorado squawfish reported here had a ratio of approximately 4.23:1. Our fish were of similar size but had an average predator-prey ratio of 5.8:1. On the average, our predators were considerably larger relative to their prey and this may be why no catfish became lodged in the esophagus of the Colorado squawfish. With only eight attempts at feeding on CCSP on fish that were small relative to the predator, data were insufficient to make conclusions about the probability of the Colorado squawfish chok- ing on channel catfish in the wild. However, we believe there is ample evidence to support the conclusion that Colorado squawfish do not prefer channel catfish as prey and would feed on other species of fish whenever possible. The low number of Colorado squawfish with catfish lodged in their esophagus in the numerous collections made by state and federal agencies supports this conclusion. Three Colorado squawfish captured live by the third author from the Green River in 1982 and 1983 had channel catfish lodged in their throat. One of these, a 645-mm fish, was captured on 16 July 1982 at Km 235, with a channel catfish lodged firmly in its throat. The channel catfish was not removed for fear of killing the squawfish and no exact length was obtained for the approxi- mately 150-nun catfish. The other two Colorado squawfish were obtained on 25 and 29 April 1983; one (431 min) was choking on a 95-mm channel catfish at KM 341 and the other (535 mm) was choking on a 135-mm channel catfish at Km 246. These two squawfish were released in apparent good health after the catfish were removed. Water temperatures taken at the point of capture of these fish were 23, 12.5, and 11.5 C, respectively. Data obtained from the three Colorado squawfish reported here and one from McAda (South- western Nat., 28:119.120, 1983) captured choking on channel catfish in nature suggest the pheno- menon might be associated with environmental conditions that make the prey more vulnerable to attack. Three of these fish were captured in cold water (about 12 C) in December and April. These three fish might have taken channel catfish that were lethargic from the cold and more vulnerable to predation than otherwise. Wanjala and Tash (Southwestern Nat., 28:380, 1983) reported a sim- ilar instance where a number of largemouth bass Micropterus salmoides died from attempting to swallow oversized tilapia Tilapia aurea affected by a sudden drop in water temperature. This hypothesis does not explain why the fourth Colorado squawfish captured in 23 C water attempted to eat an oversized channel catfish, but perhaps the channel catfish in that instance was sick or otherwise more vulnerable to predation.