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400 BERRY AND PIMENTEL TABLE 2.-Prolonged swimming performance of several freshwater fishes, expressed as body lengths (BL) per second and its equivalent absolute swimming speed. Temper- Prolonged speed T otallength ature Species (cm) ("C) BUs cm/s Source Atlantic salmon \~ Salmo salar 23 15 2.1-3.2 50-76 Kutty and Saunders (1973) Largemouth bass Micropterus salmoides 14-27 20 2.3-3.0 45-63 Beamish (1970) Smallmouth bass Micropterus dolomeiu 5-6 20 3.3-5.4 19-31 Larimore and Duever (1968) Coho salmon Oncorhynchus kisutch 6-9 20 4.6-5.4 31-41 Brett et al. (1958) Rainbow trout Salmo gairdneri 6 20 4.1-7.1 25-43 Butler and Milleman (1971) al. 1958). Our data for the humpback and bony- tail chubs are consistent with this generalization (Fig. 1). The small Colorado squawfish per- formed better at 260C than at 20oC, but their intermediate performance at 140C confounded the trend. Large Colorado squawfish did not im- prove performance with increase in tempera- ture-an unexpected result that may have been related to the small sample size or to the possible inaccuracy of estimates made without fatigue data between 0 and 100% (Stephan 1977). We noticed great variation among individuals in swimming ability, especially at intermediate velocities. Such variability has been noted by others (McNeish and Hatch 1978; Kovacs and Leduc 1982) and probably represents different levels of motivation, stress, and ability within each species. Discussion These rare Colorado River fishes have biolog- ical traits that may confer survival advantages in swift, sometimes turbulent waters (Minckley 1973). The narrow caudal peduncle and high as- pect ratio (square of the maximum fin height divided by the fin area) of the caudal fin of the humpback and bonytai1 chubs suggest superior swimming ability compared with that of other species (Lighthill 1969). Although the ability of these two species was greater than that of the Colorado squawflsh of similar size, their pro- longed swimming performance at 200C was sim- ilar to that of some other fishes (Table 2). A downward bias to our data may have resulted from the use of unexercised fish (Webb 1975) and from our use of a sudden velocity increase, instead ofthe gradually increasing velocities used in other tests. The final temperature preferendum for these species is 24-250C (Bulkley et al. 1981). It is generally assumed that the final preferendum is the optimum temperature for most physiological functions of fish. Hence, the swimming perfor- mance of these species at 260C was probably near maximum, although this conclusion is specula- tive because stamina was not evaluated at higher temperatures. Maximum prolonged swimming of other eurytherma1 species usually occurs at temperatures between 25 and 300C (Beamish 1978). Hydroelectric plants and other industrial com- plexes remove water at structures that are po- tential sites for entrainment and impingement of young fish. Recommended approach and screen- face velocities at intakes are about 15 cm/s (Barnes 1976). The 120-min FV50 for small fish exceeded 35 cm/s at all temperatures; conse- quently the fish tested in our study should be able to avoid entrainment. However, larvae may be the most vulnerable life stage, and there is no published information on their swimming abil- ity. Fish passage must be provided to preserve mi- grating species such as the potamodromous Col- orado squawfish, which undertakes spawning migrations of several hundred kilometres (Tyus 1983). Fish ladders have been suggested as a pos- sible conservation measure because the northern squawfish Ptychocheilus oregonensis readily uses fish ladders on the Columbia River (Clay 1961; {