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SPAWNING MOVEMENTS OF COLORADO SQUAWFISH TABLE 1.-Extended. Reach 6 Reach 7 Year and Seine Larvae Seine Lan ae month hauls S L hauls S 1 1982 Jul 22 (2) 4 0 15 (0) 0 0 Aug 53 (8) 12 23 24 (2) 0 2 1983 Jul 18 (0) 0 0 1 (0) 0 0 Aug 39 (0) 0 0 13 (0) 0 0 Sep 20(3) 0 26 5(0) 0 0 1984 Jul 9(0) 0 0 1 (0) 0 0 Aug 31 (4) 1 8 9 (0) 0 0 Sep 19(0) 0 0 5(0) 0 0 1985 Jul 30 (1) 0 1 9 (0) 0 0 Aug 25 (3) 1 6 7 (0) 0 0 Sep 18(l) 0 16 4(0) 0 0 squawfish spawned in widely scattered locations and in every reach of the Colorado River study. Each reach contained areas of cobble substrate like those used for spawning by Colorado squawfish in the Yampa River (Tyus and McAda 1984). Although the distances moved by larval Colo- rado squawfish are unknown, movement of larvae could confound use of their occurrence in samples to indicate nearby spawning and to estimate spawning time. Tyus (1986) hypothesized that Colorado squawfish larvae in the Green River ba- sin drift downstream as far as 100-160 km. Sup- port for this hypothesis was based on the obser- vation that the most extensively used spawning and rearing habitats were separated by such dis- tances. The downstream movement of larval Col- orado squawfish has been further evidenced by their collection in drift nets (Haynes et al. 1985; Nesler et al. 1988). However, spatial separation of the most extensively used spawning and rearing areas does not necessarily mean that larvae hatched from eggs in the former are reared in the latter, and simply collecting larvae in drift nets does not demonstrate that the larvae moved long distances downstream. In addition, more spawning areas than are presently recognized for Colorado squaw- fish may occur in the Green River basin. Of the 68 adult Colorado squawfish radio-tagged in the Green River basin from 1980 to 1983 (Tyus et al. 1987), 23 (34%) moved to one or the other of two major spawning sites during the spawning season (Tyus 1985). If other spawning sites exist in the 343 Green River basin, production of Colorado squawfish larvae at those sites would confound interpretation of data such as that used to support the current hypothesis on long-distance drift of larvae. In the Colorado River, the regular occur- rence of two larval size-classes (6-10 and 11-22 mm TL) in reaches 2-6 suggests that extensive downstream movement does not occur among all larvae. With the exception of the short-lived aggrega- tion of adult Colorado squawfish in reach 7 in July 1982, we observed no aggregations of radio-tagged Colorado squawfish in the Colorado River during the estimated spawning period. This differed markedly from the behavior of adult Colorado squawfish in the Green River system, where many radio-tagged squawfish moved 65 to 160 km to aggregate at two spawning sites (Tyus and McAda 1984; Tyus 1985). Because our sample of radio- tagged fish was small, however, aggregations of these and untagged fish may not have been de- tectable by our techniques. Variation in the degree of movement among populations of a fish species or among individuals of one population is common and is often related to differences in the distribution and availability of suitable habitats (Mahon 1984). In the Colo- rado River the rarity of spawning aggregations of radio-tagged Colorado squawfish might be ex- plained by widespread spawning habitat. The pre- TABLE 2.-Range of spawning dates for Colorado squawfish in major reaches ofthe Colorado River, 1982- 1985, estimated from the size of collected larvae. River reach Total larvae Spawning dates 1982 1 3 Jul 3-7 2, 3 54 Jul 9-Aug 2 4, 5 33 Jul 9-Aug 2 6, 7 41 Jul 10-Aug 4 1983 1 0 2, 3 5 Jul 24--Sep 15 4, 5 6 Jul 21-Aug 25 6, 7 26 Aug 19-28 1984 1 6 Jul 5-Aug 2 2, 3 28 Jul 14-Sep 8 4, 5 3 Jul 17-30 6, 7 9 Jul 16-Aug 4 1985 1 0 2, 3 49 Jun 27-Aug 7 4, 5 35 Jul 3-Aug 13 6, 7 24 Jun 28-Aug 11