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DISCUSSION Native species High catch rates of native fishes reported in 2004 can be partially attributed to very low turbidity during the sampling period (Figure 5). Recent investigations of the effects ofturbidi~ on hoop net catch rates have revealed that at turbidities < 180 NTU catch rates increase significantly (Stone 2004). We hypothesize that chub use the nets as cover in clear water. The mean CPUE of humpback chub 2:150 mm shows severe declines from 1987 to 1994 and has remained relatively stable since about J.9.941.l<t;h~e 9). It may be that the pre-1987 population of humpback chub represented individuals that were born prior to or during the time in which Lake Powell was filling when mainstem Colorado River water temperatures were warmer and the mainstem Colorado River was humpback chub habitat. Since about 1994 the number of hum pack chub has been relatively stable at a lower level. This may indicate that the present chub population represents the carrying capacity of the Little Colorado River alone and the higher pre-1987 chub population represented the carrying capacity of the mainstem Colorado River and the Little Colorado River (Figure 13). The ongoing trout removal efforts near the confluence of the Little Colorado River should help to address the question of whether or not the mainstem Colorado River is actually humpback chub habitat. If chub numbers do not increase as a result of these efforts it may be that the mainstem Colorado River is still not humpback chub habitat possibly because of the cold water temperatures, even after predators are removed. The majority of the humpback chub recaptured in 2004 were either tagged within the last 3 years or were tagged 9-15 years ago (Figure 13). One explanation for this finding is that fish tagged 9-15 years ago did not stay within the Little Colorado River. If they had stayed within the Little Colorado River they should have been caught in many of the subsequent years and the large gap in recaptured tags from 4-8 years ago would not exist. Another explanation might be that older fish become trap shy and avoid hoop nets. Figure 13 also highlights the fact that during the extensive tagging efforts of 1991-1993 (AGFD, FWS, ASU), many chub were tagged. There were 10 adult chub recaptured in 2004 which were previously tagged in the mainstem Colorado River (Appendix). All ofthese fish were tagged within 5 miles of the LCR confluence indicating little long distance movement for humpback chub within the mainstem Colorado River. 9 J~IP) P_ r! l~ r-'- 9J I Q ~