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~ T ~ \-\~ l q~ n(ll, -r~I{Ct-'<J J .~'., 138-144 - IN: Issuesend Technology in the~'dnagemen_f Impacted Western Wildlife. Proceedings of a Nat'i 1'.11 Symposium. Thorne Ecological Institute Technical Publication No. 14. Boulder, Co. LOSS O~ STREAM PASSAGE AS A FACTOR IN THE DECLINE OF THE ENDANGERED COLORADO SQUAWFISH va.. e ~ '1 ...1 .......... OJt!>ol 1984. HAROLD M. TYUS, U.S. Fish and Wildl ife Service, Vernal, Utah 84078 Abstract: Long distance spawning migration and seasonal movement patterns of Colorado squawfish, (Ptycho_cheilus lucius) have recently been documented. These life history strategies which formerly benefitted the fish now appear to be a major factor in its decline. Provision of fish passage and carefully designed habitat manipu.1ations are imp1 icated as major features necessary for recovery of the Colorado squawfish. The Colorado squawfish (Ptychocheilus lucius), top carnivore of the Colorado River System, was once so abundant it supported a commercial fishery (Minck1ey 1973). A steady decline in Colorado squawfish range and abundance has occurred since the early 1900's (Tyus et al. 1982a) and it is now classified as endangered by the U.S. Fish and Wildlife Service (1974) (FWS), persuant to provi- sions of the Endangered Species Act of 1973. The Colorado squawfish now exists in only about 25% of its original range (Figure 1). The decline of the Colorado squawfish has been linked by most researchers to water resource development. The Colorado River Fishes Recovery Team (1978) cited these workers and related its decline in numbers and range to several causes re- sulting from the construction of dams and their resu ltant changes in ri veri ne habi ta t. These changes included the conversion of riverine to lacustrine habitat; cold, fluctuating tailwaters; temperature, turbidity and salinity changes; and the overall alteration of seasonal and daily flow patterns. Although congregations and move- ments of Colorado squawfish have been observed (Sigler and Miller 1963, Minckley 1973) and hypo- thesized to be associated with spawning (Holden and Stalnaker 1975) there was insufficient data to support the existence of spawning migrations (Joseph et ale 1977) until 1981, when FWS studies (Miller et a1. 1983a), identified long distance spawni ng mi grati ons and ill umi nated general move- ment patterns of Colorado squawfish. These mi- grations and movements appear to be a critical factor in the life history requirements of the fi sh. MIGRATION OF COLORADO SQUAWFISH The spawning grounds of the Colorado squaw- fish were unknown until 1981, when eight fish were radiotracked by FWS workers from the upper ~een River (Tyus et al.1982b). Collections in this location produced 35 ripe Coloradosquawfish includ- ing three recaptured fish previously tagged in the upper Yampa and Green Rivers. Successful repro- duction was substantiated by the collection of larval Colorado squawfish (9-13 mm TL) by FWS and Colorado Division of Wildlife personnel (CDOW). This finding was confirmed by field studies by FWS, CDOW and the National Park Service (NPS) in 1982. The spawning migration is evidently his- toric, since Holden and Stalnaker (1975) and Seet- haler (1978) also noted increased numbers of ripe Colorado squawfish at this location from 1968-1975. Information gained by FWS in 1981 (Tyus et al. 1982b) indicated Colorado squawfish moved both up- stream and downstream to reach the spawning site and traveled an average one-way distance of 78 to 163 km. Further studies by FWS in the Green River (i-liller et ale 1983b) and the Utah Division of Wild- life in 1982 suggested that only two spawning sites may exist in the upper Green River basin, with the fish traveling distances 'of up to 250 km to reach them. Spawning sites appear to be widely separated and located in restricted canyon habitats (Figure 2) . Long distance spawning migration of freshwater fishes (potomadromy) is uncommon in North America, although examples of other cyprinid fishes that mi- grate as much as 1000 km (Nikolsky 1961) are known from Asia. Potomadromous spawning migrations of Colorado squawfish are difficult to observe since the fish lives and migrates in the same large river system year-round, and only very large specimens (over 500 mm) migrate. From the preceding, it is not surprising that migration of ColoradO squawfish was not understood until radiotracking technology made this possible. In addition to the spawning migrations of the adult Colorado squawfish, recent studies in the Green River (~i11er et a1 1983a, Tyus et a1 1982c) support the concept of larval movement from the spawning grounds downstream into more favorable nursery habitat and a long term upstream movement of juveniles (Figure 2). These life history strat- egies appear to offer adaptive advantage for coping with historic flows of the Colorado River mainstem. Under this strategy spawning could occur in well oxygenated waters with suitable substrate in the comparative absence of predators and the young fish could drift downstream into shallow, highly productive nursery areas. As the juveniles mature, they appear to move into upstream areas and ascend tributary streams - in this fashion repopulating upstream habitats. Unfortunately, artificial habi- tat manipulation may now cause strategies once so adapted to the changing river environment to work against the ColoradO squawfish. The decline of the Colorado squawfish is no doubt related to changes in its native habitat brought about by stream alteration. Until now these effects have been considered insidious. The recent discovery of long distance potomadromous migration and the possibilityof seasonal movements offer a more direct cause. Blockage of stream passage may be the prime factor for the loss of 138