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<br />squawfish display both upstream and downstream movement (positive and <br />negative rheotaxis) in migrating to these locations. A fidelity to the <br />Yampa" River spawning site was demonstrated in 1983, when two ripe Colorado <br />squawfish tagged there in 1981, and one tagged there in 1982, returned <br />to the same location for spawning. Included in the Yampa spawning <br />collections were four recaptures, including one recapture (3286) radiotagged <br />in 1982 that did not migrate in 1982 (Wick et al 1983). Since that fish <br />migrated and spawned in 1983, it is possible that the fish: (a) was not <br />mature in 1982, (b) spawned in alternate years, or (c) "was physiologically <br />stressed by surgery or handling and its spawning therefore inhibited. <br />This recaptured fish is a good example of the "difficulty encountered <br />when working with such a highly mobile animal and dictates caution when <br />interpretating migratory behavior of Colorado squawfish. <br /> <br />Potamodromous migrations (Myers 1949) of North American riverine <br />fishes have received little study in comparison with the information <br />developed for anadromous forms. Until now, homing, a regular movement <br />to a specific site (Gerking 1959) has been most dramatically demon- <br />strated in salmonids (Harden-Jones 1981). The movement patterns of <br />Colorado squawfish also demonstrate long distance homing and that squawfish <br />behavior is similar in many ways to that observed in migrating salmonids, <br />including positive and negative rheotaxis in orienting to their spawning <br />grounds (Hasler and Scholz 1983). Although there is no experimental <br />evidence to date that confirms or disproves the existence of an olfactory <br />imprinting mechanism for Colorado squawfish, conditions at the two <br />spawning grounds suggest that such a mechanism could exist (Tyus 1983). <br />Field observations of the two known spawning grounds in the Green River <br />Basin indicate that these canyon areas are geohydrological1y different <br />from those immediately upstream, and Colorado squawfish appear to spawn <br />in areas that receive freshwater imput from sandstone/limestone seeps. <br />The fish may use the input of groundwater in these locations as a means <br />of orientation, and the Groundwater Seepage Hypothesis, proposed for <br />other species by Harden-Jones (1981), should be considered a possible <br />mechani sm. <br /> <br />Spawning <br /> <br />Spawning Fish Collected <br /> <br />Collections at two major spawning sites located in the Yampa and <br />Green rivers (Figure 7) prOduced 70 ripe Colorado squawfish from 1981- <br />1983 (Table 5). These fish were classified IIripell if milt or eggs could <br />be expressed from the vent with light hand pressure on the abdomen. The <br />average size of these fish was about the same in the Yampa and Green <br />rivers, with 44 Yampa males averaging 557 mm in length. Only 9 ripe or <br />spent female Colorado squawfish have been positively identified by the <br />expression of eggs. The average length of these females was 696 mm <br />which is significantly larger than the males taken with them. Since <br />only two ripe females have been taken in the Green River, no comparison <br />was made between the Yampa and Green rivers with respect to the size of <br />females. <br /> <br />17 <br />