Late season reproduction by big-river catostomidae in Grand Canyon (Arizona)

To estimate residency and population sizes, endemic fishes were marked and released during October 1998 at confluence of Colorado River and Havasu Cre ek (Grand Canyon National Park, Coconino County, Arizona). Ripe Catostomus (Panlosteus) discobolus females and newly hatched fry confirmed late season (i.e., early October) spawning bl:this species. Reproduction by other cato stomids was inferred through physical examination of adult C. latipinnis an d by capture of a possible Xyrauchen texanus larva. The latter is an endang ered species not considered a constituent member of the endemic Grand Canyo n fish community; its potential presence in Grand Canyon will influence ada ptive management of endemic and introduced fishes. Results of ANCOVAs (with fishing effort as covariate) indicated numbers of aggregating C. latipinni s were significantly associated dth hows in Havasu Creek but not those in t he mainstem Colorado River. The ecology of big-river endemic fishes in Gran d Canyon on is clearly linked to tributary outflows, and environmental cues which elicit spawning of native fishes are discussed.