Ecology of spawning humpback chub, Gila cypha, in the Little Colorado River near Grand Canyon, Arizona

The morphologically unique and endangered humpback chub, Gila cypha, is fou nd in canyon-bound reaches of the Colorado River and its tributaries. Now l imited to six isolated reproducing populations, this species is believed to have been once distributed over a large portion of the mainstem river. Bec ause the species inhabits remote canyon areas, little is known about its sp awning ecology. The largest remaining population occurs in the lower Little Colorado River (LCR) near Grand Canyon, where we conducted a three-year st udy of spawning ecology during spring (March-June) 1993-1995. We analyzed s easonal patterns of movement, population density, relative condition, spawn ing scores, and frequency of ripe condition and fin abrasions and compared these data with seasonal discharge and water temperature to determine spawn ing phenology and ecology. Spawning commenced in late March, peaked in mid- April, and waned in mid-May. A high proportion of males remained ripe over this period, whereas ripe females were relatively abundant only in April. I ncreased densities of adult fish in March-April and rapid declines in May-J une coupled with recaptures of 18.4% of these adults in the Colorado River suggest that a portion of the population migrated from the Colorado River i nto the LCR to spawn and then returned. Ripe males aggregated in areas of c omplex habitat structure with high angular variation in bottom profiles (ma trix of large boulders, travertine masses combined with chutes, runs and ed dies, 0.5-2.0 m deep) and were associated with deposits of clean gravel. Ri pe females appeared to move to these male aggregations to spawn. Near-spawn ing (including gravid) females and non-spawning fish used similar habitats and were segregated but close (<50 m) to habitats occupied by aggregations of ripe males. Abrasions on anal and lower caudal fins of males and females suggest that spawning involves contact with gravel substrates, where semi- adhesive eggs are deposited and fertilized. The findings of this study shou ld aid recovery efforts for humpback chub by identifying spawning habitat w ithin the historic distributional range where additional spawning stocks co uld be established.