INVADING SPECIES IN THE EEL RIVER, CALIFORNIA - SUCCESSES, FAILURES, AND RELATIONSHIPS WITH RESIDENT SPECIES

We examined invasions of non-native fishes into the Eel River, Califor nia. At least 16 species of fish have been introduced into the drainag e which originally supported 12-14 fish species. Our study was prompte d by the unauthorized introduction in 1979 of Sacramento squawfish, Pt ychocheilus grandis, a large predatory cyprinid. From 1986 to 1990, we conducted growth and diet studies of squawfish, conducted intensive s urveys of the distribution and habitat associations of both native and introduced species, and examined the nature of species-habitat and in terspecies relationships. We found no evidence for increased growth or expanded feeding habits, compared to native populations, of Sacrament o squawfish as they invaded the Eel River drainage. Ten of the introdu ced species were well established, with four species limited to a rese rvoir and six species established in streams. The success or failure o f introductions of stream species appeared to be a function of the abi lity of a species to survive the fluctuating, highly seasonal, flow re gime. The present mixture of native and exotic species has not formed stable fish assemblages but it seems likely that four habitat-associat ed assemblages will develop. The overall effect of the successful spec ies introductions has been to assemble a group of species, with some e xceptions, that are native to and occur together in many California st reams. The assemblages now forming are similar to those found in other California streams. The assemblage characterized by squawfish and suc kers is likely to be resistant to invasion, in the absence of human ca used habitat modifications.