Introgressive Hybridization between Gila orcutti and Hesperoleucus symmetricus (Pisces: Cyprinidae) in the Cuyama River Basin, California: II. Ecological Aspects

Abstract

Ecological relationships between the native Monterey western roach, Hesperoleucus symmetricus subditus, and the introduced arroyo chub, Gila orcutti, and their hybrids were investigated in relation to distribution within the Cuyama River Basin, to breeding, and to feeding. Interactions with four other species of fishes with the hybrid complex, are also reported. Gila-like individuals preferred standing waters in Twitchell Reservoir, whereas Hesperoleucus types were found primarily in flowing waters. Fluctuations in the water level of the reservoir forced both types into close proximity in Alamo Creek, where competition occurred. Over an eight month period the numbers of Gila types were reduced and the Hesperoleucus types increased, while the intermediate Fl's remained constant. The reduction of Gila types was the result of competition with Hesperoleucus types in Alamo Creek. The number of Gila types in the reservoir was reduced by selective predation by centrarchids. The reduction of Gambusia affinis and Gasterosteus aculeatus was correlated with predation by Lepomis cyanellus in Alamo Creek. The effects of fluctuating selective pressures are discussed in relation to reinforcement of premating isolating mechanisms and development of niche specificity.