Lower Colorado Region

Non-Native Species

Native fish in the Gila River basin have been declining over the last century and a half. Early declines were principally a result of habitat destruction and alteration. However, the establishment of non-native fishes is now the most significant factor preventing persistence and recovery of native fishes in the southwest.

Non-native fish are widespread throughout the Gila River basin. Reasons for their introduction range from intentional stockings of forage and sport fish to the release of unwanted aquarium fish and leftover bait fish. Most of these introduced fish are piscivores (fish eaters) and will consume or outcompete native fish for food and habitat. Since native fishes in the Gila River Basin co-evolved with only a single piscivore, (the Colorado pikeminnow Ptychocheilus lucius), they often lack the behavioral mechanisms to cope with or avoid the wide-array of non-native predators introduced into their habitats. In most cases, introduced fish will consume or outcompete native fish to the point where native fish populations decline or become extirpated (no longer present) from a stream. In addition to predation and competition, non-native fishes can also hybridize with similar native species, as well as introduce new pathogens and diseases to the ecosystem. For more information on the effects of non-native fishes on native fishes of the Gila River basin, click here for a list of scientific articles on the interactions between native and non-native aquatic species.

Western Mosquitofish Gambusia affinis

Green Sunfish Lepomis cyanellus

Red Shiner Cyprinella lutrensis

Smallmouth Bass Micropterus dolomieu

Partial List of Established Non-Native Fishes in the Gila River Basin