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Native Aquatic Species of the Gila River Basin
in Arizona and New Mexico

Nonnative Species Impact

Native fish in the Gila River basin have been declining over the last century and a half (Table 1). This is due to development of water supplies, physical habitat alteration that favors nonnative fishes, and introduction and establishment of nonnative fishes and other aquatic animal and plant life. Early declines were principally a result of habitat destruction and alteration. However, contamination by nonnative fishes now is the most consequential factor preventing persistence and recovery of imperiled native fishes in the southwest, and perhaps globally. It is now apparent that presence of nonnative fishes cancels any benefits from habitat protection and restoration. No amount of habitat restoration can successfully advance native fish recovery unless preceded or accompanied by elimination of nonnatives.

Bass eating native fish Nonnative fish are now nearly everywhere in the Gila River basin, having been introduced as sport species, as forage for sport fish, as bait or aquarium releases (Table 2). Effects of nonnative fishes on natives result from interactions among life histories, behaviors, and habitat use. The introduced species are comprised of mostly piscivores (fish eaters), while native species are mostly generalists, primarily relying on aquatic insects and algae for their foods. Native fishes of the region are considered predator-naive, and lack behavioral mechanisms to cope with or avoid the array of predators introduced into their habitats. In the Gila River basin, native warm water fishes co-evolved with only a single piscivore, the Colorado pikeminnow - Ptychocheilus lucius, while most introduced fishes evolved within drainages containing many (e.g., Mississippi River basin).

Native fish attacked by catfish Introduced fishes typically are more evolved forms that possess sophisticated life history and behavioral traits that allow them to persist within intensely competitive fish communities. For example, most nonnative fishes afford some degree of active protection to their young via nest building or other behavioral traits, while native forms are mostly broadcast spawners with no parental care, and generally do not possess such sophistication of life history and behavior.

One result of these differences is that native fishes typically fail to recruit (survive to adulthood) young in the presence of nonnatives. Predation on natives by introduced forms during early life stages is the most likely mechanism resulting in failure of natives, but other avenues such as competition also contribute. Nonnative fishes such as green sunfish - Lepomis cyanellus, western mosquitofish - Gambusia affinis, and red shiner - Cyprinella lutrensis are ubiquitous even in shallow, near-shore habitats used as nursery areas by larval native fishes, where they consume or harass natives to decline or extirpation (extermination from a locality).

Green Sunfish Mosquito Fish Red Shiner

In addition, nonnative fish may have lost much of their co-evolved parasite and disease load due to their often small founding populations that may not have included their native pathogens. At the same time, any novel parasites and diseases that are introduced by the nonnative fish have a more detrimental effect upon native fish that have no co-evolved resistance.

Only in rare instances have natives persisted among introduced forms over a long history. However, these situations are largely unstudied, and proposed mechanisms that might allow coexistence are speculative. Disturbance, especially flash flooding that is common to the southwest, has been suggested as a mechanism that in some cases may allow persistence of native fishes when they co-occur with introduced species. However, the presence of nonnative fishes across the region ensures that the impacts of predation, competition, or parasitism are ever-present factors that limit successful completion of native fish life cycles. The fact is, where nonnatives become established, natives invariably wane or disappear.

A list of studies that link nonnative species negative impact on native fish populations


Status of Federal and State Listed Warm Water Fishes of the Gila River Basin, with Recommendations for Management

Status of Unlisted Native Fishes of the Gila River Basin, with Recommendations for Management

Analysis of Recovery Plan Implementation for Threatened and Endangered Warm Water Fishes of the Gila River Basin

Last Reviewed:
June 25, 2009

Joseph J. Billerbeck - jbillerbeck@usbr.gov