Flathead catfish upstream expansion as a threat to native fish conservation in the lower Colorado River

Authors: Chase Ehlo, Kirk Young

Year: 2022

Abstract

The Lower Colorado River harbors eight species of native fish with many special morphological and life history traits in response to this unique desert aquatic ecosystem. Four of these species are endangered, primarily due to habitat alterations from dams and predation and competition by nonnative aquatic species. One such nonnative species, Flathead Catfish, is historically native to the Great Lakes and Mississippi River Basin. The species is a voracious predator with liberal gape width limitations; as a result, this introduces new challenges to the conservation efforts of endangered fish in the Lower Colorado River below Davis dam and potentially upstream. It was stocked in the Southwest as early as the 1940s and has since expanded into many important desert stream systems such as the Gila and Salt rivers in eastern and central Arizona and the Lower Colorado River in Western Arizona and California. It is difficult to pinpoint how or when Flathead Catfish were first detected in Lake Havasu but it is suspected that it was a result of angler introductions in the 1980s from below Parker Dam. Nevertheless, the species slowly expanded throughout the lake and has begun to encroach on native fish populations, particularly Razorback Sucker, in the riverine sections upstream of Lake Havasu. Arizona Fish and Wildlife Conservation Office biologists, with partners, identified an upstream threshold in the river with the intention of preventing further upstream movement of the species. However, recent targeted surveys in 2021 and 2022 have detected around 40 individuals immediately downstream and up to 20 river miles upstream of this threshold. Specimens ranged from 202 – 775 mm indicating evidence of recent recruitment and multiple size classes. If the species continues to expand upstream, and over or through major dams it could not only threaten years of conservation work in Lake Havasu but upstream waters such as Lake Mohave, Lake Mead, and the Grand Canyon.