Nonnative removal and food web interactions in the Gila River, New Mexico

Authors: Keith B. Gido, David L. Propst, Yvette Paroz, Tyler J. Pilger

Year: 2009 (xli)

Abstract

Although stream flows in the upper Gila River basin in New Mexico have remained relatively unaltered by human activities, many populations of native fish are unstable and appear to persist only through connectivity with source populations that exist elsewhere in the stream network. The source of this instability appears to be an interaction between long-term climatic fluctuations and negative interactions with nonnative fishes. Beginning in 2006, efforts began to remove nonnative fishes from a 4.8 km reach of the Gila River using intensive backpack electrofishing and seining multiple times each year. Removal efforts were coordinated with efforts to evaluate temporal variation in food web structure through the use of stable isotope and diet analysis. Changes in fish assemblages and food web structure in the removal reach were contrasted with one or two reference sections using a before-after-control-impact (BACI) design. Preliminary data show that proportion abundance of nonnative fishes in the removal reach has decreased from a high of 19% in 2007 to 6% in 2009. However, there was a high degree of interannual variation in flows that confounded our ability to evaluate the efficacy of the nonnative removal and potential changes in food web structure. It is likely that tracking these management actions over long time periods is necessary to evaluate the response of the native fish community in highly dynamic systems. Moreover, the success of these actions is particularly important in systems like the upper Gila River, which provides refugia for several critically imperiled species.