Influence of species, size class, environment, and season on introduced fish predation on native fishes in the Verde River System, Arizona

Bonar, Scott; Leslie, Laura; Velez, Cristina

Nonnative fishes were introduced throughout the Western United States for sport, food and biological control primarily in the early part of the last century and in the late 19th century. Currently, most sportfisheries in the West are supported by introduced species, which have greatly benefited the economy of the region. For example, recreational fishing was worth over 6 billion dollars to the economies of 11 Western states in 2001 alone. Unfortunately, introduced fish have also been implicated in declines of native fish assemblages in many western rivers, along with water diversion and habitat loss. Because of these declines, many native species of western rivers are listed as threatened and endangered under the U.S. Endangered Species Act. Arizona river systems, such as the Verde, Gila and Salt historically supported rich populations of native fishes, and recovery of these fishes is of high priority. The relative importance of the effects of nonnative aquatic predators on the native fish community of the Verde River system, Arizona was identified as a sensitive element in 2000 for the Arizona Game and Fish Department IIPAM Heritage Program. Previous research projects determined that non-native species can affect native fish by predation in Arizona river systems; however, there is much less information about exactly where and when impacts occur, and which introduced species are responsible for a majority of the impact. Knowledge of where and when specific bottlenecks occur which limit native fish production is useful to managers when designing introduced species control programs. Introduced species control programs can then be focused in habitats, at times of year, and on specific introduced species to maximize their efficacy, with the least impact to valuable sport fisheries. The primary goal of our study was to examine the impacts of total populations of introduced fishes (predation rates of introduced fish species x population size) and size groups within these populations to identify where control programs might be targeted in the Verde River system for maximum benefit. In addition, much still remains to be learned about the basic ecology of desert fishes. The density and standing crop that fishes can obtain in various environment types are basic measures of productivity commonly used to manage species and understand their ecology. Estimates of fish densities and standing crops have been made for various lakes, reservoirs, and rivers throughout the world, but there is little information available regarding the densities and standing crops achieved by fishes in Southwestern desert rivers. A secondary goal of our study was to provide information on the basic biology of native and nonnative fishes in the Verde River such as their distribution, standing crops and densities. (Executive Summary copied verbatim from the report.)