Distribution of Nonnative Aquatic Vertebrates in Western U.S. Streams and Rivers

Abstract

Nonnative aquatic vertebrates have been widely introduced across the western United States. We analyzed data from the Environmental Monitoring and Assessment Program's western pilot study to determine the occurrence of nonnative fish and amphibians and the proportion of stream length they occupied in streams of 12 conterminous western states. A total of 1,361 sites (including both probability sites and candidate reference sites) were sampled from 2000 to 2004. Of these, 711 probability sites had sufficient sampling effort and vertebrates present, representing a total assessed stream length of 213,600 km. Native and nonnative species numbers and proportionate abundances based on this stream length are reported for the entire study area, for three large-scale ecoregions (Mountains, Xeric, and Plains), and for each state in the survey area. Sites with insufficient sampling effort or collection permit restrictions or in which no vertebrates were collected could not be assessed and represented an additional 90,000 km of stream length. An estimated 52 ± 4% (mean and 95% confidence interval) of the assessed stream length contained nonnative vertebrates. Nonnatives represented more than 50% of the individuals in 22 ± 3% of the assessed stream length. Nonnative vertebrates were present in 59 ± 10% of the assessed length represented by fourth-order rivers and in 83 ± 6% of the assessed length represented by fifth-order and larger rivers. From 30 to 33 nonnative species were found in each of the three ecoregions. Brook trout Salvelinus fontinalis, brown trout Salmo trutta, and rainbow trout Oncorhynchus mykiss were the most common nonnative aquatic vertebrates found in the study area based on the proportion of assessed stream length occupied. Of the 12 states surveyed, California accounted for the greatest number of nonnative taxa we collected (26) and Idaho the fewest (4).