Surface Area Estimates of Streams and Rivers Occupied by Nonnative Fish and Amphibians in the Western USA

Abstract

Statistically robust, broad-scale measures of the portion of an aquatic resource (e.g., a stream and river network) occupied by nonnative fish and amphibian species should be useful to resource managers but with a few exceptions have not been available. We used data from the western stream and river survey of the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program to estimate the surface area of flowing waters occupied by nonnative fish and amphibian species at three spatial scales. For the 12-state survey area, an estimated 75% of the surface area of the target stream network had one or more nonnative fish or amphibian species, compared with 52% of the stream length. At all spatial scales (except the state of Washington), the estimates of the proportion of surface area with nonnatives were larger than those for estimated stream length. The relative extent of occupancy for individual species often differed depending on how they were assessed. For example, when assessed by length of network occupied, brook trout Salvelinus fontinalis was the most widespread nonnative fish (17%) but dropped to ninth position when assessed by surface area occupied (7%), while common carp Cyprinus carpio went from fourth position (10% of stream length) to second position when assessed by surface area occupied (27%). Estimates of the surface areas of flowing waters occupied by nonnative species are useful and easily calculated additions to the ways in which probability-based surveys can assess the condition of the resource.