Introduced fish in northeastern USA lakes: Regional extent, dominance, andeffect on native species richness

We assessed the effects of nonnative fish on native fish biodiversity, usin g assemblage data collected during 1991-1996 from 203 randomly selected lak es in the northeastern USA by the Environmental Monitoring and Assessment P rogram (EMAP) of the U.S. Environmental Protection Agency. An estimated 74% (+/-17.6%, 95% confidence interval) of the region's 10,608 lakes between 1 and 10,000 ha contain at least one introduced species. Based on our sample s, nonnative individuals outnumbered natives in an estimated 31.5% (+/-11%) of lakes. Regression models indicated that native, introduced, and total s pecies richness were associated with lake surface area, elevation, and lake depth (0.31 less than or equal to R-2 less than or equal to 0.81). The int ensity of human disturbance in the watershed was positively associated with introduced species richness but not associated with native species richnes s. The number of nonnative species was a significant variable in the native -species regression models for the entire Northeast and for only one of fiv e subregions, the Northeast Coastal Zone ecoregion. Of the types of fishes that have been introduced, littoral predators-primarily Micropterus-appeare d to have the greatest negative effect on native species richness. Small or soft-finned species appeared to be the least tolerant of these introduced predators. Native brook trout and minnow assemblages, typical of northern l akes in the Northeast but now rare in the Adirondacks, appeared to be at th e greatest risk from continued introductions in northeastern New England. C urrent among-lake (beta) species diversity was associated more with regiona l diversity of lake types than with extent or dominance of nonnative specie s. Without quantitative historical data, it was not possible to demonstrate a homogenizing effect of introductions on lake fish assemblages.