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FISH SPECIES OCCURRENCE DENSITIES IN NORTHEASTERN LAKES AND THE EXTENT OF NON-NATIVES
Citation:
Whittier, T R. AND R D. White. FISH SPECIES OCCURRENCE DENSITIES IN NORTHEASTERN LAKES AND THE EXTENT OF NON-NATIVES. Presented at Northeast Natural History Conference, Albany, NY, April 24-26, 2002.
Description:
A species' occurrence density is the proportion or number of habitat units (lakes in this case) in a region in which it is present. Reliable estimates of occurrence density should be useful to discussions and decisions about biodiversity, rare species, and non-native invasions. The Environmental Monitoring and Assessment Program (EMAP) used a probability-based design to survey fish assemblages in 203 lakes in the Northeast, during summers of 1991-1996. The survey design permits statistically rigorous estimates of the regional occurrence density of individual species. It is straightforward to calculate these estimates. However, one needs to assume that some under-sampling occurs, and some species will be difficult to document consistently. Thus, the direct-from-data estimates are likely to underestimate species' occurrence density. I present analyses of repeat-visit sampling to illustrate possible adjustments to single-visit sample estimates of occurrence density. In northeastern lakes, single-visit estimates appeared to be reliable for some species (e.g., bluegill, rock bass), but underestimated others (e.g., brown bullhead) by up to 23% of lakes in the region. I discuss these results in context of the extent of non-native fish, especially smallmouth and largemouth bass, in northeastern lakes in relationship to potentially at-risk species, such as brook trout and minnows.