American Fisheries Society, Hartford, CT, August 1998.

Identifying areas of high native fish diversity and examining change over time. A data rich GIS study on the Gila River system in Southwestern North America.

P.J. Unmack & W.L. Minckley

Southwestern North America provides some stark examples of a depauparate and unique native fish fauna which has been subjected to great environmental change and displacement by introduced non-native species. The native fish fauna has declined to the point where most species are threatened. Due to the restricted nature of water in this arid region and concern for the existence of the fauna, the region is one of the most intensively sampled for its size anywhere on the Continent. Hence, excellent records exist on which to build a comprehensive database of fish occurrences.

Individual fish records for the Gila Basin have been compiled based upon museum records, primary, and “gray” literature. Data were geographically referenced using GIS to identify areas of high native fish diversity. A first aspect of the project was to look at changes in fish communities over time. A second was to overlay land ownership and other geographic features so that conservation efforts could be prioritized (i.e., via land acquisition, special management areas on public land, etc.).

Several problems arose from the analysis. How should a stream that once had seven native species but now have only four native fishes and many introduced species rank relative to one that has only ever had two native fishes and no exotics? The distribution of early collection records is biased by access limitations rather than covering representative areas of the basin. In recent years there has been a strong trend away from keeping specimens, furthermore introduced species are rarely kept. Hence, most recent fish records tend to be in “gray” literature whose records cannot be confirmed, checked for misidentifications, or re-examined when taxonomic changes occur. Another problem is the lack of negative data, it is not possible to know if species weren’t collected because they were not present, or if they were not sampled for adequately.

GIS provided an excellent means by which this type of data can be analyzed. The assimilation of data into a GIS allowed ease of display and high query capability. More importantly, GIS allowed for spatial analysis of relations among both biotic and abiotic factors, including fish occurrence relative to permanency of water, geologic structure, stream gradients, altitude, land use, species associations, etc. An overview of techniques will be given including delineation of some pit falls to avoided.