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Distribution of Fishes in the Red River of the North Basin on Multivariate Environmental Gradients

Species-area Relationship

Simple regression analyses indicated that the species richness of streams was best predicted by average annual discharge at gauging stations near the mouths of streams (r2=0.60) and was predicted to a lesser degree by total stream length (r2=0.57) and watershed drainage area (r2=0.50) (Figure 13). These results were similar to that presented by Angermeier and Schlosser (1989), who correlated habitat volume with species richness of riffle fishes in streams in Minnesota (r2=0.45), Illinois (r2=0.39), and Panama (r2=0.61). Watters (1992) determined that drainage basin area was the best predictor (r2=0.81) of species richness in the Ohio River basin. Similarly, Eadie et al. (1986) found that species richness was significantly correlated with drainage areas (r2=0.60), stream lengths (r2=0.45) and stream surface areas (r2=0.44) of 21 Ontario rivers.

Results similar to those obtained for the Red River basin were obtained by Livingstone et al. (1982) for African rivers. Discharge explained 74% of the variation in fish species richness, while stream length and watershed area were not significant in multiple regression models. These results were explained by the fact that over the study area, mean annual precipitation varied greatly (Livingstone et al. 1982). Streams of equal watershed area and/or length were receiving different amounts of annual precipitation. Since the discharge of a river is a function of rainfall per unit area, it was the best predictor of species richness in the African rivers. This rationale may also be applicable to the Red River basin. Because of an east-west gradient in annual rainfall, drainage basins of comparable size and streams of comparable length do not receive equal precipitation and therefore do not have comparable rates of discharge. An example can be noticed when comparing the Buffalo River (eastern basin) with the Goose River (western basin). The watershed area of the Buffalo River was 1189 mi2 and that of the Goose River was 1280 mi2 (Table 7). The stream length of the Buffalo River was 150 mi and that of the Goose River was 186 mi. However, the discharge of the Buffalo River was 132 cfs whereas the discharge of the Goose River was only 71 cfs. Similar comparisons can be made between eastern and western streams in the basin, such as the Tamarac or Middle Rivers compared to the Rush River, or as an extreme case, the Red Lake River compared to the Sheyenne River.

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