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

Conclusions

The Red River basin supports a diverse assemblage of fishes. From records of surveys conducted from 1892-1994, the species richness of the Red River basin was 84, which included 77 native and seven introduced species. Three of the native species, the longnose gar, lake sturgeon, and pugnose shiner, likely no longer exist in the basin. The longnose gar has not been reported since it was first collected in 1892. The lake sturgeon has not been reported since the 1950s, and the pugnose shiner has not been reported since 1976. The flathead chub, known only from a single specimen collected in the Red River in 1984, probably also does not occur in the basin in the United States, at least not in any great numbers. It is possibly an introduced species or a recent immigrant from the Nelson River drainage in Canada. Of the six other introduced species, five are sport fishes.

A comparison to neighboring basins indicated that the fauna of the Red River was most like that of the Minnesota and upper Mississippi Rivers, and it least resembled the fauna of the Missouri River in North Dakota, indicating that the most likely postglacial dispersal route utilized by Red River fishes was the River Warren outlet of glacial Lake Agassiz.

Streams were grouped into four clusters by CA, based on fish presence or absence. The clusters corresponded to elements of total species richness of streams, the sizes of the streams, in terms of discharge, length, and watershed drainage area, and the geographical location of the stream in the basin. Fishes from each of the four stream clusters were placed into assemblages based on the number of times each species pair occurred together at sites. Assemblages of fishes in streams of different regions of the basin varied because of differences in species richness of streams and the CA method used, which forced pairing of species.

Distribution patterns of fishes indicated apparent preferences of several species for certain portions of the Red River basin. Fifteen species were found either entirely or mostly restricted to the eastern half of the basin, and two species were restricted primarily to the western half, while several other species were determined to have widespread distribution patterns. These patterns appeared to correlate with an extreme east-west environmental gradient which occurs across the Red River basin. Regression analyses could not adequately explain distribution patterns, although discharge accounted for 60% of the variation in species richness of 26 streams in the basin.

In many cases, species exhibited ecoregion preferences, so to determine the factors most responsible for controlling the distributions of stream fishes, the streams were divided into "reaches" based on ecoregion. Data regarding the species composition of each reach, as well as environmental information such as water quality and flow, were collected for each of 46 different stream reaches. The correlation between species and environment was analyzed using CCA, a direct gradient analysis technique. The average annual discharge and CV of mean monthly flow of reaches, as well as levels of conductivity, hardness, and residue of waters at reaches, were determined to be the most important variables for explaining fish assemblage composition.

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