Spatiotemporal variation of fish assemblages in Montana prairie streams

Abstract

Fisheries biologists must be certain that their samples represent true parameters to make sound management decisions. A large-scale effort was conducted in the past decade to sample Montana prairie stream fish assemblages. This effort focused on sampling as many streams as possible, mostly during summer. Longitudinal sampling within drainages and replication of sites across time was minimal. Consequently, the spatiotemporal variation of prairie stream fish assemblages in Montana is not well understood. The objectives of this study were to 1) determine the influence of spatial, temporal, and abiotic variables in structuring prairie stream fish assemblages and 2) determine the survey design that best characterizes prairie stream fish assemblages. To assess spatiotemporal variation, lower, middle, and upper sites were sampled on five tributaries of the Yellowstone River during each season over a two-year period. Spatial position explained the greatest amount of variation (18.5%), followed by proportion of fine substrates (13.9%). Season (i.e., temporal variation) was not significant in explaining the overall variation in the fish assemblage. Fish species were associated with lower, middle, and upper spatial positions and along the fine substrates gradient based on abundance by species.

A more detailed spatial analysis of fish assemblage variation was conducted by sampling sites arrayed from the confluence to the headwaters on each of the five streams during June and July 2005 and 2006. Species richness varied longitudinally and decreased from downstream to upstream sites. Proportion of fine substrates increased and pool area decreased from downstream to upstream sites. Longitudinal changes in species richness were often associated with longitudinal changes in the proportion of fine substrates. Large river fishes were limited to lower and middle reaches and were only occasionally found in upper reaches. Spatiotemporal and longitudinal findings were consistent among streams of varying watershed sizes. At least lower, middle, and upper sites are required to adequately characterize the fish assemblages of prairie streams and more samples are needed at lower sites than middle or upper sites. Given logistic and monetary constraints, biologists should design their surveys to maximize spatial coverage to adequately characterize fish assemblages of prairie streams.