Ecology of sympatric catostomid fishes in a glaciated riverine system: Habitat, food, and biogeography

Several fundamental hypotheses derived from theoretical community ecology were tested using habitat and diet patterns for six species of the catostomid fishes of the Assiniboine River, Manitoba. Specifically, I examined expected assemblage patterns based on the equilibrium-nonequilibrium continuum that are based on competition as a structuring mechanism. The catostomid assemblage showed patterns characteristic of both equilibrium and nonequilibrium assemblages. Habitat utilization was not proportional to habitat availability for depth, velocity, and substrate indicating that habitat selection was occurring. These patterns are influenced by the spatial distributions of soft Lake Agassiz sedimentary deposits and harder glacial till-plain derived habitats and therefore localized in nature. Species co-occurred in relation to their abundance indicating that negative associations assumed in competition-based theory were not apparent. In addition, species from the same subfamily co-occurred more often than expected by chance, indicating most within-group comparisons were positive, except for silver redhorse, which did not co-occur differently than expected from random. Species from the same feeding group co-occur most frequently with conspecifics, indicating species have specific habitat patterns. Benthic invertebrate distributions and fish distributions were positively correlated with fish diet. Within-feeding-group comparisons indicated species that co-occurred most frequently consumed the same food items, but showed subtle differences in abundance of diet items. The most common diet items varied among white sucker, silver redhorse, golden redhorse, and shorthead redhorse, indicating that, although these species co-occur, subtle differences in specific feeding behavior may account for differences in relative abundance and frequency of diet items. Quillback and bigmouth buffalo shared a few core food items, but these two species co-occurred infrequently. Inter-specific interactions showed silver redhorse diets were a subset of white sucker diets, white sucker diets were a subset of golden redhorse diets, shorthead redhorse diets were a subset of silver redhorse and golden redhorse diets, while bigmouth buffalo diets were a subset of quillback diets. The lower richness and prevalence of organisms in quillback and bigmouth buffalo diets were due to highly aggregated prey items. Benthic invertebrate distributions were aggregated and dependent on the predictable patterns of substrates, based on hydraulic sorting in the meandering along the river. Large-scale redundancy of species-habitat associations was correlated with historical (phylogenetic) or adaptive (morphological) constraints on habitat selection. Stream habitats are constrained by regional factors of slope and sediments, but also determined by stable, repetitive and predictable local processes of erosion, transport, and deposition (meandering). The fish-habitat associations of catostomids in the Assiniboine River provide an example of interaction between the abundance of the component species, the phylogenetic constraints on the niche, and the deterministic nature of the spatial distribution of habitats. Within-feeding-group pairs showed that local habitat overlap is positively correlated with geographic overlap, while local diet overlap is negatively correlated with geographic overlap. Two species pairs (silver redhorse and golden redhorse and bigmouth buffalo and quillback) were concluded to have symmetric overlaps at the geographic scale and stable interactions. Using the functional niche concept ecological theory provides a link between local ecology and biogeography of sympatric species. The multi-analytical approaches used in this study provide insights into the structuring of north temperate prairie river fish communities, through hypothesis testing and correlation that have application beyond Prairie Rivers.