Typescript. Vita. Includes bibliographical references.

The sicklefin chub, Macrhybopsis meeki, historically inhabited the mainstem Missouri River and Mississippi River downstream from the Missouri's confluence. Sicklefin chub declined from 70% of their historical range in the Missouri River following construction of reservoirs and river channelization. I studied two aspects of its autecology that could be useful in conservation and recovery efforts; phenotypic variation among isolated populations and factors associated with its distribution in the Missouri and Lower Yellowstone rivers. Phenotypic variation reflects processes of local adaptation and evolutionary flexibility and can be used to identify distinct sub-populations or stocks that may require legal protection or specific management actions. Forty-two phenotypic traits were quantified and examined with principal components analysis, a multivariate statistical technique. Intra-population variation exceeded interpopulation variation in all traits examined. However, a group of sicklefin chubs in the lower channelized river exhibited more stream-lined and hydrodynamic body morphologies. This partial divergence of body morphologies is postulated to be a response to increased water velocities in the channelized river. Identification of habitats associated with sicklefin chub distribution can aid habitat creation and maintenance in restoration programs. Habitat associations were examined at segment and site spatial scales using logistic regression. Sixty-seven variables postulated to influence sicklefin chub distribution were examined. These included aspects of physical habitat, water quality, flow regime, and predation. Sicklefin chub presence was most highly predicted in segments far downstream from reservoirs, where turbidity in late summer-early autumn exceeded 80 NTUs and August flows were low, being generally <10% of the total annual flow. Smaller-scale sites where adult sicklefin chubs were present were characterized by faster column velocities, a higher percentage of gravel, and a lower percentage of silt than sites where juveniles were present. Adult sicklefin chub presence at sites was most highly associated with current velocity and gravel substrates. These patterns are proposed to be related to conditions necessary for reproduction and successful recruitment and to moderation of predation and inter-specific competition.