Cold-Water Prey Fish Monitoring and Ecology in Lake Oahe

In Lake Oahe, a large mainstem Missouri river reservoir that spans 375 river kilometers in central North and South Dakota, naturalized Rainbow Smelt are the primary forage species for a regionally important Walleye Sander vitreus fishery. Walleye condition in Lake Oahe is highly correlated to Rainbow Smelt relative abundance through time, the latter species being typified by erratic abundance trends since accidental introduction in 1971. Cisco Coregonus artedii were introduced in the early 1990s to augment the cold-water prey base. We evaluated Lake Oahe cold-water prey base monitoring and dynamics through: 1.) a comparison of two hydroacoustic survey designs, 2.) an evaluation of survey timing and effort for the cold-water prey base, 3.) an evaluation of Rainbow Smelt spawning ecology, 4.) an evaluation of Rainbow Smelt larval life history, and 5.) an analysis of Rainbow Smelt and Cisco recruitment dynamics with extensions to Lake Oahe Walleye. We demonstrated that an abbreviated survey technique using short, longitudinally oriented transects generated comparable data to the more time-intensive historic monitoring plan. We also documented relatively consistent estimates regardless of survey timing. The number of transects required to achieve an acceptable degree of accuracy reflected current practices. Rainbow Smelt spawning was strongly related to water temperature, consistent with observations elsewhere in their range. Steeply sloping and deeper habitats were associated with greater spawning abundance, becoming more pronounced as spring warming proceeded. We also documented faster larval Rainbow Smelt growth at these sites, despite cooler water temperatures. Rainbow Smelt recruitment was related to heating degree days, indicating stronger year class production in earlier springs. Recruitment was also related to lower reservoir elevations, perhaps linked to greater availability of preferred adult spawning habitat. Greater Cisco recruitment was observed following Rainbow Smelt population collapses, consistent with other systems with concurrent populations. Walleye condition, historically responsive to Rainbow Smelt abundances, may have recently decoupled with the availability of alternative prey. Although beneficial to the management of Lake Oahe, the work presented here contributes to the general understanding of Rainbow Smelt and Cisco interactions and recruitment, and offers a simplified methodology for the monitoring and estimation of cold-water prey abundances