Hydroacoustic Assessment of the Abundance and Distribution of Rainbow Smelt in Lake Oahe

Abstract

Hydroacoustic surveys with dual-beam processing and echo integration techniques were conducted on Lake Oahe, South Dakota, in the summers of 1983-1985 to assess the abundance and distribution of rainbow smelt (Osmerus mordax). Rainbow smelt are the primary coldwater forage fish for the developing salmonid fishery in this large Missouri River impoundment. Numerical estimates of pelagic fishes were 98,180,000 ± 5,550,000 (95% confidence limits), 95,430,000 ± 28,600,000, and 106,697,000 ± 21,880,000 for 1983, 1984, and 1985, respectively. Over 98% of the fish collected by midwater trawling and gillnetting during the surveys were rainbow smelt, suggesting that most of the fish sampled acoustically were also rainbow smelt. Biomass estimates for sampled portions of Lake Oahe were 157,100 kg (2.8 kg/hectare), 477,100 kg (8.5 kg/hectare), and 202,700 kg (3.6 kg/hectare) for 1983, 1984, and 1985, respectively. The accuracy of these biomass estimates is unknown because of the indirect method of calculating them. The geographical and vertical distributions of rainbow smelt were temperature dependent. Rainbow smelt occupied the colder water (5-14° C of the hypolimnion in stratified areas. Only about one-third of Lake Oahe's 152,000 surface hectares (at maximum pool) contained suitable coldwater habitat in the summer. Rainbow smelt made diel vertical migrations that were also temperature-limited. During the day most rainbow smelt were within several meters of the reservoir bottom but moved upward to the thermocline at night. Some age-0 rainbow smelt moved slightly into the epilimnion. The consistency of the acoustic estimates of rainbow smelt abundance over the 3-year period indicated that the developing salmonid stocks have not yet significantly reduced the abundance of rainbow smelt.