Distribution of Larval Gizzard Shad (Dorosoma cepedianum) in Lake Carl Blackwell, Oklahoma

Year-class formation in fishes is poorly understood because of the difficulty of estimating abundance of fish larvae. The purpose of the research was to describe the temporal and spatial distribution of gizzard shad larvae (Dorosoma cepedianum) in Lake Carl Blackwell (LCB), Oklahoma, to provide a basis for future efforts at sampling larvae of this important forage fish in reservoirs. Larvae were sampled with a net (mouth 0.20 m2) towed in front of a boat at night at depths of 0 (surface), 3, 5 and 7 m between April and July 1977. Wind direction and velocity, cited by other workers as decisive in determining fish larval distribution, were related to patterns of larval abundance. Larvae were captured by the gear at a length of ca. 5 mm, but were not captured after they reached slightly more than 15 mm about 10 weeks later. Larval density was highest ca. 100 m-3 during late May and early June. Larvae were captured near the surface at the outset and were apparently passively distributed by winds. However, the rest of the time they were most abundant at 3 m, near the compensation point, suggesting maintenance of a preferred position in the water column. Estimates of larval shad density taken offshore were the same as estimates nearshore in areas of the lake with either very high or low densities. The dendritic configuration of the lake basin precluded prediction of horizontal or vertical patterns of distribution of shad larvae. We conclude that accurate estimates of density will be possible only by intensively sampling all areas of the lake throughout the season. Therefore, it is likely that stratified random sampling programs for shad larvae are not now reasonable for dendritic lakes such as LCB.