Spatial and Temporal Distribution of Threadfin Shad in a Southeastern Reservoir

Abstract

The spatial and temporal patchiness of limnetic fishes is not well understood but has important implications for biological interactions as well as for sampling strategies. We used hydroacoustics to assess the distribution of threadfin shad Dorosoma petenense in Lake Norman, North Carolina, a large, multipurpose reservoir. We sampled three transects on four consecutive days and nights in April, July, and October to enable us to characterize threadfin shad on both daily and seasonal time scales. We used a patch recognition algorithm to analyze echo-integrated hydroacoustic data in order to define patches and related patch characteristics (numbers, density, area, and mean depth). Threadfin shad in Lake Norman were patchily distributed and exhibited strong, generally consistent patterns in patch number, fish density, size, and depth. During most surveys we observed 12–16 patches/km with fish densities exceeding twice the average background density, and 1–2 patches/km with fish densities 50 times the average background density. Most patches were small (<50–100 m2 in the vertical plane); less than 10% of the area contained 50% of the fish. In October night surveys, patches tended to be larger and less numerous and to have lower fish densities than at other times. Depth distribution of patches varied among seasons and was influenced by temperature, by oxygen concentration, and possibly by vertical distribution of predators and prey. Temporal changes in patch distribution suggested that fish composing the patches were highly mobile, spatially integrating threadfin shad impacts on their zooplankton prey. Large differences in day and night average density estimates in October suggested strong diel habitat shifts by threadfin shad during fall.