DONALD DEGAN AND SANDRA HOWIE, Aquacoustics, Inc, 816C Brawley School Road, Mooresville, North Carolina 28115, USA

DAVID G. COUGHLAN, Duke Power Company, 13339 Hagers Ferry Road, Huntersville, North Carolina 28078, USA

Hydroacoustics has become an accepted method for sampling limnetic forage fish populations in Southern reservoirs, however equipment limitations have resulted in varying success because clupeids are often surface oriented. Previous work with hydroacoustics was based on a sampling methodology where the transducer was mounted off the front or side of a boat and below the water surface 0.5 to 1 meter. Transducer limitations extend this unsampled area above the transducer 0.5 to 1.5 m below the transducer resulting in the top 1 to 2.5 meters of the water column unsampled. We sampled the limnetic fish population in 9 Southeast reservoirs with both single and dual beam acoustic systems to estimate the population size and distribution of threadfin shad Dorosoma petenense, gizzard shad D. cepedianum, and blueback herring Alosa aestivalis throughout the water column by sampling with the transducer mounted vertically to sample 2 meters below the surface to near bottom, and horizontally to sample near the surface. Sampling the near surface strata of a reservoir required a transducer with low side lobes and low winds for sampling. Fish densities were calculated be scaling the total reflected voltages by the mean fish size. This scaling factor was different than the scaling constant for the vertically aimed transducer, and is due to the different aspect of the transducer to the clupeid air bladder. Clupeid density in the top 2 meters of the water column ranged from 1% to 60% of the total fish population in the water column. Fish densities in the top 2 meters of the water column were similar to densities at 2-3 m depths in most reservoirs. Differences in vertical distribution of clupeids among and within reservoirs may be explained by water clarity, thermocline depth, or lunar phase. A larger database will provide clues to vertical distribution patterns by clupeids in Southern reservoirs.