Sampling and Statistical Considerations for Hydroacoustic Surveys used in Estimating Abundance of Forage Fishes in Reservoirs

—An understanding of the spatial distribution of forage fish resources is required to make informed fishery management decisions. We used mobile hydroacoustics to assess the distribution and abundance of forage fish in Badin Lake, a reservoir in central North Carolina. By sampling a series of cross-channel and longitudinal transects and analyzing the data using geostatistics, we characterized both largeand small-scale spatial patterns in forage fish density. Forage fish were observed in higher densities in upstream regions of the reservoir and were seen only in surface waters during July 2000 owing to the existence of a strong thermo–oxycline and in two layers (surface and near bottom) during mixed conditions in December 2001. We observed differences in the scale of patchiness (200–700 m) in forage fish distribution depending on the region of the reservoir where sampling took place, and we infer that these patterns are governed by prevailing limnological conditions. Modeling the spatial variation in the acoustic data using geostatistics resulted in similar average densities (July 2000: 0.56 0.28 [mean SD] fish/m2; December 2001: 0.57 0.49 fish/m2) and improvements in the precision of abundance estimates based on approximated variance (July 2000: 7.05 106 8.30 105 fish; December 2001: 7.07 106 2.10 106 fish) when compared with arithmetic averaging and extrapolation (July 2000: 0.60 0.61 fish/m2, 7.5 106 4.61 106 fish; December 2001: 0.60 0.69 fish/m2, 8.00 106 6.34 106 fish). We found that sampling along longitudinal transects is a more efficient way to characterize the spatial patterns of forage fish distribution and to estimate systemwide abundance and biomass than using data collected with both a longitudinal and cross-channel sampling design in this system. The management of sport fisheries in inland reservoirs is improved through a knowledge of ecosystem processes, particularly the interactions between piscivorous fishes and their forage base (Ney 1981, 1990; Noble 1981). Reaching this goal requires an informed understanding of how organisms are distributed over the broad range of spatial scales at which these interactions occur. Much of * Corresponding author: chris [email protected] Received December 9, 2003; accepted April 5, 2004 Published online February 28, 2005 the research effort in reservoir fishery studies has focused on large-scale spatial patterns, with an emphasis on physical and biological gradients (e.g., temperature, water flow, turbidity, conductivity, chlorophyll) typically oriented along the longitudinal axis of the water body (Tisa et al. 1987; Tisa and Ney 1991; Michaletz and Gale 1999). Less attention has been focused on smaller scale spatial patterns in these ecosystems (Schael et al. 1995). These smaller scale dynamics can give rise to spatial structuring in fish communities that undoubtedly have influence on higher order ecosystem pro-