Seasonal and diel changes in sampling conditions for acoustic surveys of fish abundance in small lakes

Abstract

Acoustic techniques have difficulty detecting fish that are near the surface or near the bottom. This limitation may be particularly critical in small, shallow lakes where the pelagic zone is relatively small. The feasibility of acoustic sampling was tested in 9 small lakes (5–52 ha) in the Adirondack region (New York) and northern Wisconsin during summer and autumn 1985. We report here on the effects of seasonal and diel changes in the distributions of fish and the dipteran, Chaoborus, on acoustic measures of abundance. Fish abundances were measured using echo integration of signals from echo sounders operating at three frequencies (70, 120 and 420 kHz). Fish distributions were compared with water temperatures, dissolved oxygen concentrations and water depth. During summer stratification in the lakes, fish were concentrated in the metalimnion. Water temperatures at the depths occupied by the fish matched their reported thermal preferences in some, but not all, lakes. Fish density was highest immediately above the anoxic hypolimnion. Midwater aggregation by the fish in the metalimnion produced good conditions for acoustic sampling. Following autumn overturn, fish distributions were generally not favorable for acoustic sampling because fish moved to shallow water or to the bottom. Acoustic scattering in the lakes was often much stronger at night than during the day, owing largely to Chaoborus larvae, which vertically migrate from the sediments at night. When Chaoborus was abundant, acoustic detection of fish at night was difficult. Acoustic techniques proved useful in small lakes, but distributions of fish and plankton can affect data reliability and must therefore be examined carefully. Seasonal and diel variations in fish and plankton distributions are somewhat predictable and can be used to develop more efficient strategies for acoustic sampling.