Day-night differences in hydroacoustic estimates of fish abundance and distribution in Lake Laojianghe, China

Abstract

Hydroacoustic surveys were performed in an oxbow lake (Lake Laojianghe, China) along the Yangtze River. The study took place during the day and night in October 2012 using the Simrad EY60 split beam echo sounder. A clear day-night difference in fish densities was observed in selected transect areas. The mean density estimate at night was three times higher than during the day, while at night densities were similar in the two adjacent areas. Significantly positive relationships were found between fish density and average depth for the night and daytime transects, however, the regression slope for the daytime density estimates was much smaller than for the surveys at night. Small fish dominated the acoustic size structure during both day and night, but were more accessible at night. The average biomass estimate from the daytime transects was nearly 60% lower than those from the corresponding night transects. These results indicate that an entire lake estimate of a fish population may be severely biased if fish tend to aggregate temporarily in the bottom or littoral zone or display diurnal behaviour. The conclusion is that acoustic sampling in Lake Laojianghe is best done at night. Diel differences in the lake should be further investigated by carrying out additional day-night comparison experiments and also by using control catches.