Bias in Hydroacoustic Estimates of Fish Abundance due to Acoustic Shadowing: Evidence from Day–Night Surveys of Vertically Migrating Fish

Abstract

We investigated the hypothesis that fish schooling behavior leads to underestimation of fish abundance when assessed by acoustic methods. Current methods for acoustic quantification of fish abundance rely on the assumption that fish biomass and numerical fish abundance are linearly related to acoustic scattering under all natural fish densities. However, cage experiments as well as field observations have indicated that acoustic shadowing effects occur at very dense and large aggregations of fish. Acoustic surveys of the pelagic fish community of Lake Memphremagog, Quebec, were conducted when fish were aggregated and dispersed. These differences in aggregation were related to diel responses to light levels. We found that estimates of fish abundance, as measured by echo integration, were consistently and significantly lower when fish were aggregated in dense schools. This bias was not due to diel changes in average echo level per fish, which exhibited no relationship to diel changes in vertical and hortizontal distributions. We conclude that the reduced abundance estimates obtained when fish were aggregated resulted from acoustic shadowing. Our data suggest that this bias may be as large as 50%.

Target-strength measurements of hake and jack mackerel

“…Such measurements have been described by many other authors, for example, , MacLennan and Simmonds (1992), Guillard (1991), and Appenzeller and Legget (1992).…”

Diel patterns of behavior in juvenile walleye pollock,Theragra chalcogramma

“…Schooling behavior can influence the efficiency of trawling and acoustic methods of population assessment . Appenzeller & Leggett (1992) found that acoustic shadowing resulted in a 50% underestimate of rainbow smelt, Osmerus mordax, abundance during daytime sampling, when smelt were aggregated in dense schools, compared to nighttime sampling, when smelt were dispersed .…”

Effect of light on juvenile walleye pollock shoaling and their interaction with predators

“…Although other senses, such as the lateral line system, may play an important role in the spatial and angular orientation of individuals within a school (Partridge & Pitcher 19801, it is unlikely that in the wild many species are capable of schooling in the absence of sufficient light. Thls impression is supported by observational (Emery 1973, Helfman 1979 and bioacoustical studies which generally show that shoals and schools disperse at night (Appenzeller & Leggett 1992, Brodeur & Wilson 1996. Since ambient light in marine and aquatic habitats may vary greatly according to depth, turbidity, season, lunar stage and cloud cover (McFarland 1986), the meaning of 'night', relative to light levels, 1s rather ambiguous.…”