Estimating the sampling efficiencies of a framed midwater trawl and ring net for zooplankton using an acoustic method

Abstract

It is well known that there is bias using different sampling gear, such as a framed midwater trawl (FMT) and a ring net, when measuring the density and length distribution of target species. This limit is characterized by the sampling efficiency of the gear. Acoustic monitoring can be used to determine the sampling efficiencies of this gear, as its noninvasive, wide-range sweeps provide more reliable estimates of absolute abundance of the target species. The density measured by the gear can then be standardized by multiplying the initial density measurement by the derived sampling efficiency. In this study, we compared the estimated densities of the dominant zooplankton in the sound scattering layer (SSL) from acoustic monitoring to the densities of the same species measured using the FMT and ring net. The respective sampling efficiencies of this gear for zooplankton categories was then determined using linear regression models. For small Euphausiidae and Copepoda, the sampling efficiency of ring net was higher than that of FMT. In contrast, the ring net was less effective than FMT for large Euphausiidae. These results highlight that the entering and retention rates of the species depend on the type and characteristics of the survey gear, as well as the size and swimming ability of the target species.