Target strength and density structure of Hawaiian mesopelagic boundary community patches

Abstract

A broadband sonar system and digital camera with strobe lights were mounted on a vertically profiling frame with a depth sensor. The target strengths and densities of animals within individual mesopelagic boundary community patches were investigated as a function of depth. Simultaneous echosounder surveys permitted comparison of density estimates from echo‐energy integration and echo‐highlight counting. Target strength values suggest nearshore boundary community animals are primarily myctophid fishes which was confirmed by preliminary photographic evidence. Target strength varied significantly as a function of distance from the shoreline and time. Echo‐energy integration estimates of density made with these revised target strengths compare well with those made with echo highlight counting. These density measures show that previous density estimates were too low but do not change the conclusions of these studies. Vertical microstructure in density was apparent but animal size and compositional structure was not evident within a patch. Patch edges were abrupt, with no differences in the density or target strength from patch interiors. These edges were generally straight, with a sharp drop in density to the background density of zero. Estimates of animal size as a function of time provide information about the diel migration patterns of these mesopelagic animals.