Diel migration and swimbladder resonance of small fish: some implications for analyses of multifrequency echo data

Many fish with swimbladders exhibit diel vertical migrations (DVM). Ascents and descents of hundreds of metres occur, and altered swimbladder volume and buoyancy can result from incomplete secretion and resorption of gas. When acoustic observations are made near the resonance frequency of the swimbladder, the estimated fish biomass can be positively biased. When multiple-frequency echosounders are used, the frequency response of the backscatter might vary temporally and spatially and compromise the effectiveness of conventional target-identification methods. In this paper, variations in backscatter from mesopelagic fish are studied using data collected west of the British Isles with a five-frequency echosounder (Simrad EK60). Two acoustic layers, one dominated by pearlsides (Maurolicus muelleri) and the other by myctophids (Myctophidae), were monitored during their DVM. The frequency responses of the layers changed systematically, mainly characterized by increases in the nautical-area-backscattering coefficient (s(A)) values at 18 kHz relative to those at 38 kHz. This could have been caused by changes in the resonance frequencies of fish swimbladders, as they expanded and contracted during ascent and descent. Two s(A) maxima in the myctophids layer suggest the presence of two types of target with different scattering characteristics. Models of sound scatter from myctophid swimbladders suggest that these peaks have resulted from resonance scattering. The s(A) at 18 kHz attributed to M. muelleri also peaked, but at the maximum depth of their distribution. Spatial and temporal changes in the frequency responses of fish should be taken into account when pelagic fish communities are surveyed with multiple-frequency echosounders.