On Baltic herring morphometry and its impact on the backscattering properties.

Abstract

Previous studies, dedicated to backscattering properties of Baltic herring, have shown the different target strength (TS – which is a measure of fish capacity to scatter sound) values, for the same species in different regions and seasons. The intraspecies differentiation in fish physiology and morphology as well as fish swimbladder morphometry between herring aggregations, occupying various parts of the Baltic Sea, has been supposed as one of the reasons for the variability.The paper addresses analysis of herring swimbladder morphometry and its impact on TS of individuals from ICES subdivision 26, one of the areas where Poland is responsible for herring biomass estimation. The collection of the X-rays images for 74 herring individuals, sampled in this subdivision, was created. The two-dimensional digitized dorsal images of herring swimbladder and body, as well as the angles between the swimbladder and the body longitudinal axis, were used to compute the target strength. The differentiation of herring morphometry within particular fish size classes was analysed and its consequences for the averaged target strength within the class was discussed. The difference from the previous numerical studies, in which the simplified herring morphometry was used, was also demonstrated. The computational results were considered in regard to the available in situ measured data on Baltic herring TS. The study of the Baltic herring target strength is important for increasing accuracy of acoustic biomass estimation of this ecologically and economically important species.