A study on European anchovy (Engraulis encrasicolus) swimbladder with some considerations on conventionally used target strength

Abstract

Hydroacoustic surveys are one of the prime methods to assess the commercially top-ranked small pelagic stocks. The methodrelies on acoustic scattering from a fish, which is largely controlled by the size and morphology of the swimbladder. In this study, thechanges in the size of the European anchovy swimbladder sampled in the Black Sea were investigated. Ventral cross-sectional area (byphotographing the ventrally dissected fish) and volume (by dorsal and lateral X-raying) of the swimbladders were estimated. Comparisonof areas showed that the stomach fill and presence of viscera did not have a statistically significant impact on the swimbladder size whilethe hepatosomatic index showed significant impact. Although the vertical distribution of the anchovy is naturally not very wide due toabsence of sufficient oxygen below 100 m, sampling depth showed significant impact on the volume of the swimbladder. However, it wasalso observed during X-ray imaging that a considerable number of fish (87%) had deflated swimbladders. The reasons for this variability,which may have significant implication on the acoustic estimations and stock assessment, were also discussed. The importance ofacclimatization of the fish at surface conditions in studies addressing changes in swimbladder morphometry was underlined.