Acoustic correction factor estimate for compensating the vertical diel migration of small pelagic species

Differences in acoustic estimates of small pelagic fish biomass, due to data acquisition during daytime and night-time surveys, have been recognized as a problem in acoustic surveys for many years. In the absence of a single rule for all species and for all locations, some expert groups have identified specific time intervals for acoustic data acquisition in relation to the schooling behavior of the target species. In the Mediterranean Sea, the research groups working in the MEDIAS (Mediterranean International Acoustic Survey) agreed on the necessity of acoustic sampling being conducted only during daytime. Only when available time does not permit to complete the survey during daytime, data collection might be extended. In this case, working on data collected during both daytime and night-time, a bias may occur in the biomass estimates. To evaluate and correct such bias, specific experiments were performed in some geographical subareas of the Mediterranean Sea. The data analysis allowed the estimation of a mean correction factor for the Strait of Sicily, where five surveys were carried out in different years. The correction factor was also estimated for the Adriatic Sea, Tyrrhenian Sea and northern Spain; the observed variability among areas highlighted the importance of the spatial and temporal coverage of the survey area to obtain reliable estimates of the correction factor. Further studies are necessary to improve the interpretation of the obtained estimates in relation to area-related peculiarities such as zooplankton composition and abundance along with small pelagic fish community structure.