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Acoustic surveys represent the standard fishery-independent method worldwide for evaluating the biomass and spatial distribution of small pelagic fish populations. Considering the peculiarities of the spatial behaviour of pelagic fishes, the efficiency of the survey design in determining their biomass and spatial distribution is related to its ability to capture the portion of the patches accounting for larger part of the total biomass. Yet, the spatial structure of the patches could be strongly influenced by ecosystem characteristics as well as by changes in total biomass related to a density-dependent mechanism. This is of particular interest for anchovy and sardine which are known for their wide fluctuations and high sensitivity to the environment. In this study, we analysed the efficiency of acoustic surveys, targeting European anchovy (Engraulis encrasicolus) and European sardine (Sardina pilchardus), in 10 different areas of the Mediterranean Sea across three years of different biomass levels. Using the geostatistical Coefficient of Variation (CVgeo) of the average occurrence probability of high/medium density values, we showed different patterns in terms of survey design efficiency among areas and species. Anchovy usually showed lower CVgeo than sardine, but in the Alboran Sea. In 4 out of 20 cases, CVgeo values showed a consistent decrease with increasing biomass while in the remaining cases the CVgeo did not follow any clear pattern suggesting the presence of important environmental effects. Higher survey design efficiency was found in high productive sectors influenced by river run-off, letting us to hypothesize that higher productivity along with the presence of well-localized enrichment mechanisms could favour a spatially consistent distribution and coherent organization of fish population leading to higher precision estimates with a given transect design. While most surveys displayed CVgeo close to 10% or less even at low biomass, evidencing generally good performances of the survey design, a few areas exhibited higher CVgeo, yielding discussions about a potential need to decrease the inter-transect distance, always keeping in mind that survey should be as synoptic as possible.

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.

The working group was held in remotely, from 12th to 20th Sept 2020. The meeting was attended by 14 experts in total, including two STECF member and two JRC experts. The EWG had two observers who attended part time. The objective of the Mediterranean Methodology EWG 20-15 was to carry out assessments and provide draft advice for stocks identified in the ToR supplied by STECF. An initial plenary session commenced at 09:30 on the first day. The ToRs were discussed and examined in detail. Stocks were allocated to participants based on expertise. An ftp repository was created ad-hoc to share documents, data and scripts and prepare the report. The stocks were evaluated by the GSA groups identified in the ToRs. Most of the work was concluded by Tuesday 20 Sept, after 7 full days of work, and some additional work at the weekend. Over the 7 working days plenary sessions were held each day to monitor progress and share results. The overall conclusions for each stock were discussed and finalized in plenary on the Tuesday. European Commission European Union, Joint Research Centre Published Refereed