Effects of sampling intensity and biomass levels on the precision of acoustic surveys in the Mediterranean Sea

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