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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 the ability to capture the portion of the patches accounting for a larger part of the total biomass. However, 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 anchovies and sardines, 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 anchovies (Engraulis encrasicolus) and European sardines (Sardina pilchardus) in 10 different areas of the Mediterranean Sea spanning 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. Anchovies usually showed a lower CVgeo than sardines 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 highly productive sectors influenced by river run-off, allowing 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 populations, leading to higher precision estimates with a given transect design. While most surveys displayed CVgeo close to 10% or less even at low biomass, i

Data on Sardinella aurita (round sardinella) and Sprattus sprattus (European sprat) in the Mediterranean Sea are limited due to their scarce commercial interest, at least in European countries. Yet, these two small pelagic fish, sitting at opposite ends of the thermal range, could be interesting sentinel species to monitor the effects of climate change in the basin. Using the Mediterranean International Acoustic Surveys (MEDIAS) - the most extensive source of information on these species - we analysed their biomass in several geographical subareas of the central and western Mediterranean Sea in relation to satellite-derived environmental parameters. Our findings highlight that the S. aurita biomass responded to temperature, salinity, chlorophyll concentration and sea level anomaly, depending on the GSA examined, whereas the S. sprattus biomass correlated significantly with salinity in GSA 6, with salinity and chlorophyll concentration in GSA 7 and with sea level anomaly in GSA 17. These data widen our knowledge of the factors that contribute to the ecology of these species. Further studies of their spatial distribution and of their interactions with other small pelagic species, predators and prey are needed to depict a more comprehensive scenario.

Size at first sexual maturity (L50) represents an important life-history trait that needs to be considered in the development of management measures as it provides fundamental information for avoiding the exploitation of younger individuals. L50 is known to display variability due to fishing pressure, geographical gradients, and environmental features. In this study, to investigate L50 variability among areas in the Mediterranean and Black Seas, maturity ogives of anchovies (Engraulis encrasicolus) were estimated by considering samples collected during the anchovy spawning period in the framework of the MEDiterranean International Acoustic Survey (MEDIAS) program. Anchovy size and sexual maturity data from several geographical subareas (GSAs), i.e., northern Spain, Gulf of Lion, Tyrrhenian Sea, Strait of Sicily, Adriatic Sea and Black Sea, were gathered according to a standard methodological protocol. Maturity ogives were estimated by a logistic regression considering total length, condition factor, sex and GSA. The obtained results showed a significant effect of the condition factor, in that fish in better condition reached maturity earlier, and the results also indicated differences in L50 values among the areas and between the sexes, with males reaching maturity at lower lengths than females. Even though the obtained L50 estimates are relative to the spawning period only, the variability observed at the Mediterranean basin scale highlights the importance of explicitly considering specific habitat characteristics when providing management advice based on an ecosystem approach for fisheries.

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 the ability to capture the portion of the patches accounting for a larger part of the total biomass. However, 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 anchovies and sardines, 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 anchovies (Engraulis encrasicolus) and European sardines (Sardina pilchardus) in 10 different areas of the Mediterranean Sea spanning 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. Anchovies usually showed a lower CVgeo than sardines 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 highly productive sectors influenced by river run-off, allowing 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 populations, leading to higher precision estimates with a given transect design. While most surveys displayed CVgeo close to 10% or less even at low biomass, i

Data on Sardinella aurita (round sardinella) and Sprattus sprattus (European sprat) in the Mediterranean Sea are limited due to their scarce commercial interest, at least in European countries. Yet, these two small pelagic fish, sitting at opposite ends of the thermal range, could be interesting sentinel species to monitor the effects of climate change in the basin. Using the Mediterranean International Acoustic Surveys (MEDIAS) - the most extensive source of information on these species - we analysed their biomass in several geographical subareas of the central and western Mediterranean Sea in relation to satellite-derived environmental parameters. Our findings highlight that the S. aurita biomass responded to temperature, salinity, chlorophyll concentration and sea level anomaly, depending on the GSA examined, whereas the S. sprattus biomass correlated significantly with salinity in GSA 6, with salinity and chlorophyll concentration in GSA 7 and with sea level anomaly in GSA 17. These data widen our knowledge of the factors that contribute to the ecology of these species. Further studies of their spatial distribution and of their interactions with other small pelagic species, predators and prey are needed to depict a more comprehensive scenario.

Size at first sexual maturity (L50) represents an important life-history trait that needs to be considered in the development of management measures as it provides fundamental information for avoiding the exploitation of younger individuals. L50 is known to display variability due to fishing pressure, geographical gradients, and environmental features. In this study, to investigate L50 variability among areas in the Mediterranean and Black Seas, maturity ogives of anchovies (Engraulis encrasicolus) were estimated by considering samples collected during the anchovy spawning period in the framework of the MEDiterranean International Acoustic Survey (MEDIAS) program. Anchovy size and sexual maturity data from several geographical subareas (GSAs), i.e., northern Spain, Gulf of Lion, Tyrrhenian Sea, Strait of Sicily, Adriatic Sea and Black Sea, were gathered according to a standard methodological protocol. Maturity ogives were estimated by a logistic regression considering total length, condition factor, sex and GSA. The obtained results showed a significant effect of the condition factor, in that fish in better condition reached maturity earlier, and the results also indicated differences in L50 values among the areas and between the sexes, with males reaching maturity at lower lengths than females. Even though the obtained L50 estimates are relative to the spawning period only, the variability observed at the Mediterranean basin scale highlights the importance of explicitly considering specific habitat characteristics when providing management advice based on an ecosystem approach for fisheries.