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Mesopelagic fish, being in the middle of the trophic web, are important key species for the marine environment; yet limited knowledge exists about their biology and abundance. This is particularly true in the Mediterranean Sea where no regional assessment is currently undertaken regarding their biomass and/or distribution. This study evaluates spatial and temporal patterns of mesopelagic fish biomass in the 1994–2011 period. We do that for the whole Mediterranean Sea using two well-established statistical models, the Generalized Additive Model (GAM) and Random Forest (RF). Results indicate that the bathymetry played an important role in the estimation of mesopelagic fish biomass and in its temporal and spatial distribution. The average biomass over the whole time period reached 1.08 and 0.10 t/km2, depending on the model considered. The Western Mediterranean and Ionian Seas were the sub-regions with the highest biomass, while the Adriatic was the area with the lowest. Temporal trends showed different trajectories with steep decrease and a fluctuation, using respectively RF and GAM. This study constitutes the first attempt to estimate the biomass and the spatial temporal patterns of mesopelagic fish using environmental variables as predictors. Given the growing interest in mesopelagic fish, our study sets a baseline to further develop mesopelagic fish biomass assessments in the region. Our results stress the need to improve data collection and quality in the region while identifying appropriate tools to better understand and assess the processes behind mesopelagic fish dynamics in the basin.

The length–weight relationships (LWRs) of 52 species (14 never reported before) of fishes, crustaceans and cephalopods living on the shelf and upper slope off Southern Sicily are provided. Data were collected in the framework of the International bottom trawl survey in the Mediterranean (MEDITS) in the South of Sicily (Central Mediterranean), covering a time frame ranging from 2012 to 2019. Linear regressions were significant for all species (p < 0.05) with R2 values ranging from 0.86 to 0.99. The intercept (a) of LWRs ranged from 0.0003 to 0.4677, while the slope (b) ranged from 2.1281 to 3.306. The Welch t-test, used to evaluate differences between the obtained LWRs with those reported in the literature, revealed that most of the LWRs (about 55%) reported in this study are in disagreement with those obtained previously by other authors from the Strait of Sicily. It is expected that the results obtained from this study will contribute to filling the knowledge gap of fish populations in this area and also assist fisheries scientists in future stock assessment studies.

Illegal wildlife trade is considered one of the most serious threats to biodiversity worldwide, along with habitat loss/degradation and overfishing of wild stocks. Seahorses are considered at high risk as these fish represent an important component of traditional Chinese medicine but are also sold as curios and ornamental fish. On a worldwide level, illegal trade is controlled by numerous laws and regulations, but it seems to continue by assuming more dynamic routes. In the Mediterranean Sea, Hippocampus guttulatus formed one of the largest populations at Mar Piccolo di Taranto in South-Eastern Italy. During the routine monitoring of this population in 2016, a dramatic density decrease was observed. By using questionnaires and long-term datasets, the present study determined possible causes of this decline by investigating habitat changes, temperature trends and the existence of seahorse trafficking while also examining abundance trends during the last decade. The results indicated a sharp density decline starting from 2015, co-occurring with the period of high temperatures, while habitats remained almost constant. However, interviews with main stakeholders described both illegal and legal fishing activities as the main drivers for the declining seahorse density. Indeed, at one of the studied sites, which was under strict military control, seahorse abundance started to decline only after the intensification of fishing pressure in the basin. The study suggests that Mar Piccolo di Taranto could be one of the sources for international seahorse trade, thus highlighting the need for more intense and effective actions to prevent and combat illegal poaching, while threatened populations are requiring continuous and close monitoring. Due to unfavorable socio-economic conditions, a viable and thriving seahorse population at Mar Piccolo di Taranto could contribute to the revitalization of the coastal economy and the development of environmental awareness.