Diel variations in demersal assemblages in the Strait of Sicily, central Mediterranean Sea

Day-night variations in catch of demersal species were examined by a six-day trawl survey carried out in September 2017 on the shelf-break (125–155 m depth) off the southwestern coast of Sicily. A total of seven hauls per day were performed, distributed between full daylight and complete darkness. Two main assemblages separating nocturnal and diurnal hauls were identified using ANOSIM and PERMANOVA analysis. The diurnal assemblage was characterised by higher species richness, dominance, and low diversity, with the main species consisting of visual bentho-pelagic predators, such as Illex coindetii and Merluccius merluccius, whereas lower richness but higher evenness and diversity were observed during the night, with species such as Chlorotocus crassicornis, Alloteuthis spp., and Lesueurigobius suerii characterising the assemblage. Among the main commercial species caught by bottom trawling in the area, Parapenaeus longirostris showed high CPUE values during daylight hours, while Trachurus trachurus displayed a clear diel pattern with a peak around midday and a minimum at night. In contrast, M. merluccius did not exhibit any significant variation in catch rates over the daily cycle. Understanding diel variations in shelf-break assemblages and commercial species may contribute to the regulation of fishing activities by guiding the implementation of technical measures to minimize unwanted catch and promote more sustainable demersal fisheries.