Impact of the Adriatic-Ionian Bimodal Oscillating System (BiOS) on the biodiversity patterns of larval fish community in the north-eastern sector of the Straits of Sicily (Central Mediterranean, Malta Channel)

Bernardo Patti ^1* Marco Torri ^2,3* Francesco Placenti ¹ Angela Cuttitta ^2,3

• ¹ Institute for the Study of Anthropogenic Impacts and Sustainability in Marine Environment, Department of Earth System Sciences and Technologies for the Environment, National Research Council (CNR), Rome, Italy
• ² Istituto di studi sul Mediterraneo (ISMed) CNR, Naples, Campania, Italy
• ³ CNR-ISMED, Palermo, Sicily, Italy

Off the southern coast of Sicily, the frontal region south of Cape Passero (Malta Channel) provides favourable conditions for fish larvae survival and development and hosts a biodiversity hotspot for larval assemblages. In this area the analysis of ichthyoplankton data collected in 16 oceanographic summer surveys carried out every year over the period 2001-2016 shows a cyclical pattern in the yearly average larval biodiversity, which appears to be linked to the alternating cyclonic/anti-cyclonic surface circulation of North Ionian Gyre (NIG) associated to the Adriatic-Ionian Bimodal Oscillating System (BiOS). Specifically, the cyclonic mode of NIG, in enhancing the advection of Modified Atlantic Water (MAW) towards the southern Levantine basin and reducing the deflection toward the Adriatic, is supposed to intensify the frontal thermohaline structure, so inducing higher retention/survival rates for fish larval stages, and definitively higher biodiversity. The association of total fish larval density and biodiversity with available environmental data, namely satellite SST and in situ temperature, salinity and surface current speed, corroborates this hypothesis. At last, the observed changes in the biodiversity of larval fish community would result from increased/reduced retention time for fish larvae (and accordingly slower/faster larval dispersal) across the frontal area as induced by the alternating cyclonic-anticyclonic modes of NIG. These results pay the way for future investigations on the role of surface circulation patterns on the dynamics of fish populations, with special emphasis on the effects of retention processes of fish larval stages.