On the abundance and spatial distribution of Illex coindetii (Cephalopoda: Ommastrephidae) and Eledone moschata (Cephalopoda: Octopodidae) in the Sardinian Seas (central-western Mediterranean) - A preliminary and qualitative investigation with special attention to some environmental constraints

Abundance indexes and frequency of positive hauls data obtained for Illex coindetii and Eledone moschata from eight Mediterranean experimental trawl surveys carried out in the Sardinia Seas were analysed with an explorative methodology. Two sets of yearly surveys (homogeneous among them but different when considering the two groups) were thereby identified and the relative species density (N/km2) GIS distribution maps computed. A trawl survey, "anomalous" if compared to the others of the considered historical series, was put in evidence, the anomaly resulting in a very high recruitment pulse for Illex coindetii. Given the role of environmental factors (e.g., water temperature, currents regimes, up-welling events, etc.) on cephalopods life cycles, Sea Surface Temperature (SST) images for each month of a historical series of years, i.e. 1993-2001, including those of the trawl surveys considered, were gathered and qualitatively analysed to investigate SST relationship with the observed abundance/frequency fluctuations. Basic information available on the currents systems characterizing Sardinia waters was also collected and analysed. Results did not show a clear qualitative relationship between SST and the observed general variability, but evidenced a good correspondence between both species distribution and the analysed hydrological patterns. Very interestingly, however, the unusually high pulse of recruitment observed for I. coindetii during 1997, corresponded to unusually high SST values recorded during the period November 1996-April 1997, indicating a hydrological situation different from the usual pattern. These anomalous values could reflect the shifting toward north of the Algerian current and, consequently, of the frontal zone off the western Sardinian coasts, occurred from 1996 to 1998. This, in turn, could have generated/determined optimal conditions for the eggs development and hatching (e.g., salinity, nutrients and temperature favourable conditions), thus the recruitment pulse observed.