Biology and fishery of Eledone cirrhosa in the Ligurian Sea

Biological information for Eledone cirrhosa has been reassessed through two series of trawls in the framework of the Research Projects MEDITS (summer) and GRUND (autumn) over a 10-year period of surveys in the Ligurian Sea. This information covers aspects such as life span and growth and mortality rates. The sum of samples obtained in the two different seasons allowed the analysis of consistent length frequency distributions and resulted in distinguishing recruits/juveniles from adults of two different ages, i.e. E. cirrhosa is a relatively long living cephalopod, whose reproduction occurs in the second or third year of life. By sampling in different bathymetric strata, different densities on shelf and slope fishing grounds were measured and their displacements with changing season were monitored. The most important stratum in terms of summer densities was in the range 100–200m depth, which includes the shelf edge, at 150 m. In a relatively narrow strip of seabed, covered by the shelf-edge detritic assemblage and adjacent muddy bottom assemblage VTC (80–150 m), the recruitment of E. cirrhosa occurs from January onwards, while till recent times a specific trawling targeting juveniles used to be carried out in late spring and summer months. In autumn E. cirrhosa is mainly concentrated in shallower waters where trawling activity does not specifically target this species. Trends in numbers and biomass derived by the two series of surveys, and in particular those during autumn series have revealed a significant decrease over the studied time interval. Therefore, the possible relationship between large-scale environmental factors and/or exploitation, was investigated and a positive relationship between summer biomass and winter NAO index was found, even if the cause of such relationship remains unexplained. However, trends of E. cirrhosa landings reported in other studies of the north-western Mediterranean resulted in a similar relationship, thus reinforcing the hypothesis of climatic factors influencing the success of the fishery of this biological resource. A tentative stock assessment, based on relative yield-per-recruit functions, taking into account the two different lengths at first capture and corresponding to mesh sizes of 20 or 40mm (juveniles and adults nets), showed overexploitation and underexploitation, respectively. A preliminary study of density indices by vital phase also demonstrated a correlation between summer spawners and autumn juveniles of the following year, thus providing evidence of a stock–recruitment relationship.