Aspects of the ecology of three demersal elasmobranches : Scyliorhinus canicula, Galeus melastomus and Raja clavata in the central Mediterranean

Abstract

Selected population parameters, feeding biology, habitat preferences, sexual segregation and the determination of potential nursery and spawning grounds of Scyliorhinus canicu/a, Galeus melastomus and Raja clavata within the GFCM's Geographical-Sub Area (GSA) 15 were studied since there are no studies on any of these aspects for GSA 15, and in some cases for the Central Mediterranean as a whole. Samples of the three elasmobranches were collected during the period 2003 - 2008 from 82 stations as part of the ongoing MEDITS (Mediterranean International Trawl Survey) annual bottom trawl survey. Total length, weight, sex and maturity stage were recorded for each specimen and stomach contents were analysed for a sub-sample of each species. Bottom sediment samples from 43 stations were collected by grab for granulometric analysis. All three species were abundant and inhabited mostly muddy bottoms; the preferred bathymetrical ranges are similar for S. canicula and R.c/avata (200 to 300 metres and 250 to 350 metres respectively), but G. melastomus was mostly found in deeper waters (350 to 650 metres). The three species showed positive allometric growth and different 'a' and 'b' coefficients were obtained for males and females. Length-frequency distributions had a plurimodal pattern. Females were more abundant than males for G. melastomus and R. clavata but males were more abundant for S. canicula. The lengths at 50% maturity were higher for females than for males in all cases. The three elasmobranches showed a generalised diet and may be considered as generalist feeders. Scy/iorhinus canicula and Raja c/avata showed a preference for fish followed by crustaceans while Galeus me/astomus showed a preference for crustaceans, followed by fish and cephalopods. Scyliorhinus canicu/a is probably a benthic feeder while the other two species have the ability to foraqe at different depths in the water column. Different food preferences were observed between males and females for Scyliorhinus canicula but not for the other species. The foraging zones used by males and females did not vary for any of the three species.