12. Distribution and Abundance of Spiny Dogfish in the Mediterranean Sea Based on the Mediterranean International Trawl Survey Program (MEDITS)
Fabrizio Serena, Costas Papaconstantinou, Giulio Relini, Luis Gil De Sola, and Jacques A. Bertrand
Abstract.—Data on two shark species, collected in the frame of the European Union program Mediterranean International Trawl Survey Program, are analyzed and reported. Indices of summer abundance per standardized area (per km2) in weight and number are available for both species since 1994 along the European Mediterranean coasts (from the Alboran to the Aegean seas). The studied area, with depths ranging from 10 to 800 m, was divided into five depth strata. Data of 10,000 hauls were analyzed and 44 elasmobranch species identified. The estimated comprehensive standing biomass of elasmobranch fishes within the explored area was 55,000 mt (mt); spiny dogfish Squalus acanthias was one of the most abundant species with an estimated biomass of 6,700 mt, while longnose spiny dogfish S. blainvillei, with about 1,500 mt, represents only 3% of the total biomass. The mean density of spiny dogfish was significantly different between the Mediterranean eastern basin (22.7 kg/km2) and western basin (only 0.8 kg/km2). On the contrary, longnose spiny dogfish shows higher density in the western basin (6.6 kg/km2) than in the eastern one (1.7 kg/km2). However, the spatial distribution of both species is fairly confined; spiny dogfish was caught in only 5% of the tows and longnose spiny dogfish in 3%. Even if the depth range of presence for both species spreads from less than 50 m to more than 700 m, the abundance indexes suggest a major presence in the coastal areas for spiny dogfish. This statement is based primarily on the high densities of the mentioned species in shallow waters of the northern Adriatic Sea; elsewhere the main concentrations are always positioned in the 200–500 m depth range.
Regarding likely changes in mean individual weight by depth, no clear depth effect was observed for the species. Specimens with weights over 2 kg do not represent more than 10% of the individuals of either species. A possible hypothesis to explain the almost mutual exclusion of the Squalus spp. is based on the presence of lesser spotted dogfish Scyliorhinus canicula in many of these samples.