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An introduction to Mediterranean deep-sea biology* - page 21 / 32





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FIG. 4. – Deep-sea shrimp landings (Aristeus antennatus + Aristaeomorpha foliacea) by different ports in the western Mediterranean (VRS, Villareal; ALM, Almeria; SPO, Sta. Pola; VIL, Vila Joiosa; BCN, Barcelona, BLA, Blanes; PAL, Palma; SML, Sta. Margarita; SLE, Sestri; PER, Porto Ercole; FRM, Formia; SAL, Salerno; SMC, Sta. Maria; CMR, Marina di Camerota; TRS, Terrasini; TRP, Trapani; ORS, Oris- tano; STT, Sant’Antioco; CGL, Cagliari; ARB, Arbatex; GAL, Gallipoli; MZV, Mazara; PCP, Portopalo; SCC, Sciacca; BIZ, Bizerte; ALG, Algiers; CHE, Cherchell; NAD, Nador); from Sardà (2001).

The deep-water shrimps A. foliacea and A. antennatus show a similar depth distribution pattern on the two sides of the Ionian Sea, with the former species in shallower waters than the latter. However, their relative abundance changes between Italian and Greek waters. A. foliacea is far more abundant than A. antennatus off Greece, while this latter species shows a higher density in Italian waters than in Greek ones. This could be related to the different water masses on the slope of the Ionian basin, which along the Greek coasts are warmer and have a high- er salinity, while along the Italian ones they are cold- er and less saline (Robinson and Golnaraghi, 1992; Theocaris et al., 1993, Rabitti et al., 1994). A. foli- acea would be mainly linked to the former, whereas A. antennatus would be mainly linked to the latter, according to Ghidalia and Bourgois (1961) and Bombace (1975). Although the hydrographic hypothesis of Ghidalia and Bourgois (1961) needs to be confirmed, the Mediterranean distribution of the two species would seem to support it (Relini and Orsi Relini, 1987; Murenu et al., 1994; Ragonese, 1995). However, there are no studies that have established other specific hypotheses on the differ- ent distribution of these two companion species.

A study on the selectivity of the otter trawl was carried out by Colloca et al. (1998) in the central Tyrrhenian Sea. For a mesh size of 16-18 mm, the length of first capture ranged between 24 and 31.6 mm, while for a mesh size of 20-22 mm CL was comprised between 31.5 and 38.6 mm. In the south-

ern Tyrrhenian Sea the size at first capture and the respective age was calculated by the selection ogive for a 32 mm stretched mesh size (Spedicato et al., 1995): L50 = 32.76 mm CL and T50 = 0.9778. Ragonese et al. (1994) in the Sicily channel carried out some studies of selectivity and suggested a mesh size of up to 28 mm side to reduce the juveniles in the catches.

A. antennatus seems to support a sustainable fishery along the Italian coasts despite the high fish- ing pressure. This might be due to the very wide depth distribution in bathyal waters and to the fact that part of the population is not vulnerable to trawl- ing (Sardà, 1993). Another reason could be the high fecundity of this species. On the other hand, A. foli- acea exhibits a marked overexploitation due to its greater vulnerability to fishing, since both juvenile and adult A. foliacea are almost exclusively distrib- uted at depths at which bottom trawling occurs (Matarrese et al., 1997; D’Onghia et al., 1998a).

In the Italian Ionian Sea there are some other local small-scale fisheries using longline and target- ing hake and other deep-sea species such as rock- fish, greater forkbeard and bluntnose sixgill shark (Hexanchus griseus). Another small-scale fishery is that targeting Hexanchus griseus, which is carried out with longlines in the central and southern Aegean Sea at depths of 600 to 1500 m. The species has a low commercial value, but the catch is quite high and the fishery is profitable. The length of the longlines is about 15-20 km. The duration of each


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