engleski

Temporal and spatial distribution of the loggerhead sea turtle, Caretta caretta, in the eastern Adriatic Se : a seasonal migration pathway?

We analyzed the data on the loggerhead sea turtle by-catch in the eastern Adriatic Sea (Slovenia and Croatia), in the period 1992-2002, with two objectives: (1) to provide information on interactions of marine turtles with the different fishing gears, and (2) to present additional analyses of distribution patterns related to environmental conditions. For the spatial distribution analysis, the study area was divided into five regions: (1) Slovenian waters, (2) W Istria waters, (3) Kvarner Archipelago, (4) Zadar Archipelago, and (5) Split-Dubrovnik Archipelago. Based on the differences in temperature regimes of the Adriatic Sea the year was divided into two periods: the “ warm period” (May - October), and the “ cold period” (November - April). Out of 132 turtles analysed, 44.69% were caught in gill nets, followed by trawls (40.15%) and longlines (0.76%), while 14.4% of turtles were caught in unidentified types of net. Direct mortality was 54.9% for gill nets (n=51), and 12.5% in trawls (n=32). A higher frequency (60%) of adults (CCL > 70cm) were caught in trawls (chi-sq = 30.6, p<0.001, d.f.= 3). On the other hand, a significantly higher proportion of juveniles with CCL = 30 - 50 cm are affected by gill nets (chi-sq = 8.33, p < 0.03, d.f.= 3). There is a high significant association between certain kinds of fishing gear and a particular period (G-test = 14.94, P < 0.001, d.f. = 1). By-catch in gill nets is associated with the warm period (May-October)(chi-sq = 4.33, p = 0.03, d.f. = 1), while captures in trawls are associated with the cold period (chi-sq =10.87, p < 0.001, d.f. = 1). Trawls and gill nets are the fishing tool with the major level of interaction with sea turtles in the eastern Adriatic Sea. Direct mortality in trawls seems to be low and comparable to the 0-10% registered in the Mediterranean. In contrast, high direct mortality (54.9%) has been recorded in gill nets. Temporal and spatial distribution analysis show that loggerheads are not homogeneously distributed in five regions between the warm and the cold period (G-test = 34.59, p < 0.001, d.f = 4). If we analyze by-catch data in the warm vs. the cold period separately, we find that during the warm period a significantly higher number of turtles were recovered in regions 1 and 2, while lower numbers than expected were caught in regions 3 and 4 (chi-sq = 17.65, p = 0.001, d.f. = 4). The frequency of by-catch is also not equally distributed among the study regions (chi-sq = 19.35, p < 0.001, d.f. = 4) in the low temperature conditions. More loggerheads are caught in regions 3, 4 and 5, and low numbers are recovered in the regions 1 and 2. These suggest the existence of a seasonal loggerhead movement in the Adriatic Sea. When sea temperature decreases, loggerheads seem to migrate from the northernmost part of the Adriatic Sea to regions south of about 45° N, where they overwinter. With the increase of the sea temperature turtles become more active, and a part of the overwintering population returns to foraging and developmental habitats in the northern Adriatic.

sea turtles; by-catch; mortality; trawl fishery; critical habitats; over-wintring; migrations; Adriatic; Mediterranean

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