Sea turtles of the Mediterranean Sea : population dynamics , sources of mortality and relative importance of fisheries impacts

Two sea turtle species nest in the Mediterranean, the green turtle and the loggerhead turtle. Leatherbacks are widely distributed in the Mediterranean, although they do not have any permanent nesting area. Two other species, the hawksbill and the Kemp’s ridley turtle only occur occasionally. Available information on sea turtle populations in the Mediterranean Sea, including distribution, population abundance estimates and population dynamics is presented. Main sources of sea turtle mortality are reviewed, with special emphasis on threats due to fishing activities, with the aim at assessing, where possible, the relative importance of these on overall sea turtle mortality. INTRODUCTION The Mediterranean region is an important breeding area for two marine turtle species: the loggerhead (Caretta caretta), with nesting beaches in ten countries (Cyprus, Egypt, Greece, Israel, Italy, Lebanon, the Libyan Arab Jamahiriya, the Syrian Arab Republic, Turkey and Tunisia), and the green turtle (Chelonia mydas), which nests predominantly in Cyprus and Turkey (where 99 percent of recorded nesting occurs), but also on beaches in Lebanon, Israel and Egypt. Another species distributed across the whole region is the leatherback (Dermochelys coriacea), although regular reproduction has not been observed. Some other turtle species, such as the hawksbill (Eretmochelys imbricata) and Kemp’s ridley (Lepidochelys kempii) have also been observed occasionally. The distribution range of the loggerhead populations extends from the eastern to the western Mediterranean limits, including the Black Sea. Green turtles, on the other hand, are restricted to the eastern basin and are regarded as critically endangered. Leatherbacks are distributed throughout the Mediterranean Sea. Most of the relevant international agreements, such as the Convention on International Trade in Endangered Species (CITES) and the Bonn and Bern Conventions, include marine turtles in lists of endangered species or in annexes listing species to be protected. All Mediterranean marine turtle species are listed as threatened or endangered in the IUCN (World Conservation Union) Red List of Threatened Species. OBJECTIVES The general objectives of this document are expressed in the terms of reference of Theme 1 of the Expert Consultation on Interactions between Sea Turtles and Fisheries within an Ecosystem Context and, in particular, are to “identify sea turtle populations, report on population abundance estimates, on various sources of sea turtle mortality, including habitat degradation, pollution, natural causes, fishing etc., define sustainable population levels, 28 model the effect of various sources of mortality and finally determine the contribution of fishing to overall sea turtle mortality”. BACKGROUND The impact of fisheries on marine turtle populations is an important issue in the Mediterranean. In 1989 the Barcelona Convention adopted the Action Plan for the Conservation of Mediterranean Marine Turtles. This plan defines some priorities for their protection, namely the banning of exploitation and minimization of accidental catches. Gerosa and Casale (1998) reviewed available information on interactions between sea turtles and fisheries. A Council of Europe document provides a baseline for future assessment (Groombridge, 1990). A report (Laurent, 1996a) includes a chapter on fisheries impacts on turtle. Another document regarding the eastern Mediterranean focuses on green turtles, summarizing the main fisheries affecting this endangered population (Oruç, Demirayak and Sat, 1997). The journal Zoology in the Middle East published a volume dedicated to the marine turtles in the eastern Mediterranean including a review of the green turtle population (Kasparek, 2001a). Other documents concerning different aspects have recently appeared (Miranda, 2001; Tudela, 2000; WWF, 2003; Margaritoulis et al., 2003). GEOGRAPHICAL CONSIDERATIONS The Mediterranean Sea occupies an area of around 2.5 million km, is about 3 800 km from east to west, and its maximum north–south extent is around 900 km, between France and Algeria. The eastern basin is connected to the Black Sea. The Mediterranean waters include Atlantic surface water masses in the western Mediterranean and slightly hypersaline warm–temperate surface water masses in the Levant basin. The Mediterranean Sea and the Black Sea make up FAO Fishing Area 37. The Mediterranean region is vast and includes countries where fisheries represent an important form of revenue and countries where only artisanal fisheries exist. The bathymetry of the Mediterranean Sea determines the distribution of marine turtles and their abundance. The continental shelf and slope, which constitute the main adult turtle feeding areas, are relatively narrow in most areas. In some places, such as the Gulf of Gabes (Tunisia/Libyan Arab Jamahiriya) and the northern part of the Adriatic, the continental shelf is larger and turtles use these areas as resting or wintering areas. SEA TURTLE SPECIES FOUND IN THE MEDITERRANEAN Currently, two marine turtle species nest in the Mediterranean – the loggerhead, Caretta caretta (Linnaeus, 1758) and the green turtle, Chelonia mydas (Linnaeus, 1758). The main nesting concentrations of the loggerhead are found in Greece and Turkey, Cyprus and the Libyan Arab Jamahiriya, while those of the green turtle are restricted to Turkey and Cyprus, with minor activity in Israel and Lebanon (Figure 2, Annex 1). A third species, the leatherback (Dermochelys coriacea), is observed throughout the region all year round (Camiñas, 1998). Two other species, Eretmochelys imbricata and Lepidochelys kempii, are occasionally present (Camiñas, 2003). These Mediterranean species were established recently by the migration of a very small number of females from the Atlantic. This may have occurred after the last glacial 29 period (Bowen et al., 1994). The isolation of different populations within the Mediterranean must have produced genetic differentiation (Kaska, 2000).