World Status, Exploitation and Trade - WIDECAST

INTRODUCTIONvirtually eliminated in 100 years (King, 1982). On Aves Island, nestingnumbers probably dropped by two orders of magnitude between 1947 and 1979(Pritchard and Trebbau, 1984). The harvest on the Pacific coast of Mexicobuilt up from very low levels in the 1950s to a peak in 1968, followed by asharp fall until the present, caused by a population decline. The export ofcalipee from the Seychelles declined by two orders of magnitude between 1907and 1970. In the Ogasawara Islands (Japan), the harvest fell from nearly2000 in 1880 to about 200 in 1920 (Kurata, 1979). A decline in the nestingpopulation on Aldabra was reversed after the atoll received protection as anature reserve (Mortimer, 1985a). The huge harvest of turtles for the saleof meat in Bali has resulted in the fishermen having to travel furtherafield to find large turtles: good evidence of the severe depletion of localstocks within the space of ten years (Anon., 1984c).It has frequently been pointed out that the harvest of turtles on, or justoff, nesting beaches is particularly damaging as it takes primarily maturefemales. Turtles are particularly vulnerable during nesting because theyemerge onto land, and every female turtle which is to contribute to the nextgeneration must do so. Thus by killing before it lays its eggs every femalethat emerges, by no means an impossible or unusual achievement, the ultimateextinction of the local population can be assured. For the rest of theirlife-cycle, the turtles disperse into the sea and are much less easy to huntwith the same lethal efficiency, although they may congregate over thefeeding grounds, and in this respect their capture is similar to theexploitation of marine fish. Marine fisheries are notoriously hard tocontrol, as the fish populations are difficult to monitor and they are oftenexploited by a number of different nations. This history of fisheriescontrol is depressing, in that many stocks have suffered severe declines asa result of over-fishing, but no marine fish is known to have become extinctas a result of fishing, probably because remote fishing techniques areinherently inefficient. Turtles combine the worst attributes of terrestrialanimals and fishes, in that they are easy to hunt and the fishery Isdifficult to control. It would be expected that fishing for turtlesoffshore, outside the breeding season should be less damaging than catchingthem near the breeding beaches. There are few documented examples whereoffshore fishing is not combined with beach collection, and so it isdifficult to test this hypothesis. The nesting populations of Europa andTromelin are not exploited, except for the ranch on Reunion, but femalestagged on these islands have been caught around Madagascar and Mauritius,both of which countries operate a sizeable fishery. So far as is known, thebreeding populations do not appear to have been depleted by this activity.In a similar way, the turtles nesting on Ascension are caught off the SouthAmerican coast, and yet no population decline has been recorded. One of thelargest offshore turtle fisheries operates in the Torres Strait, where boatsfrom both Papua New Guinea and the Australian islands catch turtles butagain no decline to the nesting population in Australia has been recorded.A note of caution sounds from studies in the Caribbean, where the C . mydaspopulation nesting on Tortuguero, Costa Rica, is exploited on feedinggrounds off the Nicaraguan coast. A marked decrease in abundance of turtlesoff Nicaragua has been reported (Nei tschmann , 1982), and although there hasbeen no apparent decline in the numbers nesting at Tortuguero between 1971and 1985, Bjorndal (1980) inferred increased mortality of tagged turtles.The conclusion must be that sustained harvesting of nesting C. mydas carriesa high risk of over- exploitation , and almost inevitably leads to localpopulation depletion and quite possibly extinction. The effects ofharvesting foraging populations offshore are less well known, but logic28