World Status, Exploitation and Trade - WIDECAST

INTRODUCTIONHawksbill - Life historyWhilst some elements of the life history of C. mydas are well established,much less is known of the Hawksbill. The species, circum-tropical indistribution, generally has been regarded as relatively sedentary, nestingand feeding on small beaches and coral reef shallows in close proximity toeach other, and only rarely nesting in numbers. The species is essentiallycarnivorous, browsing over reef surfaces on sponges and other invertebrates,and over most of its range is more rare than C. mydas . Extensive cyclicmigrations, as in C. mydas , have not been demonstrated, although some longdistance movements are known. It seems likely that significant long rangedispersal movements do occur, perhaps intermittently, or prior to adopting amore sedentary existence once suitable habitat is found. Because Hawkrbillnesting is typically diffuse, and may take place at small and isolatedbeaches little used by other sea turtles, as well as in the midst of moredense nesting by other species, nesting numbers are particularly difficultto monitor. At some sites, turtles seem reluctant to move away from coralreef shallows into deeper waters, and what appear to be the same individualsmay be seen in the same reef area for periods of many months; thus protectedarea management in coral reef zones may be an effective means of conservingHawksbill populations (Goodwin and Reid, undated).Tables 1 and 2 summarise selected size and productivity data frompopulations of both species.Estimation of population sizeThe difficulties inherent in estimating population size in sea turtles haveoften been discussed (Meylan, 1982), and yet remain intractable. Thefemales spend almost their entire lives at sea, emerging on land only tonest on sandy beaches, frequently three or more times in one season andsometimes (perhaps often) for only one season in their lives. The malesgenerally never leave the water (but for one or two exceptional situations,such as in Hawaii and the Galapagos, where small numbers of turtlessometimes bask on land during the day). In general, only those turtles thatforage in shallow inshore waters can be encountered at sea with anyregularity, and C. mydas . which often occurs in aggregations overshallow-water seagrass pastures or other suitable feeding grounds, isnear-unique among turtles in being amenable to study at some localities fromthe air or by boat (e.g. the Masirah Straits in Oman: Ross, 1985; aroundHeron Island, Australia: Limpus and Reed, 1985a).With the exceptions given above, only parameters that can be measured on thenesting beach can be used to make an estimate of population size, and infact the "population" being assessed is only the segment of the total maturefemale population that is nesting during any given season. Because femalesrarely nest every year, but more usually at two , three- or four-yearintervals, and because it is not possible to determine what proportion ofthe total mature female population is at the nest beach in any given year,significant error will be introduced by attempting to extrapolate from dataon one season's nesting numbers to the total mature female population.A further complication is the fact that long-term monitoring at severalsites has shown that nesting numbers can show extreme variation from oneyear to another: at Heron Island (Queensland) more turtles nested in 1974-75than ever before in living memory, but very little nesting occurred nextseason (Limpus, Fleay and Guinea, 198A); at Tortuguero (Costa Rica), around13