World Status, Exploitation and Trade - WIDECAST

INTRODUCTIONMethods ofturtle exploitationVarious general strategies of turtle exploitation have been practised indifferent parts of the world and these demonstrably have had, ortheoretically are expected to have, different effects on turtlepopulations. Wild turtle populations may be exploited by the collection ofeggs, by the killing of adults and sub-adults, or by the collection of eggsor hatchlings for ranching operations. The most technologically advanced,and as yet unrealised scheme involves closed-cycle captive breeding.Egg harvestingIn many parts of the world, egg harvesting is carried out on a subsistencelevel by the coastal inhabitants, and consequently the past or presentlevels of collection are impossible to determine. Furthermore, it isusually combined with the capture of nesting females, and so its effects areimpossible to monitor or separate from those of the adult harvest. However,there are a few localities where the egg harvesting is well documented overa very long period and the harvest of adults is of minimum significance.These examples give us not only the best evidence of long-term fluctuationsin nesting populations but also of the effects of egg harvesting.The harvest on Diamond Island off Burma has the earliest documentedhistory. Maxwell (1911) recorded the harvests from 1883 to 1898, and showedthat they were in the region of two million eggs a year. He considered thatabout 9A% of eggs laid were collected, and said that this intensity ofharvest appeared to have continued unchanged since at least 1870 andprobably earlier. The next documented record of harvests on Diamond Islandwere from 1977 to 1982, which showed that they had declined to between152 000 and 238 000 a year, the collection regime and the intensity ofharvesting apparently having changed hardly at all since 1883 (Salter,1983). This represents a decline of about an order of magnitude over ahundred years equivalent to only about 2% a year. Maxwell (1911) consideredthat the harvests in the late 19th century had remained approximatelyconstant over about 15 years, but a re-examination of his data shows adecline of 2.4% a year. Thus it appears that this low rate of decline mayhave persisted for the next 100 years under harvesting intensity of between90 and 95% of all the eggs laid. Corroborative evidence for this theory wasfound from the egg harvest at South Moscos Island, which declined at asimilar rate from 60 000 in 1939 to 21 000 in 1977 (Salter, 1983). Maxwellmarvelled at the apparently insignificant effect that a near-total eggharvest for many years had had on the numbers nesting, and concluded thatthe nesting population must represent turtles immigrating from breedingbeaches elsewhere. We now know that a large egg harvest should have littleeffect on the nesting numbers for at least 30 years, but we also know thatMaxwell's immigration theory was probably incorrect, and our surprise shouldbe all the greater that the harvest, continuing for over 100 years, shouldhave caused the population to decline so slowly. There seem to be only fourexplanations which could account for this: a) the level of harvest has beengrossly over estimated; b) the harvest has not remained constant over thewhole period; c) the age at first breeding is considerably more than 30years and that the females continue breeding to an age of over 100 years; d)that the natural mortality of hatchlings is highly density-dependent, sothat the majority survive if only a few hatch.25