World Status, Exploitation and Trade - WIDECAST

INTRODUCTIONBut harvesting is rarely quite so intense for quite such long periods, andlate maturation can in a sense be said to protect populations fromextirpation (Bjorndal, 1985) because if harvesting is variable through time,there may be sufficient numbers of various age classes surviving harvest tomaintain the population. Furthermore, present population data are not onlya reflection of the nesting success of the previous generation, perhaps 25years ago, but also of conditions in the feeding and developmental habitatsused by immatures and sub-adults. A contemporary reduction in artificalmortality suffered on foraging grounds by near-adults could, in principle,result in an increase in nesting numbers in the very near future.While late maturation may serve to buffer populations from extirpation, italso simultaneously both conceals the effects of exploitation, which mayeventually turn out to have been excessive, and conceals the effects ofconservation action at the nesting beach, which may be abandoned before anybenefits could have been expected. Masking of the effects of exploitationcan be particularly misleading, as shown by the following statementregarding Green Turtles at Assumption, made by the Director of Agriculturein the Seychelles in 1929 (cited by Mortimer, 1985:9): "It is wonderful,however, to think, that after 19 years of constant fishing the resources inturtle have not been depleted except to a slight extent; 1000 turtles arestill captured per annum". As Mortimer points out, this "constant fishing"eventually resulted in the decline of the Assumption nesting population fromseveral thousand females annually to only around 200 annually. A similarbelief, that the turtle resource is inexhaustible, has been widely recordedamong coastal peoples who utilise turtles.Genetic isolation and dispersalThe strong philopatry that has amply been demonstrated in a number ofC. mydas populations, coupled with the fact that mating often occurs justoffshore from the nesting beach, seems to be the basis for thefrequently-cited statements to the effect that "each nesting colony istherefore a separate reproductive unit that does not demographicallyreinforce any other" (Carr and Stancyk, 1975:171). Similarly, "each seaturtle nesting population is genetically isolated and distinct and cannotreplenish other such populations", and "each turtle population must betreated as a discrete entity for the purposes of conservation" (Ross,et al. . 1979).While it is reasonable to accept this as a working hypothesis, it is not yetfully confirmed by available evidence. For example, in the present absenceof any standard method for marking hatchlings, there is no evidencewhatsoever that a mature female on her first nesting emergence has returnedto her natal beach. It is an assumption, and should be recognised as such,no matter how plausible it may seem. If she has not returned to her natalbeach, then significant between-population gene flow will have occurred.Recent electrophoret ic work does not provide evidence for the high level ofbetween-population genetic diversity in C. mydas that would be expectedunder conditions of strict genetic isolation. A study of 16 enzyme loci inGreen Turtles from Costa Rica and Florida demonstrated polymorphism in four,with only phosphoglucomutose showing highly significant genotypic andallelic differences between the samples (Kochinsky and Menzics, 1983).Similarly, an electrophoret ic survey of 23 loci in four Green Turtlesamples, from the three main tropical ocean basins, demonstrated very littleheterozygosity, suggesting that rates of enzyme evolution may be very slow19