World Status, Exploitation and Trade - WIDECAST

..INTRODUCTIONIf the details of homing behaviour were learned rather than inherited, aflexible response to changing nesting opportunities (consequent upon islandformation, erosion, sea level changes, colonisation by predators, etc.)would be possible. As Bowen et al , point out, "a new migrational circuitcould be established by a single female in a single generation". On asimilar theme, Hendrickson some time ago (1958:A62) proposed thatnewly-matured females may constitute a "pioneering fringe" of the mainpopulation, able to range widely through a region of potential nestinghabitat, but able in subsequent nesting years to home in, by means of alearned response to certain environmental cues, to whatever beach firstprovided them with suitable nesting conditions. Gyuris and Limpus (1988)also suggest that social facilitation and learning should be considered asan alternative to the natal beach imprinting model as a basis for sea tu.'tledispersal and migration (and see Owens et al . 1982).It might be the case that immatures and sub-adults from different nestingbeaches may intermix while at their foraging grounds, or may associate ontheir first nesting with experienced nesters from different nesting beachesthan their own. This is purely speculation, but combined with observationsand ideas outlined above, suggests a model predicting significantinter-population gene flow up to the near-mature phase, but subsequentlyminimal gene flow between each nesting colony owing to a learned homingbehaviour demonstrated by adults. This model predicts that nestingpopulations based on discrete beaches will not be completely isolatedgenetically one from another, but that the degree of genetic isolation willbe largely a function of the degree of physical separation between nestbeaches, and the opportunities afforded for intermixture of pre-adults intheir developmental and foraging habitats.Population structure: some consequences for conservationEven if nesting site populations are not completely isolated genetically,and even if, over evolutionary time, significant powers of dispersal areevident, it remains true that populations appear to have little abilitydemographically to reinforce one another. Once a population is extirpated,there is no direct evidence that it can in practice be restored by turtlesderived from neighbouring populations thus they should still be treated asdiscrete entities for conservation purposes (Mortimer, in litt . , 31 December1987).The nearby islands of Assumption and Aldabra (Seychelles) provide aparticularly relevant example (Mortimer, i n litt . , 1987). Both historicallysupported very large nesting populations. Nesting numbers at Assumption arenow very low after many years of exploitation earlier this century; Aldabrastill supports a reasonable population which may be recovering as a resultof local protection since 1968. Although some Aldabra females may nest onAssumption, the Assumption population has remained in a severely depletedstate since at least the early 1970s. Similarly, the Cayman Islands andBermuda populations, both reduced to vestigial levels by tho end of the lastcentury, have not been restored by turtles derived from the large andflourishing Tortuguero population (Costa Rica) even though all three mayhave shared the same feeding grounds (Mortimer, in litt . , 1987). However,although turtle colonies no longer nest on Reunion and Mauritius, feedingturtles occur offshore and some nesting attempts have been recorded on bothislands; disturbance may deter increased nesting.21