World Status, Exploitation and Trade - WIDECAST

INTRODUCTION5 000 C. mydas nested in 1979 and about 50 000 in 1980 (Carr et al . 1982).In other words, data from one or two years will be of limited use in anattempt to assess the mean size of the annual nesting population.Limpus and Reed concluded from an assessment (by ovarian examination) of thereproductive condition of females on feeding grounds in the southern GreatBarrier Reef (1985a), and of those from the Raine Island colony stranded offtheir feeding grounds by Cyclone Kathy (1985b), that observed broad annualfluctuations in nesting density may reflect conditions on the foraginggrounds, in particular the number of females that have been able to preparethemselves for breeding, rather than annual fluctuations in the totalpopulation of mature females. Preparation for breeding, including thelaying down of fat deposits and vitellogenesis, starts more than 12 monthsbefore nesting, and in northern Australia climatic fluctuations during thisphase are directly correlated with fluctuations in nesting numbers two yearslater (Limpus and Nicholls, 1988). Limpus and Reed further point out(1985a) that because differing proportions of mature females breed indifferent years, and individual females do not breed annually, a very largefeeding ground population, including all size classes from small immaturesto mature adults, would be necessary to sustain even a moderate nestingcolony of several hundred nesting females a year.In the country accounts comprising the main body of this report, we haveattempted to cite field-workers' basic data, such as emergences or nests perlength of beach per unit time, or nesting numbers per night at peak, season,and, when available, have quoted their estimates of total nesting numbers ina given season. Many authors have derived an approximation of annual femalenumbers from egg harvest data, by dividing the total egg harvest by the meanclutch size (figures of 100-120 are often used) and by the mean number ofnests per female per season (often around three, but see above for otherpossibilities) .Estimates of nesting numbers of Eretmochelys will generally be even lessreliable than for C. mydas because of the general tendency of the former tonest singly or in small numbers at scattered nesting beaches. Except inunusual situations where this species is the predominant nester and beachescan be regularly monitored (e.g. at Cousin Island, Seychelles), Hawksbillnests will often go unrecorded.Reproductive remigrationHughes (1982a) introduced yet another element of uncertainty into attemptsto assess population size, namely, the fact that in all cases where suitabletagging programmes have been in operation, fewer than half the femalesnesting in any given season can be identified as remigrants, and they oftenform a minor proportion of total numbers nesting (sometimes less than 1% andrarely more than 20%). Although Bjorndal ( in litt. . 20 June 1987) haspointed out that Green Turtle remigrants recorded at the well-moni toredTortuguero beach (Costa Rica) have averaged 35% of total nesters over thepast decade, and believes that the apparent losses are largely an artefactof incomplete beach coverage, even this high figure is lower than might beexpected if remigration was predominant.Hughes' evidence could be taken to suggest that a large proportion ofnesting females nest in only one season. This assumes that the females willreturn to the same beach on each active nesting season, in order formonitoring of tag-bearing females to be possible. Whilst such philopatry14