Assessing the composition of green turtle (Chelonia Mydas) foraging grounds in Australasia using mixed stock analyses

  • Michael Paul Jensen

    Student thesis: Doctoral Thesis


    Understanding the population dynamics in both breeding and foraging habitats is a vital part of assessing the long-term viability of any species, especially those that are highly migratory. This is particularly true for green turtles, Chelonia mydas, which are long-lived marine turtles that undergo migrations for several years as post hatchlings until they select foraging grounds, and as adults, migrate between their foraging grounds and nesting beaches. Monitoring of populations at the foraging grounds may help detect early signs of population trends that would otherwise take decades to be observed at the nesting beach. In order to gain such insights the connectivity between nesting and foraging habitats must be established. Genetic analysis of rookeries to define discrete populations (stocks),in combination with Mixed Stock Analysis (MSA) based on data from molecular markers, provides an effective approach for estimating the origin of turtles sampled away from their nesting beach. In this thesis, new investigations into the genetic structure of green turtle populations in Australasia were conducted using longer (~780 bp) mitochondrial (mt) DNA sequences, larger sample sizes and new locations. This information provided the baseline data used in Mixed Stock Analyses of the composition of foraging grounds in three regions of Australasia including Western Australia, the Great Barrier Reef (GBR) and Malaysia. In chapter 2,I review what has been learned since the first MSA studies in marine turtles more than a decade ago. Since the early 1990‟s,numerous studies used this method to elucidate the rookery origins of young pelagic stage turtles and of older turtles in benthic foraging grounds, in fisheries by-catch and in strandings. These studies have all shown how Mixed Stock Analysis has provided valuable new insights into the distribution of marine turtles, although in most cases the estimates are affected by large uncertainty. Several issues in the effective use of MSA need to be addressed concerning study design, sample sizes and the resolution provided by the genetic marker. Nonetheless, Mixed Stock Analysis holds great potential for monitoring population trends at oceanic and coastal foraging grounds for all size classes. Comparisons of adults and juveniles provide an opportunity to pick up early signs of shifts in the contributions of populations that may indicate population decline (or increase) (e.g., Chapter 5). Recent increases in industrial development of coastal island and offshore habitats in Western Australia (WA) have highlighted the need to better understand the dynamics of marine turtle populations in these areas. An analysis of previously sampled populations (Management Units; MUs) and four new rookeries identified two possible new Management Units in this region at Cobourg Peninsula and Cocos (Keeling) Island and grouped Browse Island with the existing MU at Scott Reef and Barrow Island to the large North West Shelf MU. These analyses used a 780 bp sequence of the mtDNA control region that encompassed the 386 bp sequence used in a previous study. The longer sequence, larger sample sizes and new locations revealed more than doubled the number of haplotypes (n = 39) than previously observed. However, this made little difference to the population genetic structure as common haplotypes were still shared among population. MSA showed that the majority (>90%) of turtles foraging at Shark Bay were from neighbouring North West Shelf rookeries, while the Cocos (Keeling) foraging ground was composed of turtles mainly from Cocos (~70%),but with some contributions from North West Shelf and possibly Scott Reef MUs. In an investigation of foraging populations in Malaysia, mtDNA sequence data were analysed from 81 immature green turtles at two foraging grounds at Mantanani Island and Layang Layang Island located northwest of Sabah, Malaysia. Previously published data from 17 Australasian green turtle populations were used as the baseline data for tracing back the origin of turtles at the two foraging grounds. The majority of these turtles originated from major rookeries in the Malaysia and Philippine Turtle Islands (~30%),and Sarawak (~60%) in north-western Borneo. These same rookeries have a long tradition of using unshaded beach hatcheries that has resulted in the production of mostly female hatchlings. This may have contributed to the 1:4 female biases seen at the two foraging grounds. The implications of hatchery practises at nesting beaches are discussed and the importance of continued monitoring and research at these foraging areas is highly recommended to improve the management of marine turtles in the region. Detailed MSA of green turtle aggregations at six major foraging grounds along the east coast of Australian were combined with data from more than 30 years of mark–recapture efforts along the Great Barrier Reef. Overall, the MSA in combination with the mark-recapture data supports a model in which the foraging aggregations are composed of individuals from the two Great Barrier Reef stocks (nGBR,sGBR) with small contributions from other stocks. The north/south transect of foraging grounds analysed spanned ~2300 km. Along this transect the main contributor shifted from being predominantly the nGBR stock at foraging grounds in Torres Strait, Clack Reef and the Howicks Group to predominantly the sGBR stock at Edgecombe Bay, Shoalwater Bay and Moreton Bay. At the most northern foraging ground in the Torres Strait, significant shifts in haplotype frequencies between juveniles and adults resulted in major shifts in the estimated stock contributions for these groups. Fewer juveniles originated from the nGBR stock and higher proportion originated from the sGBR and „other‟ stocks in comparison to adults. This trend was apparent in the four most northern foraging grounds, even in Edgecombe Bay, which had a predominance of turtles from the sGBR stock. Point estimates of contributions from the nGBR stock dropped from 0.89 in adults to 0.53 in juveniles in Torres Strait, from 0.69 to 0.49 at Clack Reef, from 0.66 to 0.49 in the Howicks Group and from 0.10 in adults to 0.01 in juveniles at Edgecombe Bay. In contrast, at the Shoalwater Bay foraging ground the opposite was observed, with a drop in contribution from the sGBR stock from 0.98 in adults to 0.84 and 0.85 in juveniles and sub-adults, respectively, and an increase in contributions from „other‟ stocks in juveniles and sub-adults. The observed patterns at the various foraging grounds likely resulted from several causes and four possible explanations are explored, the mostly likely of which were that (i) juveniles have shifted foraging grounds as they mature, or that (ii) reduced hatching success from the main nGBR rookery at Raine Island for more than a decade has resulted in reduced recruitment into the nGBR foraging ground. The later possibility suggests a need to take action to conserve the nGBR population The combined strength of data derived from mark-recapture studies, demographic studies to determine sex, maturity and breeding status of the turtles, genetic studies to determine stock composition and satellite telemetry, are needed to provide informed assessments of foraging populations necessary for guiding sustainable management of marine turtles.
    Date of Award2010
    Original languageEnglish
    SupervisorNancy Fitzsimmons (Supervisor) & Stephen Sarre (Supervisor)

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