As green sea turtle (Chelonia mydas) populations in the Caribbean recover from historical overexploitation, growing environmental obstacles pose threats to the recovery of this species. The invasion of Halophila stipulacea seagrass in previously Syringodiumfiliforme and Halodule wrightii -dominated beds drastically alters the composition of green turtle foraging habitat. This change in forage supply for juvenile and adult green turtles in the Caribbean could impact their future habitat use and resource partitioning, information that conservation and management agencies use to implement protective guidelines for this species. We conducted a fine-scale tracking study of green turtles’ space use and movement patterns in Brewers Bay, St. Thomas to investigate their foraging selectivity in the mixed-species seagrass beds. The fine-scale positioning system(FPS) acoustic receiver array was deployed across ~1.5 km2 of the bay, which includes seagrass, coral reef, and sand/rock benthic habitat. Seventeen individual juvenile green sea turtles were tracked with acoustic transmitters with an estimated precision of ± 2meters. The native and invasive seagrass composition was mapped in the highest trafficked daytime area to pair with the turtles’ foraging locations. Turtles displayed typical diel patterns of movement with higher activity levels in shallow mixed-seagrass habitats during the day and lower activity levels in shallow reefs and rocky habitats at night. These movement results were linked to seagrass composition within the sampling grid using resource selection functions (RSF) to estimate turtle selection towards each seagrass species in Brewers Bay. Turtles were actively selecting the two native species, with no selection towards the invasive seagrass despite its high abundance. Interestingly, three individuals utilized foraging areas outside the sampling grid and in deeper water with monotypic invasive seagrass. This pattern of space use has not been observed in past tracking and observational studies in Brewers Bay, implying that part of this population has started modifying its foraging patterns to incorporate H. stipulacea.
Green turtles living in coastal foraging areas often occupy distinct home ranges within which they visit resting and foraging sites. Knowledge about the size of home ranges and movement patterns within these areas is important for sea turtle conservation. However, few data are available for the wider Caribbean. This study measured the movement pattern of five juvenile green sea turtles in Brewers Bay, St. Thomas, in the US Virgin Islands. Each turtle was fitted with a V13 acoustic transmitter and tracked from 90 to 214 days. Turtles were tracked with a fixed array of 30 acoustic receiving stations placed ~200-260 meters apart throughout Brewers Bay. Minimum convex polygon (MCP) and kernel density estimator (KDE) techniques were used to measure home range size. Home ranges were split into days vs. night times and compared by using utilization distributions (UD). Habitat classification was done in areas of high turtle activity and overlapped with home ranges. A general linear model was used to explore the relationship between the home range size (95% UD), core area size (50% UD), and potential predictors: mass. Average KDE day home range size was 63.3 Ha and average day core area was 6.9 Ha. Average KDE night home range size was 35.9 Ha and average night core area was 5.1 Ha. All five incremental area plots became asymptotic, indicating that the home range estimates are robust. There was a statistically significant relationship between core area size and predictor variable (P-value= 0.002; mass). Tracking results showed that all five turtles remained in Brewers Bay for 98% of the tracking duration. During the day turtles were located in seagrass beds and at night they moved to resting areas associated with natural and artificial coral reefs. Core areas for foraging overlapped with seagrass beds dominated by Syringodium filiforme; turtles occurred less frequently in seagrass beds with the invasive Halophila stipulacea. During the night there is less activity when compared to day time hours. Our data confirm that Brewers Bay is an important foraging and resting habitat for juvenile green sea turtles and that their foraging movements center on areas with S. filiforme. These areas in return should receive focused management for both seagrass and reefs to protect green sea turtles. With this management these areas should receive protection from recreational boaters and tourists to prevent the spread of the invasive H. stipulacea
Green sea turtles, Chelonia mydas, utilize coastal areas as foraging grounds for the majority of their lives. Human development of coastlines is increasing, but the effects of the urbanization of foraging grounds on green turtles are poorly understood. I used both manual and automated acoustic telemetry to determine the home ranges, movement behavior, and temporal patterns of site visitation of green turtles during 2009-2011 in San Diego Bay, California, a highly urbanized temperate foraging area. The home ranges of all tracked turtles were restricted to the southern portion of San Diego Bay, where eelgrass (Zostera marina) is abundant and where human activity is the lowest within the bay. Core activity areas coincided with eelgrass distribution or occurred adjacent to the warm water-effluent outfall of a waterfront power plant. Automated monitoring of sites throughout south San Diego Bay confirmed this finding, showing that green turtles most frequently visited the outfall of the power plant and areas known to contain eelgrass. This method also elucidated that turtle presence at the power plant was strongest during the winter and at night, whereas visitation to eelgrass areas was strongest during the spring and in the daytime. Turtle visitation to a high boat traffic shipping terminal was rare but occurred almost exclusively during the daytime, the period during which human activities in the area are also the highest. Manual tracking of green turtles similarly demonstrated that individuals ranged across larger portions of south San Diego Bay during the day, during which they exhibited high swimming speeds but highly non-linear movement. Turtle activity at night was primarily restricted to the power plant's effluent outfall channel and adjacent jetty. Nighttime movement was characterized by long periods of inactivity sporadically interrupted by brief, linear movements to new resting locations. Collectively, the results of this study paint a robust picture of the spatial, diel, and seasonal patterns of habitat use by green turtles in San Diego Bay. All data support the hypothesis that south San Diego Bay serves as important turtle habitat within the bay. Further, a combination of manual and automated acoustic telemetry enables a more complete understanding of turtle spatial ecology that would not have been possible with exclusive use of one technique. Future monitoring and modeling is required to document the potential effects of changing environmental conditions, including power plant closure, on green turtles resident to San Diego Bay. This study helps to assess the data gap of how turtles use urbanized foraging areas and changing coastal ecosystems, a currently novel scenario that will likely become commonplace in the face of increasing coastal development worldwide.
The diel foraging behavior of juvenile green turtles (Chelonia Mydas L.) ~ 7 to 8 kg in size was studies by visual observations from a fixed point and by acoustical tracking of three tagged animals, two of which were followed for >1 wk. Green turtles fed in a shallow seagrass-covered bay most commonly by day. There were usually two feeding bouts, one in the morning and one in the afternoon and each turtle visited a characteristic feeding area. Turtles were inactive at night and usually in mid-day in characteristic resting sites in coral reefs separated from the feeding sites by 0.2 to o.5 km. Pre-sunrise feeding activity was noted consistently in one tagged turtle while the other entered the feeding area later in the morning and remained there much of the day. Approximately 9 h per day were spent on the feeding sites by both turtles where the major food of one was determined by fecal examination to be turtle grass Thalassia testudinum König. Turtles were occasionally observed in groups of up to three, but they appeared to be generally solitary rather that social in their behavior.
For the endangered green turtle, Chelonia mydas, a fundamental component of recovery and conservation is an understanding of its foraging ecology. Foraging optimality models suggest animals will select resources of high quality over those of low quality. For green turtles, this behavior is important, as sufficient quantities of nutritionally adequate forage items are necessary for growth and reproduction. One intrinsic element in the understanding of green turtle foraging ecology is to identify and document the availability and quality of forage resources preferred by green turtles.
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 1990s, 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.
Recent developments in open water research have refined our understanding of green turtle, Chelonia mydas, foraging ecology, but diet characterization among populations remains understudied. Previous hypotheses state that once young green turtles recruit to shallow water habitat they shift rapidly from an omnivorous to herbivorous diet. Supporting evidence has primarily been derived from traditional gut content analysis that only provides a small window in time to perceive the diet of an animal. In contrast, stable isotope analysis explore show a consumer uses its resources over a broad temporal scale. We tested the dietary shift hypothesis using gut content and stable isotope analyses to assess the nutritional ecology of a juvenile green turtle aggregation in the northern Gulf of Mexico. We examined the gut contents of 65 green turtles collected from 2008 and 2011 hypothermic stunning events in St. Joseph Bay, Florida. Gut contents were evaluated using volume, dry mass, percent frequency of occurrence, and index of relative importance (IRI). Juvenile green turtles showed omnivorous feeding behavior, feeding on a variety of animal and vegetal items with a bias towards seagrass and tunicates. In addition, we evaluated feeding consistency by stable isotope patterns from epidermis tissue. We measured the stable carbon (delta13C)and nitrogen (delta15N)isotope values in epidermis of 43 green turtles, ranging from 22.5 to 72.7cm in curved carapace length (CCLmin), and eight known prey items (e.g., algae, seagrasses, invertebrates) collected in 2011. Our study provides a foundation for characterizing the foraging ecology of green turtles in St. Joseph Bay and highlights the value of utilizing isotopic ecology for further foraging studies.
Recent studies show that sea turtles use both magnetic and visual cues to successfully orient. Juvenile green sea turtles from the near shore reefs of Palm Beach County, Florida were brought to the lab to determine whether the sun could serve as a visual orientation cue. When tethered during the day in a large outdoor tank west of the ocean, the turtles oriented east to northeast. To determine whether the sun's position was used to maintain their heading, I altered the turtles' perception of time by entraining them to a light cycle advanced by 7 h relative to the natural cycle. When tested afterward in the same outdoor tank the turtles oriented northwest, the predicted direction after compensating for the sun's movement over 7 h across the sky. Orientation was unchanged when the turtles bore magnets that negated the use of magnetic cues. These results are consistent with the hypothesis that the turtles used the sun for orientation.
The green turtle, Chelonia mydas, is a circumglobal species that exhibits several important developmental or ontogenetic shifts throughout its life history. The first major shift occurs when juvenile turtles migrate from pelagic habitat, where they forage as omnivores, to coastal neritic habitat, where they become primarily herbivores, foraging on algae and seagrass. Anecdotal evidence and gut-content analyses suggest that juvenile green turtles in south Texas bays, such as the lower Laguna Madre and Aransas Bay, undergo an additional ontogenetic shift during this important life history stage. Evidence from stable isotope analysis (SIA) of scute tissues of green turtles from Texas' lower Laguna Madre and Aransas Bay supports an intermediate stage between this species' shift from pelagic waters to seagrass beds in neritic waters; this additional shift comprises an initial recruitment of post-pelagic juveniles to jetty habitat located on the channel passes Gulf-ward of adjacent bays before subsequently recruiting to seagrass beds in these bays. Examination of stable carbon ([delta]13C) and nitrogen ([delta]15N) isotopes in microlayers of scute tissue from several size classes of green turtles from the lower Laguna Madre and Aransas Bay was used to confirm the occurrence of two ontogenetic shifts. Smaller green turtles (35 cm SCL) exhibited more depleted [delta]13C signatures and more enriched [delta]15N signatures, consistent with jetty habitat, compared to those of larger counterparts ( 45 cm SCL) that displayed enriched [delta]13C signatures and depleted 15N signatures, consistent with seagrass habitat. Changes in the isotopic composition between these size classes indicate distinct shifts in diet. Post-pelagic juveniles first recruit to jetty habitat and forage primarily on algae, before subsequently shifting to seagrass beds and foraging primarily on seagrass. These findings indicate the use of a characteristic sequence of distinct habitats by multiple life history stages of green turtles in Texas bays, a conclusion with broad management implications for this endangered species.
