Post-hatchling sea turtle biology

The post-hatchling stage of a sea turtle's life history has often been referred to as the 'lost years', reflecting the lack of understanding about this phase in their life. Obtaining information on where post-hatchlings go, or for how long, is significantly hindered by the elusiveness of a post-hatchling in its natural environment and the limitations of tagging technologies to track a hatchling as it leaves its nest. Consequently, much of what is understood of the post-hatchling life stage has been derived from indirect methods. As a result, our current understanding of post-hatchling biology is based on information gathered from stranded animals, opportunistic reports of sightings at sea, studies of hatchling behaviour, and more recently genetic based studies. Although knowledge on the post-hatchling stage has progressed considerably in the last few years, studies have been limited primarily to loggerhead turtles in the northern Atlantic Ocean and northern Pacific Ocean. Thus there are substantial gaps in our knowledge of the life history of sea turtles for many regions of the world. The aim of this study is to increase the understanding of the ecology of loggerhead and green post-hatchling sea turtles in the southwest Pacific Ocean. The information acquired on the post-hatchling phase of sea turtle life history will help direct future regional management of these animals by providing region-specific information on the migratory routes and habitats occupied during the posthatchling stage. This study also informs our global understanding of the sea turtle posthatchling biology. This study employed a multidisciplinary approach, incorporating ecological information from spatial and temporal distributions, diet and stable isotopes, and genetic methodologies. Post-hatchlings were sourced from strandings and from the stomachs of dolphin fish (Coryphaena hippurus). In addition, records were collated from the Queensland Environmental Protection Agency's database of marine wildlife strandings and deaths. Data on the spatial and temporal distribution of post-hatchlings in relation to rookery location and oceanographic features compiled in this study provides evidence that loggerhead and green post-hatchlings from populations in the southwest Pacific region become entrained in oceanic currents and live a pelagic existence. Occupancy of an oceanic and pelagic habitat is supported by stable isotope signatures. In addition dietary investigations that show post-hatchlings in the southwest Pacific Ocean, from both of the investigated species, derive nutritional sustenance primarily from neustonic animal matter. The spatial and temporal data on the two species of post-hatchlings however, indicates that the two species do not take the same migratory route after departing from the same coastal waters. The data provides strong evidence that loggerhead post-hatchling undergo trans-Pacific migrations within the southern Pacific sub-tropical gyre. This is suggested by; (i) incremental post-hatchling size increase in direction of this current away from nesting beaches, (ii) reports of loggerhead post-hatchlings are in New Zealand waters and on the eastern side of the southern Pacific, and (iii) loggerhead post-hatchlings larger than 13.7 cm CCL are not documented in the southwest Pacific Ocean. Although the current resolution of the genetic stocks in the southern Pacific does not allow differentiation between stocks on a regional scale, there is discrimination at the oceanic scale. Analysis of the haplotypes of the loggerhead post-hatchlings shows that all specimens investigated in this study originated from southwest Pacific rookeries. Whereas the data implies that loggerhead post-hatchlings embark on trans-Pacific migrations, it suggests that green post-hatchlings do not. Whilst this species also occupies offshore oceanic waters, it appears they remain in the southwest Pacific region. This is indicated by; (i) green post-hatchlings occupying waters around offshore seamounts (whereas loggerhead post-hatchlings appear absent), (ii) the absence of green post-hatchlings in New Zealand or southeast Pacific waters, and (iii) the occurrence of larger size classes of green post-hatchlings stranded on eastern Australian coast. Mixed stock analysis (using SPAM & TURTLE) performed with haplotypic information from post-hatchlings calculated that green post-hatchlings originate from the SGBR (60%), Coral Sea (27%) and New Caledonia (13%) rookeries. This study is the first to describe the route that loggerhead and green post-hatchlings from the Australian region are taking. I demonstrate that these two species are undertaking significantly different migrations during this stage of their life. The principal findings of this study support the currently accepted view on the sea turtle's post-hatchling stage, for most species, is that of a pelagic oceanic existence.