Flatbacks at sea: understanding ecology in foraging populations

To assess the current conservation status of a species, we first need to have a good understanding of how the individuals are distributed in time and space. The latter is the focus of fields such as biogeography and conservation biogeography when applied to inform conservation decision-making. The distribution of a species will be influenced by innumerable factors, from large-scale processes such as dispersal and migration, to the local variability in environmental parameters, inter- and intra-specific interactions and ultimately the effect and intensity of natural and human-induced changes. In the marine realm, currents and wind promote connectivity among sometimes far-spread habitats. While large physical barriers are limited, the distribution of marine species is often influenced by physic-chemical barriers (i.e. thermoclines, photic zone). As a result, marine habitats tend to be highly dynamic in time and space, which in turn influences the diversity and distribution of marine species. In tropical environments and in particular in the tropical shallow waters of the Indo-West Pacific region, habitats are characterised by very high diversity of species with relatively broad geographic ranges. This is particularly true for migratory marine megafauna, such as whales and marine turtles, which can distribute across 100 to 1000s of km. Such large-scale movements pose a great challenge to monitor their movements and identify habitats used by the populations. However, the advancement of tracking technologies (i.e. acoustic and satellite tracking), animal borne-videos, accelerometers and molecular techniques such as stable isotope analysis, have enabled great advances in the understanding of marine megafauna biogeography. The general biogeography of marine turtles has been extensively studied worldwide. The distribution of nesting grounds of all marine turtle species is very well described, and while less is known on their foraging distribution and ecology, there is still an extensive body of work in this field. As migratory species, marine turtles make use of a great variety of coastal and oceanic habitats throughout their life, which vary between and among species and populations. Ontogenic, seasonal and reproductive changes in habitat use have been widely described, and are a common feature in all species. In general, biogeography of marine turtles is influenced by local and oceanic currents, distribution and abundance of prey, presence of predators, availability of shelters and seasonal shifts in water parameters (i.e. temperature). Nevertheless, improving the knowledge on biogeography of local populations, especially on the distribution of non-reproductive turtles, is still one of the global research priorities for marine turtles. In Australia, the foraging distribution and ecology of green, loggerhead and hawksbill turtles has been well described, because these species inhabit shallow coastal habitats, typically with clear water and relatively easy access for research. In contrast, knowledge on the foraging ecology of leatherback, olive ridley and flatback turtles in Australia is limited. Some aspects of the foraging ecology of leatherback and olive ridley turtles can be inferred from studies on other populations in the world, and recent advances have been published on the migration and foraging distribution for flatback turtles in Western Australia. In my thesis, I provide new and novel information on the distribution of non-reproductive flatback turtles in eastern Australia. In particular, the aim of my thesis was to improve our understanding of the biogeography and ecology of flatback turtle across different life-stages, with the intention to generate scientific information to improve the state of knowledge of the species and provide relevant outputs to inform management actions for conservation. First, in Chapter 2, I assessed a key process in understanding the biogeography of marine turtles: the early dispersal of post-hatchlings. In particular, I examined the potential mechanisms that underpins the neritic dispersal of post-hatchlings flatback turtles. Long-term records of post-hatchling flatback turtles are evidence of the neritic dispersal of the species, and studies on hatchling swimming behaviour and particle simulation have provided insights on the evolutionary adaptations of this species to turbid coastal waters. To explain the lack of oceanic dispersal of this species, I employed a hydrodynamic advection-diffusion model (called SLIM) to simulate the dispersal of virtual post-hatchlings under different scenarios of passive drift and active swimming behaviour. The results of my simulations suggest that, under the conditions I tested, the retention of flatback turtles in neritic waters of the Great Barrier Reef (GBR) depends on three main factors: (a) the location of the nesting beaches: flatback turtles nest in inshore islands and mainland of eastern Queensland; (b) the local currents, wind-driven waves and the tidal phase when post-hatchlings were released; and (c) swimming behaviour of post-hatchlings, with higher swimming effort dispersing turtles into neritic habitats of the GBR. In this chapter, I also provide future directions for research in the area of early dispersal of marine turtles, and potential approaches to test the cues that induce directional swimming in marine turtles. Next, in chapters 3 and 4, I examined two other key biogeographical processes: migration and habitat use of foraging adult turtles. Chapter 3 focused on describing the spatial distribution of 44 flatback turtles tracked with high accuracy GPS-linked satellite tags, from eight different nesting beaches to their respective foraging grounds. Home ranges of flatback turtles in eastern Australia were relatively larger than other coastal marine turtle species in the region. I also describe common patterns in the migratory and foraging strategies displayed by the tracked turtles. In this sense, flatback turtles in eastern Australia displayed direct and multi-stop migrations, and flexible and dynamic foraging behaviour (i.e. using more than one foraging ground). To supplement the understanding of the spatial ecology of foraging turtles, in Chapter 4 I further examined the distribution of the tracked turtles in relation to the environmental parameters of the seabed habitats which they inhabit. Such studies are not common in marine environments, given the challenge that represents comprehensively surveying the habitat used by individuals. However, data on the biotic (prey) and abiotic characteristics of the seabed habitats used by flatback turtles in the Great Barrier Reef were available and accessible through the GBR Seabed Biodiversity Project. Employing a Random Forest analysis, I was able to assess how the tracked turtles respond to abiotic parameters in the environment, as well as to the distribution and abundance of potential prey groups. The results confirm that flatback turtles inhabit inshore subtidal habitats, with a preference for mud-rich inshore environments. In addition, the tracked flatback turtles were associated with distribution of soft-bodied invertebrates such as sea pens and soft corals; however, flatback turtles might as well be associated with a wider variety of benthic prey than previously reported. The flexibility observed in foraging behaviour, combined with the large size of home ranges and the association of the tracked turtles to multiple habitat types and prey items suggest that flatback turtles in eastern Australia are generalist and opportunistic feeders. Nevertheless, some turtles displayed high affinity for just one of the three habitat types, which might suggest some degree of individual specialisation within the population, a hypothesis that warrants further research. Finally, in chapter 5 focused on quantifying the exposure of flatback turtles to different threats and the level of protection in their foraging habitats within the Great Barrier Reef Marine Park (GBRMP). To achieve this, I performed a spatial analysis overlapping the foraging area identified for the tracked flatback turtles (Chapter 3) with the distribution of marine protected areas, as well as with two potential threats known to occur in the GBRMP, shipping and trawling. I also considered the cumulative exposure of flatback turtles to the combined threats. The results indicate that half (52.2%) of the foraging area of the tracked turtles is within a marine protected area, 47.3% was located in "General Use" areas, and 0.5% were located within ports. In addition, the overall exposure of the tracked turtles to the individual and cumulative effect of shipping and trawling was low, with some foraging locations in the northern section of the GBRMP displaying medium exposure to the threats. I strongly suggest that future research should aim to include the synergistic effect of threats, and include other human-related stressors, such as water quality and marine debris. Overall, my thesis provides detailed and novel information which improves the knowledge base on the spatial distribution of flatback turtles in eastern Australia, including aspects on their dispersal, migratory and foraging behaviour and ecology. My thesis is relevant to several priority action areas required to maintain/recover the eastern Australia flatback stock. In this sense, I have already been able to provide copies of my data, shapefiles and written work to the GBR Marine Park Authority and the Australian Government and my data has informed the revised Marine Turtle Recovery Plan 2017. In conclusion, my study provides a comprehensive baseline on the spatial and movement ecology of flatback turtles at sea, and will hopefully provide the guidelines to address further gaps that need to be addressed to gain a better understanding of the status, vulnerability and adaptive capacity of flatbacks of the eAus stock.