Hawksbill and green turtle distribution and important areas

AIMS acknowledges the Traditional Owners of Country throughout the northern coast of Western Australia where this Northwest Shores to Shoals work was undertaken. We recognise these People’s ongoing spiritual and physical connection to country and pay our respects to their Aboriginal Elders past, present and emerging.

Global populations of green (IUCN listing endangered) and hawksbill (IUCN listing critically endangered) turtles are declining due to a range of threats. Australia supports some of the largest rookeries (nesting sites) for these turtles in the Indo-Pacific. Even though they've been much studied, most data that shows where these turtles spend their time around Australia remains unpublished. This is a problem because there are many coastal and offshore activities such as mining, commercial fishing and pollution that may threaten these species. In order to protect them, we need to know the areas that are important to them; the areas where they spend time during the nesting season, their migratory routes and the areas where they forage (feed). In the absence of hard data, the Australian Government have previously designated Habitat Critical Areas as part of the Recovery Plan for Marine Turtles and Biologically Important Areas based on expert scientific knowledge. Here, we set out to quantify and map the important areas that turtles use to help refine these protected areas and assist with turtle conservation management (read the full paper here).

Tracking turtles with satellite tags

We addressed this by collaborating with the Department of Biodiversity, Conservation and Attractions and range of other turtle researchers to compile and analyse all available existing satellite tracking data for adult, female green (n = 76) and hawksbill turtles (n = 22) from Western Australia (WA) combined with data from targeted new deployments of satellite tags (20 for each species) in the region. The satellite tracking devices were attached to turtles while they were ashore for nesting (as shown in the photos below), with the compiled data covering 10 green (see table) and 5 hawksbill (see table) turtle rookeries across northwest WA.

Photographs courtesy of Dr. Michele Thums.

Photographs courtesy of Dr. Michele Thums.

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Watch the animations below to see where turtles who were tagged went over the time that the tags sent data back to us.

Finding the areas most important to turtles

We used this data to consider what turtles were doing while they were moving and calculate how much time they spent in each location, doing each key activity. These activities are: inter-nesting (the period of time female turtles spend in the vicinity of the nesting beach during nesting season), migrating (travelling to where they feed) and foraging (feeding). For each activity, we identified key areas used by most of the turtles, most of time (read a detailed explanation here).

We found that Habitat Critical Areas that the Australian Government has previously defined as a 20 km radius around each turtle rookery (shown as dashed line polygons on Map 1 and Map 2) largely protects the nesting areas we calculated for both green (see Map 1) and hawksbill (see Map 2) turtles. However, not all of the inter-nesting distributions for hawksbill turtles were completely encompassed by the 20 km Habitat Critical buffer and for some, our sample size was small and therefore some cation is needed with interpretation (e.g., Varanus Island in the Lowendal Island Group) (see Map 2). The tracking data shows us that turtles move far beyond these areas to feed (see Map 3, Map 5). We find that only 35% of areas where we calculated that green turtles spend time foraging (see Map 4) and 23% for hawksbill (see Map 6) turtles are protected within Marine Parks (blue outlined polygons on the maps), and the existing BIAs do not include much of the important foraging areas for the turtles (see Map 4, Map 6).

See the metadata - Read the full paper

AIMS’ North West Shoals to Shore Research Program is proudly supported by Santos as part of the company’s commitment to better understand Western Australia’s marine environment. We also thank data providers Department of Biodiversity, Conservation and Attractions, INPEX, Woodside Energy Ltd, Pendoley Environmental.

Detailed explanation

Methods

We used data for hawksbill turtles tagged from five different locations. Those we tagged ourselves are marked with a *.

We used data for green turtles tagged from 10 different locations. Those we tagged ourselves are marked with a *.

The satellite tags provided information on turtle movement during the inter-nesting period (i.e. the period of time female turtles spend in the vicinity of the nesting beach during nesting season) and post-nesting which includes migration to foraging grounds. To quantify the spatial distribution and important areas, we split turtle movement data into these components (nesting, migration and foraging) based on the understanding of the timing of the nesting season and using simple metrics underlying the track such as speed and turning angle. We then used time (and number of turtles) in area analysis to calculate two measures of turtle spatial use: relative proportion of time spent in each grid cell across the study area (index of occupancy) and the number of turtles using each grid cell. To represent spatial use for inter-nesting, migration and foraging, we ranked the summed grid cell values (both occupancy index and number of turtles) from largest to smallest and determined the spatial distribution as the cells encompassing the top 95% (for inter-nesting) and 75% (for migration, foraging) of the cumulative frequency distribution based on the method described by Soanes et al. (2013) and akin to a 75% and 95% utilisation distributions (UD). The 75% distribution was used instead of the 95% for foraging and migration to exclude rare large migrations with low/no individual overlap. In addition to undertaking the analysis at the scale of each of the rookeries where the tags were deployed, the analysis was also done at the scale of genetic stocks of green turtles encompassed by our dataset (Northwest Shelf (NWS) stock and Scott-Browse stock) with the former split into management regions (Pilbara and Kimberley).

Results

Both species exhibited mostly coastal post-nesting movements to foraging grounds in shallow, coastal waters, with green turtles (see Map 3; displacement distance = 765.9 ± 706.0 km) exhibiting larger migrations than hawksbills (see Map 5; displacement distance = 421.4 ± 366.3 km).

Some turtles (14% of greens and 3% of hawksbills) did not migrate but remained resident in the vicinity of their rookery for the duration of their deployment. The post-nesting distribution of Western Australian hawksbill turtles was contained within north-western Western Australia (see Map 5), however the green turtle distribution expanded to Shark Bay and the Northern Territory (see Map 3; 75% UD). The turtles had relatively direct movements to many disparate foraging grounds with little foraging recorded en-route (see Map 3, Map 5). Turtles used two main migration corridors, with green turtles from the Pilbara rookeries (NWS stock-Pilbara) and hawksbill turtles using a corridor from Barrow Island to Eighty Mile Beach and green turtles from Kimberley rookeries (NWS stock-Kimberley and Scott-Browse stock) using a corridor from Eighty Mile Beach to the Tiwi Islands (see Map 3, Map 5).

The spatial extent of the inter-nesting areas calculated here for both species was for most rookeries encompassed by the Habitat Critical areas designated by the Australia Government (20 km buffer around rookeries). Although 35% of the foraging distribution for green turtles and 23% for hawksbill turtles calculated here occurred within MPAs, the existing BIAs are largely underestimating the important foraging areas for the turtles (green turtles - Map 4; hawksbill turtles - Map 6).

Maps

AIMS’ North West Shoals to Shore Research Program is proudly supported by Santos as part of the company’s commitment to better understand Western Australia’s marine environment.