Marine turtles and dugongs of the Torres Strait - Spatial models of dugong and turtle distribution and relative density of aerial surveys from 1987 - 2013 (NERP TE 2.1, JCU)

This dataset shows (1) a raster spatial model of the distribution and relative density of dugongs (Dugong dugong) in the Torres Strait region based on an aggregate of 27 years (1987 - 2013) of systematic aerial surveys; and (2) a raster spatial model of the distribution and relative density of marine turtles (green turtles, Chelonia mydas) in the Torres Strait based on an aerial survey conducted in 2013.

Aerial surveys were conducted using the strip transect method described by Marsh and Sinclair (1989). The survey region was divided into blocks containing systematic transects of varying length, which were typically perpendicular to the coast across the depth gradient. Tandem teams with two observers on each side of the aircraft independently recorded sightings of dugongs and marine turtles, including information on group size and calf numbers. Transects were 200 m wide at the water’s surface on either side of the aircraft.

For the dugong spatial model, spatial data from all the aerial surveys in the region (1987, 1991, 1996, 2001, 2006, 2011, and 2013) were corrected for differences in sampling intensity and area sampled between surveys. For details on sampling intensities and areas, please refer to the reports and publications for each individual survey (list below). For the turtle model, spatial data were used from the survey conducted in 2013. The corrected data were then interpolated using universal kriging over the spatial extent of the aerial surveys.

Planning units were 2km * 2km and were classified as low (1), medium (2), high (3) and very high (4) dugong/turtle density on the basis of the relative density of dugongs and turtles estimated from the models and a frequency analysis. Low density areas: 0 animals per square km; medium density areas 0.0015 - 0.25 animals per square km; high density areas 0.25 - 0.5 animals per square km; very high density areas > 0.5 animals per square km.

Note: A project is currently under way (funded by AMMC) to create an integrated database for all aerial survey data which will host raw data.

Limitations:

The modelled abundance and distribution in this dataset shows the relative density of dugongs and turtles (areas where there are more or less animals). It does NOT show the absolute dugong and turtle density. The observations only count a small percentage of the total number of dugongs. Some dugongs will be missed by the observers in the survey plane (perception bias) and some dugongs will be diving and can't be seen due to water turbidity (availability bias). Corrections for these under estimates can only be estimated at the spatial scale of entire surveys (thousands of square kilometres) and so when they are allied to the dataset they are assumed to be constant over the entire survey area. However these correction factors would vary spatially and the absolute numbers of the dugongs at the fine scale of this dataset (in a small area) are unlikely to be accurate. Nonetheless, the relative densities among regions should be approximately comparable (H. Marsh, personal communication).

Format:

The spatial model is 794x680 pixels with a spatial reference of WGS84 UTM Zone 54S. The original dataset is stored in ESRI GRID format (60 KB), which was converted to a GeoTiff for use in the eAtlas (22 kB). The GeoTiff was then converted to a shapefile.

Publications for aerial surveys in the Torres Strait:

- Marsh, H, Harris, ANM, & Lawler, IR. (1997) The sustainability of the Indigenous dugong fishery in Torres Strait, Australia/Papua New Guinea. Conservation Biology 11:1375-1386.
- Marsh, H, Lawler, IR, Kwan, D, Delean, S, Pollock, K, & Alldredge, M. (2004). Aerial surveys and the potential biological removal technique indicate that the Torres Strait dugong fishery is unsustainable. Animal Conservation 7: 435-443.
- Marsh, H.D., Hodgson, A., Lawler, I., Grech, A., & Delean, S. (2007) Condition, status, and trends and projected futures of the dugong in the Northern Great Barrier Reef and Torres Strait; including identification and evaluation of the key threats and evaluation of available management options to improve its stautus. Marine and Tropical Sciences Research Facility Reports Series. Reef and Rainforest Research Centre Limited, Cairns (77pp.)
- Marsh, H., Grech, A., & Hagihara, R. (2011) Aerial survey of Torres Strait to evaluate the efficacy of an enforced and possibly extended Dugong Sanctuary as one of the tools for managing the dugong fishery. Report to the Australian Marine Mammal Centre and the Torres Strait Regional Authority (33pp.)