Torres Strait Dugong distribution and relative density - Spatial model of aerial surveys from 1987 - 2011 (NERP TE 2.1, JCU)

Brief description

This dataset shows a raster spatial model of the distribution and relative density of dugongs (Dugong dugong) in the Torres Strait region based on an aggregate of 24 years (1987 - 2011) of systematic aerial surveys. 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, including information on group size and calf numbers. Transects were 200 m wide at the water’s surface on either side of the aircraft. The spatial data from all the aerial surveys in the region (1987, 1991, 1996, 2001, 2005, 2006, and 2011) were corrected for differences in sampling intensity and area sampled between surveys. The corrected data was then interpolated using universal kridging over the spatial extent of the aerial surveys. The modelled abundance and distribution show the relative density of dugongs (areas where there are more or less dugongs) and NOT the absolute dugong density as corrections for perception bias (animals that are available to, but missed by, observers) and availability bias (animals that are unavailable to observers because of water turbidity) can only be applied at the spatial scale of entire surveys (thousands of square kilometres), making them inappropriate for the spatial scale for this dataset. Nonetheless, the relative densities among regions should be approximately comparable (H. Marsh, personal communication). Planning units were classified as low (1), medium (2), high (3) and very high (4) dugong density on the basis of the relative density of dugongs estimated from the models and a frequency analysis. Low density areas: 0 dugongs per square km; medium density areas 0.0015 - 0.25 dugongs dugongs per square km; high density areas 0.25 - 0.5 dugongs per square km; very high density areas > 0.5 dugongs per square km. The spatial model is 134x118 pixels with a pixel size of 2kmx2km and 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 (26 kB). Both datasets are available under a creative commons attribution license.

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Statement: The modelling methods used to develop this datasets were developed as part of Alana Grech's PhD.