Dugong distribution, the seagrass Halophila spinulosa, and thermal environment in winter indeeper waters of eastern Shark Bay, WA

Abstract

Five aerial survey transects in eastern Shark Bay, along which dugongs (Dugong dugon) had been reported in deep water in July 1989, were flown on 3 and 14 July 1992. As in 1989, dugong sightings were concentrated between 9- and 15-m depth contours. Thermal patterns shown on images obtained from NOAA satellites, and direct measurement of temperatures in the water column, confirmed that temperatures were at least 19°C where dugongs were concentrated. Although no seagrass had previously been reported in this zone, examination of benthic communities using a remotely controlled underwater video system revealed an apparently monospecific stand of Halophila spinulosa. Dugong behaviour, observed from a 10-m catamaran, indicated that the animals were diving to the bottom. Long, deep dives may be more costly in time and energy than the shorter dives characteristic of feeding in shallow water. H. spinulosa has large fleshy rhizomes similar to those known to contain abundant starch in other members of the genus and it is suggested that carbohydrate-rich rhizomes might compensate for the higher costs of foraging in deep water. Previously known stands of preferred forage species appeared inadequate to support the large dugong population of Shark Bay, but if temperatures are favourable throughout the winter and if the H. spinulosu observed in this study is available throughout the year, this H. spinulosa community may be an important resource for the Shark Bay dugong population.