Studies of Dugons at Shark Bay, Western Australia I. Analysis of Population Size, Composition, Dispersion and Nabitat USe on the Basis of Aerial Survey

Abstract

In midwinter dugongs concentrate in western Shark Bay, adjacent to Dirk Hartog I., although scattered individuals or groups remain in other regions. Up to 496 dugongs were counted in intensively surveyed western areas in July 1979. At least 553 individuals are calculated to have been present in the region, and a minimum population of over 900 individuals estimated for the Bay as a whole. The population seems to be a well defined isolate, and is relatively undisturbed. Conditions are very favourable for more detailed study. A method for estimating the number of animals overlooked in the blind zone directly beneath the aircraft is described. Aerial counts of dugongs varied greatly with conditions. Cloud cover had a large depressing effect: surface turbulence and subsurface turbidity were less significant. Dugong activity is concentrated on the sublittoral platform. Sites of major aggregation may not be the same in successive years. Locally favoured habitats are at the seaward margins of the sublittoral platform, and on offshore shoals. Vegetation patterns at favoured sites may reflect dugong grazing. Persistent use of favoured localities, and local shifts in concentration were observed. Distribution of other large vertebrates, and of boats, relative to that of dugongs was examined and a significant correlation between sightings of large sharks and larger aggregations of dugongs was found. Analysis of dispersion revealed that most dugongs were in groups although most sightings were of single individuals. Cows with calves were less likely to be in the company of other dugongs than were adults without calves. From ll to 12% of individuals were calves.

Distribution and abundance of the dugong in New Caledonia, southwest Pacific

“…The predominance of sightings of single dugongs in New Caledonia is consistent with reports from Australian populations (Marsh et al 1981, Anderson 1982, Marsh and Saalfeld 1989, Marsh et al 2004. Groups of more than three dugongs were uncommon during our survey and only one large group of forty-five individuals was observed.…”

“…The predominance of sightings of single dugongs in New Caledonia is consistent with reports from Australian populations (Marsh et al . 1981, Anderson 1982, Marsh and Saalfeld 1989, Marsh et al . 2004).…”

Can measures of prey availability improve our ability to predict the abundance of large marine predators?

“…The presence of large tiger sharks near dugong aggregations in Shark Bay has been noted (Anderson 1982), and Table 1 Generalized linear models of tiger shark abundance for each of four size categories (divisions based on total length (TL; cm). Independent variables are sea surface temperature (T; °C) and the density of cormorants, dugongs (D), sea snakes, and sea turtles.…”

Summer survey of dugong distribution and abundance in Shark Bay reveals additional key habitat area

“…5). Given that these shallower waters are known to contain extensive seagrass beds (Anderson 1982(Anderson , 1998Walker et al 1988;Walker 1989) of species known to be preferred dugong forage (e.g. Halodule uninervis and Halophila ovalis), it would appear that the presence of suitable food is the primary determinant of dugong distribution in summer.…”

“…Some attempts have been made to conduct complementary summer surveys, but these have been hampered by the difficult survey conditions and have not been able to cover the entire bay, focussing principally on an area on the eastern side of Peron Peninsula (Fig. 1) known to support significant numbers of dugongs (Anderson 1982). Concentrations of dugongs have been reported in the south-west of the bay in summer (Prince et al 1981;Anderson 1986), but there have been no data available with which to assess the significance of the area to the population as a whole.…”