of Dugongs in the Western Gulf of Carpentaria. P. BaylissA ... The seasonal distribution and relative abundance of dugong over 27 216 km2 of coastal waters.
Aust. Wildl. Res., 1989, 16, 141-9
Seasonal Distribution and Abundance of Dugongs in the Western Gulf of Carpentaria
P. BaylissA and W. J. Freeland Conservation Commission of the Northern Territory, P.O. Box 496, Palmerston, N.T. 5789, Australia. Present address: Queensland National Parks and Wildlife Service, P.O. Box 5391, Townsville, Qld 4810, Australia.
A
Abstract Aerial surveys of coastal waters (27 216 km2) in the western Gulf of Carpentaria during the dry season of 1984 and wet season of 1985 indicated no major seasonal changes in distribution and relative abundance of dugongs. Minimum population size in the dry season was estimated as 16 816k2946 (standard error), with a relative density of 0.62 k 0.11 km-2, and that for the wet season 16 846 + 3257, with a relative density of 0.62k0.12 km-2. The estimates exclude 5% of observations which could have been either dugongs or Irrawaddy dolphins, and were corrected for submerged dugongs and those missed on the surface. Dugongs were unevenly distributed over the study area, and neither mean group size nor proportion of calves varied between seasons. Dugong abundance was correlated with area of available seagrass. The catch rate of dugongs by Aboriginal people off Numbulwar decreased six-fold between the 1960s and 1985 (60 to 10 p.a.), possibly due to excessive hunting.
Introduction
Although the dugong, Dugong dugong (Miiller) has apparently experienced a reduction in its Indo-Pacific range and is listed as vulnerable to extinction (Thornback and Jenkins 1982), recent surveys in northern Australia indicated that relatively large populations still exist. Marsh and Saalfeld (1989) estimated 8106+ 1125 (all estimates +s.e.) dugongs in 31 288 km2 of northern sections of the Great Barrier Reef. Bayliss (1986) provided a minimum estimate of 2953 +530 for 28 746 km2 of coastal water between the Daly River and Milingimbi, Northern Territory. However that study used a different method for correcting visibility bias to Marsh and Saalfeld (1989), and an appropriate adjustment for comparison gives an estimate of 13 800+2683. Both these studies demonstrated patchy dugong distributions, with highest densities on the Great Barrier Reef occurring in known seagrass areas. Marsh and Saalfeld (1989) showed seasonal redistribution of dugongs on a relatively local scale. In contrast to the above reports of 'abundance', other studies suggest that there is no room for complacency. Prince (1986) reported relatively high harvest rates of dugongs in parts of north-western Australia and expressed concern for their future, whilst Marsh (1986) reported only 1455 +276 dugongs for Torres Strait (25 243 km2), and provided evidence of a population decline and considerable hunting pressure. The western portion of the Gulf of Carpentaria is known to maintain populations of dugongs (Elliott 1981). Potential threats to dugongs in the area include a commercial barramundi fishery which causes incidental mortality (Marsh et al. 1986), and possible overharvesting by Aboriginal people (Bertram and Bertram 1973). 0310-7833/89/020141$03.00
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P. Bayliss and W. J. Freeland
We present results of aerial surveys of coastal waters in the western Gulf of Carpentaria as a record of dugong population status and distribution in the area during 1984-85. The effects of season and seagrass distribution on dugong distribution and abundance are examined.
Methods and Analysis The seasonal distribution and relative abundance of dugong over 27 216 km2 of coastal waters in the Gulf zone of the Northern Territory (Fig. 1) was determined by standardised low-level aerial surveys. Surveys were flown at a height of 137 m, a speed of 185 km h-', and observers counted dugongs in transects 200 m wide either side of the aircraft. Details of the survey technique are found in Bayliss (1986). Surveys were conducted in the mid-dry season of 1984 (August) and mid-wet season of 1985 (February). Each survey took approximately two weeks and averaged 6 h day-'. Surveys were flown without regard to tide (Bayliss 1986) and weather conditions were consistently favourable. Hence no corrections were made for variable weather (see Marsh and Sinclair 1988).
OF
CARPENTARIA
Fig. 1. Map of the survey area showing location of blocks and transects.
For estimation of regional relative densities, the survey area was divided into eight blocks based on geographical features (large islands, bays, river mouths and other coastal features). The blocks provided a basis for analysis of seasonal density shifts between regions. Each block was sampled by east-west transects systematically spaced 5 km apart, providing a mean sampling rate of 7.9% (Fig. 1, Table 1). Each transect was divided into 5 km units to aid the mapping of dugong distribution. Data were recorded continuously on a transecthnit basis and comprised: the number of dugong, dolphin or turtle groups, their group sizes, behaviour (resting, diving, swimming) and position in the water column (surface, submerged), surface water condition, and weather conditions (see Bayliss 1986 for details). If an animal could not be classified as a dugong or dolphin it was recorded as unidentified. Dugongs and Irrawaddy River dolphins (Orcaella brevirostris) at first glance appear similar, the only striking difference being the small dorsal fin low on the dolphin's back. However after several observations,
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most observers had little difficulty differentiating the two species under favourable viewing conditions. Misidentification is not considered a problem because only 0.1% and 0.6% of transect units (total of 834) in the wet and dry seasons respectively were recorded as having both dugongs and Irrawaddy dolphins occurring together. Overall only 5.1% and 4.3% of total observations of dugongs plus dolphins could not be classified in the wet and dry seasons respectively. Calves were identified by their small size and proximity to larger adults. Table 1. Areas of the survey blocks and sampling intensities Block number
Area (km2)
Sampling intensity (%)
1 2 3 4 5 6 7 8 Total
mean sample rate = 7.9%
Accuracy of the group counts was improved by correcting for the proportion of dugongs missed at the surface of a transect using the double count technique (Bayliss 1986), and the proportion of dugongs which were submerged and not counted at the time of survey (called perception and availability biases respectively; see March and Sinclair 1988 for details). That method was chosen in an attempt to standardise marine surveys in Australia. Bayliss (1986) only accounts for perception bias; minimum population sizes and density estimates using his method are provided (Appendix 1) to allow comparison with published data within the Northern Territory. Survey-specific correction factors (CF) for each observer, used to adjust for perception and availability biases ( + s.e.), are summarised in Appendix 2. The same three observers were used on both surveys, one providing double counts by alternating aircraft position. One factor ANOVA showed no significant difference in the mean group size (MGS) of dugongs between observers (dry season F=0.36, d.f. = 2/l95; wet seaon F=0.58, d.f. =2/167), hence MGS (+s.e., Appendix 2) was estimated by pooling all observations. Corrected counts/transect were obtained by multiplying MGS by the number of groups/transect and the appropriate survey-specific group CFs (details in Marsh and Sinclair 1988). Estimates of population size and density are still referred to as 'minimum' and 'relative' respectively, because the estimate of availability bias is most likely underestimated (Marsh and Sinclair 1988). The Ratio Method (Jolly 1969, Caughley 1979, Bayliss 1986) was used to estimate minimum population sizes and relative density estimates per block. The method of estimating population variance incorporating CF and MGS errors is detailed by Marsh and Sinclair (1988). An overall minimum population size and estimate of relative density were calculated by adding block estimates and merging their variances (Caughley 1979). Differences in relative density between seasons and blocks were tested by a two factor ANOVA (block by season). An unweighted means analysis was used because the number of transects varied by blocks of different sizes (Winer 1971, Caughley and Grigg 1981). Input data were corrected densities (km-') per transect and both factors were fixed. Transects were treated as replicates within surveys, not as a formal blocking factor (Caughley and Grigg 1981), because they could not be exactly replicated between surveys. Nevertheless, both forms of analyses are approximations to reality, and produced the same results.
Results Although blocks differed in relative dugong density, distribution (block by season interaction) and abundance (factor season) did not change significantly between seasons (Table 2). Blocks 2, 3, 6 and 7 (Blue Mud Bay, western coast of Groote Eylandt, Rose River mouth and Sir Edward Pellew Islands respectively) had higher dugong numbers and densities than the other blocks (Table 3, Fig. 2). Dugong rank abundance was correlated with area of seagrass (Figs 2, 3 and 4;Spearman Rank r1=0.83,n = 8 , PcO.05). Block 3 (Rose River mouth) had the largest area of
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seagrass (Fig. 3), yet ranked only fourth in dugong abundance. Removal of that anomalous data point from the correction analysis resulted in a Spearman Rank r ' of 0.96 (n =7, P6% p.a. will force populations into a continuous decline, and harvest rates
