RESEARCH PUBLICATION NO. 77 Dugong distribution and abundance in the northern Great Barrier Reef Marine Park - November 2000. Helene Marsh and Ivan Lawler RESEARCH PUBLICATION NO. 77 Dugong distribution and abundance in the northern Great Barrier Reef Marine Park - November 2000. Helene Marsh and Ivan Lawler School of Tropical Environment Studies and Geography, James Cook University, Townsville. PO Box 1379 Townsville Qld 4810 Telephone: (07) 4750 0700 Fax: (07) 4772 6093 Email: [email protected] www.gbrmpa.gov.au © Great Barrier Reef Marine Park Authority 2002 / James Cook University 2001 ISSN 1037 – 1508 (Print) ISSN 1447 – 1035 (On-line) ISBN 1 876945 15 X. Published July 2002 by the Great Barrier Reef Marine Park Authority. This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from the Great Barrier Reef Marine Park Authority. Requests and inquiries concerning reproduction and rights should be addressed to the Director, Information Support Group, Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville Qld 4810. The opinions expressed in this document are not necessarily those of the Great Barrier Reef Marine Park Authority. Accuracy in calculations, figures, tables, names, quotations, references etc. is the complete responsibility of the authors. This publication will also be available on the Great Barrier Reef Marine Park Authority’s website, www.gbrmpa.gov.au National Library of Australia Cataloguing-in-Publication data: Marsh, Helene. Dugong distribution and abundance in the northern Great Barrier Reef Marine Park, November 2000. Bibliography. ISBN 1 876945 15 X. 1. Dugong - Queensland - Great Barrier Reef Marine Park. 2. Dugong - Conservation - Queensland. 3. Marine mammal populations - Queensland - Great Barrier Reef Marine Park. I. Lawler, Ivan. II. Great Barrier Reef Marine Park Authority. (Series : Research publication (Great Barrier Reef Marine Park Authority (Australia)) ; no. 77). 599.55909943 PO Box 1379 Townsville Qld 4810 Telephone (07) 4750 0700 CONTENTS SUMMARY............................................................................................................................................1 INTRODUCTION ..................................................................................................................................4 METHODS .............................................................................................................................................5 Survey Methodology.......................................................................................................................5 Correction Factors...........................................................................................................................9 Population Estimation..................................................................................................................10 Statistical Analysis.........................................................................................................................10 Estimating the Size of a Sustainable Dugong Catch in the nGBR..........................................11 RESULTS............................................................................................................................................12 Survey Conditions.........................................................................................................................12 Group Size and Composition.......................................................................................................12 Dugong Distribution and Abundance........................................................................................16 Northern GBR Region..............................................................................................................16 Cape Bedford to 17°25’S..........................................................................................................18 Estimating a Sustainable Level of Human-induced Mortality for Dugongs in the Survey Regions ...........................................................................................................................................20 Northern GBR...........................................................................................................................20 Minimum Population Estimate Nmin..............................................................................20 Maximum Rate of Increase Rmax.....................................................................................20 The Recovery Factor...........................................................................................................21 Cape Bedford to 17°25’S...............................................................................................................23 DISCUSSION .....................................................................................................................................24 Northern GBR Region...................................................................................................................24 Status..........................................................................................................................................24 Changes in Distribution Between Surveys...........................................................................24 Strategies for Protecting the Dugong in the nGBR..............................................................26 Cape Bedford to 17°25’S...............................................................................................................28 CONCLUSIONS ................................................................................................................................29 ACKNOWLEDGEMENTS.................................................................................................................34 REFERENCES ...................................................................................................................................35 APPENDICES.....................................................................................................................................39 Appendix 1. Raw data used for calculation of correction factors for dugongs for the 2000 survey...........................................................................................................39 Appendix 2. Beaufort sea state and glare for each transect .................................................40 Appendix 3. Raw data detailing sightings of dugong groups for each transect in each block used for population estimates....................................................49 i FIGURES Figure 1. Map of the survey region showing the positions of the survey blocks and transects...........................................................................................................7 Figure 2. Histograms showing the frequencies of groups of dugongs of various sizes and numbers of calves sighted in November 2000; (a) south of Cape Melville; (b) north of Cape Melville; and (c) entire Northern GBR survey region.............................................................................14 Figure 3. Locations where dugongs were sighted in the Northern GBR on the November 2000 survey.).....................................................................................15 ± Figure 4. Temporal changes in the numbers of dugongs ( standard error) in each of the survey blocks in the Northern GBR which supported an estimated 500 dugongs or more on at least one of the aerial surveys..........19 Figure 5. Dugong sightings on the 2000 aerial survey of the Northern GBR in relation to the Marine Park Zoning of the area between Cape Bedford and Bathurst Bay. …............................................................................................30 Figure 6. Dugong sightings on the 2000 aerial survey of the Northern GBR in relation to the Marine Park Zoning of the area from Princess Charlotte Bay to Lockhart River..........................................................................................31 Figure 7. Dugong sightings on the 2000 aerial survey of the Northern GBR in relation to the Marine Park Zoning of the area from Lockhart River to Shelburne Bay.......................................................................................................32 Figure 8. Dugong sightings on the 2000 aerial survey in the region between Cape Bedford and south of Yarrabah in relation to the existing Marine Park Zoning of the area.........................................................................33 TABLES Table 1. Areas of survey blocks and sampling intensities..............................................8 Table 2. Details of mean group size estimates and correction factors used in the population estimates for dugongs in the 2000 survey of the northern GBR region and the region between Cape Bedford and Cooktown and 17º25'S.........................................................................................10 Table 3. Weather conditions encountered during the survey compared with those encountered on previous surveys of the northern GBR......................12 Table 4. Estimates of dugong abundance in each of the survey blocks in 2000 in comparison with the corresponding data for 1985, 1990, and 1995.........17 Table 5. Summary of Split-plot Analysis of Variance comparing dugong densities among surveys.....................................................................................18 Table 6. Estimates of the maximum rate of population increase Rmax for dugong populations for combinations of life history parameters................21 Table 7. Estimates of the total sustainable anthropogenic mortality (Potential Biological Removal sensu Wade 1998) for various estimates of dugong population size in the entire northern GBR and the region south of Cape Melville for a range of estimates of Rmax and Recovery Factor values of 0.5 and 1..................................................................23 Table 8. Evidence of significant changes in the spatial distribution of dugongs as suggested by standardised aerial surveys in Australia.............25 ii SUMMARY The waters of northern Australia are internationally recognised as the stronghold of the dugong (Dugong dugon) which is listed as vulnerable to extinction by the IUCN (2000). Significant populations persist in Australian waters, and dugongs are specifically cited as one of the World Heritage values of the Great Barrier Reef World Heritage Area. Much of the information used to manage dugong populations in this region has been provided by aerial surveys using standardised techniques. We report here on aerial surveys conducted in the following regions in November 2000 to assess the status of the dugong: (1) the Northern Great Barrier Reef Region between Hunter Point (11°15’S — south of Cape York) and Cape Bedford (15°30’S — near Cooktown); and, (2) the region between Cape Bedford and 17°25’S (near Innisfail). Northern Great Barrier Reef Region The estimated size of the dugong population for this region in 2000 was 9081 dugongs (± s.e. 917). Statistical comparison of this result with those from previous surveys indicates that overall dugong numbers in the region are stable at the scale of the region as a whole. The survey technique is designed to detect regional scale trends and cannot accurately detect changes at a local scale even if they are occurring. The dugongs were distributed differently from previous surveys. The surveys in 1985, 1990 and 1995 indicated that Princess Charlotte Bay supported between 37% and 56% of dugongs in the northern GBR region. The corresponding proportion for 2000 was 24.5%. At the same time, the area south of Cape Melville supported 59% of the dugongs in the region, a proportion about twice as high as the percentages of between 25% and 32% in 1985, 1990 and 1995. The results of the 2000 survey add to a growing body of evidence from aerial surveys and satellite tracking that dugongs undertake large-scale movements. The reasons for such large-scale movements are not generally known, but at least sometimes appear to be associated with disturbance to their seagrass habitat. In some instances, large-scale movements are associated with large-scale episodic disturbance to habitat caused by cyclones and floods. The distribution and abundance of dugongs has influenced the placement of highly protected areas within this region. Great Barrier Reef Marine Park (GBRMP) zoning protects dugongs in the coastal waters of Shelburne Bay, the Friendly Point region, Corbett Reef and much of the region between Cape Melville and Lookout Point from extractive activities, especially fishing impacts including incidental capture in commercial gill nets and habitat damage from trawling. Such protection has been enhanced by the rezoned Far Northern Section of the Great Barrier Reef Marine Park, which provides increased protection for dugong and their habitats in Temple Bay, the Cape Direction region, parts of Princess Charlotte Bay and Bathurst Bay. Nonetheless, dugongs and dugong habitats are not specifically protected in intertidal areas in the Northern Great Barrier Reef Region that are outside the Great Barrier Reef Marine Park. The planned introduction of zoning which complements that introduced as 1 a result of the rezoning of the Far Northern Section of the Great Barrier Reef Marine Park and the review of fisheries’ management arrangements by the Queensland Government, will help ensure threats to dugong in this region are minimised. The potential biological removal method was used to estimate sustainable anthropogenic mortality from all causes for a range of estimates of dugong life history parameters based on empirical data from various wild populations using values of both 0.5 and 1 for the recovery factor. The justification for using a recovery factor of 1 was that the temporal series of aerial surveys suggests that population numbers are stable at a regional scale. We also used a recovery factor of 0.5 because the dugong is listed as a threatened species in Queensland and the default value is 0.5 for such stocks. The middle value for the maximum rate of increase R max (=0.03) suggests that the following total annual anthropogenic mortalities should be sustainable: • for the whole NGBR region: 63 dugongs (RF= 0.5); 125 dugongs (RF=1) for Blocks 1-4; • the region south to Cape Bedford based on the population’s distribution in 2000: 36 dugongs (RF= 0.5); 72 dugongs (RF=1) for Blocks 1-4; and • the region south to Cape Bedford based on the population distribution in 1995 (worst case scenario): 13 dugongs (RF= 0.5); 26 dugongs (RF=1). Given that the population estimates in this report are relative estimates, these estimates of sustainable anthropogenic mortality should be revised when absolute population estimates become available. Cape Bedford to 17°25’S The survey was extended south from Cape Bedford to 17°25’S, between Cairns and Innisfail. Poor weather had prevented us from surveying this region as scheduled in 1999 as part of a survey of the Southern Great Barrier Reef (southern GBR). Unsuitable weather once again prevented us extending this survey to be contiguous with the northern boundary of the 1999 survey of the southern Great Barrier Reef region. However, the gap between the two surveys (17°25’S to 17°45’S) was only 35km so that we surveyed 87% of the planned survey area between Cooktown and the northern boundary of the 1999 survey. The number of dugongs sighted south of Cooktown was too low to calculate a population estimate for this region, a result similar to those obtained in 1987 and 1992. This inability to calculate a population estimate for this region made it impossible to estimate a sustainable level of human-induced mortality. Marine Park zoning currently provides little protection of dugong habitats in the region between Cape Bedford and Innisfail. 2 Options For Management Northern Great Barrier Reef Region Optimisation and Recognition of Measures to Protect Dugongs We suggest that consideration be given to extending the seaward boundary of the highly protected zones between Cape Melville and Lookout Point and the northern boundary of the National Park Zone around Friendly Point when this region is rezoned again as part of the Representative Area Program. We suggest that the Queensland Government should introduce zoning above mean low water mark (for which it has jurisdication) which compliments the zones in the Far Northern Section of the Great Barrier Reef Marine Park. The introduction of such zoning is essential to adequately protect of dugong and dugong habitats in the Northern Great Barrier Reef Region. We suggest that Great Barrier Reef Marine Park Authority (GBRMPA) and the Queensland Parks and Wildlife Service (QPWS) negotiate with the Queensland Fisheries Service and local Aboriginal communities with a view to improving the protection of dugongs from drowning in mesh nets set in the intertidal reaches of rivers and creeks and in coastal intertidal areas. We suggest that, in view of the generally high level of protection given to dugongs in the northern GBRMP, especially under the Far North Section Zoning Plan, GBRMPA consider producing public education material outlining the protection given to dugongs throughout the GBR World Heritage Area. To date such material has featured the southern GBR region. Cape Bedford to 17°25’S Protected Areas for Dugong Conservation We suggest that GBRMPA should consider the following through the Representative Areas Program: (1) improving the protection of seagrass habitats in this region; (2) establishing a highly protected area in the Port Douglas region. Management of Dugong Mortality In view of the low numbers of dugongs sighted in this region, all sources of dugong mortality should be minimised. In particular, we suggest that GBRMPA and QPWS negotiate with Yarrabah community with a view to developing protocols for minimising the risk of capturing dugongs in the mesh net fishery operated by the community in adjacent Mission Bay. 3 INTRODUCTION The waters of northern Australia are internationally recognised as the stronghold of the dugong (Dugong dugon). As the only surviving member of the family Dugongidae (Marsh et al. 1999), the dugong is a species of high biodiversity value. The dugong is listed as vulnerable to extinction by the IUCN (2000), along with the other three species in the order Sirenia, the manatees (family Trichechidae). Anecdotal evidence suggests that dugong numbers have decreased dramatically throughout most of their range (Marsh et al. 2002), but significant populations persist in Australian waters, which are now believed to support most of the world's dugongs. In Australia, dugongs occur along much of the coast from Shark Bay in Western Australia to Moreton Bay in Queensland. Consequently, Australia has an international obligation to ensure their conservation (Bertram 1981). In addition, dugongs are specifically cited as one of the World Heritage values of the Great Barrier Reef World Heritage Area (GBRMPA 1981). Much of the information used to manage dugong populations in Australia has been provided by aerial surveys using the standard techniques developed by Marsh and Sinclair (1989a; 1989b). The northern Great Barrier Reef (GBR) region between Hunter Point (11°15’S — south of Cape York) and Cape Bedford (15°30’S — near Cooktown) was surveyed using this technique in 1985, 1990 and 1995. The aim of these surveys was to provide a temporal series of information on the distribution and relative abundance of dugongs in a region where Indigenous hunting and incidental capture in commercial fishing nets are considered to be the major impacts (Marsh et al. 2002). In this report, we present the results of the aerial surveys conducted in 2000, five years after the last survey, to again assess the status of the dugong in the northern GBR region. The results indicate that overall, dugong numbers in the region are stable. However, the dugongs were distributed differently from previous surveys with more animals in the inshore region south of Cape Melville and fewer animals in the Princess Charlotte Bay region, providing further evidence that dugongs undertake large-scale movements. The survey was also extended south from Cape Bedford to 17°25’S, between Cairns and Innisfail. Poor weather had prevented us from surveying this region as scheduled in 1999 as part of a survey of the southern Great Barrier Reef. Unsuitable weather once again prevented us extending this survey to be continuous with the 1999 survey of the southern Great Barrier Reef region. However, the gap between the two surveys was only 35km. The number of dugongs seen south of Cooktown was too low to calculate a population estimate for this region, a result similar to that obtained in 1987 and 1992. 4 METHODS Initially we had intended to survey the entire northern Great Barrier Reef Region (referred to hereafter as the northern GBR) and to extend the survey coverage southwards to at least 17°45’S to overlap with the northern part of the southern GBR survey, conducted in 1999. We were able to complete the northern GBR survey to as far north as transect 506 (11°35’S), rather than the planned transect 508 (11°30’S). The survey coverage extended as far south as the Innisfail region (17°25’S), which was 35km north of the limit of the 1999 survey. Poor weather prevented us completing the intended survey coverage, therefore we ended up surveying approximately 96.8% of the intended survey area in the northern GBR; 87% of the planned survey area south of Cooktown. Survey Methodology The surveys were conducted in November 2000. The aerial survey method used was the strip transect technique detailed in Marsh and Sinclair (1989a; 1989b), which involved flying twin-engine Partenavia B aircraft fitted with a GPS at a speed of 100 knots, and a height of 137 m Above Sea Level. We did not use line transect methodology for two reasons: (1) we wished to retain the same methodology as had been used in the other surveys in the temporal series; (2) we had been advised to retain the strip transect methodology in a review of dugong survey methodology conducted by Professor Ken Pollock and other experts in 1997. Transects were flown in an east-west direction as this reduces the interference of glare with the observations. The transect positions and lengths were modelled on previous surveys of the region (northern GBR in 1995 Marsh & Corkeron 1996; southern GBR in 1992 Marsh et al. 1996) (see Figure 1 for details of transect and block positions). Transects 200 m wide on the water surface were demarcated for the observers using fibreglass rods attached to artificial wing struts on each side of the aircraft. Each transect was divided into four longitudinal sub-transects of equal width. Tandem teams comprising two observers on each side of the aircraft recorded their sightings independently onto separate tracks of an audio-tape. These independent sightings were then used to develop survey specific correction factors (see later section). Each sighting was designated as being made in one of the longitudinal sub-transects within its transect to enable us to decide if simultaneous sightings by the members of the same group of tandem observers were of the same group of animals. Other large marine vertebrates (especially sea turtles and cetaceans) were also recorded during the survey and will form the subject of a second report. As the window of opportunity of suitable weather for aerial survey is small, we used two aircraft flying concurrently with separate teams of observers. One team (in an aircraft equipped with a pressure altimeter) surveyed the region from Princess Charlotte Bay northwards, while the other surveyed southwards from Cape Melville to Cape Bedford (in an aircraft equipped with a radar altimeter). The region north of Princess Charlotte Bay 5