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Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland
Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995
ISBN 0 642 17399 0
H. Marsh1, P.J. Corkeron2, C.J. Limpus3, P.D. Shaughnessy4 & T.M. Ward1.
There is considerable concern for the status of many marine mammals and reptiles. The giant Steller's sea cow, Hydrodamalis stelleri, was exterminated from the north Pacific by hunters in the 18th century. Of the seven extant species of sea turtles, five are listed by the International Union for the Conservation of Nature (1990) as endangered and one as vulnerable to extinction. Although humans have not caused the extinction of any species of whale or dolphin, some are now severely threatened by human activity ( Cooke 1991).
All major groups of marine mammals and reptiles are represented in Australia. There are over 30 species of sea snakes, six species of sea turtles, one species of sirenian or sea cow, three species of fur seals and sealions, and more than 40 species of whales and dolphins. This paper, which is adapted from Marsh, Corkeron, Limpus, Shaughnessy and Ward (1994), reviews the biology and management of the marine mammals and reptiles of Australia as a basis for an assessment of their status.
The saltwater crocodile, Crocodylus porosus, is not included in this review. It is found in estuaries, large streams, lakes, and swamps as well as marine environments. We regard it more appropriate to consider the status of this species in the context of a review of the terrestrial environment.
Most sea snakes are venomous, benthic piscivores. All species are viviparous (ie bear live young) and display numerous adaptations to the marine environment (eg sublingual salt gland, paddle tail, nasal valves). However, hydrophids, aipysurids and P. platurus also
display significant differences in morphology (eg ventral scales), ecology (eg habitat) and biogeography, and may not be as closely related as was assumed by Smith (1926).
Sea snakes (Hydrophiidae) and kraits (Laticaudidae) are restricted to the Indian and Pacific oceans ( Heatwole 1987). Most species occur in the warm, shallow seas of the IndoMalaysian Archipelago, northern Australia and Oceania. Three groups within the Hydrophiidae (hydrophiids, aipysurids and Pelamis platurus) are commonly found on the northern Australian continental shelf (McDowell 1972, Marsh et al. 1994). No breeding population of laticaudids (McDowell 1967) has been found in Australian waters, but these snakes occasionally strand on the north-east Queensland coast.
Five genera of hydrophiids (Acalyptophis, Astrotia, Disteira, Enhydrina, Hydrophis) inhabit the northern Australian continental shelf. Hydrophiids have narrow ventral scales and typically use soft bottom (interreefal) habitats. Most species are specialist piscivores, feeding on eels (eg Gymnothorax species) and slender fishes such as gobies.
The two genera of aipysurids, Aipysurus and Emydocephalus, are characterised by their wide ventral scales. They are typically associated with coral reefs ( Cogger 1989). The seven known species of Aipysurus are distributed between the Sahul Shelf and the Gulf of Carpentaria, and five of them are endemic to the northern Australian continental shelf. Most aipysurids are generalist piscivores. However, the olive sea snake, A. laevis, also eats cephalopods, crustaceans and fisheggs; and the teeth and venom apparatus of A. eydouxii and the endemic Emydocephalus annulatus are substantially reduced as these species feed exclusively on fish eggs (McCarthy 1987).
The yellowbellied sea snake, P. platurus, differs from other sea snakes in not being part of a benthic community. It is a pelagic species most commonly found in groups associated with `drift lines', where it feeds on surface fish such as mullet, trevallies and anchovies ( Kropach 1975). Pelamis platurus inhabits tropical and subtropical seas from the western Indian Ocean to the eastern Pacific Ocean. Individuals are occasionally found stranded on beaches throughout Australia (see Cogger 1975, Guinea 1981).
Sea snakes are smaller than other marine reptiles. The heaviest sea snake is Astrotia stokesii which can weigh more than 2 kg. The longest sea snake inhabiting Australian waters is Hydrophis elegans which can reach almost 3 m.
Olive sea snakes (A. laevis) from the Keppel Islands and Swain Reefs are estimated to reach sexual maturity at 3 years of age (males) and 4-5 years (females) and live for at least 10 years (Burns 1984, 1985). Recent data indicate that the hydrophiid sea snake, Lapemis hardwickii, may have a much longer lifespan.
Sea snakes tend to have smaller broods and larger offspring than have terrestrial snakes of a similar size ( Lemen & Voris 1981, Shine 1988). The mean number of offspring in 15 broods of A. laevis from the southern Great Barrier Reef was 2.6 ( Burns 1984). Brood sizes for P. platurus range from about two to six (see Visser 1967, Pickwell 1972, Kropach 1975).
Population sizes and trends
Inter-reefal sea snakes (hydrophiids) are probably carried large distances by currents. Populations of interreefal sea snakes are not amenable to mark recapture studies because of the low rates of recapture (see Heatwole & Burns 1987). Demersal trawls provide estimates of relative abundance, but difficulties in estimating depth:time budgets and rates of net avoidance/escape complicate the calculations of population density and size.
The reefal (site attached) habits of aipysurids make them amenable to mark recapture studies. Marking techniques include freeze branding and removal of subcaudal scales ( Burns 1984). Because of logistical difficulties associated with sampling the enormous expanse of ocean through which P. platurus travels, the development of reliable estimates of the population sizes of this species is restricted.
There are few records of seasonal movements by hydrophiid sea snakes. The possibility of an inshore migration during the summer (wet season) was suggested by Shuntov (1971). Seasonal variation in the species composition of the sea snake bycatch of prawn trawlers was documented by Heatwole and Burns (1987). Lapemis hardwickii in the Gulf of Carpentaria were caught more frequently by prawn trawlers in shallow, inshore waters during spring, and in deeper waters, further offshore, during autumn (Wassenburg et al. 1994).
Markrecapture, sonic tracking and visual mapping studies of olive sea snakes (A. laevis) from the Swain Reefs indicated that these snakes generally confine their movements to particular sections of reef (home ranges of about 0.18 ha for females and 0.15 ha for males).
Surface currents carry individuals of P. platurus over large distances. These probably account for the wide distribution of the species ( Kropach 1975).
The major impact on the habitats and populations of interreefal (hydrophiid) sea snakes on the northern Australian continental shelf is prawn trawling. Between 10% and 42% of sea snakes caught in prawn trawls die. The proportion killed is correlated with duration of the trawl ( Heatwole & Burns 1987). Products made from sea snakes obtained from prawn trawlers (such as wallets, belts and bikinis) have been sold in markets and souvenir shops throughout northern Australia for at least 20 years.
The Queensland Department of Primary Industries has issued three licences for the collection of 10 000 sea snakes per annum from prawn trawlers on the east coast of Queensland (two licences) and the Gulf of Carpentaria (one). This harvest includes several species and its sustainability has not been assessed.
No information is available.
Sea snake products have been traded internationally since the 1930s but there are no data suggesting that any species of sea snake is endangered or threatened and no species is currently listed by CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora). The Commonwealth's Wildlife Protection (Regulation of Exports and Imports) Act 1982 was invoked to provide legislative support for CITES. This Act restricts the export of sea snake products from Australia (eg by tourists).
Reefal habitats in northern Australia are generally well protected and there are few impacts on Australian populations of aipysurids. Some olive sea snakes have been illegally collected from reefs in the Great Barrier Reef Marine Park for use in the aquarium trade (unpublished data). The low population densities and fecundity of aipysurid (reefal) species make them unsuitable for commercial harvest. Conservation of reef species should continue to be conducted in the context of largescale habitat management (eg Great Barrier Reef Marine Park).
Except for the pelagic P. platurus, the shallow-water habitat (less than 100 m depth) of sea snakes impedes their crossing of international boundaries. Thus they can be managed within national conservation strategies. Responsibility for managing the sea snakes of the northern Australian continental shelf is shared between six State and two Commonwealth fisheries management and wildlife conservation departments. The development of national management plans for the commercial use of Australian hydrophiids caught in prawn trawls should be a high priority.
Of the world's seven species of sea turtle, six occur in Australia. They represent all extant families and genera:
Dermochelys coriacea, leatherback turtle
Caretta caretta, loggerhead turtle
Chelonia mydas, green turtle
Eretmochelys imbricata, hawksbill turtle
Lepidochelys olivacea, olive ridley turtle
Natator depressus, flatback turtle
Leatherback, loggerhead, green and hawksbill turtles have pantropical distributions. The olive ridley is widely distributed in the tropical and subtropical IndoPacific. Unlike the other species, the flatback turtle has a limited distribution and is effectively endemic to Australia (Walker & Parmenter 1990). It has not been recorded nesting outside Australian territorial waters, and its feeding grounds extend only to the Indonesian Archipelago and the Papuan coast.
Sea turtles are amongst the largest extant reptiles. Breeding adults range in size from approximately 40 kg for olive ridley turtles to over 300 kg for leatherback turtles. Sea turtles are exceptionally long lived and breed only occasionally. For example, it is estimated that, on average, female loggerhead turtles migrate to breed on about five occasions at intervals of several years ( Limpus et al. 1992).
All sea turtles have a similar life history ( Carr 1968, Bustard 1972). They spend most of their life at sea but return to land to lay eggs on traditional nesting beaches. Fecundity varies with species and ranges between about 50 eggs per clutch in flatback turtles to 130 per clutch in hawksbill turtles. There is no parental care of the eggs or young and sex is determined by the temperature at which the eggs are incubated ( Limpus, Reed & Miller 1985).
The hatchlings of all species have a pelagic dispersal phase mediated through ocean currents and lasting for several years. Leatherback turtles remain pelagic; they feed on jellyfish. The other species of sea turtles recruit to shallow benthic feeding areas after they reach 35 cm or more in curved carapace length (more than 6 kg). They spend the rest of their life in that habitat except for intermittent breeding migrations. Their diet varies with species ( Bjorndal 1982): green turtles eat algae and seagrasses; loggerhead turtles molluscs and crabs; and hawksbill turtles sponges and algae.
The absolute size of some sea turtle breeding units is in the tens of thousands to hundreds of thousands. For example, the olive ridley turtle forms massive breeding aggregations of up to 150 000 individuals ( Harris 1994). An estimated 11 500 green turtles were recorded nesting at Raine Island on a single night in 1984 (C.J. Limpus, unpublished data).
Population models ( Crouse, Crowder & Caswell 1987) have shown that sea turtles will not withstand significant increases in mortality above natural levels. Crouse, Crowder and Caswell found that the most sensitive life history parameter is the survivorship of large prereproductive and adult females, the very size/sex classes targeted for harvest.
The status of most sea turtle breeding units is difficult to assess for the following reasons:
Despite their apparent abundance, declining populations of green, loggerhead, hawksbill and leatherback turtles (Figures 1 and 2) have been reported from Australia (Pritchard 1969, Hirth 1971, Marquez, Villanueva & Penaflores 1976, Witzell 1983, Dodd Jr 1988, Groombridge & Luxmoore 1989, Limpus & Reimer 1991).
Breeding migrations are typically for hundreds of kilometres (km) and may exceed 3000 km ( Bjorndal 1982). Tag resightings and recoveries of 118 loggerhead and 273 green turtles from eastern Queensland rookeries have documented migrations between nesting beaches and feeding areas of between 8 km (green turtle) and more than 2600 km (green and loggerhead turtles). Both of these species are capable of high-speed sustained migration in excess of 30 km per day for several weeks. They demonstrate fidelity in their choice of both feeding and nesting areas ( Limpus et al. 1992). Recent genetic studies support the hypothesis that turtles home to their natal beach at least at the regional level ( Gyuris & Limpus 1988, Meylan, Bowen & Avise 1990). Hatchlings from rookery regions within eastern Australia colonise feeding areas that encompass regions spanning 10-30o in latitude and 30-37o in longitude. Thus turtles from widely separated rookeries co-habit the same feeding area ( Limpus et al. 1992).
Figure 1: Population decline as evidenced by the trend in the number of nesting loggerhead turtles recorded during the annual tagging census conducted during the peak of the nesting season (last two weeks of December at Wreck Island in the southern Great Barrier Reef since 1977. Wreck Island is the most significant breeding site for this species in the south-west Pacific.
Impacts on the sea turtle populations which feed or breed in Australian waters are widespread and varied but generally unquantified ( Bjorndal 1982, Groombridge & Luxmoore 1989, Limpus et al. 1989). The eggs of all species are harvested at nesting beaches. The green turtle is favoured for its meat and is the species generally targeted by traditional and commercial hunters, although loggerhead ( Limpus et al. 1992) and hawksbill turtles are also eaten in some areas. Vitelogenic females (those preparing to breed) are favoured because they are fattest. Kwan (1991) monitored the harvest of turtles between 1984 and 1987 by the inhabitants of Daru Island off the Papuan coast. He found that adult female green turtles comprised up to 83% of the catch. Most turtles were caught coincident with the migration of green turtles through Torres Strait to rookeries in eastern Australia. The greatest threat to the hawksbill turtle is the tortoiseshell trade. The present rate of harvest of green and hawksbill turtles throughout the IndoPacific region appears unsustainable.
Figure 2: Top: Known rookeries of each species of sea turtle in Australia and Oceania at the beginning of western colonisation. Note the large number of rookeries (x) at which significant declines in the number of nesting females has occurred and/or where major harvests of eggs and adults are continuing. Bottom: Known home ranges, based on recoveries or tagged females while nesting; of green turtles that breed at the rookeries at the northern and southern ends of the Great Barrier Reef region ; of green turtles that breed in French Polynesia; and of flatback turtles that breed in the southern Great Barrier Reef region.
Some sea turtles drown when they are caught in trawl gear. The flatback turtle is the main species caught in prawn trawls in northern Australia (43%), followed by loggerhead (19%), olive ridley (15) and green (4%) turtles (Poiner, Buckworth & Harris 1990). The rate of mortality varies with the duration and depth of the trawl. An estimated 4114 ± 1369 turtles are estimated to have been caught in the Australian northern prawn fishery in 1988, of which an estimated 247 ± 90 drowned (Poiner, Buckworth & Harris 1990). Although Poiner, Buckworth and Harris concluded that the impact of trawling on sea turtle populations in the northern prawn fishery is not of immediate concern, their view is challenged by some other scientists such as Parmenter (1994), who considered that the mortality rate from fishing has been underestimated. Turtles are also drowned in gillnets and shark nets set for bather protection (Paterson 1990), but the extent of the mortality in gillnets is largely unquantified. Other causes of indirect and direct anthropogenic mortality include: feral fox predation on sea turtles nests on mainland beaches in northern Australia; destruction of nesting beaches by urban developments; and turtles being killed as a result of collisions with boats. The unintentional killing of turtles and their eggs as a result of human activities in the Australian region is threatening the survival of regional populations of loggerhead and olive ridley turtles.
Genetic studies by Bowen, Meylan & Avise (1989) and Norman et al. (1994) demonstrated that the major sea turtle breeding aggregations are essentially genetically discrete and therefore should be regarded as separate breeding units (stocks). For example, green turtles that breed in the Gulf of Carpentaria, northern Great Barrier Reef and southern Great Barrier Reef are separate stocks. Not all the members of an individual breeding unit follow the same migratory paths. Turtles from different breeding units can be mixed in the one feeding area; eg green turtles breeding at rookeries in Java, Western Australia or the northern Great Barrier Reef have been caught in the one feeding area in northwestern Arnhem land ( Limpus et al. 1992).
All species of sea turtle are listed on Appendices I and II of CITES. The International Union for the Conservation of Nature (1990) lists the leatherback, green, hawksbill and olive ridley turtles as endangered and the loggerhead as vulnerable. Kennedy (1990) listed flatback turtles as potentially vulnerable. All species of sea turtles occurring in Australian waters - except the flatback turtle - are listed under the Commonwealth's Endangered Species Protection Act: the loggerhead turtle as endangered the others as vulnerable.
All sea turtles are protected in all States of Australia, although State and Commonwealth laws allow Aborigines and Torres Strait Islanders to hunt them subject to various restrictions. Management practices in neighbouring countries frequented by sea turtles are more limited. Indonesia protects selected species (loggerhead, olive ridley and leatherback turtles); Papua New Guinea has some protected areas and bans export; the Solomon Islands protect certain size classes; and Fiji has a closed season. As a shared international resource, sea turtles are particularly relevant for inclusion in international agreements addressing the conservation of migratory species. Such an agreement between the countries bordering the Arafura and Coral seas (Australia, Indonesia, Papua New Guinea, Solomon Islands, Vanuatu and New Caledonia) is urgently required.
The dugong, Dugong dugon, the only strictly marine herbivorous mammal, is the only member of the mammalian Order Sirenia (sea cows) present in Australia. Its range (Figure 3) extends throughout tropical and subtropical coastal and island waters from east Africa to the Solomon Islands and Vanuatu, and between about 26o and 27o north and south of the equator (Nishiwaki & Marsh 1985). Over much of this range, dugongs are believed to have been reduced to relict populations. These populations are separated by large areas where dugongs are close to extinction or are extinct. This assessment is, however, almost entirely based on anecdotal information and the actual extent to which their range has contracted is unknown. Australia appears to be the dugong's stronghold, as a significant proportion of the dugong stock is believed to be present in northern Australian waters between Moreton Bay (Queensland) and Shark Bay (Western Australia).
Figure 3: The distribution of dugong in south-east Asia, Australia and Oceania.
Adult dugongs reach a length of 3 m and weigh up to about 420 kg (Spain & Heinsohn, 1975). Individuals may live for 70 years or more, but a female does not have her first calf until she is at least 10 years old, and then only bears a single calf every three to five years after a gestation period of about 13 months (Marsh 1980, Marsh, Heinsohn & Channells 1984, Marsh, Heinsohn & Glover 1984, Marsh, Heinson & Marsh 1984, Marsh 1986). Population simulations (Marsh 1986) indicate that dugong numbers are unlikely to increase at more than about 5% each year, even if all the females in a population are breeding to the maximum.
The only quantitative information on dugong population size comes from dedicated aerial surveys. Such surveys conducted in Australia indicate that dugongs are the most abundant marine mammal in the inshore waters of northern Australia (Marsh, unpublished data). Even though not all areas of suitable habitat have been surveyed, the population estimates sum to more than 80,000 ( Bayliss 1986, Marsh & Saalfeld 1989b, 1990, Bayliss & Freeland 1989, Marsh, Saalfeld & Preem 1990, Marsh, Saalfeld & Prince 1991, Marsh et al. 1994, Marsh & Lawler 1992).
The major problem in determining the status of the dugong is the difficulty of detecting trends in abundance. Marsh and Saalfeld (1989b) considered a hypothetical situation in which the estimated 8000 dugongs in the northern Great Barrier Reef region were declining at 5% a year. Using power analysis, they calculated that it would take at least 10 years of annual surveys before it could be determined within the usual limits of statistical error whether the population was declining or not. By that stage, numbers could have declined to about two thirds of their value at the time of the first survey. The difficulties of detecting such a trend at a more localised level are much greater than this. Using present techniques, it is impossible to detect trends at spatial and temporal scales which are useful to management.
Marsh and Rathbun (1990) and Preen (1993) studied dugong movements and habitat preferences using satellite radio-tracking techniques. They found that most movements of the 18 dugongs fitted with satellite transmitters were local and in the vicinity of seagrass beds. However, one individual moved between two localities a straight line distance of 140 km apart three times in six weeks, indicating that the management of dugongs is an international matter in areas such as Torres Strait.
Humans have been hunting or netting dugongs for thousands of years, mainly for their meat and/or oil. Dugongs are still killed for food by Aborigines in many parts of northern Australia. Contemporary hunting statistics are almost nonexistent. Harris et al. (1994) estimated that the 1991 dugong catch in the Australian parts of Torres Strait was approximately 1000 dugongs. It is not known whether this harvest is sustainable (Marsh & Lawler 1992). In contrast to the estimated Torres Strait harvest, the estimated annual catch of dugongs by members of the two main Aboriginal communities on the east coast of Cape York was less than 100 (Smith and Marsh 1990) - much less than the estimated sustainable yield.
An unknown number of dugongs are killed incidentally in commercial gillnets in northern Australia (Marsh 1988). A total of 576 dugongs were killed in shark nets set for bather protection in Queensland between 1964 and 1988 (Paterson 1990).
Most of the dugong's habitat in Australia is remote from human settlements. Loss of seagrass beds because of coastal residential, industrial and tourist developments is a potential threat along the eastern Queensland coast south of Cooktown (Morissette 1991). In 1992, more than 1000 km2 of seagrass was lost from Hervey Bay - approximately 25% of the total known area of seagrass along the eastern Queensland coast - as a result of two floods and a cyclone (Preen, Lee Long & Coles, unpublished). Consequently, the regional dugong population dropped from an estimated 2206 (±420) in August 1988 (before the die off) to 600 (±126) in November 1993 (21 months after the floods; Preen & Marsh, submitted). At least 99 dugong carcasses were recovered from Hervey Bay and surrounding areas following the seagrass loss, the peak of mortality occurring 68 months after the floods (Preen and Marsh, unpublished data). The subsequent decline in dugong population was probably due to emigration. Some dugongs moved at least 900 km from Hervey Bay (Preen & Marsh, unpublished data).
The only information available on dugong stock identity in Australia is a morphometric study by Spain and Marsh (1981). They demonstrated that the skulls of dugongs from Mornington Island in the Gulf of Carpentaria can be statistically distinguished from those from Townsville on the east coast, approximately 1850 km away by sea.
The dugong is listed as vulnerable to extinction in the IUCN red list of threatened animals ( International Union for the Conservation of Nature 1990). Trade in dugong products is either regulated or banned (depending on the dugong population involved) by CITES.
In Australia, relevant State and Commonwealth laws allow Aborigines and Torres Strait Islanders to hunt dugongs subject to various restrictions. The management of dugongs in Torres Strait is an international matter for Australia and Papua New Guinea and dugong hunting is a nominated fishery under the Torres Strait Treaty for bilateral management.