Thursday, March 15, 2012

Pinnipeds and Australian Sea Lion

This article focuses on pinnipeds in the South‐west Marine Region, specifically the family Otariidae, which contains the fur seals and sea lions. Pinnipeds are aquatic carnivorous mammals, with a streamlined body specialised for swimming and limbs modified as flippers. Of the 10 species of seals and sea lions that occur in Australian waters (including the Australian Antarctic Territory, Heard Island and MacDonald Island), three are known to occur in the Southwest Marine Region: Australian sea lion (Neophoca cinerea), New Zealand fur seal (Arctocephalus forsteri) and Australian fur seal (Arctocephalus pusillus). Another seven species have been recorded in the region and are considered vagrant species. Attachment 1 lists the pinnipeds that are known to occur in the region and those that may occur infrequently.

Pinnipeds tend to be highly mobile and opportunistic predators that utilise a wide range of benthic and pelagic foraging habitats, and base their foraging strategies on prior experience and situational decision‐making (Baylis, Page & Goldsworthy 2008; Staniland, Boyd & Reid 2007). They forage in shallow coastal waters, across the continental shelf and in oceanic waters beyond the shelf edge. All pinnipeds found in Australian waters are protected under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) as listed marine species. Australian sea lions are also listed as a threatened species in the vulnerable category and, as such, are a matter of national environmental significance.

Australian Sea Lion
Australian sea lions are endemic to Australia and occur in coastal habitats, waters and islands offshore from South Australia and Western Australia. The species is almost entirely confined to the South‐west Marine Region and adjacent state waters, islands and coastal areas. Although its range extends to the Houtman Abrolhos Islands in Western Australia, most of the population is found in South Australia. Australian sea lions have an estimated population of approximately 14 700 individuals (DEWHA 2010a). Based on pup numbers, 86 per cent of the population (3105 pups) is found in South Australia, and 14 per cent (503 pups) is in Western Australia. Australian sea lion populations have not recovered from past hunting, and it appears that populations at some breeding sites may be in decline (DEWHA 2010a).

Australian sea lions are unique among pinnipeds in having large numbers of small breeding colonies, low reproductive rates, an unusually long breeding cycle of 17–18 months (the timing varies between breeding sites that are close to each other different colonies breed at different times), high site fidelity and poor dispersal. Genetic research into the population structure of Australian sea lions has found evidence of a strong sex bias in dispersal, with females breeding at the colony where they were born (natal site fidelity) and males dispersing between colonies over a range of 200 km. This is highly unusual for pinnipeds (DEWHA 2010a). These findings indicate that at least some Australian sea lions are genetically isolated, and that recolonisation may not occur readily if colonies are extirpated (entirely removed) (DEWHA 2010a).

Australian sea lions feed on the continental shelf in the region, most commonly in depths of 20–100 m (Shaughnessey 1999). They appear to be mainly benthic foragers (Goldsworthy, Campbell & McKenzie 2006) and eat a wide variety of prey, including fish, small sharks, invertebrates (such as rock lobster), cephalopods (octopus, squid, cuttlefish and nautilus) and occasionally seabirds. Recent studies (e.g. Campbell 2008; Costa & Gales 2003; Goldsworthy, Peters & Page 2007) using satellite tracking and time depth recording techniques at a number of sites have provided detailed information about the areas where Australian sea lions forage.

Generalisations are difficult because different classes of animals and animals of different colonies vary considerably in their foraging and diving behaviour. A study at Dangerous Reef and islands in the Nuyts Archipelago (Goldsworthy et al. 2009) showed that females and sub‐adult males spend about one day foraging at sea, followed by a day of rest, whereas adult males typically spend longer at sea up to 2.5 days per trip. The study showed that sea lions consistently dive to the ocean floor, with the deepest dives at just over 100 m, suggesting that they are benthic foragers.

The species ‘hauls out’ (or rests) and breeds on rocks and sandy beaches on the sheltered sides of islands, although there are some small colonies on the Australian mainland. As detailed in the technical issues paper to the Australian Sea Lion Recovery Plan, breeding colonies are generally defined as sites where pups have been recorded, and haul‐out sites are defined as sites frequented by seals where pups are not born (DEWHA 2010a). Detailed information on methods for estimating the abundance of colonies is in this publication (DEWHA 2010a).

Australian sea lion pups have been reported at 76 sites (48 in South Australia and 28 in Western Australia) over the past 20 years. However, only 58 of these sites have enough pups recorded to be classified as breeding sites (DEWHA 2010a). Most of the known Australian sea lion colonies (42 colonies, 62 per cent) are small, producing fewer than 25 pups per breeding season. Eight breeding colonies produce more than 100 pups each year, and these are all in South Australia (Goldsworthy, Hamer & Page 2007):

• Dangerous Reef and Lewis Island (Southern Eyre Peninsula)

• North and South Page islands (which are adjacent to the South‐west Marine Region)

• West Waldegrave Island (Western Eyre Peninsula)

• Seal Bay (Kangaroo Island)

• Purdie Island (Nuyts Archipelago)

• Olive Island (Western Eyre Peninsula).

Because of the closed breeding patterns of Australian sea lions and their conservation status as a threatened species, all breeding sites for Australian sea lions are considered significant.


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