The Australian Sea-lion has a blunt snout, with small tightly rolled external ears. Males have dark blackish to chocolate brown fur with a whitish crown of the head and nape of the neck, whilst females are more silvery-grey above and yellow to cream below. Males can become very large, 185–225 cm in length and weighing 180–250 kg. Females are smaller, 130–185 cm in length and weighing 65–100 kg. Pups are chocolate brown in color with a pale fawn crown until they molt at about two months of age. After molting, a juvenile’s coat is similar to that of an adult female (Van Dyck & Strahan 2008).
The Australian Sea-lion is the only pinniped endemic to Australia (Strahan 1983). The breeding range extends from Houtman Abrolhos, Western Australia (WA), to The Pages Island, east of Kangaroo Island, South Australia (SA). The species has also been recorded at Shark Bay, WA; the New South Wales coast; southern Tasmania; and Victoria (Kirkwood et al. 1992, 1999; Ling 1992; Llewellyn et al. 1994; Warneke 1995b).
Breeding colonies occur on islands or remote sections of coastline. Lone or small numbers of animals will regularly visit known haul-out sites and occasionally visit other locations. The widespread distribution of small colonies may offer the advantage of minimising competition in areas for limited trophic resources (Shaughnessy 1999). Overall, 66 breeding colonies have been recorded to date: 28 in WA and 38 in SA (Shaughnessy 1999).
The Australian Sea-lion exhibits high site fidelity and little movement of females between colonies has been observed. There is little or no interchange of females between breeding colonies, even between those separated by short distances (Campbell et al. 2008). Also, it has been suggested that each breeding colony on the west coast of Australia could be considered a distinct management unit due to the low gene flow between even quite closely located colonies (Campbell 2003, 2005). Site fidelity has implications to the risk of local extinction, especially at sites with low population numbers.
Historical records indicate that the former range of the species extended to Bass Strait, particularly Clarke Island and adjacent islands in the Furneaux Group (Warneke 1982). The small population on the Abrolhos Island off the west coast of WA is thought to have been more extensive before the arrival of Europeans. Also, the north and east coasts of Kangaroo Island, SA, and islands near Perth and Albany, WA, previously had breeding sites (Flinders 1814; Gales et al. 1994).
Surveys of known breeding sites of the Australian Sea-lion between 1987–1995 by Gales and colleagues (1994) and Dennis and Shaughnessy (1996) estimate an overall population of between 9900 to 12 500 animals with a mean of 11 200. Of these, 2590 were pups. The estimated population numbers make the Australian Sea-lion the rarest pinniped in the world (Campbell et al. 2008). About 30% of the population occurs at sites in WA and 70% in SA. Forty-two percent of the total known population occurs at the three largest colonies east of Port Lincoln, at the eastern limit of the species known range (Gales et al. 1994).
The Australian Sea-lion is neither increasing in population numbers or expanding its range (DAFF 2007b). Due to the species long breeding cycle (17.6 months) the time required to increase population size is longer than for species with shorter breeding cycles (Orsini & Newsome 2005). In addition, there is evidence of a reduction in the numbers of breeding sites along the greater Perth metropolitan coastline area (Campbell 2005), and along Victorian annd Tasmanian coastline (Campbell et al. 2008).
An analysis of pup production at the Seal Bay colony on Kangaroo Island, SA, indicates a rate of decrease of 0.77% per year (12% decline between 1985–2003) (Shaughnessy et al. 2006). There is also evidence of declining pup production at some of the smaller colonies in WA and SA (Shaughnessy et al. 2005), though the extent of decline is unknown. It is recorded that at 60% of the breeding sites, fewer than 25 pups are produced annually. This figure is indicative of the historical subpopulation declines (DEWHA 2010p).
Smaller populations are highly vulnerable to extinction especially in the context of loss to fisheries bycatch and the high site fidelity of females (Goldsworthy et al. 2010).
Australian Sea-lions use a wide variety of habitats (Gales et al.1994) for breeding sites (called rookeries) and, during the non-breeding season, for haul-out sites (rest stops, which are also useful for predator avoidance, thermal regulation and social activity) (Campbell 2005). Onshore habitats used include exposed islands and reefs, rocky terrain, sandy beaches and vegetated fore dunes and swales. They also use caves and deep cliff overhangs as haul-out sites or breeding habitat (Dennis & Shaughnessy 1996, 1999).
Most colonies occur on islands, however several small colonies occur on the mainland, including:
- Point Labatt, SA (King & Marlow 1979)
- Baxter Cliffs, west of Twilight Cove, WA (referred to as Thundulda by Warneke 1982)
- Bunda Cliffs, Great Australian Bight, SA and WA borders (nine small breeding colonies discovered at the base) (Dennis & Shaughnessy 1996).
Australian Sea-lions prefers the sheltered side of islands and avoids exposed rocky headlands that are preferred by the New Zealand Fur Seal (Arctocephalus forsteri). Islands used on the southern coast of WA and SA are comprised either of igneous or metamorphic rock, or of igneous platforms below limestone caps. An important feature of colony sites used for breeding are shallow, protected pools in which pups congregate. On the west coast of WA they breed on low-lying limestone islands which are well protected by perimeter reefs (Gales et al. 1994).
Shelter, in the form of holes in rock or vegetation, is important for adult females to hide their pups. Bushes such as Nitraria schoberi are preferred if available (Gales et al. 1994). However, the largest colonies (Dangerous Reef and The Pages Island) occur where there is little protection available and where most pups are born on open ground.
Over much of the Australian Sea-lion’s range, the marine environment is characterized by shallow on-shelf waters (less than 200 m deep) of low productivity. It is primarily influenced by the Leeuwin Current which feeds warm, nutrient impoverished waters southwards along the west coast of Australia and then eastward along the south coast. This current acts as a barrier to the rich subantarctic waters and the region has been described as one of the most nutrient-poor marine environments in the world (Gales et al. 1994). During winter, the prevailing winds along southern Australia are westerly and enhances the strength of the Leeuwin Current, allowing it to reach its eastern extremity. During summer, the high-pressure weather systems that dominate the southern coast of Australia cause consistent southeast winds that have the effect of blocking, and in some cases reversing, the flow of the eastward moving Leeuwin Current. This blocking and reversing of flow facilitates minor upwellings of relatively nutrient-rich, cool water. These influences of seasonal changes in current result in more productive waters in the eastern part of the sea-lion’s range. The bias in population density of the sea-lion towards the east is also seen in the New Zealand Fur Seal, which has a similar overall range in Australia (Shaughnessy et al. 1994).
Australian Sea-lions commonly reach 8–9 years of age, with a maximum age of 12+ years (Stirling 1972). Females show a high level of natal site fidelity, only breeding at the site where they were born (DEWHA 2010p). In captivity, females first enter oestrus and mate when 4–5 years old (Langdon 1987) and a male has been recorded as successfully mating at 3–3.5 years old (Ling & Guy 2007).
The species has an asynchronous 17.5-month breeding cycle across its known range (Campbell 2003). The pupping season can extend for between five and seven months (Gales et al. 1992b; Shaughnessy et al. 2006). Associated with the longer pupping interval present in this species is a longer period of embryonic diapause of four to five months, and a prolonged post-implantation period of up to 14 months (Gales et al. 1997). Adult females haul-out a day or two before giving birth and leave 10 days later to forage at sea (Higgins & Gass 1993). They have their first pups on an average of 4.5 years of age (Higgins 1990).
A strong bond is established between a female and her pup, sometimes lasting from a year (Strahan 1983) to 40 months (Higgins & Gass 1993). Both males and females are very territorial during the breeding season, often becoming aggressive. When this aggression is directed towards pups it can contribute significantly to their mortality (Strahan 1983).
At birth, pups weigh between 6.4–7.9 kg, and are 62–68 cm long (King 1983; Walker & Ling 1981). Females nurse pups for approximately 15–18 months until the next pup is born. If however, a pup is not born in consecutive breeding seasons, many females will nurse existing pups for longer. Females forage for up to two days away from pups, and foraging trips increase in frequency gradually during lactation. Shore attendance bouts were about 1.5 days. This pattern continues until pups are weaned (Higgins & Gass 1993).
Mortality of pups
Mortality of pups can be high and is related to environmental conditions at colonies, attacks by males of the species, nutritional stress (malnutrition), disease or illness (Campbell 2005). For pups on islands on the west coast of WA, the mortality rate for the first five months varied from 7.1% to 24.3%, depending on whether pupping occurred in summer or winter, respectively. It has been suggested that this higher winter mortality rate is linked to the Leeuwin Current, as the flow is strongest during winter and brings warmer water and less food supply (Gales et al. 1992b). At Seal Bay, attacks on pups by territorial bulls accounted for 19% of pup mortality during two breeding seasons (Higgins & Tedman 1990). In the first two years of life, mortality is estimated at 40–50% for Australian Sea-lion pups (Higgins 1990).
Adult male behavior
Some male Australian Sea-lions congregate in bachelor colonies on islands adjacent to the Perth metropolitan region during the non-breeding season and migrate up to 280 km north each breeding season (Gales et al. 1992b). There is little or no movement of females between breeding colonies. Males defend harems of a few females at high-density breeding sites, but only one at a time at less dense colonies (Van Dyck & Strahan 2008). The location of all-male or ‘bachelor’ haul-out sites are known.
Australian Sea-lions feed on a wide variety of prey, including cephalopods, fish, sharks, rock lobsters and sea birds (Gales & Cheal 1992; Ling 1992). There is little quantitative information on their diet as only a few hard parts are normally found in the feces of this species (Gales & Cheal 1992), although the species is known to ‘feed’ at fishing boats on scraps or by taking fish off lines. Australian Sea-lions in western WA spend more time foraging compared to those in SA due to the less productive conditions of the Leeuwin Current (Lowther et al. 2013).
Radio transmitter and time-depth recorder studies of Australian Sea-lions at Seal Bay found that nursing females were benthic feeders on the continental shelf approximately 20–30 km offshore, in depths less than 150 m (Costa et al. 1988, 1990). While at sea, females and juveniles dive almost continually through the day and night. Young sea lions (approximately 7–18 months old) have been recorded foraging in depths up to 60 m and range up to 10 km from their birth colony (Fowler & Costa 2004 cited in Campbell 2005). Less is known about males’ feeding behavior, but they are recorded to dive deeper. The inshore breeding and foraging habitat of this species is responsible for interactions with fisheries and aquaculture (Gales 2008).
Dispersal of young appears to be self limited in this species, as females show strong natal site fidelity to maximise breeding potential due to the asynchronous nature of their breeding cycles (Campbell et al. 2008). Females’ movements appear to be no greater than 60 km from their natal site (Campbell et al. 2008). Males disperse approximately 200 km from natal sites (Campbell 2003). Dispersal mode is reflected in the high levels of genetic differentiation found in colonies of Australian Sea-lions over relatively short distances (Campbell 2003).
Adult females have been recorded to move pups away from the natal area to other haul-out areas to continue nursing when pups, at approximately 2–3 months of age, can make short distance movements (Higgins & Gass 1993).
Migration of adult and juvenile males has been recorded on the west coast of WA between breeding colonies in the Jurien Bay area and non-breeding sites on islands near Perth (Gales et al. 1992b).
Historic hunting and sealing
Historically, the main anthropogenic threat to the Australian Sea-lion was hunting and overharvest through sealing activities (AFMA 2010). Although this activity was stopped in the 1920s, the sea lion population has not recovered to pre-exploitation levels (AFMA 2010). Early sealers left few records of the identity, distribution and abundance of sea lion colonies from which to draw comparisons with the sea lion colonies today (Ling 2002 cited in AFMA 2010).
Commercial fishing impacts
Interaction with bottom-set gillnet fisheries
South Australian populations of the Australian Sea-lion occur entirely in the Gillnet and Shark Hook Sector of the Southern and Eastern Scalefish and Shark Fishery (SESSF). Detailed ecological risk assessments have been undertaken in the SESSF to assess the risks that fishing poses on ecological sustainability of the marine environment (AFMA 2010). It was found that of the seal species in the SESSF, the Australian Sea-lion was at greatest risk due to its small population size, complex breeding and high mortality rate of populations (AFMA 2010).
Evidence that supports the bycatch mortality of Australian Sea-lions in the SESSF gillnet fishery, includes (Goldsworthy et al. 2010):
- anecdotal reports from fishers of bycatch
- high incidence of Australian Sea-lion entanglement in gillnetting material at Seal Bay
- overlap between historic and current fishing effort with modeled Australian Sea-lion foraging distributions
- the very limited ability of Australian Sea-lion subpopulations to withstand additional mortality rates
- a detailed assessment of the potential risk posed by Australian Sea-lion populations from bycatch in gillnet fisheries
- increasing pup production and population recovery at Dangerous Reef that coincides with the closure of the Commonwealth gillnet fishery in southern Spencer Gulf in 2001.
Goldsworthy and colleagues (2010) completed an assessment of the risks to the Australian Sea-lion from the gillnet sector of the SESSF. The study estimated that approximately 374 Australian Sea-lions are removed as bycatch each breeding cycle. Population viability analysis indicated that the capacity for the species to recover would increase if the bycatch of adult females was reduced (Goldsworthy et al. 2010). This mortality rate was modelled from an observed 12 mortalities (AFMA 2010). The majority of observed interactions within the gillnet sector (73%) occurred within a 12.5 km range of the colonies, however modelling predicted that most interactions would be within a 60 km range with some interactions occurring as far as 130 km from colonies (Goldsworth et al 2010).
The Australian Fisheries Management Authority’s (AFMA) Shark Resource Assessment Group have raised concerns regarding the modelling that underpinned the bycatch estimates that Goldsworthy and colleagues produced (AFMA 2010). There were particular concerns about observer effort occurring in low fishing effort areas, rather than the high fishing effort areas (AFMA 2010). Similarly, an AFMA observer program recorded substantially lower rates of 0.004 sea lions per kilometre net set compared to a 0.013 rate observed by Goldsworthy and colleagues (2010) (AFMA 2010).
Drowning in lobster pots
Young sea-lions have been recorded to drown in Southern Rock Lobster (Jasus edwardsii) pots (Gales et al. 1992b) and are attracted to bait, caught lobsters and discarded bait (Goldsworthy et al. 2010). There is limited quantitative data on the level of mortality of Australian Sea-lions through entrapment (Goldsworthy et al. 2010). Published reports suggest that most incidental mortality occurs around breeding colonies, haul-out sites and breeding colonies, particularly in shallow water (Campbell 2005; Goldsworthy et al. 2010).
Entanglement in fishing gear
Entanglement in fishing gear (and in other man-made debris) can cause drowning. In a review of the problem in SA, Robinson and Dennis (1988) refer particularly to entanglement in monofilament netting of 150 mm mesh, which is used in the bottom-set gill-net shark fishery. Australian Sea-lions (and New Zealand Fur-seals) also interact with nets at tuna farms near Port Lincoln, where some become entangled in nets. Modifications to existing nets, including increasing tension on them, and adding bottom nets and top nets were suggested as methods to alleviate this impact (Pemberton 1996b). Page and colleagues (2004) found that industry and government attempts to reduce the problem did not succeed, and calculated that annually between 64 and 146 Australian Sea-lions died from entanglement, though this figure could be much higher due to observational methodology limitations (Page et al. 2004).
Due to the broad diet of Australian Sea-lions, direct competition with commercial fishers is probably limited. However, Australian Sea-lions rob lobster pots and nets set for schooling shark, and take Australian salmon and herring from nets set from shore on the south coast of WA (Shaughnessy 1999). This can lead to illegal shooting of “trouble” seals by commercial fishers (Campbell 2005).
Other threats to the Australian Sea-lion include:
- Tourism, centered on nine known haul-out sites in SA and WA, was found to increase the state of vigilance in Australian Sea-lions and to invoke a retreat from sites with increased human disturbance (Orsini 2004). Long-term effects of human disturbance include physiological stress, excessive time spent at sea, and possible abandonment of the haul-out sites. These effects have been displayed in other pinnipeds (Orsini 2004).
- The White Shark (Carcharodon carcharias) viewing industry attracts sharks to known colonies with the potential to increase the mortality of Australian Sea-lions (Shaughnessy 1999).
- Occasional epidemics (i.e. morbillivirus) have occurred where up to 50% of an entire pinniped population has died (for example, European Harbour Seals (Phoca vitulina)) (Dietz et al. 1989 cited in Campbell 2005; Jensen et al. 2002 cited in Campbell 2005).
- Large scale mortality events have also occurred in populations of Galapagos Islands pinnipeds during El Niño weather conditions, due to increased water temperature and decreased prey abundance, although it is unknown what climate conditions would impact the Australian Sea-lion (Trillmich & Limberger 1985 cited in Campbell 2005).
- While the population size of the Australia Sea-lion is stationary or possibly decreasing, the New Zealand Fur-seal has been increasing (Shaughnessy et al. 1994), thus indicating potential competition of resources.
- Pollution from oil spills, sewage, land run-off and toxic contaminants that may bioaccumulate