Issues Paper for the Australian Sea Lion
|
|  Yüklə 0.65 Mb.
|
|
(Source: Walker & Ling, 1981; Gales, et al., 1992; Ling, 1992; McIntosh, et al., 2006). 3.4 Life history, breeding strategies and population genetics The Australian sea lion is a large-bodied marine mammal that is slow to mature, with females having few young over their lifetime, to which they commit extended maternal care (Gales & Costa, 1997). Females become sexually mature at 3.5–6 years of age (Higgins, 1993; McIntosh, 2007), which is similar to most other otariids (Wickens & York, 1997; Dickie & Dawson, 2003). The maximum age at which a female has been observed giving birth is 24 years (McInstosh, 2007), which is also similar to other otariids, such as the New Zealand fur seal (McKenzie, et al., 2007). However, Australian sea lion breeding cycles are atypically extended, being 17.4–17.8 months in duration (Shaughnessy, et al., 2006), which reduces reproductive opportunities by approximately one third when compared with other annually breeding pinnipeds. Breeding is expected to occur at all times of the year across a 24 year period, based on a 4.6 month pupping season (Shaughnessy, et al., 2006; Figure 5), inferring that prey availability must be relatively stable throughout the year (Lowther, et al., 2011). Pups are typically nursed for 15–18 months, typically being weaned one month prior to the birth of the next pup (Higgins & Gass, 1993). There is evidence that pups learn to forage during this time, suggesting that pups are able to make a slow transition to nutritional independence, which may be necessary for learning the skills to survive in an environment where prey are patchy and scarce (Lowther, et al., 2011). The only genetic investigation into the population structure of the Australian sea lion utilised mitochondrial (mtDNA) and nuclear (microsatellite) DNA markers to investigate the degree of population sub-structuring and sex-biased dispersal throughout most of its range. Samples were collected from eight colonies in Western Australia (Abrolhos Islands, Beagle Island, North Fisherman Island, Buller Island, Hauloff Rock, Red Islet, Six Mile Island and Spindle Island) and from two colonies in South Australia (Dangerous Reef and Seal Bay; Campbell, 2003; Campbell, et al., 2008a). This study provided evidence of strong sex-biased dispersal in the populations, manifested primarily in extreme female natal site fidelity (or philopatry). This means that females will typically breed in the same colony in which they were born, with genetic differences in the female line being evident in colonies as close as 20 kilometres apart. The marked foraging site fidelity recently observed in lactating Australian sea lions (Lowther,
cycles, with the peak in breeding seasons among breeding colonies being several months apart, even between those in close proximity where foraging ranges overlap (Shaughnessy, et al., 2011). Secondly, effective decoupling in the timing of breeding has resulted in a temporal barrier to female immigration, because the short window of oestrus (i.e. seven days immediately after giving birth; Higgins, 1990) may not be synchronised with adjacent colonies. Consequently matrilineal genetic separation (involving mitochondrial or maternally inherited DNA) has occurred at the colony or regional (i.e. clusters of colonies) level (Campbell, et al., 2008b; Lowther et al. 2012; Figure 6). In essence, some breeding colonies, or clusters of breeding colonies, are unique populations. Figure 5: Predicted timing of breeding for the Australian sea lion, depicting the seasonal drift in the peak of breeding across the entire year at Seal Bay between 2001 and 2020. 
(Source: DEWHA, 2010). Figure 6: Geographic representation of genetic differentiation among Australian sea lions depicting mitochondrial DNA differentiation between sampled colonies, by halotype distribution and proportion. (a) Minimum spanning tree, where distribution is colour coded with colony name, size is related to their frequency of occurrence, and extended circles indicate sharing between colonies. 
a (b) Pie charts depict relative frequency/proportion in sampled colonies, with dotted lines indicating clusters of colonies where halotype combinations are statistically similar 
b (Source: Campbell, et al., 2008a [a]; Lowther, et al., 2012 [b]). 4 Conservation Status The Australian sea lion was listed as endangered under the IUCN Red List in 2008 (Goldsworthy & Gales, 2008). In Australia, the Australian sea lion was listed as vulnerable under the EPBC Act in 2005.
In South Australia, the Australian sea lion was listed in 2008 as a threatened species and is protected under the South Australian National Parks and Wildlife Act 1972 and the South Australian Fisheries Management Act 2007. In Western Australia, the Australian sea lion is specially protected as threatened fauna under the Wildlife Conservation Act 1950 — Wildlife Conservation (Specially Protected) Fauna Notice 2003. A number of breeding and haul out islands are protected as nature reserves, and existing marine parks further protect marine areas of Australian sea lion habitat. The current conservation status of the Australian sea lion in Australia is detailed in Table 2. Table 2: Current conservation listings for the Australian sea lion in Australia
5 Immediate and Known onservation Threats Historically, the main anthropogenic threat to the Australian sea lion was hunting
Life history characteristics of the Australian sea lion are suspected to have contributed to lack of recovery of the species post-commercial sealing. Slow maturation and low fecundity, an extended breeding cycle and philopatry are all indicative of adaptations to improve breeding and foraging success in a nutrient poor environment, where benthic prey availability is low but stable,. These are also characteristics that make this species highly vulnerable to extinction. The prevalence of philopatry among female Australian sea lions is especially problematic for the species, because it effectively negates dispersal. In situations where breeding colonies are very small or have gone extinct, immigration of pregnant or sexually mature females to facilitate growth or recolonisation is not possible. In instances where extinction has occurred, it is possible the Australian sea lion will not recolonise those sites. The extirpation and non-recovery of several breeding sites within, and to the east, of the current geographic range of the species are cases in point. Given the genetic diversity recently demonstrated among populations of the Australian sea lion, it is likely that historical and future extinctions may irreversibly diminish matrilineal genetic diversity. A range of anthropogenic factors have been identified which may be impacting on the recovery of the Australian sea lion. The cumulative impact of many of these threats varies across the range of the species. Fisheries bycatch and entanglement in marine debris appear to pose the greatest threat to the Australia sea lion at present, while secondary threats include habitat degradation and interactions with aquaculture operations; human disturbance to colonies; deliberate killings disease; pollution and oil spills; noise pollution; prey depletion and competition and climate change. 5.1 Primary threats 5.1.1 Fishery bycatch 5.1.1.1 Demersal gillnet fishing for shark Recent research confirms that interactions with commercial gillnetting operations have been a significant cause of mortality of Australian sea lions and are likely to be limiting population growth (Goldsworthy, et al., 2010). Gillnets are mesh nets that are designed to target a particular fish species through the use of net size in which target species get stuck while smaller species swim through.Species too large to push their heads through the mesh as far as their gills are not retained. Shark gillnetting in southern Australia commenced in the late 1960s, firstly to target school shark (Galeorhinus galeus) and later gummy shark (Mustelus antarcticus), and has remained largely unchanged since that time (Kailola, et al., 1993; Larcombe & McLoughlin, 2007). The most significant demersal gillnet fishery is the Commonwealth managed gillnet sector of the Southern and Eastern Scalefish and Shark Fishery (SESSF), which operates from the Western Australia-South Australia border to the Victorian-New South Wales border. In South Australia, a bilateral agreement between the Commonwealth and South Australian governments permits fishing across shelf waters under both jurisdictions, from near shore coastal waters to a depth of 183 m. Annual fishing effort peaked in 1987 at 43 000 km of net-lifts, declining to and remaining steady at about 17 000 km of net-lifts since 2000 (DSEWPaC, 2010). In 2009/10 and 2010/11, gillnetting effort increased again to around 37 000 km net-lifts and 40 000 km net-lifts in each year, respectively (Woodhams, et al., 2011). In South Australia, effort has been concentrated along the west coast of the Eyre Peninsula and along the south coast of Kangaroo Island (DSEWPaC, 2010). A lower level of demersal gillnetting for sharks also occurs across Western Australian shelf waters under similar bilateral agreements between the Australian and Western Australian governments, managed by the Western Australian government (McAuley & Leary, 2010). The gillnets used across the range of the Australian sea lion typically involve a mesh size that can also entangle large species, including the Australian sea lion. Once caught, Australian sea lions often drown before the nets are retrieved or may tear out a section of the net or be cut free by fishers (Gales, et al., 1994; Page, et al., 2004). Concerns about the impact of demersal gillnetting on the Australian sea lion are longstanding and were initially motivated by the regular occurrence of entanglements observed on individuals ashore at breeding colonies and haul-out sites (Shaughnessy & Dennis, 2001; Shaughnessy & Dennis, 2002; Page, et al., 2004). Two anecdotal reports from shark fishers indicate high levels of bycatch by individual gillnetters, with one report of 20 animals being killed each year during the 1990s (Shaughnessy, et al., 2003). Reliable estimates of Australian sea lion mortality from gillnetting operations have only recently become available, as fishers have not historically been required to keep records of interactions with this species. Together with limited observer coverage of commercial fishing operations, this has led to only small numbers of interactions being reported and it is likely that reporting has under-represented the actual bycatch (Hamer, 2007; DEWHA, 2010). Achieving a reliable estimate of bycatch is further complicated by drowned animals dropping out of gillnets, due to being minimally entangled and subsequently not being detected by the operator or observers (Goldsworthy, et al., 2010). For example, in a study of bycatch mortality rates undertaken over 24 months in South Australia, Goldsworthy, et al. (2010) reported that 10 (83 per cent) of the 12 Australian sea lion bycatch mortalities dropped out of the gillnet before or on making contact with the net roller, as the net was hauled out of the water. In addition, other animals may escape and die later from injuries (Hamer, et al., 2011). A study undertaken by the South Australian Research and Development Institute (Goldsworthy, et al., 2010) estimated that Australian sea lion mortality off South Australia, as a result of interactions with the gillnet fishing sector, was around 374 animals per breeding cycle (17.5 months) of which, it was estimated, approximately 197 would be female. The study concluded that this level of mortality equated to about 3.9 per cent of the overall female population being removed as bycatch mortality each breeding cycle, which was further estimated to be an approximate 35 per cent increase from natural mortality levels. Fisheries closures around Dangerous Reef (South Australia) provide a case study of what might happen if gillnets are removed from the foraging range of Australian sea lion colonies. As previously noted, the Dangerous Reef population is the only Australian sea lion population known to have undergone a recent recovery in numbers and the explanation for this is likely to be linked to the restriction of gillnet fishing in the region following closures in 2001 (Goldsworthy, et al., 2007b). The majority of the Australian sea lion population found in waters offshore of South Australia is contained within the boundaries of the Shark Gillnet and Shark Hook Sectors of the SESSF. The SESSF, including these commercial gillnetting operations, is an approved Wildlife Trade Operation (WTO) under the EPBC Act (until 28 February 2013) and is accredited for interactions with protected species. The first environmental assessment of the SESSF was conducted in 2003 and highlighted the potential for operational interactions between Australian sea lions and demersal gillnetting. This prompted the Australian Government environment department (currently DSEWPaC) to recommend that the Australian Fisheries Management Authority (AFMA), the managers of the SESSF, to (i) “establish a robust reporting system”
Perhaps most importantly, independent monitoring of fishing effort from 2006 to 2007 reported levels of observed bycatch that were higher than anticipated. Specifically 12 bycaught and drowned animals were reported from a total of 994 km of gillnet observed hauled from 234 fishing events or sets (i.e. 2.9 per cent of the combined length of gillnets set across South Australian waters during the two year monitoring period; Goldsworthy, et al., 2010). Based on the calculated annual length of gillnet set in the fishery (i.e. 17 355 ± 852 km) and the bycatch rate observed from monitored fishing, it was estimated that 293–324 Australian sea lions were becoming bycaught and drowning each breeding cycle across South Australian shelf waters (Hamer, et al., 2013). The bycatch rate is likely to be similar, or at least at the same order of magnitude, across a substantial portion of South Australian shelf waters because the absolute minimum overlap between Australian sea lion foraging and fishing effort has been estimated to be 68.7 per cent (Hamer et al., 2013). Thus, a simple extrapolation based on kilometres of net and observed interactions is appropriate. Based on a more sophisticated ‘equal sample size’ approach, a similar estimate was obtained of 374 (272–506 ± 95 per cent confidence limit) Australian sea lions being bycaught and drowning each breeding cycle across South Australian shelf waters (Goldsworthy, et al., 2010). Although 12 drowned animals were observed, 10 (83 per cent) dropped out of the fishing gear as they were raised to and above the surface of the water, raising concerns about the proportion of drowned animals that go unobserved and unreported, and are thus excluded from calculated estimates of bycatch (Hamer, et al., 2013). The two animals that were hauled aboard the vessel were small juveniles, suggesting the weight of larger animals may cause structural failure of the gillnet meshes as the dead animal is hauled through the water, or as the full effect of gravity occurs as they are hauled from the water. Additionally, others may drop out of the net as it is hauled off the benthos, or may become temporarily bycaught and then escape with an entanglement. Whatever the case, the mortality estimates based on observed bycatch should be viewed as a minimum. The fact that entangled animals are frequently observed on land (refer: section 5.1.2) further supports this conclusion. Formal measures to protect the Australian sea lion are stipulated in the SESSF WTO approval, with closures and an Australian Sea Lion Management Strategy for the fishery implemented by AFMA as conditions of this approval. Further information regarding management of the SESSF is available at: www.afma.gov.au/managing-our-fisheries/fisheries-a-to-z-index/southern-and-eastern-scalefish-and-shark-fishery Recent research on the interaction between commercial fisheries and the Australian sea lion (Goldsworthy, et al., 2010) has informed deliberations on what measures are required to afford a sufficient level of protection to the Australian sea lion within the area of impact of the SESSF. Following this research and further work undertaken by AFMA, revised management arrangements have been put in place for the SESSF, including modified zone boundaries, lowered maximum bycatch trigger limits for each management zone, increased observer coverage and new identification and reporting requirements. The revised triggers permit a maximum bycatch level of 15 Australian sea lions per fishing season. Consistent with these measures, as of February 2012, areas of the fishery that have exceeded the Australian sea lion bycatch trigger for their zone have been closed for 18 months, so that the period of closure will encompass one full breeding cycle for that zone. More information on Australian sea lion bycatch is available at:www.afma.gov.au/australian-sea-lion-management-strategy-reset-maximum-bycatch-trigger-limits An observer program is not currently in place in the Western Australian temperate demersal gillnet fisheries, therefore the impacts of possible bycatch off the Western Australian coastline are currently unquantified. However, an observer program that operated from 1994 to 1999 recorded only one dead Australian sea lion over the six years of the program (McAuley & Simpfendorfer, 2003). While there is less demersal gillnet fishing effort in Western Australia than in South Australia, Campbell (2011) reviewed information on Australian sea lion foraging effort and gillnetting activity in Western Australia and found there is almost complete spatial overlap. The Western Australian Department of Fisheries is currently reviewing the monitoring arrangements for Australian sea lions in its temperate demersal gillnet fisheries. Fisheries bycatch (including in demersal gillnet fisheries and rock lobster fisheries) has been assessed in the Commonwealth’s marine bioregional plans as a pressure ‘of concern’ for the Australian sea lion in the South-west Marine Region. More information on the South-west Marine Region is available at: www.environment.gov.au/coasts/marineplans/index.html
The diet of juvenile Australian sea lions includes rock lobsters (Gales & Cheal, 1992; Ling, 1992; McIntosh, et al., 2006). The use of rock lobster pots in areas close to Australian sea lion breeding colonies, or in areas where animals forage, may result in occasional bycatch related mortality, as the animals may become caught inside the lobster pots as they attempt to depredate on the caught lobsters. Quantitative studies are scarce, although in Western Australia, annual surveys provided estimates of zero to 12 interactions over five years, in waters less than 20 m deep and within 30 km of breeding colonies (Campbell, et al., 2008b). Satellite tracking of animals from colonies in areas where rock lobster fishing occurred indicated that the foraging area of the tracked animals overlapped spatially with the rock lobster fishing effort (Campbell, et al., 2008). No quantitative data on bycatch rates are available from the South Australian Rock Lobster Fishery (Goldsworthy, et al., 2010). However, Goldsworthy, et al., (2010) suggested that the bycatch impact from the South Australian Rock Lobster Fishery was likely to be smaller than from the SESSF because there was less overlap in fishing effort with Australian sea lion foraging effort (i.e. two-thirds of the fishing effort occurred in areas with little foraging); fishing was restricted to eight months of the year and bycatch was likely to be restricted to pups and juveniles. In addition, the use of sea lion exlusion devices (SLEDs) provides a relatively simple management solution for sea lion interactions with rock lobster pots, while management solutions for gillnet fishing are more complex. In Western Australia, mitigation was effected by modifying rock lobster pots to include a
In South Australia, anecdotal reports suggest spikes have been used for some time by fishers to exclude depredating Australian sea lions and New Zealand fur seals. However, there is no formal management framework that regulates the use of spikes in areas close to Australian sea lion colonies, in waters of a certain depth, or in any other critical marine habitat. Nonetheless, the effectiveness of the spike in deterring depredating Australian sea lions and their effect on commercial catch rates has been explored to determine the practicality of implementing or regulating their use in South Australia. Experimental trials were carried out in South Australia to determine the most effective height
|