Species description

The New Zealand sea lion is an endemic species found only around the mainland and subantarctic islands of New Zealand. It was named 'Hooker's sea lion after Sir Joseph Hooker, a botanist with a British Antarctic expedition which arrived in the Aucklands in 1844.

Although the New Zealand sea lion is similar in size and appearance to its geographical neighbour the Australian sea lion (Neophoca cinerea), the two species are not, in fact, closely related genetically (Wynen et al., 2001). Adult New Zealand sea lions are markedly dimorphic. Adult males are heavily built, especially in the forequarters, measure between 2.0-3.25m in length and weigh 318-410 kg, while adult females measure 1.8-2.0m in length and weigh 136-230 kg (reviewed by Walker and Ling, 1981). . Males are black or dark brown, and mature bulls have a mane of rough hair about their shoulders (see photo below, left). Females are silver-grey dorsally and creamy ventrally (Bonner, 1994), although they appear pale brown when dry towards the end of the year. The pups are born with a thick coat of light or chocolate brown fur and measure a little less than 1m at birth. Male pups are born at an average 10.6 kg, while females are a little smaller, at an average of 9.7 kg (Chilvers et al., 2007). Most females become sexually mature at 3-4 years of age, males at 5 years. Males do not reach social maturity however until they are at least 8 years of age. Males may live as long as 23 years, females as long as 18 years.

Population and distribution
Archaeological evidence indicates that the sea lions were once present all around the New Zealand coastline, but hunting - first by the Polynesian settlers and then by the European sealers - resulted in their extinction from the mainland (Childerhouse & Gales, 1998).

Today the sea lions haul out mainly in an area ranging from the Australian subantarctic Macquarie Island to the southeast coast of South Island, New Zealand. Breeding has been almost exclusively on New Zealand's subantarctic islands. The main breeding colonies (a total of 71% of the total population) are currently on Enderby and Dundas Islands at the north of the main Auckland Island (King & Marlow, 1974; Childerhouse & Gales, 1998) with some breeding colonies (29% of the total) also on Campbell island (Robertson & Chilvers, 2011). The total population of the species was estimated at 11,600 - 15,200 in 1996 (Childerhouse & Gales, 1998).

However, epizootics caused by bacterial infections resulted in the deaths of 53, 32 and 21% of pups during the first month of life in 1998, 2002 and 2003 seasons, with at least 74 adult females also dying in 1998 ). Since then pup production in the Auckland Islands has fallen from 3,000 in 1998 to 1,500 in 2011 (Robertson & Chilvers, 2011;  New Zealand Sea Lion Trust, 2013). Female NZ seal lions (in common with other otariid species) generally show a high degree of philopatry, i.e. more than 60% of females return as adults to their birth site to give birth themselves (Chilvers & Wilkinson, 2008). The pupping decline at the Aucklands is directly linked to femakes not returning to the breeding areas (Robertson & Chilvers, 2011). The reason for this is not known, but up to 140 adult females a year are known to die as by-catch in the trawl fishery for arrow squid. It is also possible that the squid fishery has disrupted the ecosystem and food chain around the Auckland islands (Chilvers & Wilkinson, 2008), possibly causing foraging difficulties for lactating females .

In order to reverse the declining population trend, the sea lions need to found new distinct breeding areas away from the present main sites (Chilvers & Wilkinson, 2008) - but female philopatry tends to discourage such expansion of the breeding range unless there is physical overcrowding in existing colonies or the local population reaches its carrying capacity. Since the early 1980s some immigration into the Otago Peninsula on SE NZ mainland has been recorded, those recorded more recently being mainly from juvenile males tagged on the Aucklands (Hawke, 1986; Beentjes, 1989; McConkey et al., 2002a). However, one juvenile female immigrant from the Aucklands, recorded in 1991, behaved atypically, and instead of returning to her natal site to breed hrself, gave birth in the 1993/4 season to the first recorded NZSL pup on the NZ mainland since the complete elimination of the species by humans 150 years ago. Since then this female (named 'Mum') has given birth almost every year at Otago, and her daughters have remained and also started to breed (McConkey et al., 2002b). All these pups have been tagged and monitored, and the family tree can be viewed on the NZSL Trust website. Mum gave birth to 11 pups up to 2007, her last recorded. Three of Mum's daughters (Katya, Leone and Suzie) have given birth to 7 daughters between them who have also become mothers up to the 2012/13 season. Since 2006, another unrelated female 'Marea', born in the Aucklands in 2000, has begun to breed at Surat on the Catlins coast to the south of Otago.  Up to the 2012/13 season 10 pups had been born, at least two of which seem to be second generation.

    Photo: Nathan McNally, University of Otago                                       Photo: Irina Trukhanova, Seal Conservation Society                  Photo: Michael Bryden, University of Sydney                                      

Life cycle and breeding behaviour
New Zealand sea lions prefer sandy beaches for both hauling out and breeding. They breed in colonies between November and January, the pups being born from early December until late January.

In November each year the bulls arrive at the rookeries and fight, in a largely ritualistic manner, for territory before the females come ashore a few weeks later. In contrast to its compatriot, the New Zealand fur seal, NZ sea lions like to be in close contact. King and Marlow (1974) describe pregnant females emerging from the sea as first surveying the scene and then hurriedly galloping towards a group of females 'as if she were late for an appointment'! Breeding bulls who establish territory eventually have about 8 to 25 females on their territory. However, the bulls do not herd the females or try to prevent them from moving between adjacent territories (Walker & Ling, 1981), but sit on their chosen patch of beach and 'huff' loudly at their neighbours!

The females give birth about two days after arrival at the breeding site (Chilvers et al., 2007) and come into oestrus 6-7 days after giving birth. Most mating takes place on the beach or in the surf zone. The bull initiates most matings. He mounts from the rear while the female lies prone or on her side (Walker & Ling, 1981). 

About 8-9 days after the pup is born, its mother makes her first foraging trip to sea. Her first foraging trip averages only 2 days, after which she returns to nurse her pup. While their mothers are away, young pups collect into pods close to mother-pup pairs. Some pups try to take shelter from the weather in the entrance to rabbit burrows, may get stuck and die (King and Marlow, 1974), although we understand that rabbits have been cleared from Enderby Island and this is no longer a major threat to pups. The pups actually lose weight (on average 48g/day) during the first week after birth, but gain an average of 151 g/day over the first three months.

Males that do not succeed in establishing breeding territories loiter on the periphery of the colony and intercept mothers on their way to the sea. These males may cause injury to females (84% of females were found to bear scars from male bites) and may also cause delays in mothers reaching the sea or returning to their pups (Chilvers et al., 2005a). 

After mating, the females start to lead their pups away from the breeding beach and disperse to locations further inland, sometimes into the forest, where they are left during the mothers' foraging trips (King & Marlow, 1974). Despite this dispersal pattern, females continue to form small groups inland (Auge et al, 2009). These authors consider that this pattern of extensive dispersal is unusual among the otariid pinnipeds. They suggest that one driving factor for this post-mating dispersal may be avoidance of the unwanted attentions of the males that do not hold breeding territories. Another benefit of the dispersal may be avoidance of hookworms, which tend to be abundant where there are high densities of animals, and pose a threat to pup survival.

The pups start to explore freshwater pools by 4-6 weeks. The mothers also start leading the pups into the sea as far as neighbouring islands, and by mid-February (when the pups and 2-3 months old)  the natal beach becomes almost deserted (Walker & Ling, 1981).

The growth rate of pups on Enderby island (Aucklands) seems to be less than for other sea lion species, possibly because mothers have to forage further from their breeding colonies and over greater distances than any other sea lion (Chilvers et al., 2007; see discussion below). The mother continues to nurse her pup for about 9 months, with pups thought to be weaned in August-September (Chilvers et al., 2007). 

In the 2nd week of December 2013 the present author visited a beach on the Otago peninsula and encountered what appeared to be a young adult male and female sea lion. This pair were alone except for one other female sleeping much further along the beach. The pair were first seen resting together, head-to-head. They then awoke and engaged in mutual gentle courtship, which seemed to be equally iniated by both animals.

Photo sequence: Sue Wilson, Seal Conservation Society

The pair were first seen resting together (and we would like to commend the group of adults and children in this clip, who behaved with great respect towards the sea lions and took great care not to disturb them). The pair then woke up and engaged in a mutual gentle courtship sequence. We flimed this in four short clips (courtship-1, courtship-2, courtship-3 and courtship-4). No mating was observed, and we left the pair resting together after about an hour's observation.  Unlike in the mating behaviour described for the breeding colonies, the female of this pair was initiating much of the interaction, sometmes in a 'flirtatious' manner. The male behaved quite contrary to the harassing behaviour described above for non-territorial bulls in that he was extremely gentle and made no attempt to bite or otherwise intimidate the female. In fact he often displayed a 'relaxed open mouth' face (see above photos), often indicative of play. 

Below are some more links to NZ sea lions

Male-female courtship ('Love at first bite')

Sea lions underwater at Campbell Island

New Zealand sea lions at Waipapa Point

Sea lions on a beach on the Otago Peninsula

Migration, diving, foraging behaviour and diet

Studies of movement ranges of male and female NZ sea lions have found females to be highly philopatric. This philopatry is year-round, unsurprisingly, since females return to their pups' natal site to nurse them for most of the year. Juvenile males tend to disperse more widely - up to 700 km to the NZ mainland - athough they, too, tend to return to the natal site to breed when they reach sexual and social maturity (Chilvers and Wilkinson, 2008). 

A satellite telemetry study of 26 nursing mothers found that these females travelled an average of 423 km on each round trip. They preferred foraging on the continental shelf and its edge, and there was evidence of inidividual preference for particular foraging areas and some partitioning between individuals. The female foraging areas overlapped with the commercial squid fishery (see below) (Chilvers et al., 2005b). A further study of females from the south of the Auckland Islands, where pup production has declined almost twice as rapidly as in the northern part of the islands, found that these females foraged over the entire area of the Auckland islands shelf and there was extensive overlap with the squid trawl fishery (Chilvers, 2009).  A four-year study of individual females' foraging site fidelity found a high degree of site fidelity between years. This suggests that NZ sea lion females may have limited ability to cope with anthropogenic environmental changes (Chilvers, 2008a).

The NZ sea lion is a benthic forager. having the deepest and longest duration dives for any otariid (Chilvers, 2008; Chilvers et al., 2008). It is an opportunistic feeder, the majority of the diet consisting of cephalopods (such as squid and octopus), crustaceans, many species of small fish such as flounder, and wandering male sea lions have been known to feed - probably when food-stressed - on Gentoo penguins on Macquarie Island (Walker & Ling, 1981). A recent study of the stomach contents of sea lions by-caught in the squid trawl fishery found that their prey (by weight) was predominantly squid and octopus (48%) with other small fish, particularly opalfish, making up the remainder (Meynier et al., 2009). A study of dive depths and times of adult females found that while at sea, the sea lions dived almost continuously, spending just over half their time submerged. Their average dive depth was 130m and lasted an average of 4 min. The authors concluded these animals may have been diving beyond their estimate aerobic dive limits on 68% of all dives, possibly because they were having difficulty finding sufficient food (Chilvers et al., 2006). However, a study of NZ sea lion blood physiology found that these animals have the highest blood volume yet reported for an otariid. Their blood volume would support an aerobic dive limit for adult sea lions of 5.5-7.8 min. This would suggest that the females in the 2006 study were probably diving well within their normal aeriobic limit, and that these sea lions are uniquely adapted to deep diving (Costa et al., 1998).

A very recent study comparing the foraging behaviour of juvenile female sea lions aged 2-3 from the Auckland Islands and the Otago peninsula found that the Auckland females spent more time at sea, foraged over greater areas and dived deeper and longer than the Otago females. These results suggest that Otago may provide a more optimal foraging habitat than the Auckland island area (Leung et al, 2013), and that there is some validity in the hypothesis that the Auckland island sea lions may be relatively food-stressed at the present time. 

Photo: Nathan McNally, University of Otago

Historic and current threats
Although the New Zealand sea lion was extensively killed for its hide and oil in the 19th century and, to a much lesser extent, the early 20th century, commercial killing of the species has been illegal in New Zealand since 1893. The New Zealand sea lion was designated as a Threatened Species under New Zealand's Marine Mammals Protection Act in August 1997, but is now listed as 'Nationally Critical' owing to the continued decline of breeding success at the main sites in the Auckland Islands. The species is also listed as Vulnerable on the IUCN Red List (Gales, 2008). In 1995 a 20km (12nm) Marine Mammal Sanctuary was declared around the Auckland Islands. New Zealand's subantarctic islands were granted UNESCO "World Heritage" status in 1998 and in 2003 the sanctuary became a 'no take' reserve (Chilvers, 2008).

The continuing problem of bycatch in the NZ arrow squid fishery 

A major threat to the species is from entrapment and drowning in the nets of the squid trawl fishery which has operated within the sea lions' range since the late 1970s. The first reported sea lion bycatch was in 1978, when 10 sea lions were killed in 58 research tows (cited by Chilvers, 2008b). A public consultation was held in 1991 (Anon, 1991a), to which Greenpeace publicly responded with suggested fishery management options (Anon, 1991b).Since 1992 government observers have been placed on a proportion of fishing boats to determine the seal lion bycatch numbers.

Since 2001 sea lion exclusion devices (SLEDs) have been used. trials carried out in 2001 estimated a 91% ejection rate of sea lions through the escape hatch, but half the animals that did escape suffered traumatic internal and head injuries that would have compromised their survival.

Detail of 'codend' of squid trawl net showing SLED (From Chilvers, 2008; reproduced by NZ Sea Lion Trust )

Between 1992 and 2007, 58% of 226 sea lions caught in the trawls were female. Since SLEDs have ben used full-time, the proportion of females caught has been 82%. For each female killed in these nets, an unborn foetus and a dependent pup onshore will also die (Chilvers, 2008b). Quotas for sea lion total allowable by-catch have ranged from 62 to 150 in different years between 2003-2007, with the Ministry always arguing for a higher allowable by-catch. Sea lions are also by-caught in other NZ fisheries, including the scampi and blue whiting fisheries.

The 'no-take' reserve in the 12nm area around the Aucklands does not protect the whole foraging area of any female sea lion so far investigated. An alternative extended no-take zone (Marine Protected Area) around the Aucklands has been put forward, which - if implemented - would protect the entire foraging areas of 70% of the females so far investigated, and would still allow more than 50% of current fisheries activities to continue (Chilvers, 2008b). However, this has not been implemented. The use of alternative nets, 'jiggers', which do not entrap sea lions has also been suggested, but not implemented.

At present, the Government management plan for the NZ sea lion, which is to reduce human-induced mortality to allow the sea lion to achieve  non-threatened status as soon as reasonably practical, and in any event within 20 years seems a long way from being fulfilled, since the national status has in fact turned in the opposite direction, to 'Nationally Critical'.

The impact of the commercial fisheries is not only to kill directly an unsustainable number of animals as by-catch, but also to change significantly the sex-ratio of adult animals in the population resulting in more males on the fringes of the breeding grounds, which could exacerbate the problem of male harassment and injury of females. A further impact of the commercial fishery is to reduce fish stocks in the females' natural foraging range close to the Aucklands and cause females to forage further afield, dive to extreme depths in search of food, and potentially have to feed on lower-calorie fish. This in turn may be the reason why female NZ sea lion milk currently has the lowest fat content of any sea lion species, and the reason why NZ sea lion pup growth is relatively slow (Chilvers, 2005; 2008b). 

Interestingly there may be some parallels between the problems of the NZ sea lion and the critically endangered Hawaiian monk seal. In both species the animals, particularly females (juveniles in HMS, adults and possibly also juveniles in NZSL), may be food stressed due to ecosystem changes resulting in low stocks of prey species. In both species the main colonies in the outer islands are continuing to decline, while small numbers of animals are succeeding in re-establishing and breeding on the mainland islands in proximity to human activity. Also in both species there is a problem of male harassment of females (again juveniles in HMS and adults in NZMS), possibly due in part to an imbalanced sex ratio in favour of males, in turn due to lower female survival.