What’s killing the sea-lions?
New Zealand sea-lion numbers have plummeted. Is it fishing pressure, climate change, the mysterious disappearance of octopuses, or a deadly new virus?
A female sea-lion hauls herself through the shallows, eyes wide with exertion, her golden flanks heaving. In the ocean, she is capable of reaching dazzling speeds, but here on land it’s a different story. She makes only 20 or 30 metres in each burst before slumping to the ground.
A large bull is closing in on her, his thick fur blood-crusted from wounds inflicted by the competitors he has fought off on the beach below.
This female has arrived on Enderby Island early—hauling up on the beach before any other females have gathered there. Alone, she is forced to run a gauntlet of sex-starved males that have been waiting for the breeding season to start.
She heads for the bush—the best strategy for avoiding unwanted attention under these circumstances. Before she can get there, though, the male catches her up and pins her to the ground. In frustration, she twists her neck to bite, but it’s futile, he’s nearly twice her size.
Afterwards, with the two animals slumped motionless in the grass, I turn off my camera, somewhat shaken. Giant petrels soar overhead, with primeval caws, and over the ruffled waters of Port Ross, the bulk of the Auckland Islands are wreathed in cloud. Southern royal albatrosses bank across the sea, weightless on the westerly.
I’ve been sent here by NHNZ (formerly Natural History New Zealand) to film for the next season of TVNZ’s Our Big Blue Backyard series. Now 450 kilometres from mainland New Zealand, the two scientists with whom I share the hut and I are the only people in the Auckland Islands—at this moment perhaps the most isolated human beings in New Zealand. We are the solitary witnesses to the beginning of this natural drama, one that few people have seen—the gathering of New Zealand sea-lions to breed.
By mid-December, I will barely turn an eye to events like the one I’ve just described. By now the beach is crowded with several hundred adult sea-lions, the females huddled together for protection and warmth, with each ‘harem’ controlled by a single large male.
Pups are being born constantly, and each time one emerges, a shrieking choir of skuas descend to pull the afterbirth from the sand.
The mothers leave their pups as soon as they can to forage in the open ocean—they’ll spend a day or two feeding, then return to suckle their pups and rest before doing it all over again, sometimes later that same day. Meanwhile, the bulls jealously guard their females from neighbouring harem-holders and the lurking pretenders that shadow the fringes of the colony.
All day they bluff-charge, roar, stamp and fight—grabbing their foes by the mane and shaking hard enough to rip skin, before slumping back, wide-eyed and panting, jaws hanging open to reveal missing teeth and blood-soaked mouths.
I witness pups being trampled seconds after emerging from the womb, at least one killed by the weight of a male that sits on him, oblivious to the tiny life he is crushing.
At first glance the sea-lions’ world seems chaotic and violent, but away from the stress of the breeding arena, I see another side to these creatures. Young sub-adult males—or ‘Sammys’, as we affectionately refer to them—are a constant presence outside our hut, spending much of their day play-fighting, sleeping or lolling around in the surf.
The more time I spend with them, the more their intimidating facade slips away. Where once I saw aggression in their loud barks and fearless advances, I now see only a mischievous and exuberant curiosity. After weeks of filming, I find myself moving freely among the harems on the beach—cautiously at ease beside the wild animals.
Sea-lions do not fight because they’re violent by nature but because they must in order to breed—not every male will hold a harem, and to win that right, they must battle.
Like all wild animals, sea-lions are slaves to the genetic economics that drive their existence—an existence that may now be hanging in the balance.
The sea-lion colony at Enderby Island is one of the last outposts of a population once found all around mainland New Zealand. They were extensively hunted by early Maori and may have been extirpated from much of the mainland by 1450 AD, following the same path towards extinction as the country’s avian megafauna, including the moa.
Sea-lions in the subantarctic islands were killed in great numbers by European sealers, although only after fur seals were depleted, as sea-lion pelts were not as valuable. However, unlike fur seals, which have since returned to mainland rookeries in good numbers, it appears that sea-lions are still struggling to recover. Around 85 per cent now breed at one of three sites in the Auckland Islands, with another, smaller, population found on even more remote and inhospitable Campbell Island.
With only an estimated 10,000 animals in existence, they are one of the world’s rarest pinnipeds, listed in New Zealand as ‘nationally critical’. In 2015, sea-lions made it onto the Red List of the International Union for the Conservation of Nature with the same threat ranking as tigers and giant pandas.
While the small Campbell Island colony appears to be stable or even increasing in size, concerned researchers have watched pup numbers at the Auckland Islands halve since 1998, and the number of breeding females there has also dropped significantly over that time. It’s a dramatic and alarming decline.
The possible reasons for the decline are multifarious, uncertain and hotly contested—the case of the missing sea-lions is a tangled mystery, compounded by the remoteness of their habitat and the difficulties of conducting studies on an elusive marine mammal in the Southern Ocean. And yet there’s a single question on everyone’s lips… ‘What’s killing our sea-lions?’
SUSPECT NUMBER ONE: KILLER NETS
Sea-lions have been recorded diving to over 600 metres, deeper than any other eared seal—the Otariid family of which they are members. Their hunting forays may take them hundreds of kilometres from shore, and it’s these wide-ranging habits that bring them into contact with trawl nets.
Each year, thousands of tonnes of arrow squid are hauled off the Auckland Islands shelf edge by a fleet of mainly foreign charter fishing boats. Sea-lions feeding amid the squid shoal are occasionally caught up in the trawl nets, where unless they are able to escape, they will drown.
Female sea-lions mate within a week of giving birth, which means that the death of a single sea-lion can result in the loss of three animals—the adult, her pup on land and the unborn foetus inside her.
While a marine mammal sanctuary and marine reserve both protect an area around the Auckland Islands, sea-lions regularly range outside this area into fishing grounds where hundreds of animals are known to have died in squid nets.
A control known as an FRML (fishing-related mortality limit) annually sets an ‘acceptable’ number of sea-lion deaths in the fishery. Current regulations permit 68 sea-lions to be incidentally killed each year; modelled on fishing effort and expected ‘strike-rate’. Ninety per cent of fishing boats have observers on board.
In the late 1990s, sea-lion exclusion devices (SLEDs) were developed for the squid fishery. These comprise a steel grille built into the net that prevents sea-lions from getting trapped in the cod end, and a hatch in the top of the net that allows the animal to escape. Since the implementation of SLEDs, recorded sea-lion deaths have steadily fallen. The government accepts this as evidence that the devices work, and has allowed the industry to fish more intensively as a result. But not everyone is convinced.
University of Otago researcher Bruce Robertson has argued that SLEDs are not as effective as many claim them to be. “I think it’s incredibly optimistic to say they’ve solved fishing by doing this,” he tells me. “You don’t have to dig very far and there are huge uncertainties.”
Robertson believes sea-lions may be sustaining fatal injuries on their way out of the trawl nets, through collision with the SLED or oxygen starvation, leading to brain trauma. “All of this occurs in the dark at 200 metres,” he says. “If an individual got in late in their dive, they may not make it out in time.”
Robertson also believes that SLEDs allow drowned sea-lions to fall out of the net before it’s pulled up, evading detection by observers. But Wendi Roe, a veterinary pathologist at Massey University who has researched sea-lion deaths for 10 years, disagrees.
“It’s possible that it might happen every now and then, but I certainly don’t think hundreds of sea-lions are drowning and falling out of the nets,” she says. Roe’s work has shown that supposed evidence for SLED-related injuries was in fact caused by freezing of the carcasses prior to study. “My opinion is that SLEDs work,” she says. “And I came into this area thinking the exact opposite.”
Dead sea-lions being hauled up in trawl nets is a red rag to conservation groups, who argue that sanctioning the death of even a single individual of a protected native species is unconscionable. Forest and Bird has called for an extension of the marine mammal sanctuary around the Auckland Islands and for the fishing industry to shift to jigging for squid, a method that would prevent sea-lion deaths. The industry, however, argues that conditions in the Southern Ocean make this unfeasible, that the mitigation in place is effective, and that the decline of sea-lions is the result of other forces at play.
SUSPECT NUMBER TWO: FISHING PRESSURE
Sea-lions are opportunistic feeders that will eat anything from barracouta to fur seals. In the Auckland Islands, their diet consists mainly of opalfish, rattail, hoki, red cod, squid and octopus. It has been suggested that fishing pressure on squid and hoki stocks there may be forcing sea-lions to work much harder for their food, and perhaps shift their foraging towards less-nutritious species.
The timing of the squid fishery (February–May) coincides with the period that female sea-lions are still nursing their pups on shore and need every bit of sustenance they can get.
Sea-lions may need as much arrow squid as the trawlers are taking, in which case indirect fishing competition is of significant concern, although another recent study suggests that these squid form only a minor part of sea-lion diet.
“It’s very difficult to assess the effect of fishing,” says NIWA’s Jim Roberts, who is currently heading an investigation into the threats to sea-lions, due for release later this year. “The local availability of prey can also relate to changes in local climate, and that has to be compared with the foraging of sea-lions. You need some really fine scale information to disentangle that.”
Almost every diet study on Auckland Island sea-lions has found that a large part of their diet is made up of the elusive endemic yellow octopus, Enteroctopus zealandicus. Scientists know very little about these cephalopods as fewer than 100 have ever been observed, although it is thought that Auckland Island sea-lions devour more than a million of them every year.
In February, Roberts and other NIWA scientists made a trip to the Auckland Islands and Stewart Island to conduct trawl surveys of key sea-lion prey species to shed more light on the biology of this enigmatic species and enable scientists to better understand its contribution to sea-lion diet. Understanding what sea-lions are eating and how that is changing is crucial to deciphering the unfathomable decline in their number.
SUSPECT NUMBER THREE: DISEASE
In 1998, the year the decline in pup production began, a bacterial epidemic swept through the colony on Enderby Island. Researchers on the beach piled up 1600 dead pups and 70 dead adults, while nursing females fled the beach, leaving their pups to starve. Then, in 2002 and 2003, another epidemic, Klebsiella pneumonia, again ravaged the colony. For Wendi Roe, tackling such disease outbreaks is a key consideration in turning around the Auckland Islands decline.
“Just looking at the number of pups that die, the biggest thing by a long, long way is bacteria,” she says. “From my perspective, we need to understand much more clearly how the disease is working and what we might be able to do about it.”
Other researchers argue that pup survival does not necessarily impact on the viability of the population as a whole, and that disease is therefore not a major factor. Many pinniped species sustain a naturally high infant mortality rate. Up to half the pups at the rugged Campbell Island colony die each year from natural causes—falling off steep drop-offs or being crushed by rampaging bulls—and yet that population remains stable and may even be increasing. The Campbell Island population also seems to withstand disease much better than that at the Auckland Islands.
Disease, after all, is an inevitable part of life in a crowded pinniped colony, something Roe doesn’t dispute. “Even if 75 per cent of the pups in a season die, it may not have an effect on the population long term,” she says. “The thing is, if you add that to other things that are happening, it becomes important.”
What can be done about it is another story—while developing a vaccine is possible, Klebsiella is a fast-moving infection that’s very difficult to treat, and if it is surviving in the breeding environment through winter, may be almost impossible to get rid of.
SUSPECT NUMBER FOUR: THE FATAL COMBINATION
Simon Childerhouse began researching the sea-lions at Enderby Island in 1996, and for the past four years has been leading the annual Department of Conservation survey there.
“The discussion is polarised into several camps,” he says. “I think we need to get out of those and recognise that if we want to do best for the sea-lions we have to consider all of these different threats and think about how to manage them.”
In his opinion, SLEDs, while not perfect, are an effective mitigation tool. “The focus has always been on fishing because that always has been the most easily identifiable contributor to the decline,” he says. “But what the recent modelling has shown is that there’s quite a few other things going on as well.”
Jim Roberts also believes it’s time to cast the net, so to speak, wider. “I think in the past there’s been a single fact bias,” he tells me. “There’s a bit of a tit for tat that’s been played out through the media, that the most important thing is adult survival and that might relate to fishing mortality, or it’s pup survival affected by bacterial disease. But the research that we’ve been doing says that you can’t explain it by one of those, it’s a combination of the two, and there are other threats as well that may be implicated.”
Every year, pups drown in muddy holes behind the beach at Enderby Island. The Department of Conservation has installed ramps to help them climb out again, a relatively simple thing that may save dozens of pups each year.
Natural variation in food supply is also certain to have an impact on sea-lion survivability, especially in a marginal environment like the Auckland Islands. The fall in pup numbers since 1998 coincided with the disappearance from the area of a formerly common fish species, Chilean jack mackerel, perhaps as a result of severe El Niño conditions that year. The prevalence of hoki in the Auckland Islands also dropped markedly during the mid-2000s.
Other factors potentially affecting sea-lion survivability include predation (particularly by great white sharks), a genetic bottleneck effect as a result of reduced numbers, and toxic phytoplankton blooms. And then of course there is human-induced climate change, the effects of which remain to be seen.
Some or all of these factors working in concert may therefore be creating a perfect storm for the sea-lions, which cannot extend their range or move their colony to somewhere more convenient.
Overarching all of this is uncertainty about just how many sea-lions we ‘should’ be seeing in the subantarctic. “We’re lacking information about what we’re trying to conserve the species back to,” says Roberts. “We don’t know if the current population size is roughly as high as it ever was, or if there is some further growth potential that it should naturally attain.” A recent genetic study estimates New Zealand sea-lions could have numbered from 17,000 to 205,000 animals in prehistoric times, a wide target.
Encouragingly, there are signs that the population slide may have now slowed, as pup numbers at Enderby Island have remained relatively constant for the past seven years.
“My personal opinion, which is not shared by everyone, is that pup production has stabilised now at a lower equilibrium than it was previously,” Childerhouse tells me. This year, he and his team recorded a 10 per cent increase in pups born, 15 per cent higher than the lowest recorded year in 2009.
I prepare to depart Enderby Island in early January, just as the intensity of the breeding arena is starting to ebb. On my last evening, I walk out on the beach to take a few photographs. The cloud is breaking up over Port Ross and a rare burst of sunshine spills over the colony. A trio of yearling females come scampering out to greet me, one swinging her head side to side in a wildly comic show of mock aggression, the rich sunlight lighting up her hide a deep golden-brown.
These young females have avoided the beach until now—the breeding arena is a dangerous place for them—and so they are the last characters in the New Zealand sea-lion family I will come to know on this trip. They are delightful animals—full of play, the epitome of sea-lion spirit.
Back in Dunedin two weeks later, I find the grind of urban life seems offensive after months in such a wild place. I long for the simplicity and daily drama of my life among the sea-lions, but the ocean connects everything, and in Otago at least, sea-lions are never far away.
In 1993, a female named “Mum” hauled up at Taieri Mouth to give birth to her pup, the first recorded sea-lion birth on the mainland since the days of sealing.
Sea-lions exhibit strong philopatry, which means that to breed, they will almost always return to the place where they were born. Mum’s daughters now return to Otago to bear their own pups, and around 70 of her descendants have now been born in the province. Another female from Enderby Island has since founded a new family line in the Catlins, while a small population of around 35 breeding females has been established at Stewart Island.
The females’ allegiance to their place of birth allows for strong genetic divergence to occur, and recent research has revealed that the subantarctic animals now recolonising the mainland are of a different genetic stock to those that were originally here.
The sea-lions we encounter on our coasts are therefore pioneers—a new breed choosing to make their home on these shores. This time, it’s up to us to make them welcome.
I open up the paper one morning to see a picture of a female sea-lion and her pup on a local Dunedin beach. I drive out to see if I can find them, but in vain—they’re tucked away somewhere amid the pine trees and campers that crowd the foreshore. After all my time on Enderby Island, it feels strange to be looking for sea-lions amid fold-up chairs, barbecue tables and caravans.
Local DOC ranger Jim Fyfe tells me that the females will often choose a spot near people—it helps them to hide from the males that would otherwise harass them.
A few days later, I accompany Fyfe out to Boulder Beach on the Otago Peninsula. There’s a pup here that he needs to tag for identification, one of 10 born in Otago this year. “I think it’s a very rare conservation story in this day and age,” he tells me. “It’s the nearest we’ve come to the return of an animal that’s been wiped out. I compare it to the return of the moa to the New Zealand mainland.”
After a fair bit of searching, we find the mother, a four-year-old, secluded amid the marram grass and scrub deep in the dunes. Females on the mainland breed earlier in life than their subantarctic relatives—a sure sign of better living conditions here. However, those that haul up on New Zealand beaches have other threats to contend with, including dogs and even direct human aggression. Tragically, sea-lions have been shot on New Zealand beaches. For the most part, though, people and sea-lions seem to coexist peacefully on the mainland. In Dunedin, pups have been born on city beaches, while breeding females are welcomed and cherished by small communities along the southern coast.
Fyfe tags the pup, a male, and when we walk past again half an hour later, he is snoozing, unbothered by the new chunk of plastic hanging off his flipper.
It’s tempting to think that in six or seven years he’ll be one of those big, belligerent bulls, huffing and puffing his way among a harem of females here in Otago, and that for the first time in hundreds of years, people walking these beaches might be able to see something of that ancient drama I was lucky enough to witness on Enderby Island—the New Zealand sea-lion, come in from the cold at last.