The mystery of the absent sea lion pups

Wednesday, 15 February 2023

An annual count of endangered sea lions on our remotest islands has revealed numbers are drastically down.

The world’s rarest sea lion is one step closer to extinction with a new survey showing worryingly low numbers of pups.

An annual count of rāpoka, or New Zealand sea lion, on the remote subantarctic Auckland Islands was 30 per cent down on the previous summer.

It means 400 fewer pups were born. And that has triggered a government review of a protection plan, and of squid fishing in their foraging grounds.

Scientists are at a loss to explain the drop – devastating to a population that numbers only 10-12,000 and is on an international ‘red list’ of endangered species.

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The New Zealand sea lion, once known as Hooker's sea lion, is endemic to New Zealand and primarily breeds on Auckland and Campbell islands

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The sea lion was hunted almost to extinction between the 13th and 19th centuries. After sealing was banned in 1893, the population began to recover.

But, about 25 years ago the population began to plummet again, with a 40 per cent drop in pup production in 1998 at the Auckland Islands, the main breeding area. Overall, the population there declined by which declined by about 50% between 2000 and 2015.

Nearly 80% of all sea lion breeding occurs on the Auckland Islands, with colonies on Dundas Island, Enderby Island and Figure of Eight Island.

The possible reasons for decline were fisheries interactions – both direct, such as accidental capture or boat strike and indirect, like competition for food resources.

Another cause may have been outbreak of disease with bacterial infection epidemics in 1997/98, 2001/02 and 2002/03.

However, the latest data has experts puzzled. Otago University zoologist Bruce Robertson says finding out why will be especially difficult because of the way the species breeds.

Pups are dependent on their mothers for milk and protection for the first year of their lives. While mothers are at sea feeding, their pups are alone.

Females give birth between December and mid-January. Within 6-10 days of giving birth, females go into “heat” (known as oestrus) for roughly one day, and a male will mate with her for next year's pup.

But she doesn’t immediately fall pregnant: the fertilised egg is ‘put on ice’ and only implants into the uterus wall after a couple of months. Gestation is about 10 months, even though the female is mated some 12 months before she gives birth.

“If you assume those individuals have mated, and they've been fertilised then those females have either reabsorbed those pups, or they’ve [the pups] died,” Robertson said. “Something's happened to them and that's not normal. We haven't had that as long as we've been recording.

Subantarctic squid, scampi trawl fisheries, and southern blue whiting fishery operate in the sea lion foraging grounds.

“It can be tied to a body condition but also environmental conditions.”

DOC science advisor Laura Boren stressed the numbers were preliminary, and the data was being finalised. But she said researchers noted that pup mortality in the rookeries was low, and that observed marine mammals appeared to be healthy.

“Those sorts of things tell us that whatever the cause is, is something that happened prior to females coming ashore to breed. Definitely, a lot of a lot of things are pointing to environmental circumstances this year. It is really sad and it is really concerning.

Sea lions were extirpated from mainland New Zealand for 300 years. In 1994, a pup was born at Taieri Mouth on the south Otago coast, and in 2022 the peninsula’s female population numbered 28.

'There could be any number of reasons. Females might have decided not to breed this year, and so they went elsewhere. Or they could have not had enough food to maintain a pregnancy,” she said.

Climate change, causing the Southern Ocean to warm, is also a consideration.

Because the number of new pups fell below 1501, that triggers an overhaul of a threat management plan.

The plan was supposed to halt the decline within five years and ensure the population is stable or increasing within 20 years.

The threshold also means Fisheries NZ must review an operational plan for fishing for squid in that area. . Industrial fishing depletes a food source, and nets accidentally trap and kill the pinnipeds.

Females limit their foraging range when nursing, making them more susceptible to colliding with trawl nets around the breeding colonies. When caught, most are nursing and pregnant, and so leave behind a starving pup.

The fishing industry says bycatch in the squid trawl fishery has been greatly reduced since government observers and sea lion exclusion devices – which allow ensnared creatures to escape – were deployed.

Marine conservationist are not convinced of the efficacy of SLEDS, or sea lion exclusion devices, and say data is limited.