Mystery of why Otago sea lions are so big and healthy

Undertaking a health check on Gem, a 3-year-old sea lion, are (from left, clockwise) University of Otago zoology PhD student Amélie Augé, Department of Conservation field officer Nathan McNally, Wellington Zoo veterinarian Katja Geschke, Doc marine mammal scientist Dr Louise Chilvers, and biologist Jacinda Amey. Photo by Peter McIntosh.

Last year, four females from the Otago Peninsula sea lion population were tagged, and their locations and the depths to which they dived recorded.

The information showed the Otago population was larger and healthier, and spent less time foraging than their Auckland Island counterparts.

They also did not dive as deep, and stayed closer to shore.

Department of Conservation marine mammal scientist, Dr Louise Chilvers, said the information showed the sea lions travelled around nearly all of Otago's beaches, from Shag Pt to Brighton, including beaches popular with the public like St Clair and St Kilda.

"It's something we never realised that females with pups would do."

This year it had been decided to attach satellite tracking tags to younger females, who were still learning how to feed, to see if they copied their mothers.

This week, a team led by Dr Chilvers captured, anaesthetised and glued the tags to the backs of seven sea lions.

The tags would be recovered at the end of May.

VHF transmitters were also put on each female so that they could be tracked on land to allow samples to be taken for diet studies.

While the sea lions were under anaesthetic, the team also took blood samples and swabs, so their health could be assessed.

One of those tagged was Gem, a 3-year-old daughter of Katya, the first pup born on the mainland in decades, who was often seen at St Clair and St Kilda beaches.

She weighed 105kg.

"That is bigger than most adult females we see in the Auckland Islands - it's a lovely size," said Dr Chilvers.

In the Auckland Islands, the main home for the sea lion population, pup numbers were down 30% this year.

University of Otago zoology PhD student, Amélie Augé, said the research would continue for another year, to help better understand the behaviour of this important and relatively new breeding population.

Doc marine ranger Jim Fyfe said the more learnt about the sea lion, the better Doc could manage potential interactions with humans along this coastline.