The markings are part of a study by University of Otago PhD student Nathan McNally, who wants to investigate the sea lions' movements between the mainland and the subantarctic and observe their behaviour at Enderby Island, where they breed.
So he could recognise each individual sea lion, he would "tag" those on Otago Peninsula or the Catlins beaches with a diluted mix of hydrogen peroxide and ammonia, applied with a "tomato sauce bottle" while they were asleep.
The identifying marks would be letters and a number on the hair on the lower back of the males, just above the hips.
"I've used this method in the past. It's harmless and rather like having your hair coloured at a salon. In fact, the majority of sea lions remain asleep while I'm applying the bleach."
The bleach mark, which could be identified from a distance, did not affect the animals' hides and disappeared during moulting the following autumn.
Last season, he did a trial run of the "tagging" and 15 sea lions were spotted returning to Otago's coast.
While on Enderby Island this summer to carry out sea lion monitoring for Doc, Mr McNally would use any downtime to observe the marked sea lions, mapping their movements and relationships to try to understand their breeding system.
"It's an excellent opportunity to monitor movements between mainland New Zealand, Stewart Island, the Snares, Auckland Islands and Campbell Island," he said.
"I'll also observe their breeding behaviour from the relative comfort of a viewing hide at Sandy Bay on Enderby Island."
The information gathered, combined with tag and microchip recording, would indicate how many males returned to breed each season, he said.
Mr McNally was also keen to hear of sightings of bleached individuals when they returned from the breeding grounds in late January. Records should include date, place and a description of the bleach mark.
