Another 250 whales reportedly stranded at Chatham Islands

A pilot whale stranding on Stewart Island in 2018. Photo / Supplied

Hundreds more pilot whales are reported to have beached themselves in the Chatham Islands – making for one of the species' worst recorded stranding events.

After more than 215 whales died after stranding at the northwest corner of Chatham Island on Saturday, another 250 are reported to have beached at Pitt Island, about 40km away.

Massey University marine ecologist Professor Karen Stockin expected the chances of survival among these whales was "nil to none".

"Pitt Island has only got one [Department of Conservation] ranger there, and they are trying to mobilise some of the team from Chatham Island in response."

When contacted for comment, a DOC spokesperson said it was dealing with a "mass stranding" at Pitt Island, and had sent a team over to "assess and handle the situation".

It has previously said it doesn't actively refloat whales at the islands, due to the risk of shark attack to people and the whales.

"We're talking about harsh conditions, a very small number of staff and few residents: this really is a case of assessing if any are still alive and likely to end in euthanasia again," Stockin said.

1/ It's been a full-on weekend with the mass stranding on the Chatham Islands, multiple calls about possible whale and dolphin strandings in Tāmaki Makaurau, a Leopard seal event in Timaru that required police intervention and a few phone calls about seals in precarious places. pic.twitter.com/OzB3AzGoDv

— Project Jonah (@ProjectJonah) October 9, 2022

Project Jonah, a not-for-profit organisation that helps rescue animals and protects marine mammals, has also described the remote islands – home to fewer than 800 people - as a "challenging spot" to mount rescues.

Strandings are "not uncommon" in the Chatham Islands it says, noting that the largest recorded event involved an estimated 1000 whales in 1918.

Why these strandings occurred – and also why they sometimes coincided with similar strandings in southern Australia – remained a largely unanswered question.

Pilot whales — which can grow to more than six metres (20 feet) long — are also highly sociable, so they may follow podmates who stray into danger.

"This time of year is the beginning the stranding season and I find it really interesting that the patterns we see in South Australia and some of our offshore islands are very much closely aligned," Stockin said.

"That in itself is a really interesting part of the work that needs to be done."

Another question was what impact this number of deaths - which she said made for "one of the largest stranding events for this species, in New Zealand and globally" - would have on local populations.

"The amount of loss to the population is significant," she said.

"What it means for population fitness and viability, we don't know, and there hasn't really been a great deal of work done."

Fellow Massey researcher Dr Emma Betty said predicting these impacts were challenging "especially when dealing with a population for which limited biological information is available".

"We do not have a good understanding of the current size of the long-finned pilot whale population in New Zealand waters, but some of our recent work has provided estimates of key reproductive parameters."

An understanding of reproduction is important, she said, since life history traits might ultimately influence population stability, growth or recovery.

"In the event of increasing stranding-related mortality, expected recovery rates can be modelled using reproductive parameters, such as calving interval and average age at attainment of sexual maturity.

"If monitored over time, changes in reproductive parameters could also help detect population-level impacts."

In New Zealand, about 300 animals beach themselves annually, according to official figures, and it is not unusual for groups of between 20 and 50 pilot whales to run aground.