Heatwaves driving whale and dolphin strandings

Increasingly warmer seas could see new types of marine mammals beaching on New Zealand's shores, experts say.

A seven to eight-metre juvenile fin whale died after getting stuck on a sandbar in the shallow estuary at Moncks Bay in Christchurch on Sunday night.

New Zealand has one of the highest stranding rates in the world, with about 300 dolphins and whales beaching each year.

Daren Grover, from the marine rescue charity Project Jonah, said while strandings would generally happen in summer, now they had to be ready for them to happen any time.

"Over the summer months, we have warmer coastal waters, and historically we've seen greater numbers of whales and dolphins coming closer to shore, passing through our waters as well," Grover said.

"We've had a series of year-on-year marine heatwaves, and that's caused our stranding season to be much longer than it used to be."

The juvenile fin whale died after getting stuck on a sandbar at Moncks Bay on Sunday night. Photo...
The juvenile fin whale died after getting stuck on a sandbar at Moncks Bay on Sunday night. Photo: Supplied
Massey University marine biology professor Karen Stockin said the warming temperatures had changed the distribution of prey in the water - meaning marine mammals not previously seen near New Zealand were coming here, and consequently could end up becoming stranded.

"Are we likely to see more blue whales? Are we likely to see more fin whales? Are we also likely to see increasingly more Antarctic species ... things such as leopard seals?"

With the weekend's Christchurch whale stranding, residents and experts alike were surprised a fin whale was in the area in the first place.

But Stockin said the species had recently been seen in a number of places around New Zealand.

"(Sunday's) fin whale off Christchurch, and the fact we've had one off Haast just a week ago, and we also had another fin whale off Wellington just in December - they're not a species regularly sighted or observed, and certainly not stranding in New Zealand, but yet here we are with three of those since December," she said.

Department of Conservation Mahaanui District operations manager Andy Thompson said there had been a lot of interest in the Moncks Bay stranding from the public, and offers of support from residents in the area.

"It seems like the second time in a few months we've had a whale almost right in the middle of Christchurch city, so a lot of people passing by, a lot of interest and of course a lot of emotion around trying to save the whale and do the right thing," he said.

Project Jonah's Grover said if people saw a stranded whale or dolphin they could phone the Department of Conservation or Project Jonah, which operates a 24-hour hotline.

Other strandings in the past few months include a pod of 45 false killer whales and one bottlenose dolphin, which had to be euthanised after becoming beached near Māhia.