A woman and her son were stunned when they came across tens of thousands of washed-up fish while walking along a Kapiti Coast beach.
A marine biologist, who has reviewed the pictures the pair took, said he had never seen anything like it describing the sight as "very unusual".
Louise Wilman and her 11-year-old son, who describe themselves as "marine nerds", took a trip from Waikanae to Pukerua Bay on Wednesday to beachcomb.
"We go there lots because there's lots of different sea life and things," Wilman said.
"We just decided to go for a walk and see what had washed up because we were aware that there was a full moon the night before, and it'd been quite stormy recently."
As they approached the beach, they spotted flocks of seagulls diving towards the water.
"We were saying to each other, 'Oh, there must be lots of good fish in the sea'."
Closer to the shore, they spotted "thousands and thousands" of shiny fish, later identified as deep-sea-dwelling lanternfish, strewn along the shoreline.
"There were just literally millions of them in every direction, also jumping in and out of the waves as the waves were rolling in.
"We just ran up and down the beach, all excited, looking at all the fish.
"My son's really into marine life. He was in the waves, trying to catch them and look at them. They were actually swimming around, and all the birds were diving, and yeah, it was crazy."
Wilman, who frequently photographs nature, called the sight "bittersweet".
"It's so sad that all these fish are dying, but it just looks amazing with the sun on them."
She said she had never seen anything like it before.
"A few years ago, we had some fish that looked like little anchovies. Hundreds of them had washed up, and then lately there's been some krill.
"But never that many like, they were thick in the sand."
'Very unusual' - marine biologist
Professor Kendall Clements, a marine biologist at the University of Auckland, said he hadn't previously heard of a mass stranding of lanternfish.
"This is very unusual," he told 1News, saying a combination of different factors could have caused the fish to wash up.
Lanternfish are among the most abundant species in the ocean and inhabit the twilight zone – the area 200 to 1000 metres below the surface, which receives only a faint amount of sunlight.
He explained that many species of lanternfish undergo what is called "vertical migration".
"You won't see them in the upper couple of 100 metres during the day. They go quite deep. They go down below 1000 metres.
"During the night, they move up towards the surface and feed near the surface on zooplankton."
Clements said it was possible the weather brought them to shore. The area has experienced large swells recently, as well as storms last weekend.
"Maybe what happened was some fish got washed into shallow water and died as a result of that.
"These are are open-ocean fish," he said. "They couldn't survive in shallow water like that."
Shallow-water predators may have also caused the fish to jump onto the shore.
"Fish will jump out of the water under those circumstances - you see strandings of things like anchovies. I've seen that in the tropics."
He said the fish had likely been caught in the "wrong place at the wrong time".
It wasn't something people should be freaking out about, however, and Clements did not think the fish had died from poisoning or disease.
"These all looked pretty healthy," he said. "Lanternfish are fairly fragile and tend to lose their scales and things like that when they get caught in nets.
"But these ones look like they were fine one minute and then not the next.
"It probably is related to the weather. This isn't some sort of harbinger of some sort of catastrophe, I don't think."
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