Odd behaviour among snapper this summer has been put down to a lingering marine heatwave that last month made the Tasman Sea the warmest on record for December.

Fishers have told the Herald of the fish n' chip shop favourite spawning more than a month earlier than normal.

Motueka commercial fisherman Chris West has been trawling for snapper since 1981.

"They're our target species - our bread and butter, I suppose you could say - and we've studied them and watched them closely for all of that time."

Advertisement

Each year, there was a consistent build-up starting in October and carrying on through November, making the end of the year the best time for fishing.

After weeks of good catches, one morning, West noticed snapper had vanished far earlier than normal.

"It was quite amazing really... they just disappeared overnight, just, see ya, buddy."

From his years of observing snapper populations, West estimated snapper had spawned around six weeks earlier than normal this season, before leaving fishing grounds earlier.

West said the change had also been much discussed by recreational fishers.

There were other reports of people catching snapper in Milford Sound and Doubtful Sound for the first time, and bluebottle jellyfish washing up on beaches a month early.

Previous New Zealand studies have indicated snapper spawning began in water as low as 15C, and as early as September, with the peak occurring around November and December.

Backing up the fishers' anecdotes with hard data wasn't straightforward.

Advertisement

To confirm those reports, scientists would have to take monthly samples of fish and look at their gonad stage, which wasn't done currently.

Scientists might also be able to infer an effect from the spatial and temporal pattern in commercial catch rates, but this data wasn't made available until months after.

But climate scientist and marine heatwave expert Dr Jim Salinger said early spawning would have been triggered by the warmest Tasman Sea in more than a century of records.

Between September and November, sea surface temperatures in the Tasman were 0.9C above average.

"Schools of kingfish have also been sighted in the Fiords," Salinger said.

In Otago Harbour, warmer waters were suspected to have been behind an increase in stingray sightings.

Having set in strongly over late spring - around the same time effects of a La Nina climate system began being felt here - the marine heatwave was lingering on, with sea surface temperatures this so far running at least 2C above average.

Waters were as warm as 16-17C off the Otago coast, and 20C off Taranaki in the west and Hawkes Bay in the east.

"The big actor in this has been a combination of global warming, with sea surface temperatures now 0.7C to 0.9C warmer than 100 years ago, and a very positive Southern Annular Mode," Salinger said.

"This has meant that the anticyclones or highs have been well south of their normal track across New Zealand - instead passing over the South Island and to the south, keeping the door shut to cold blasts up from the Southern Oceans.

"As a result unusual easterly winds along their northern flanks have kept the swells down in the Tasman Sea reducing the mixing, and that to the east of the South Island."

The warming of the ocean's top layer had prevented the normal upwelling of cold water, something known to bring up krill to the east of the South Island.

"Red billed gulls around Kaikoura have not caught the usual krill with which to feed their young, and have had to feed on jellyfish instead," Salinger said.

The marine heatwave was expected to last into next month, and then dissipate.