Warmer sea temperatures have been a factor in less sea lettuce in Tauranga's waters this summer, experts say.

Traditionally, Tauranga Harbour's shorelines are blanketed in the smelly green sea lettuce each year but officials confirm there has been a notable drop-off this summer.

However, Bay of Plenty Regional Council coastal catchments principal advisor Toby Barach said warmer temperatures were "just one of several complex factors that affect the abundance of sea lettuce".

"There have been small volumes of sea lettuce washing up at Pahoia, Kauri Point and Ongāre Point but much less than in previous years.

Advertisement

"With an increase in water temperature there has been a drop-off in growth and less sea lettuce washing up on beaches."

There is very little sea lettuce on the shoreline of Fergusson Park. Photo /Andrew Warner
There is very little sea lettuce on the shoreline of Fergusson Park. Photo /Andrew Warner

The regional council currently has a research project under way with the University of Waikato to better understand local sea lettuce growth. It has already learned the optimum temperature window for the lettuce to thrive was between 16C and 20C.

"Growth rates drop when water temp falls below 16C ... and growth generally stops in water temperature above 25C."

In Ōmokoroa the average temperature for January has been 21.4C, with the highest daily recording over 25C.

Barach said warmer seas usually attracted more jellyfish due to an increase in good food supply, however tides, currents and wind played a large role in the movement of jellyfish, as they relied on drifting with currents for movement.

"Less storms and onshore winds may mean less jellyfish have drifted into the harbour."

Barach said warmer waters also increased the growth and abundance of microscopic algae, providing more food for the food chain, but also decrease oxygen concentrations which could cause short-term stress on shellfish.

A woman among the sea lettuce at the Tay Street Beach, Mount Maunganui, in 2015. Photo / file
A woman among the sea lettuce at the Tay Street Beach, Mount Maunganui, in 2015. Photo / file

Russ Hawkins, skipper and owner of Fat Boy Charters, said there definitely appeared to be far less sea lettuce and jellyfish.

Advertisement

Hawkins said near Astrolabe Reef the water temperature was 22C and by the time he got to Okaparu Reef near Mōtītī Island, it had risen to 23.7C.

"We usually see a lot of large jellyfish out in the harbour, these are the really big ones that can be as much as two to three feet across, but I haven't seen any yet this summer."

Hawkins said a few days ago he saw a school of SkipJack Tuna near Astrolabe Reef, a fish species which likes warmer water temperatures.

Fergusson Park is one of the Tauranga spots which is usually inundated with sea lettuce.

On Thursday, Bellevue resident Gena McGregor was out walking her dog there and said she was surprised and pleased there was little sea lettuce spread across the shoreline.

"Last year the council was collecting truckloads of the stuff. When the sea lettuce decomposes it absolutely stinks, so it's really nice not have to put up with the pong."

Lack of sea lettuce surprising

A Mount Maunganui surfer who usually collects sea lettuce for his garden says he has been surprised at the lack of sea lettuce on Tauranga shores this summer.

Willie Donovan was visiting Fergusson Park on Thursday when he told the Bay of Plenty Times how little sea lettuce there was compared to the same time last year.

"I am usually collecting quite a bit for my garden but there definitely doesn't seem to be much either at Fergusson Park or at Mount Maunganui," he said.

"Some years, the council contractors collect truckloads of sea lettuce and take it to an organic orchardist in Te Puke."

Donavan said he was surfing and doing a bit of fishing down at the Mount Maunganui a few days ago and the water temperature was so warm, he didn't need a wetsuit.

He was not sure whether the balmy water temperature was linked to the reduced amount of sea lettuce but he had also not seen as many jellyfish this summer either.