He said evidence so far suggested Milky White Flesh Syndrome was due to a lack of food, which was caused by factors such as extended La Nina weather patterns and warmer waters reducing production of phytoplankton and zooplankton. Those microscopic organisms provided vital food at the bottom of the food chain.

“This is a complex issue, and is more likely to be due to the effect of recent weather patterns and natural changes in fish biology throughout the seasons. Prevalence of the syndrome appears to be declining, which supports these theories.”

Warmer ocean temperatures could also be affecting snapper metabolism, making them use more energy and requiring more food.

The discovery of malnourished snapper prompted Forest & Bird to call for a ban on bottom trawling in the Hauraki Gulf.

The conservation group said overfishing, in particular bottom trawling and bottom-contact fishing, was “decimating” the sea floor and food sources snapper depended on.

Lawrence said, however, commercial fishing did not affect the main food sources of snapper and the areas of the gulf where bottom trawling was permitted did not correlate with the areas where the syndrome was most prevalent.

Niwa carried out the research on behalf of Fisheries New Zealand, with scientists collecting a range of data, including from commercial and recreational fishers, about affected snapper and where they had been caught.

The Milky White Flesh Syndrome appears to be unrelated to the so-called “zombie fish” being reported in the Kaipara Harbour on the opposite coast.

The zombie fish term was coined by recreational fisherman Sam Erickson earlier this year, when he reported as many as eight out of 10 snapper caught near Ruawai had sunken, bloodshot, milky eyes.

Zombie fish were lethargic when caught and appeared to be missing their normal mucus layer and outer flesh.

The cause is also under investigation by Fisheries New Zealand, with five fish so far collected for investigation.

Lawrence said one snapper had been tested so far by Biosecurity New Zealand, which had ruled out exotic diseases or infectious agents.

The fish had cataracts and inflammation in both eyes, as well as internal scarring or thickening of tissue.

The four other snapper were also undergoing a full range of tests, he said.

Meanwhile, the Dargaville Ratepayers and Residents Association wants an inquiry into the state of the Northern Wairoa River – a major waterway that flows into the Kaipara Harbour, where many of the ailing fish were caught – and whether the river’s poor health is connected with the zombie fish phenomenon.

Chairwoman Rose Dixon said the Northland Regional Council rated water quality in the Northern Wairoa as “very poor”, with the river failing eight out of 12 standards between 2020 and 2023, including turbidity, total nitrogen, total phosphorus, faecal bacteria, total zinc and total copper.

Sedimentation was another serious threat to harbour health, which a major tree-planting programme was attempting to address.

The Kaipara Moana Remediation project is now approaching its two-millionth tree in the ground.

Food Safety New Zealand is also conducting its own tests to determine whether there are any risks from eating snapper displaying zombie fish symptoms.

The agency’s advice is not to eat any fish that looks unhealthy.

Fishers who catch zombie fish, or who are concerned about the state of any fish they have caught, should report it to the Biosecurity New Zealand hotline 0800 80 99 66.

The fish should be photographed and kept whole and chilled (not frozen) so it can be tested.