Other species he saw that day appeared unaffected, and larger mullet further up the estuary looked healthy, he said.

Concerned about the appearance of the diseased fish and that mullet are an important part of the wider ecosystem, Watkins reported his discovery to the Ministry for Primary Industries (MPI) and was put through to its Wellington after-hours service.

He said that “within two minutes” of hanging up, he had received a call from Biosecurity NZ, which seemed “really interested” and asked him to deliver the fish the following morning to its Whangārei office for testing.

Watkins was told results would take about four weeks.

Photographs he posted of the diseased fish to social media quickly drew numerous responses. Many people suggested the disease looked like what was discovered in yellow-eyed mullet (Aldrichetta forsteri) in the Mahurangi River at Warkworth, North Auckland, during April this year.

That outbreak was ultimately confirmed as the myxozoan parasite (Myxobolus episquamalis).

According to MPI testing at the time, Myxobolus episquamalis was previously found in grey mullet (Mugil cephalus) in New Zealand in 2013, in the same region. However, the Mahurangi case was the first confirmed in yellow-eyed mullet (YEM).

The parasite causes granular lesions across the body surface, which may lead to secondary bacterial infections, but it has not been directly linked to fish deaths.

Factors contributing to the increased prevalence of Myxobolus episquamalis are unknown. Possible causes include changes in the parasite’s secondary host life cycle or increased stress in YEM due to spawning or environmental conditions.

Mike Taylor, team manager aquatic health, Biosecurity New Zealand confirmed to the Advocate yesterday that the samples Watkins discovered would be sent to its Animal Health Laboratory for identification and testing.

People should not consume fish with lesions as it could cause illness. They should also wash their hands thoroughly if they touch the fish. Fish with lesions should also not be used as bait as this can spread the disease.

If people feel unwell after eating or touching infected fish, they should seek advice from Healthline or their GP.

Watkins said he knew of at least two school trips planned for the estuary in the upcoming weeks and wondered if it was safe for swimming.

A spokesperson for Northland Regional Council (NRC) said the water conditions at popular swimming spots, including those near the estuary but not for the estuary itself, were regularly updated at safeswim.org.nz.

Water quality data indicate the estuary is generally safe for swimming, but care should be taken after rainfall, where water quality can exceed the recreational bathing guidelines. The quality also dipped at low tide.

The Waipu discovery comes just months after something dubbed “zombie fish” alarmed recreational fishers in Northland’s Kaipara Harbour.

In May last year, Kaipara angler Sam Erickson coined the term after catching snapper so sick he described them as “like the walking dead”.

Symptoms included cloudy, cataract-like eyes, sunken faces, protruding bones, and a lack of mucus and flesh. Fisheries New Zealand received samples from Kaipara Harbour and other North Island locations, but tests failed to pinpoint a cause.

While exotic diseases and parasites were ruled out, Biosecurity NZ suggested an environmental pollutant – such as a chemical, heavy metal or toxin – was the most likely culprit. However, subsequent testing found heavy metal and chemical levels within normal ranges, and biotoxin screening also came back clear. Seasonal factors and stress remain under consideration.

Fishers are still being urged to report any snapper or other species showing these symptoms and avoid eating affected fish.

Sarah Curtis is a news reporter for the Northern Advocate, focusing on a wide range of issues. She has nearly 20 years’ experience in journalism, most of which she spent court reporting in Gisborne and on the East Coast.