Fish with milky, bloodshot eyes and lack of flesh caught in Kaipara Harbour

Improving the health of the Kaipara Harbour could help turn around the flailing fish health, a scientist says. Photo / Jason Oxenham

Snapper caught in New Zealand’s largest harbour are so unwell that one fisherman is describing them as zombie fish or “like the walking dead”.

Scientists say more testing is needed to find out what is wrong with the fish, which have been caught in Kaipara Harbour’s Northern Wairoa River.

It is likely a parasite or a version of the milky-white flesh syndrome afflicting snapper on the east coast, which is linked to chronic starvation.

Recreational fisherman Sam Erickson said he first noticed the sick fish about a month ago while fishing near Ruawai.

“They just looked like the walking dead – the eyes were sunken in the face and the eyes were bloodshot and milky all at the same time.”

Erickson said the fish appeared to be missing the outer layer of flesh, with bones sticking through, and they were lacking their normal mucus layer. The sick fish gave a lethargic fight when caught but did not appear to be starving.

Erickson said when he first noticed the sick fish, he simply threw them back, with about 80 per cent of his catch affected.

But after talking with other recreational fishers, he realised a pattern was starting to emerge in fish caught near Ruawai, but with other parts of the harbour not seeming to have the same problems.

With Erickson’s latest fishing trip on Sunday, about 12 per cent of fish were affected and he kept a sick one for further testing.

He is concerned the ill fish have been caused by a water quality issue: “I very much would like to get to the bottom of it”.

The Kaipara Harbour is the largest harbour in the Southern Hemisphere and is a nursery for 98 per cent of all snapper off the west coast of the North Island.

The Ministry for Primary Industries (MPI) is urging anyone who is concerned about the state of fish caught to report it as soon as possible to Fisheries New Zealand on 0800-80-99-66.

The diseased "zombie" snapper have sunken, milky eyes, bones sticking out and a lack of mucus on their skin.

Fisheries New Zealand director science and information Simon Lawrence said the fish should be photographed and kept whole in the fridge, with samples best when they are sent to the laboratory for testing within 24 hours.

“Without more information and samples of the fish, we cannot confirm if the symptoms you’ve described in snapper caught in Kaipara Harbour are associated with milky white flesh syndrome, or if their condition is due to another cause.”

The MPI is investigating the potential causes of milky white flesh syndrome, with Niwa research expected to be available soon, he said.

Northland Regional Council monitors water quality in Kaipara Harbour, including one monitoring station north of Ruawai and one south at Burgess Island.

The sampling has not shown any anomalies, apart from a spike in chlorophyll in March, which could indicate the start of an algal bloom.

A council spokesperson said fish are mobile creatures, so should be able to move away from a short-term water quality issue.

The spokesperson admitted the water quality in the Wairoa River is typically poor, with water quality standards exceeded for almost all parameters, and sediment and nutrient concentrations particularly high.

To help improve water quality in the harbour, the council is a proud partner in the Kaipara Moana Remediation Programme, a 10-year project focused on reducing sediment entering the harbour.

Symptoms commonly found with fish disease, scientist says

The symptoms described are common fish disease symptoms, said Cawthron Institute aquatic animal health team leader Dr Kate Hutson.

“The fact that marine diseased fish are being found near freshwater inflows matches my experience of some other marine fish disease events,” she said.

Sometimes marine fish move into areas with a high freshwater content to treat external diseases, as pathogens and parasites on the skin cannot tolerate freshwater, Hutson said.

“In this way, fish can recover from minor skin lesions or infections.”

Sam Erickson is concerned the illness of the snapper he caught in Kaipara Harbour is due to poor water quality.

Hutson said fish disease is usually caused by a combination of factors, from a complex interaction between the fish, its environment, pollutants and any pathogens present.

“Skin abnormalities may be exhibited in environments of poor water quality and/or when infectious organisms are present.”

Poor water quality can compromise fish’s immunity and can be caused by run-off, especially after heavy rain, Hutson said.

“Acid sulphate soils have been documented around Tāpora and could release sulfuric acid into nearby waterways when exposed to oxygen by clearing, draining and development. Sulphuric acid can erode cells in fish skin, making them more prone to infection.”

Pathogens and parasites can also cause skin diseases in fish, Hutson said, such as marine parasitic flatworms which naturally occur on snapper and feed by grazing on the fish’s skin.

“These parasites can cause blindness, skin irritation and lead to secondary infection by bacteria. Flatworms are transparent and often go unnoticed by fishers.

“In this case, a disease investigation may not initially reveal flatworms as a contributing culprit, as the parasites may have already died and fallen off the fish in freshwater.”

Other possible causes include infection from viruses, bacteria and fungi, she said.

More testing needed, collaboration required

Hutson also encouraged fishers catching diseased fish to report it to Fisheries New Zealand, so the MPI can do sampling.

“Definitely diagnosing the cause of this disease is the essential first step – without this information, we cannot make informed decisions to manage the disease.”

But she said any disease management would be complex and require collaboration amongst scientists, Māori, government and the community.

“Remediation with respect to land and pesticide use can help in major catchments to improve the health of freshwater and marine environments.”

The Cawthorn Institute is developing a new forensic approach for investigating aquatic diseases to enable reliable and timely diagnosis, Hutson said.

Emerging Aquatic Diseases is a five-year collaborative research programme, enabled by the Ministry of Business, Innovation and Employment’s Endeavour Fund, involving government, industry, iwi, crown research institutes and universities.

Denise Piper is a news reporter for the Northern Advocate, focusing on health and business. She has more than 20 years in journalism and is passionate about covering stories that make a difference.