Key Takeaways
- A recreational fisherman discovered diseased young grey mullet in Northland’s Waipu Estuary, prompting concerns about the health of the fish and the wider ecosystem.
- The disease is suspected to be Myxobolus episquamalis, a myxozoan parasite that causes granular lesions on the body surface of infected fish.
- The parasite has been previously found in grey mullet in New Zealand, but its increased prevalence and causes are unknown.
- People are advised not to consume fish with lesions and to wash their hands thoroughly if they touch infected fish.
- The discovery comes after a previous outbreak of "zombie fish" in Northland’s Kaipara Harbour, where snapper were found with symptoms including cloudy eyes, sunken faces, and protruding bones.
Introduction to the Discovery
Recreational fisherman Pete Watkins was horrified when he hauled up sick young grey mullet while netting for baitfish in Northland’s Waipu Estuary. The fish had visible signs of disease, including lesions on their bodies. Watkins reported his discovery to the Ministry for Primary Industries (MPI) and was put through to its Wellington after-hours service. He was then contacted by Biosecurity NZ, which asked him to deliver the fish to its Whangārei office for testing. The results of the testing are expected to take around four weeks.
The Disease and Its Effects
The disease suspected to be affecting the mullet is Myxobolus episquamalis, a myxozoan parasite that causes granular lesions on the body surface of infected fish. This parasite has been previously found in grey mullet in New Zealand, with the first confirmed case in yellow-eyed mullet (Aldrichetta forsteri) in the Mahurangi River at Warkworth, North Auckland, during April this year. The parasite can lead to secondary bacterial infections, but it has not been directly linked to fish deaths. The factors contributing to the increased prevalence of Myxobolus episquamalis are unknown, but possible causes include changes in the parasite’s secondary host life cycle or increased stress in the fish due to spawning or environmental conditions.
Response to the Discovery
Mike Taylor, team manager aquatic health, Biosecurity New Zealand, confirmed that the samples Watkins discovered would be sent to its Animal Health Laboratory for identification and testing. People are advised not to consume fish with lesions as it could cause illness, and they should also wash their hands thoroughly if they touch infected fish. Fish with lesions should 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. The Northland Regional Council (NRC) has also advised that water conditions at popular swimming spots, including those near the estuary, are regularly updated at safeswim.org.nz.
Comparison to Previous Outbreaks
The discovery of diseased mullet in Waipu Estuary comes just months after a previous outbreak of "zombie fish" in Northland’s Kaipara Harbour. In May last year, Kaipara angler Sam Erickson coined the term after catching snapper that were so sick they resembled "the walking dead". The symptoms included cloudy, cataract-like eyes, sunken faces, protruding bones, and a lack of mucus and flesh. 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, and fishers are still being urged to report any snapper or other species showing these symptoms and avoid eating affected fish.
Conclusion and Future Actions
The discovery of diseased mullet in Waipu Estuary highlights the importance of monitoring the health of fish and the wider ecosystem. The suspected Myxobolus episquamalis parasite has been previously found in grey mullet in New Zealand, but its increased prevalence and causes are unknown. Further testing and research are needed to determine the extent of the disease and its impact on the ecosystem. In the meantime, people are advised to take precautions when handling fish and to report any suspicious symptoms to the relevant authorities. The Northland Regional Council (NRC) will continue to monitor water conditions at popular swimming spots, and fishers are urged to report any affected fish to help prevent the spread of the disease.
