Date:  23 February 2024

Mass deaths of tuna/eels and other fish have been reported in the upper Whanganui River over previous years, usually in summer.

Iwi, DOC, the Ministry for Primary Industries (MPI), Horizons Regional Council, Genesis, and Fish and Game are banding together to get to the bottom of this problem.

Jane Taylor, River Restoration Ranger at DOC, says, finding the cause of the mass fish deaths is like a ‘whodunit’ investigation.

“We need teamwork to solve this mystery. It’s especially important because the longfin eel, one of the species in the catchment, is at risk of extinction.”

The public are asked to report any observations of sick, dead, or dying fish to MPI’s pest and disease hotline (0800 80 99 66).  

Dr Mike Taylor, Team Manager Aquatic Health at Biosecurity New Zealand (within MPI), asks people to make the reports as soon as possible after sighting.

“Please take photos and report the location, date, number, and type of species, and how fresh they are. This will help us consider whether there are suitable specimens to test for diseases.”

MPI will test some of the fish specimens to try and determine the cause of death, Horizons Regional Council will gather more environmental data, and mana whenua and Fish & Game will add their data, observations and local knowledge to the mix.

Chair of Te Mana o Ngā Tuna, Ben Potaka, says mana whenua have been concerned for years about the annual mass deaths of tuna in the Whanganui, but until now, nothing seems to have been done about it.

“Tuna are significant taonga. It’s time for everybody to all pull together to try and understand the cause of these mass fish deaths.”

Last summer there were no reports of the mass deaths of tuna. However, previous years have seen reports of dead and lethargic tuna from the Manganui o te Ao River and the Whanganui River, as far downstream as Pipiriki.

Maree Patterson, Senior Water Quality Scientist at Horizons Regional Council says the cause of these deaths is not clear.

“Previous mass deaths events tended to be widespread, indicating they weren’t the result of a single pollution event. Water temperatures probably aren’t a factor as temperatures in the major rivers don’t get hot enough to cause fish deaths directly.

“We’re extending our existing monitoring programmes in the Whanganui catchment, by installing new monitoring equipment at Pipiriki and Wades Landing to continuously measure dissolved oxygen levels in the water over warm, dry periods.”

The expectation is that through this monitoring work, there’ll be better information to understand the mass fish deaths and determine ways to reduce or prevent them in the future.

People are advised to exercise common sense and not take dead or dying fish to eat while the cause of their deaths is undetermined.

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