Testing to see if the mass death of more than half a million mussels on Northland's west coast may have been caused by biotoxins has come up empty - further supporting the theory that the mussels died as a result of climate change.

Northlander Brandon Ferguson out fishing with his family recently when they discovered hundreds of thousands of dead green lipped mussels at Maunganui Bluff.

It led to Dr Andrew Jeffs, a marine scientist at the University of Auckland, blaming climate change on the mass death.

Jeffs said the mussels died as a result of "heat stress" and warned they may disappear from the region altogether as global warming tightens its grip.

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"Yes they've died as a result of the recent hot weather and midday low tides. High pressure systems bringing the hot weather also forces the tide out for longer, exposing the mussels to longer periods of sunshine.

"The mussels die of heat stress. Imagine lying in the midday sun every day for four hours for the best part of a week. You'd be pretty sunburnt at the end of that," he said.

Jeffs said Northlanders may not be able to enjoy the seafood delicacy in the future as global warming ramps up a gear.

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This saw some questioning his assertion and instead saying bio toxins could have caused the deaths instead.

However, Ministry for Primary Industries director of diagnostics and surveillance services Veronica Herrera said there was no evidence of that.

"Our most recent tests haven't shown any evidence of biotoxins being the cause of this event," Herrera said.

"We keep track of these events to build our understanding of shellfish health and the state of associated fisheries. Where we have evidence, we take action to protect the fishery; for example, through adjusting fishing rules or through compliance action.

"We also test for biotoxins every two weeks, our most recent tests haven't shown any evidence of biotoxins being the cause of this event."

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However, samples of the dead shellfish were unable to be tested.

"The time window for obtaining suitable samples in cases like this is very narrow because shellfish tissues break down very quickly following such mortality events."

Furthermore she said the original notifier of the event who she did not name, advised MPI of other smaller events occurring further up the coast as well.