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New Zealand is investigating the mysterious death of thousands of eels in a stream on the country’s North Island.

An estimated 3,500 juvenile eels were found dead in the Kauritutahi stream, sparking an investigation by environment officials under the Ministry for Primary Industries.

This is the second mass death of eels in the country this year. Several thousand adult eels were found dead in a stream near Mataura in Southland in February.

While the Mataura mass death was attributed to a toxic pollutant in the waterway, it’s unclear what killed the eels in Kauritutahi. The authorities, though, suspect it may be due to a “stress event” linked to climate change.

Juvenile eels, known as elvers, migrate upstream between late November and early March when temperatures reach about 16C, according to the New Zealand National Institute of Water and Atmospheric Research. Elvers thrive when water temperatures range between 16 and 18C along with good stream flow and dissolved oxygen levels of 7-8.5 milligram per litre.

The parameters in the stream were reportedly found to be unsatisfactory when measurements were taken the day after the mass death was reported.

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Algae buildup was also observed in some stream sections with limited water flow, indicating it was warming up.

“We noticed some algae build up which generally is due to the stream water warming up,” according to Hona Edwards, a member of a local guardian group that monitors the stream.

“Then the biggest percentage of the stream was observed to range from very limited to no water flow. When there’s no flow, the dissolved oxygen reduces, and the toxins build up in the waterway,” he said.

Such algal blooms occur under when conditions like warm water temperatures and low water volumes are coupled with nutrient flows from agricultural use and toxic pollutants from human activities.

While mass deaths of freshwater fish are known to occasionally take place across the world, a study of lakes in Minnesota and Wisconsin in the United States found that such incidents have increased globally in the past decade.


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