Recent investigations have determined that a mass die-off of eels in the Waiatai and Ikanui streams was caused by a natural blackwater event, exacerbated by unusually warm temperatures and heavy rainfall in early January. The event resulted in critically low oxygen levels, making survival impossible even for resilient species like tuna.
What is a Blackwater Event?
A blackwater event occurs when substantial amounts of organic matter – leaves, grasses, and wetland debris – are washed into a stream following heavy rainfall. As this material decomposes, it rapidly depletes the water’s oxygen supply. In this instance, oxygen levels plummeted to just 1.9%, compared to the typical 80% found in healthy freshwater streams, according to findings from accredited laboratories.
The situation was worsened by long-term changes to the landscape surrounding the streams, including reduced shade, altered wetlands, and warmer water temperatures, making the aquatic system more vulnerable to rapid weather shifts.
Calls for Action
Kate Eaglesome, whose father initiated investigations and lives on Kairangi Station downstream, reported finding no signs of life in the stream at the time of the event. While a few baby eels have recently been observed, she emphasized the need for immediate action to restore the stream’s health. “It’s good now to finally have an answer,” Eaglesome said, “But I want to see some action. Let’s start planning now to give the stream life a fighting chance.”
Eaglesome highlighted autumn and winter as ideal times for planting native vegetation. She also acknowledged the support of Michelle McIlroy and Katarina Kawana from Wairoa Awa Mauri Restoration.
A post-mortem examination of one eel, conducted by Massey University’s Associate Professor Stuart Hunter, was unable to determine a specific cause of death due to advanced decomposition. Hunter noted that water testing is a more reliable method for identifying issues related to water quality.
Frequently Asked Questions
What caused the eel deaths?
The eel deaths were caused by a natural blackwater event, worsened by extreme heat and heavy rainfall, which led to critically low oxygen levels in the Waiatai and Ikanui streams.
What is being done to address the issue?
Discussions are underway regarding potential restoration efforts, including native planting, restoring wetlands and natural stream flow paths, and maintaining open fish passages.
Was a definitive cause of death determined through the post-mortem?
No, the post-mortem examination was unable to determine a specific cause of death due to the advanced state of decomposition of the eel.
How can communities best support the recovery of vulnerable freshwater ecosystems like the Waiatai Stream?
