Regional council drainage pumps have been brutally killing native eels for decades as they attempt to swim to the ocean to breed, according to documents obtained by Forest & Bird.
The reports, created for Waikato Regional Council, contain grisly images of dead eels with internal haemorrhaging due to high speed spinning, or with extensive spine damage.
Forest & Bird's fresh water advocate Annabeth Cohen says the potential scale of the crisis is horrifying and in some cases all breeding female eels died as they passed through the drainage pumps.
One report concluded that "currently ... no pumps in NZ are likely to be 'fish friendly'", or safe for fish to pass through, Forest & Bird said.
"Sadly, the heavy rain that triggers eel migration, also triggers the pumps, and it's the larger breeding females - essential to the ongoing survival of these species - that have the least chance of surviving," Ms Cohen said.
"Seventy-four per cent of our freshwater fish, including long fin eels, are heading towards extinction because of water pollution and habitat destruction. We can now add publicly owned drainage pumps to their problems."
The Resource Management Act requires that regional councils "ensure that fish passage is provided ... where it would otherwise exist", but it appears most councils are failing to do so, she said.
Forest & Bird are urging regional councils to act urgently and start replacing existing pumps with proven fish friendly versions, and returning ecologically significant areas to wetlands.
"This will mean a managed, long term project of returning waterways and wetlands to their natural state," Ms Cohen said.
There are two main types of eel in New Zealand - the short fin, and the threatened long fin. Loss of wetlands and historical commercial fishing practice has impacted heavily on their populations.
The long fin eel is one of the largest eels in the world and it is found only in the rivers and lakes of New Zealand.
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