Environment Canterbury says it assisted eel migration by creating a pathway to the sea.
Environmental authorities’ original explanation about the death of thousands of eels on a Canterbury beach was “absurd”, according to a well-known freshwater ecologist.
Birdlings Flat in Canterbury was littered with the corpses of thousands of eels last Saturday in what Environment Canterbury (ECan) described at the time as a poorly timed migration by the tuna (eels).
Victoria University freshwater ecologist Dr Mike Joy told the Herald that ECan’s original claim of the cause being natural is “impossible”.
“To blame the eels is like blaming the victim, absolutely absurd,” Joy said.
“The triggers for eel migration, the natural water quality and flow volumes have been completely altered due to ECan’s failure to limit intensification of agriculture and its associated harms.”
Dr Mike Joy.
ECan’s explanation distracted from the harm done to the lake, which has been happening for decades, Joy said.
“With the man-made alterations to that lake, it’s impossible to claim it’s natural, there is nothing natural about it.”
Joy said ECan had allowed the catchment of Te Waihora (Lake Ellesmere) to become polluted to the point where it was now classed as one of the most polluted in New Zealand.
He said this pollution was caused by excess nutrients coming from dairy farming in the lake catchment.
Thousands of dead eels washed up on a Canterbury beach. Photo / George Heard
“The resulting eutrophication of the lake combined with reduced flows would mean the eels were likely stressed from a lack of oxygen (hypoxia) and the flow alteration and many more human-induced changes would completely mess with all the natural cues for eel migration.”
Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in increased growth of organisms that may deplete the oxygen in the water. Eutrophication may occur naturally or as a result of human actions, Joy said.
“There are many other factors including the fact that the lake levels have been altered and almost all the surrounding wetlands crucial to lake health and eels’ habitat have been drained for dairy farming,” he told the Herald.
ECan told the Herald on Wednesday that eel deaths were not “a mistiming by the eels, [as] they migrate to the sea to spawn at this time of year”.
“One of the natural triggers for tuna (eel) migration is the presence of seawater. Last Thursday night seawater was washing from the sea into the lake, which likely triggered the urge to migrate.
“In absence of a lake opening, they will follow any pathway and unfortunately can find themselves stranded and perish.”
ECan says Te Waihora/Lake Ellesmere isn’t open to the sea at the moment.
At the request of Te Taumutu Rūnanga, ECan says it can assist with creating an easier path for the eels to the ocean to help migration, “which is what we did in this instance.”
The National Institute of Water and Atmospheric Research (Niwa) said it was not in a position to provide any expert comment as it did not know about the circumstances of the event but understood that ECan and the rūnanga have dug a channel out to the sea.
