In early April, a tonne of eels died trying to get to the sea from Canterbury’s Te Waihora/Lake Ellesmere, and the sight of their serpentine carcasses saddened many people. Environment Canterbury and Ngāi Tahu spokespeople said such deaths are a natural event and that another 19 tonnes of eel made it over the bar that separates lake from sea the same night.
Much is unnatural about the lake these days. It’s badly polluted and less than half the size it was, thanks to drainage and farm irrigation. Regardless, days later, another eel run described as “knee deep” slithered over the gravel into the ocean, heading for their Pacific spawning ground.
Most were shortfins, one of our two native species. Both are taonga and important for mahinga kai. Shortfins are more abundant, cope with murky waters and are also found elsewhere in the Pacific.
Not so the longfin eel, which prefers clear waters, rocky beds and shade. It’s found only in Aotearoa and the Chathams, and is the world’s largest eel, with mature females weighing 2–25kg.
Consider its challenges. Like the shortfin, eggs hatch in tropical waters and hatchlings travel here on ocean currents. Some make it up streams, most of which are now murky, so the journey inland to clean waters is long. A third of streams have culverts or other barriers, including hydroelectric dams, many of which have programmes to help the elvers (juveniles) pass. And there they grow, for at least 40 years, perhaps 80 for females.
It’s thought some longfins above dams swam there before the dams were built. Their chance of returning downriver and back to the Pacific to spawn – their life’s final act – is almost nil. They choose the highest flow areas, which is where dam turbines are. It’s a gruesome ending.
Don Jellyman, a retired eel scientist, once aged a longfin female at more than 100 years. Their long lives make them vulnerable, he says, because there’s more chance they’ll die before their single end-of-life spawning event.
“There’s a high likelihood of them being caught before they achieve that size because they could be exposed to commercial fishers for around 20 years.” That’s how long it takes them to grow from minimum to maximum catch size (tiny eels escape nets and very large ones are released).
Plus, says Jellyman, you’ve got floods, dam turbines and, out at sea, predators. Jellyman once tagged several longfins. Most ended up inside a whale, shark or marine tuna. That work was for his “holy grail” quest: finding the spawning ground. It’s somewhere between New Caledonia and Fiji. “But that’s a very big area,” he says.
The number of very small and very large longfins has become worryingly low, the then Parliamentary Commissioner for the Environment (PCE), Jan Wright, wrote in a 2013 report. The Department of Conservation classifies the species as “at risk, declining” but not threatened with extinction, whereas the International Union for Conservation of Nature (IUCN) red list says it’s endangered.
Yet eels are commercially harvested, though longfins are less popular with international eaters. Both species fall under the Ministry for Primary Industries’ quota management system, so their numbers are monitored. That monitoring is fisheries-style rather than ecology-style, an approach to sustainability the PCE criticised as “overly narrow and inadequate”.
That report triggered a review that recommended a wider range of measurements. Yet 11 years on, assessments use the same old fisheries measurements. Multiple graphs show declining catch, although that doesn’t necessarily mean stocks have dropped.
But their bizarre lifecycle means eels need special consideration, says Jellyman. “You must have a conservative catch, so that you allow a good proportion to get to the size where they will mature and go to sea – but nobody really knows what that critical mass is.”
