Closure on a fishy cold case
A PhD student has possibly cracked the case on one of New Zealand’s fish mysteries and his work could shed light on risks facing longfin eels.
It’s been an enduring whodunnit. Who and what killed New Zealand’s grayling?
The freshwater fish, about the size of a small trout was once so abundant its babies were shovelled onto market gardens as fertiliser. It also was said to make a fine meal.
The widespread, and reportedly beautiful fish disappeared shortly after European settlement.
The grayling (upokororo) is New Zealand’s only freshwater fish to have gone extinct, with the last reported catch in 1923.
Factors leading to the extinction were thought to include over-fishing, degraded habitat, and the introduction of trout, which find many of New Zealand’s freshwater fish delicious.
The mystery though, is why did the upokororo also vanish from unfished, pristine, trout-free streams?
University of Auckland PhD candidate Finn Lee may have solved the puzzle and his findings could be used to save some of the other 72 percent of freshwater fish species teetering on the edge of extinction.
“We know they were super widespread and abundant, and we know they went extinct pretty quickly relatively recently, so it’s a pretty sad story.”
Lee’s paper looks at whether the upokororo’s “dispersal habit” was to blame.
Upokororo, like many New Zealanders who came after them, were partial to embarking on the big OE.
Amphidromous fish, such as the upokororo, migrate from freshwater to the sea and back again as part of their life cycle.
What they did when they returned from their OE is the possible key to the mystery.
Unlike North American salmon, which return to the family home after their time at sea, it’s thought upokororo did not seek out the stream they were born in.
Instead Lee thinks they chose the first stream they came upon, potentially ending up in the equivalent of dangerous slums, at the mercy of ravenous trout and getting scooped up to fertilise cauliflowers.
In ecological language, these fish slums are called sinks.
“The fish that end up there, they’re either going to die without reproducing, or they might produce a few offspring, but not enough to replace the fish going in.”
If sinks are like slums, their counterparts referred to as “sources” are the leafy suburbs of ecological environments where space and food are plentiful, threats minimal and reproduction is easy.
“If you’ve got some sources and some sinks, the whole species can go extinct, even while you’ve got some good quality habitat,” said Lee.
While there were probably sinks before European settlers’ arrival, degradation, harvesting and predation may have rapidly increased the percentage of slums and swallowed up many leafy suburbs.
Lee’s detective leg-work included looking at old newspaper clippings to understand where the fish were found and in what numbers.
“There was quite a lot of interest in them back in the day. If someone happened to catch a whole bunch it would quite often make it into the local paper.”
He found around 200 articles, some with dates and locations, one item was a menu for a banquet full of species now in dire situations.
Some recounted enormous numbers of fish.
“There was one for the Hutt River, there were so many grayling going up the river, there was a mill operating – a turning wheel – and there was so many grayling going up they clogged the whole thing up.”
Collating the information in the articles helped paint a picture of distribution and quantity seen or caught over time.
He also tracked the many reasons people were suggesting might be the cause for the species' sudden decline. Trout got the most pointed fingers, followed by fishing. Forest clearance causing water quality decline was mentioned, as well as factories belching pollutants into water. Sulphur overflows from thermal springs were also put forward as a possible cause of upokororo's fading from rivers.
This information was then modelled to work out an estimated date of extinction. The date range of 1924 to 1972 arrived at lined up with the last recorded catch in 1923 and the sporadic reports of sightings which continued into the 1960s.
The result he found was if upokororo had returned to their river of birth, around 30 percent of each generation of fish would need to have been fished or eaten by trout.
“That would have had to have been happening across every river, simultaneously. That’s not very likely.”
If they had returned to rivers at random, the number of “sinks” or fish slums would not need to be high to drive the upokororo to extinction.
With five percent of rivers as slums it would take just five percent of each generation of fish to be eaten by trout or caught by humans to drive the species into extinction.
Can the past inform the future?
While there’s satisfaction in potentially closing a cold case, there’s still the question of what can be learnt for the future.
Lee said his research could shed light on the prospects of similar indigenous species which go to sea, then return to random rivers. Too many fish slums, combined with other risks such as predation and fishing could prove to be a species extinction tipping point, even if good habitat remains.
Native fish expert Stella McQueen said Lee’s research is relevant to ongoing discussion about eels.
Longfin eels are listed as at risk and declining yet are part of the Quota Management System and fished commercially. In 2013 a Parliamentary Commissioner for the Environment report written by Dr Jan Wright called for a moratorium on commercial fishing. This did not happen, although the quota has been lowered.
Scientists worry as eels can take at least 30 to 40 years before they reach breeding age, and only breed once, the effect of commercial fishing is yet to be felt.
Eels are born at sea and make their way to New Zealand. Like the upokororo, they go randomly to various rivers.
Eelers often point to the fact that 70 percent of the country is not fished.
McQueen calls this “bullshit” as a strategy to preserve eels.
“Not all the habitat would be economically worthwhile for commercial eelers to target. Not all suitable habitat is equally desirable to eels.”
So, the 70 percent, could include fish slums, and the 30 percent commercial eelers are targeting could be leafy suburbs.
“You can’t fish one catchment heavily and not fish another catchment and think you’ve got a safe area … you can’t absolutely trash the South Island, but you’ve got all the North Island eels going out to spawn. That’s not ok. The eels are coming back to the North and South Island and are just going to be decreasing in numbers.”
For whitebait species, McQueen thinks the situation could be slightly different. As they don’t go as far out to sea as eels which migrate somewhere close to Tonga, there’s a higher chance whitebait return to the river they came from.
Four of five whitebait species are threatened and like the threatened longfin eel there is still a commercial trade in them.
Lee, in his search for newspaper reports of upokororo came across an observation made in 1938 by Gerry Stokell, a keen freshwater fish citizen scientist, regarding the now extinct upokororo:
“The fact that this fish is now unprotected by law is a standing reproach on the administration of New Zealand’s wildlife. Its fate forms a melancholy illustration of the indifference with which New Zealand statesmen regard the natural resources of their country, and of the danger of placing control of indigenous animals in the hands of those whose chief interest lies in killing them.”
Upokororo was finally protected in 1951, long after the last recorded catch of 1932. Bizarrely, it’s also the only native fish to have legal protection under the Wildlife Act.