‘Canary in a coalmine’ warning on rivers
Monday, September 15, 2008
Whitebait disappearance a ‘canary in a coalmine’ warning on rivers
Whitebait disappearing from New Zealand’s waterways are an indicator of just how polluted our rivers and streams have become, Dr Mike Joy is warning.
“Even if you don’t think fish are cool or important, what this is telling us is that the state of the freshwater that we humans depend on is getting pretty bad,” Dr Joy says.
Dr Joy has spent the past 15 years researching whitebait and other freshwater fish, finding that whitebait have disappeared from around 75 per cent of their expected habitats in Manawatu and Horowhenua. The national group that monitors the fate of the adult whitebait (galaxiids) is reporting a similar level of disappearance.
“I recently attended a working group meeting in Gisborne and reports from all over the country are saying that the fish that were there 10 years ago cannot be found now. They are disappearing, and very fast.”
Dr Joy says the issue is complex, with impacts both on the quality of water in rivers affected by pollution and hill country erosion, which is sending sediment downstream. His group has tagged 150 galaxiids in the Mangahao Stream, a tributary of the Manawatu River. The Mangaghao enjoys pure, clean water from diversion of rainfall from the top of the Tararua ranges into a hydroelectricity power station.
“There is one section of stream up there and adults go there and spawn every year. We have estimated 300 or 400 galaxiids can be sustained in every 200m stretch.”
Impairing the ability of rivers to sustain the fish is sediment. Too much sediment washing into the habitat covers boulders. Galaxiids “hang out” under the boulders and in the semi-dark during the day, Dr Joy says, only emerging at night. “So it’s crucial that a stream has boulders and especially, spaces between those boulders because they are a mostly nocturnal fish.”
A key finding from the Mangahao study is that fish definitely prefer the cleaner water.
“We have taken huge 500-litre tanks of water from there and made the water flow through. When we put fish in they make a clear decision on which way to go – they have very good olfactory (smell) senses.
“The analogy is a smoke-filled hallway in a building on fire. If you were trying to run out of the building you’d pick the cleaner hallway, and that’s what the stream is like for them.”
The dwindling numbers are further affected by the many New Zealanders catching the juveniles as whitebait, and selling them for up to $150/kg, Dr Joy says.
“Not enough galaxiids are able to return to the streams because of the whitebaiting. Two of those species have the same threat ranking as a Kiwi yet selling whitebait is a crucial incentive to get people out there. In the West Coast fishing stands sell for $60,000. Clearly, it’s an industry for some people.
”The Resource Management Act mentions trout specifically – these introduced fish can’t be sold and they have so much protection -– yet endemic and endangered adult whitebait species have no protection. The trout fishery is probably the most sustainable fishery in New Zealand due to its non-commercial status. If you could get $150 a kilo for trout, there would be a whole lot more people out there fishing for them – and fishing as hard as they could.”
Dr Joy says his computer modelling, which he has focused on the greater Manawatu catchment, shows him where the galaxiids should be, including the upper Oroua, upper Pohangina and upper Manawatu rivers.
“But they are not there, we have searched and searched for them.”
Four of the five galaxiid species spawn inland in forested areas, at a spring flood. This makes them very susceptible to land use around them, Dr Joy says, while the fifth species spawns on a high spring tide around the tidal zone. In all cases, the spawn hatch and are washed out to sea some weeks later, giving them a head start on their journey in the seas around New Zealand. About six months later, the juveniles are a few centimetres long. Returning to the rivers to the upstream home where they will spend their lives, the whitebait are fished from August to November.
Dr Joy says a few simple measures could protect what is left of the stocks: prohibiting the sale of whitebait in the same way trout is protected, minimising high-country erosion and cleaning up waterways from pollutants including sewerage and run-off.
He also believes better monitoring of waterways would provide a clearer picture of their state.
“On a motorway, if you simply measure the cars going through at 11am every morning you would possibly conclude that the motorway is way too big. But you are just measuring at one point in time. In the same way, taking a water quality sample in a flowing river at a set point in time doesn’t reflect what may have been discharged over a period.
“If we don’t do something quickly we won’t have these species any more.“