Half of NZ’s rivers blocked to struggling native fish

Nearly half the country’s river network is either partly or fully inaccessible to migratory species, like the endemic and threatened New Zealand longfin eel. Photo / NZME

Aotearoa’s extinction-threatened freshwater fish face some of the most blocked waterways in the world, with nearly half the country’s river network either partly or fully inaccessible to migratory species.

That’s according to the first-ever national assessment of barriers blocking fish passage – such as dams, culverts and weirs – just carried out by Niwa researchers.

It found there were about 0.16 barriers per kilometre of river – one of the highest densities observed anywhere.

The researchers estimated that at a minimum 48 per cent of the network was at least partially inaccessible, while a further 36 per cent, still to be assessed, might also be restricted.

What’s called river fragmentation remained a key driver of freshwater biodiversity loss, at a time roughly three quarters of our freshwater fish were either threatened or at risk of extinction - a higher proportion than almost any other country.

“There are just over 50 native New Zealand fish species, with around 85 per cent of those not existing anywhere else in the world,” Niwa freshwater ecologist Paul Franklin said.

Many of our iconic fishes – such as longfin eels and whitebait species - migrated between the sea and our rivers to complete their life cycle.

“However, they often encounter different structures along the way that act as a barrier, stopping them reaching key habitats,” Franklin said.

“This ultimately leads to lower abundances and potentially loss of these species.”

Niwa freshwater ecologist Paul Franklin helped conduct a national assessment of fish migration barriers. Photo / Stuart Mackay/Niwa

In 2018, a Victoria University-led study compared land use changes and more than 20,000 freshwater fish records since 1970, to find more than three quarters of 25 analysed species were in decline.

About the same rate of decline was found in 20 native fish species - and in two thirds of cases, the drop was a significant one.

The same year, the Government introduced New Zealand Fish Passage Guidelines, recommending that all waterway infrastructure lower than 4m should be built to allow some fish species to make it to their ocean breeding grounds.

The guidelines, developed for councils by Niwa and the Department of Conservation, proved New Zealand’s first nationwide plan to help the free movement of our fish species.

The requirement applied to newly-built structures, where approval should be sought before they were constructed, and to existing structures that did not already have an approval or which could no longer meet the requirements of the approval.

Yet New Zealand still lagged far behind other parts of the world when it came to efforts to support fish passage, Forest and Bird freshwater advocate Tom Kay said.

“In the United States of America, for example, they’re ripping down dams, which are a barrier to migrating salmon,” Kay said.

“We have lots of fish that migrate in a similar way to salmon – including tuna, which only breed once in their life – and which face the prospect of being ripped to shreds in dam turbines when they try to migrate downstream.

“While we can make small barriers easier for fish to pass, there’s not much we can do to stop fish having to swim downstream - and then die - in huge dam turbines.

“We need to mitigate these barriers, start a conversation about the impact of large dams on our native fish, and stop over-engineering these places by making room for rivers. If we can do that, there’s hope for our native fish species.”

Meanwhile, Niwa scientists were studying the swimming abilities of fish to help find solutions to help them migrate.

“We put fish into the equivalent of a water treadmill to see what speed they can maintain - we want fish to be able to move upstream casually, not having to fight the whole way,” Franklin said.

“We can then assess what adjustments need to be made to structures in our river networks to help fish pass through these environments more easily.”

An example of a fish ramp, constructed in 2007, next to the Waikato River. Photo / Stuart Mackay/Niwa

Culverts, for example, are put in streams and rivers to help people cross safely, while allowing an adequate passage of water.

However, they could alter the flow of the stream or river, and therefore the ability of fish to swim through them.

“The good news is that many of the solutions are extremely easy to implement,” Franklin said.

“You can install baffles into culverts, which are basically panels placed inside the tube that help to slow water down and create resting areas, so that fish can pass through into the stream above.

“It’s small changes like this that can make a big difference. If we don’t start addressing the threats that we’ve placed into their environment, these precious fish will one day be gone.”