New Zealand river network nearly 50 per cent inaccessible to migratory fish

Hamilton-based freshwater ecologist Dr Paul Franklin. Photo / Stuart Mackay, Niwa

Nearly half of New Zealand’s river network is partially or fully inaccessible to migratory fish, a new Niwa (National Institute of Water and Atmospheric Research) study shows.

A Niwa team, including Hamilton-based Niwa freshwater ecologist Dr Paul Franklin, has conducted the first nationwide assessment of river fragmentation (structures that interrupt the natural flow of rivers, like dams) and found New Zealand has some of the highest densities of barriers to fish passage in the world: about 0.16 barriers/km.

This means, a minimum of 48 per cent of the river network is at least partially inaccessible to migratory fish, and a further 36 per cent could be restricted but has not yet been assessed.

Franklin says the fragmentation of river systems is a key driver of freshwater biodiversity loss.

“There are just over 50 native New Zealand fish species, around 85 per cent of those [don’t exist] anywhere else in the world. Many of our iconic fishes, such as eels and the whitebait species, migrate 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. This ultimately leads to lower abundances and potentially loss of these species,” says Franklin.

The current in the tank pictured can be adjusted - it's the equivalent to a water treadmill for fish - to see how long the fish can swim, to help with fish passage design. Photo / Stuart Mackay, Niwa

Niwa is now studying the swimming abilities of fish to come up with solutions to help the fish species migrate.

Franklin says: “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.”

This fish water treadmill allows the team to assess what adjustments need to be made to structures in the river networks to help fish pass through more easily.

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

Franklin says even small changes to the environment can make a big difference for the fish.

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

“If we don’t start addressing the threats that we’ve placed into their environment, these precious fish will one day be gone.”