Aquaculture for all

Scientists Discover Whitebait Can Climb Ropes

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NEW ZEALAND - In what is believed to be a New Zealand first, Environment Waikato scientists may have found a clever way to overcome resource consent barriers for farmers and possibly help to save some of New Zealands threatened native fish species.

Tiny fish climbing ropes – it might sound like a crazy idea from a Dr Seuss book, but it’s actually a real-life experiment being carried out by Environment Waikato scientists.

The scientists, Bruno David, Kevin Collier and Mark Hamer, have discovered juvenile banded kokopu, a native whitebait species, can climb up a special type of polypropylene rope.

Now they are conducting field trials to see if the fish can do the same thing in the natural environment and use their “climbing skills” to wriggle up ropes threaded through perched culverts.

Culverts are large pipes that allow water to pass underneath roads and farm tracks. “Perched” culverts hang above water level, blocking fish migration through rivers and streams.

“Fish need to be able to migrate up rivers as part of their life cycle so they can reach suitable adult breeding habitats,” Dr David said.

“Unfortunately, even well installed culverts can end up blocking fish passage over time, because the water rushing through erodes away the land away the end of the culvert. This can leave the culvert hanging above the stream, creating a drop that fish just can’t get past.”

This, along with habitat loss and substantial recreational harvesting nationally, was probably why reported whitebait catches had been steadily declining in the Waikato from an estimated average of 46 tonnes per year in the 1930s to around only three tonnes per year today, he said.

The scientists’rope-climbing idea uses the knowledge that some native New Zealand fish species from hilly or steep river habitats are equipped with “their own version of four-wheel drive”.

“Some juvenile native fish have this unique ability to climb by flattening out their bodies and wriggling up using their heads and fins,” Dr David said.

“But even four-wheel drive can’t get them past a manmade structure like a perched culvert – to do that they’d have to climb upside down to negotiate the overhang, basically defying gravity.

“We’re hoping the rope could help them climb up to the culvert entrance and get past the lip.

“Providing fish access through just one culvert could open up kilometres of habitat upstream.”

Dr David said the rope could be an extremely cost effective way of retrofitting culverts to allow fish passage, eliminating the need for expensive engineered structures like fish ladders, which could cost tens of thousands of dollars.