Freshwater fish swim their all for science

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In a secret training location on the outskirts of Hamilton, a squad of whitebait is being put through its paces by fish scientists.

The tiny inanga have been plucked from Waikato streams and held in a darkened laboratory for the last month, undertaking highly advanced testing to find the strongest, fittest and fastest fish.

NIWA freshwater fish scientists are trying to understand how long they can swim at given speeds – between rests - and how much variation there is between fish of the same species.

This knowledge will enable them to design the perfect stream conditions inside a culvert that will help the fish migrate between the sea and our streams and rivers to complete their life cycle.

Manmade obstacles in waterways are one of the biggest factors in declining native freshwater fish numbers to the point many species are now threatened.

NIWA freshwater ecologist Dr Paul Franklin says getting culvert conditions shipshape is critical. If water flows too fast, fish find it too tough to negotiate.

“We need to find ways to slow the water down or create resting spots. But very little is actually known about how well these fish can swim.

“There is an assumption that inanga are the weakest swimmers of all New Zealand’s freshwater fish but we don’t definitively know that. However, we do expect there will be differences in abilities between the five whitebait species.”

In the lab the fish are tested individually – placed in a specially designed tank where the water speed, temperature and oxygen levels are carefully regulated and the fish training session timed to see how long it takes them before they give up.

Watching their every movement under strict ethical guidelines is university student Dana Nolte. She sits in a neighbouring office watching a live onscreen video feed from the tank, taking note of its behaviour, and how it performs against a strong current.

Some canny subjects spend as much time as they can in a corner of the tank. Dr Franklin thinks this is because the flow is slightly less here, so they’re choosing the spot of least resistance. But figuring out what part of the flume the subject prefers, and how much it’s using the corners is all part of the experiment.

“We’re really trying to learn how we can create conditions the fish prefer or how we can manipulate their behaviour to improve their passage through obstacles like culverts.”

So far, the results have been mixed. Some find it too hard to maintain a position against the current and only last a few seconds. Then there are stayers who have lasted more than an hour before giving up and drifting downstream to the back of the tank. When that happens, Dana turns the current off, and the fish is returned to its holding tank to rest. “We’re finding there’s a lot of natural variation but we’ve also been wondering if the length of time we hold the fish in the lab is impacting on their performance so we’re taking note of that as well,” Dr Franklin says.

At the end of the experiment all fish are returned to the stream they came from.

After the inanga trials, the team will move on to other species to collect as much information as they can – but fish can be tricky.

“We suspect some won’t behave nicely as they’ll be able to stick themselves to the bottom of the tank and not swim at all.”

Ultimately the research will move out of the lab and into streams where the scientists will use their initial information to figure out the future for our freshwater fish in their struggle to survive.

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Research subject: Fish