Fish parasites are a real drag

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Parasites that attach themselves to reef fish cause a drag effect, limiting the fishes' ability to swim fast and putting them at increased risk of predation.

In studies on the Bridled monocle bream (Scolopsis bilineatus), researchers at the Australian National University found that parasitised fish also use more energy to swim.

Using a swim tank, which can be likened to a fish treadmill, the team made the fish swim and measured the amount of oxygen the fish was consuming.

“Oxygen consumption is a proxy for energy consumption: if a fish consumes more oxygen, it is burning more fuel and will need to eat more,” said lead author of the study, Sandra Binning.

“For fish, eating more means you may be more exposed to predation and have less time for other important activities, such as attracting mates.”

Fish without a parasite outperformed parasitised fish in all tests. When the parasite was removed, the fish was back to normal oxygen consumption only 24 hours later, suggesting the effects are quickly reversible.

To determine whether it was added drag that increased oxygen consumption, or if the parasite was somehow affecting the breams’ health, the team attached plastic models of the parasite to healthy fish.

“The model attached to the fish’s head simulates the drag caused by the parasite. When they’re not swimming fast, the fish are not affected,” said Binning.

However, at higher speeds the drag effects became more pronounced.
They found that when the fish weren't swimming quickly, they were unaffected, but at higher speeds the drag effect became more pronounced.

"It’s similar to drag on a car with a roof rack. At slower speeds, you don’t consume more petrol. But at higher speeds, the drag increases, meaning the engine has to work harder and uses more petrol. It’s the same with the fish – there is more drag at higher speeds causing the fish to use a lot more energy," said researcher Dominique Roche.

This extra drag could spell trouble for parasitised fish, with global warming creating choppier water conditions due to increased extreme weather events.

"In the future there will be more frequent and more intense wave action, which is a big stressor for coral reef fish because it increases their energy requirements. They have to swim even to stay in the same spot. This could prove too much for parasitised fish who are already working harder," said Roche.

The results also have implications for ecological tracking studies in which radio or GPS transmitters are attached to animals so researchers can understand their migration movements.

"A tracking tag is similar to an ectoparasite stuck on the surface of a bird, a turtle or a fish. What our study shows is that drag effects are really important. If we want to see natural animal movement, especially in migrating species, then we need to invest more money into developing smaller tags," said Binning.

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