Man-made nanoparticles in water slow fish down

Scientists think the nanoparticles damage the fish’s gills, causing oxygen deprivation in the internal organs including the brain.

Exposure to man-made titanium dioxide nanoparticles means trout spend less time swimming at top speed, scientists have shown. They think the particles damage the fish’s gills, causing oxygen deprivation in the internal organs including the brain and forcing them to save energy by cutting out rapid bursts of movement.

Photo credit: Gabriel Pollard

The changes won’t have much impact on the animals’ activity most of the time, but affected trout may struggle to put on the sudden bursts of speed that might be needed to flee a predator, fight a rival for the chance to mate or catch elusive prey.

Other research by the same group has shown that metals like copper can also stop the trout reliably maintaining their proper daily rhythm, so they no longer forage at dawn and dusk when food’s easiest to find and predators are less of a threat; the researchers say this may be an emerging concern for nanoparticles too The particles also seem block their nasal cavities, interfering with their sense of smell, which is one of the main ways they detect threats.

These aren’t likely to be the only harmful effects of nanoparticles on fishes, but changes in animal behaviour that affect feeding and the ability to detect threats and avoid predators could have a serious effect on their survival in the wild. This is only the latest in a series of studies suggesting that nanoparticles can have varied and sometimes significant effects on fish; these could be strong enough to influence which fish survive to reproduce in the wild. Professor Richard Handy of the University of Plymouth is an environmental toxicologist who leads the group responsible for the research. He said:

We’ve shown that nanoparticles have similar effects on fish to trace metals like copper – they produce hypoxia [oxygen starvation], which in turn leads to poor muscle performance, neurological problems and changed behavior.

Nanoparticles are being used in a growing range of products, from sunblock to antimicrobial socks. But we don’t know much about what they’ll do when they reach the wider environment. There are many possible effects, but one area of concern is the impact on wildlife health and perhaps ultimately on humans.

Handy’s group have been investigating how artificial nanoparticles affect many different aspects of fish behaviour and physiology. In their latest experiment they tested the effect on a population of rainbow trout of a fortnight’s exposure to titanium dioxide nanoparticles – thought to be one of the commonest types now being made. They then tracked the trout’s movements using video cameras and logged how much time they spent swimming at a variety of different speeds.

On average the trout ended up with a 22-fold increase in titanium nanoparticle concentrations around their gills. They spent significantly less time swimming in the study’s fastest movement category – 20 centimetres per second or more – compared to unexposed peers, although their mean speed didn’t change. Swimming is one of the biggest energy drains that trout face, accounting for some two thirds of their daily energy expenditure, and limiting their use of the top gear lets them save a significant amount of energy.

There were also physiological changes; exposed trout had less red pulp in their spleens and higher levels of blood haemoglobin, supporting the idea that they were struggling with mild oxygen deprivation. But the titanium didn’t seem to be building up in their brains or other internal organs. And when pitted in territorial scuffles with unexposed rivals, male fish didn’t do any worse on average despite their avoidance of high-speed swimming, suggesting the particles’ impact is more complex than it initially seems.

Handy says more work is needed to understand the complex effects of different kinds of nanoparticles on fish. But he thinks the likely explanation for this effect is that the fish are slowing down as a deliberate response to the damage to their gills, rather than just because they can’t go faster. He explained:

Bioenergetically speaking, swimming fast is very expensive – exponentially more expensive than swimming slowly. If fish save the energy they would be spending on locomotion by shifting to lower speeds, they can use it for tissue repair.

The paper appears in Aquatic Toxicology.

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