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Friday, 26 October 2007

How polution sepparates the fishes

A contaminant found in rivers and estuaries the world over can "rob" fish of their ability to sense each other and stay in a tight, cohesive shoal, say researchers.

The chemical, 4-nonylphenol, does this by overpowering the fish's natural smell-signatures, say researchers. And because these signatures are critical to helping the fish form in groups, the chemical effectively weakens their "strength in numbers" defence against predators.

"The loss of the ability to shoal cohesively is serious business for fish. It's a defensive strategy. If fish can't shoal properly, they are extremely vulnerable to predation," says Ashley Ward at the University of Sydney, Australia, who led the study.

Nonylphenol or 4-NP is widely used in soaps, sewage treatment, and in some pesticides. They are known to affect human and animal hormonal systems, and can "feminise" fish, causing males to produce typically female proteins.

In developed nations, the maximum concentration deemed "permissible" is between 0.5 and 1 microgram per litre of water, because fish do not show signs of stress at this level. In European rivers, typical concentrations range from 0.1 to 340 micrograms per litre.

Other experiments suggested that the reason the fish shoals were not as tightly grouped in the presence of 4-NP was that the chemical was masking the fish's own smell. "Shoaling fish develop a chemical profile based on their recent habitat and diet – they smell of what they eat and where they have been, just like us," explains Ward. "They prefer to shoal with fish that smell similar to themselves."

But 4-NP is a lipophillic compound, meaning it tends to stick to oily surfaces – a fish, for example. "It seems that it might 'coat' the fish," says Ward. This changes their individual chemical signature and breaks down recognition among the fish.

Ward and his team point out that other chemicals, heavy metals for example, damage the olfactory organs of fish. They say that in polluted waters, chemicals like 4-NP and heavy metals could both be present, one affecting the way that fish smell, the other their ability to smell.

My comment: I think it's particularly important for humans to understand the global effect of their actions on the surrounding medium. Because we're used to judje only by what we see, but in Nature everything is connected and even small deviations can lead to drastic changes. This survey is good example of which. I hope this will make people think again before polluting water with all kind of sh*t. We have to protect what we have, because we're gonna lose it!

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