Pollution far below the level seen as dangerous for aquatic life has nevertheless dramatically altered animal behaviour in North American lakes. Heavy metals are knocking out the sense of smell in organisms from bacteria to fish. Even we may not be immune.
Nathaniel Scholz, at the Northwest Fisheries Science Center in Seattle, Washington, and colleagues found that salmon lose their sense of smell if there are even low levels of copper in the water they are swimming in. The fish could die as a result, because they cannot smell chemicals that would warn of a nearby predator.
All over the world, storm water run-off shuttles heavy metals such as copper and zinc from industry, mines and built-up areas into natural water courses. The concentrations are generally low 鈥� too low for polluters to bother about, or so many of them seem to have thought. 鈥淣ow we鈥檙e going after [this] 鈥楽o what?鈥� question,鈥� says Scholz.
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Scholz鈥檚 team kept young coho salmon in tanks with different concentrations of copper for 3 hours, then measured their movements when a drop of salmon skin extract was added to the water. In the wild, the skin would be a cue that a predator may have injured a fish nearby.
Unexposed salmon stopped swimming, sank to the bottom of the tank and kept still 鈥� typical tactics for avoiding predators. But fish exposed to concentrations of copper as low as 2 parts per billion (ppb) just stopped for a few seconds, or merely slowed down, while fish exposed to 10 or more ppb didn鈥檛 notice the cue at all (Environmental Science and Technology, DOI: 10.1021/es062287r).
The US Environmental Protection Agency has set the maximum safe level of copper for aquatic life at 13 parts per billion, well above that needed to wipe out the salmon鈥檚 ability to sense chemical cues. Yet Greg Pyle, at Nipissing University in North Bay, Ontario, Canada, has found chemosensory problems at three levels of the food chain at or below 5 ppb, the limit set by Ontario鈥檚 water quality standards. 鈥淭he phenomenon is ubiquitous,鈥� he says.
Leeches lost their ability to smell food, zooplankton were unable to evade predators, and fathead minnows couldn鈥檛 recognise their eggs: the fish ate them instead of protecting them. The contamination in these lakes is much too weak to kill these organisms outright, Pyle says, yet their populations are suffering.
Metals may have the same effect in humans. The makers of the cold remedy Zicam, which contains zinc, recently settled out of court for $12 million with people who reported losing their sense of smell after spraying the product into their noses. The company maintains the remedy is safe. Studies have not been conducted to test whether zinc destroys human sensory abilities, but given what鈥檚 happening in aquatic ecosystems, Pyle believes it could. 鈥淒on鈥檛 squirt metals up your nose,鈥� he says. 鈥淭hat would be my advice.鈥�
