A contaminant found in rivers and estuaries the world over can 鈥渞ob鈥� 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鈥檚 natural smell-signatures, say researchers. And because these signatures are critical to helping the fish form in groups, the chemical effectively weakens their 鈥渟trength in numbers鈥� defence against predators.
鈥淭he loss of the ability to shoal cohesively is serious business for fish. It鈥檚 a defensive strategy. If fish can鈥檛 shoal properly, they are extremely vulnerable to predation,鈥� says at the University of Sydney, Australia, who led the study.
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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 鈥渇eminise鈥� fish, causing males to produce typically female proteins.
In developed nations, the maximum concentration deemed 鈥減ermissible鈥� 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.
Keeping their distance
Ward and colleagues decided to test whether 鈥減ermissible鈥� levels of 4-NP could disrupt social organisation in banded killifish, a shoaling fish commonly found in North American lakes.
Ward 鈥� then a researcher at Mount Alison University in New Brunswick province, Canada 鈥揼athered killifish from the nearby Morice Lake. He found that the groups of fish that were placed in aquariums with 1 microgramme per litre of 4-NP tended to stay at least twice as far from each other as those in uncontaminated aquariums.
The chemical did not appear to affect the fish鈥檚 ability to smell other substances, as they were just as able to find food hidden away in their aquarium as the fish in the uncontaminated tanks.
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鈥檚 own smell. 鈥淪hoaling fish develop a chemical profile based on their recent habitat and diet 鈥� they smell of what they eat and where they have been, just like us,鈥� explains Ward. 鈥淭hey prefer to shoal with fish that smell similar to themselves.鈥�
Fish are also thought to produce chemical signals relating to their social dominance, reproductive state, and genetic make-up. The last type of signals help them avoid breeding with relatives.
鈥楽ub-lethal鈥� effects
But 4-NP is a lipophillic compound, meaning it tends to stick to oily surfaces 鈥� a fish, for example. 鈥淚t seems that it might 鈥榗oat鈥� the fish,鈥� says Ward. This changes their individual chemical signature and breaks down recognition among the fish.
Ward and his colleagues placed single killifish in a corridor of water that had two separate currents running down it. One current was clean. The other came from a tank in which the researchers had placed killifish that had previously bathed for an hour in 4-NP contaminated water.
The team found that the lone killifish moved away from the contaminated current when the upstream fish had bathed in 4-NP at concentrations of 1 microgram per litre or higher.
鈥淚 think we need to reappraise our comfortable position that if a certain concentration doesn鈥檛 actually kill an animal, it鈥檚 OK,鈥� Ward told New 杏吧原创. 鈥淭here are subtle 鈥榮ub-lethal鈥� effects that can be devastating in the medium and long term.鈥�
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.
Journal reference: (DOI: 10.1098/rspb.2007.1283)
