If people are keeping their distance from you, looking away, or discreetly placing a hand over their noses, it might be time to check your breath. The aroma of that shrimp with garlic sauce you had for lunch just might be keeping other people away.

School of reef fish. Credit: U.S. Dept. of the Interior

Odors play an important role in the way fish congregate, too. If a fish is particularly smelly, others may keep away from it. And one way for a fish to pick up a bad smell is to swim through pollution.

Odor plays a big role in fish behavior. Hormones released in the water may attract mates. Fish that live on coral reefs may use smell to sniff out the right reefs to settle on. And when it comes to forming groups, some fish seem to prefer other individuals that smell like themselves -- fish that swim in the same waters and eat the same things.

But the wrong odors can send them swimming away.

An example came from a study last year by marine biologists in Australia and Canada.

The researchers added a chemical found in soap to a tank full of a freshwater fish. There wasn’t enough of the chemical to harm the fish, but there was enough for the fish to soak up its odor. The levels were similar to those that the fish might pick up in waters near sewer plants. When these fish were released into a tank without the chemical, the other fish stayed away from them.

The experiment suggests that even mild levels of pollution could keep fish from banding together. Groups provide extra protection, so the bad smells that keep fish apart could make life more hazardous.