Low-level acidity in streams, caused by acid rain, can impair the migrating Atlantic salmon's ability to smell its way back to the place it hatched, University of New Hampshire scientists have found.

As a result, in many streams fewer salmon are returning each year to spawn even though the acid rain affecting them is too mild to produce any other obvious bad effects.

It is known that salmon, having migrated into the open ocean and matured over several years, use their sense of smell to find the plume of odors emanating from their native stream. Each stream exudes its own essence, a mix dominated by organic compounds from plants and animals in the stream's watershed. The fish, its memory imprinted with this mix of odors, can find the scent and follow it home.

The problem was discovered when fisheries biologists wondered why salmon were declining in streams that were affected only slightly by acid rain. Too much acid water can kill salmon, but the fish in these streams looked perfectly healthy.

Winsor H. Watson III and Carl Royce-Malmgren tested the effects of low acidity by means of a simple Y-shaped underwater maze. Fish were put in the stem of the Y and organic compounds known to attract or repel salmon were released into each arm. Normally the fish would swim into the arm with the attractive scent.

When the acidity was raised slightly, however, the fish became indifferent to the formerly attractive odor and were attracted to the formerly repulsive odor. The researchers suspect the acidity alters the odors or the way the salmon perceives them.