Even low levels of common pesticides can disturb the ability of salmon to smell, possibly reducing their chances of survival, according to research by the National Marine Fisheries Service. Salmon rely heavily on smell to help determine friends, even mates, from foes, and it may be the primary sense the fish use to navigate back to their home streams from the sea. In the past, safe pesticide levels for fish have been determined by lethality tests, but environmentalists say the new research shows that the levels must be set much lower. Two enviro groups last week said they would sue the U.S. EPA unless the agency developed stricter limits. They contend that other efforts to protect salmon, such as habitat restoration and perhaps even dam breaching, might all be for naught unless the pesticide problem is resolved.