If water level fluctuates greatly, nesting may be impossible, since nests are generally near an abrupt slope in the lake bottom, allowing easy access to water in case of danger. If territorial lakes stay frozen longer than normal, loons may move to thawed lakes. For feeding, loons locate fish by sight, then dive, and chase them underwater, making water clarity crucial to their territory. They generally dive between 6.5 and 13 feet deep. In a recent study conducted at the Chippewa National Forest in Minnesota, Secchi disc readings averaged 7.2 feet on lakes with territorial loons, and only 3 feet on lakes without loons.
Pollutants also present a potential hazard to loons.
The lead shot of duck hunters and lead sinkers used in fishing are the main contributors to this problem. Hoping to ease the problem, the New Hampshire-based North American Loon Foundation promotes the use of fishing sinkers made of tin, Bismuth and recycled glass. Gostomski believes these alternatives may soon be available in Wisconsin. Studies conducted by LoonWatch and its volunteers show a gradual increase in loon population in the state, while some other Midwest states have recorded slight declines. A total of 2357 loons were counted in Wisconsin in 1985, growing to 2420 in 1990 and most recently to 3016 loons documented in 1995.