Parker, K.E. 1985. Foraging and reproduction of the common loon (Gavia immer) on acidified lakes in the Adirondack Park, New York. M.S. Thesis, SUNY-ESF, Syracuse, 144 pp.

Abstract: The effects of lake acidification on common loon foraging and reproduction were studied on 24 lakes from May through August in 1983 and 1984. The lakes ranged in size from 10.5 to 179 ha; the pH ranged from 4.5 to 6.7.

Loons on small (< 50 ha), highly acidic (mean pH < 5.5) lakes had a mean dive time 21% longer than loons on small lakes where pH exceeded 5.5. Mean dive time of loons on lakes with a low index of fish density (< 5 fish caught/100 units of effort) was also 21% longer than the mean found on lakes where the index of density exceeded that level. The ability of adults to procure food items during chick feeding sessions was not related to lake pH or prey abundance, as assessed by catch/effort minnow trapping. However, the amount of time spent feeding chicks generally increased with decreasing pH. On 3 occasions, a loon was observed flying with a fish in its bill to a nesting lake containing no fish. No significant difference (P > 0.10) in loon reproductive success was found when lakes were grouped by size or pH. Reproductive impairment of fishes in many of the highly acidic lakes is suggested by the absence of small fish while larger individuals are still found in limited numbers.

Loons seem able to adapt, at least in short-term, to food resource depletion attributable to lake acidification. However, they may not be able to cope with acidification on a long-term basis. Highly acidic lakes which currently support loons may not do so in 5 years when the remaining fish have died. Further research efforts need to address the growth and survivorship of young raised on acidic lakes, total daily foraging time for loons on acidic and non-acidic lakes, and the energetic costs of feeding and nesting at separate lakes.