Duckling Responses to Lake Acidification: Do Black Ducks and Fish Compete?

Abstract

This study focused on two pairs of ponds in Maine, USA, that were physically similar but which had markedly different acidities. Salmon Pond (pH 6.3) had fish, while 120 m away, Mud Pond (pH 4.5) was fishless. The second pair, Killman Pond (pH 6.5) and Unnamed Pond (pH 4.8), 14 km distant, were also physically similar but again the circumneutral pond had fish and the acidic pond was fishless. Imprinted black duck ducklings, Anas rubripes, kept on the acidic ponds grew faster than those on the circumneutral ponds. Differences in behavior also indicated that the acidic ponds were better duckling habitat. Ducklings on the acidic ponds spent less time searching and moving, and more time feeding and resting, than ducklings on the circumneutral ponds. There was substantial overlap in the diets of trout and ducks; two of the three indices calculated indicated that ducklings sharing a circumneutral pond with fish had diets that were more similar to the fish's diets than to the diets of ducklings on acidic ponds. Sweep-net samples of invertebrate abundance and comparisons of benthic invertebrate abundance inside and outside fish exclosure cages indicated that the presence of fish decreased invertebrate abundance. These results support the idea that ducklings and fish compete for invertehrates and that, under certain circumstances, the negative effect of acidification on fish may produce a beneficial effect for ducklings.