Shorebirds, waterfowl, and wading birds feed on flats and in the creeks and shallow subtidal areas near flats. Waterbirds also use the open waters of Casco Bay. Consequently, a wide variety and unusual aggregation of marine birds occur in Casco Bay. The number of waterbirds in Casco Bay varies seasonally from approximately 4,600 to about 32,000. Salt marshes filter stormwater from upland developments and help moderate nutrient flow to adjoining waters.
Marshes also act as giant sponges during storms and therefore reduce damage from flooding.
The 500 acres of rocky shore in Casco Bay provide habitat for a wide range of species, from seaweeds, periwinkles, mussels, barnacles, and crabs to starfish and seals. Waterfowl such as eider ducks use rocky shore habitat for feeding. For example, lobsters, crabs, and sea urchins generally live on hard bottom, whereas scallops and worms dwell in soft-bottom areas. Thirty-six species of finfish reside in Casco Bay. The most abundant year-round fish in the bay are bottom feeders such as pollock, sculpin, and skate.