Mill Creek and connecting Chelsea Creek are important habitats for both marine life and shorebirds. They are a spawning ground and nursery for smelt, American eel, Atlantic butterfish, Atlantic cod, Atlantic mackerel, Atlantic sea herring, pollock, red hake, and at least four types of flounder. Many birds who feed on the fish hatchlings, such as egrets, swans, herons, and cormorants, are therefore attracted to these estuaries to feed and breed.