Casco Bay is an open bay of the Gulf of Maine on the coast of Maine in the United States. The National Oceanic and Atmospheric Administration's chart for Casco Bay marks the dividing line between the bay and the Gulf of Maine as running from Bald Head on Cape Small in Phippsburg west-southwest to Dyer Point in Cape Elizabeth. The city of Portland and the Port of Portland are on Casco Bay's western edge.
Name origin There are multiple theories about the origin of the name "Casco Bay". Aucocisco, an Anglicisation of the Abenaki name for the bay, means "place of herons", "marshy place", or "place of slimy mud". The explorer Estêvão Gomes mapped Maine's coast in 1525 and named the bay "Bahía de Cascos", translated as "Bay of Helmets", based on its shape.
Colonel Wolfgang William Römer, an English military engineer, reported in 1700 that the bay had "as many islands as there are days in the year" leading to the bay's islands being called the Calendar Islands, based on the popular myth there are 365 of them. Estimates vary on the actual count, with the Casco Bay Estuary Partnership listing 785 if including exposed ledges. Former Maine state historian Robert M. York said there are "little more than two hundred" islands in Casco Bay.
Geography and other characteristics Casco Bay spans about 229 square miles, with its shore stretching 578 miles by one estimate and the inner bay divided into eastern and western sections by the Harpswell Neck peninsula.
In addition to Portland, Cape Elizabeth, and Phippsburg, municipalities with shorelines fronting Casco Bay include Brunswick, Cumberland, Falmouth, Freeport, Harpswell, South Portland, West Bath, Yarmouth, and the island towns of Chebeague Island and Long Island.
Researchers with the U.S. Geological Survey have dated volcanic material embedded in exposed bedrock in Casco Bay to the Ordovician period roughly 470 million years ago, predating the formation of the Atlantic Ocean by some 320 million years. The Norumbega fault developed just inland from the Maine coast, with the geologic fault running roughly parallel to the coastline, including a portion of the northern shore of Casco Bay. The Flying Point fault in Casco Bay is considered part of the Norumbega fault system, dividing bedrock formations that have distinct geological characteristics.
Around 14,000 BCE during the Wisconsin glaciation period at the end of the last glacial cycle, the Laurentide ice sheet covering the Casco Bay region began to recede, according to radiocarbon dating on marine shells and other materials. The glacier's retreat stripped bare underlying bedrock to form the rocky coast of Casco Bay's shore and islands.
According to NOAA's soundings, the bay's deepest point is about 204 feet, southwest of Halfway Rock. A Phippsburg hill called Fuller Mountain has the bay's highest elevation along the immediate shoreline, estimated at 269 feet above sea level by the U.S. Geological Survey in 1980, and 277 feet on more recent topographical maps. Sebascodegan Island has the highest elevation of any Casco Bay island at 201 feet on a hill called Long Reach Mountain, followed by Great Chebeague Island at 176 feet.
In Casco Bay's western reaches, a line of islands extends west from Great Chebeague to Cushing Island to create protected anchorages for vessels, as do the narrow peninsulas that jut into the bay's eastern section. A number of deep-water channels lead into the bay's inner sections, including Cushing Island Reach, Hussey Sound, Luckse Sound, Broad Sound, and Merriconeag Sound.
Casco Bay's shoreline creates a number of smaller bays and tidal embayments, including Harpswell Sound, Maquoit Bay, Middle Bay, Quahog Bay and New Meadows River, where depths exceed 150 feet in a narrow channel just south of Cundy's Harbor.
Casco Bay's topography produces a tidal range of about nine feet on average. Seawater circulates counterclockwise into Casco Bay via the Gulf of Maine Gyre, which is formed from cold water that passes over the Scotian Shelf off Nova Scotia, then in and out of the Bay of Fundy. In Casco Bay, tidal currents are stronger between island channels and weaker in smaller bays in the eastern section.
The Casco Bay watershed has been estimated at 986 square miles, with more than 1,300 streams in the basin. The Presumpscot River is the largest single source of non-saline water emptying directly into Casco Bay, flowing south from its headwaters at Sebago Lake, Maine's second-largest lake. The Royal River and the Stroudwater River are also considered main stem rivers emptying into Casco Bay. In addition to freshwater entering Casco Bay from those rivers and smaller watercourses along its length, lower-salinity seawater outside the mouth of the Kennebec River circulates west into Casco Bay.
Ecology
Scientists have defined a distinct Casco Bay Coast Biophysical Region as part of the larger Northeastern Mixed Forest Province, with common tree species including red maple, eastern white pine, northern red oak and gray birch.
More than 800 marine species have been identified in Casco Bay.
Water temperatures in Casco Bay rose by 3 degrees Fahrenheit over a three-decade period through 2022, with some scientists linking the change to shifting mixes of organisms and wildlife in the bay.
In a 2019 study of invasive species threatening Casco Bay eelgrass and kelp beds that other organisms and wildlife depend on, researchers found abundant evidence of the presence of several types of tunicates, bryozoa, Japanese skeleton shrimp and European green crabs. As of 2022, Casco Bay's eelgrass beds were at 28% of their coverage area two decades earlier.
Casco Bay has an estimated of intertidal habitats to include mudflats, marshes, beaches and rock formations according to the National Wetlands Inventory, supporting a range of biota and wildlife.
In a Gulf of Maine Research Institute analysis of predominant sea creatures caught in seine and jigging surveys in 2025, the most common species were alewife, American sand lance, Atlantic silverside, bluefish, green crabs, mummichog, grubby sculpin, permit and winter flounder. Catches of Atlantic herring, Atlantic silverside and green crabs hit the lowest on record.
Other species of fish found in Casco Bay include Atlantic mackerel, bluefin tuna, Atlantic pollock, Atlantic tomcod, cod, cunner, menhaden, northern pipefish, sharks, smelt, and striped bass. Shellfish include lobsters, Asian shore crabs, mussels, clams, oysters, scallops and periwinkles.
Harbor seal populations have been observed to number between 400 and 500 seals in Casco Bay. There have been a number of whale sightings in Casco Bay over the years, including the beluga whale, the North Atlantic right whale and the humpback whale.
The number of water birds in Casco Bay varies by season and migratory cycles, with studies having shown anywhere from less than 5,000 to 32,000 or more across as many as 150 species. Surveys of seabird populations in 1979 and 1980 identified nearly 5,400 nesting pairs of herring gulls across 56 colonies; close to 4,000 pairs of double-crested cormorants in 15 colonies; almost 3,000 pairs of eider ducks in 45 colonies; more than 2,100 pairs of great black-backed gulls in 37 colonies; and about 560 nesting pairs of common terns in nine colonies. Smaller numbers of horned grebes, common loons, ring-billed gulls, Bonaparte's gulls and laughing gulls have been observed.
As of 1996, scientists had identified 50 islands in Casco Bay that were used as nesting grounds for 15% of Maine's nesting seabirds, with 17 of those islands deemed to have nationally significant populations. That included nesting areas for great blue herons, black-crowned night herons, glossy ibises and snowy egrets.
In addition to eider, other waterfowl in Casco Bay depending on seasons include Canada geese, snow geese, black ducks, goldeneyes, buffleheads, greater scaup, scoters, long-tailed ducks and harlequin ducks. Migratory shorebirds that pass through Casco Bay include sandpipers, plovers, turnstones, dowitchers and greater yellowlegs. As of 2024, the Maine Department of Inland Fisheries & Wildlife had designated four locations in Casco Bay as "essential habitats" for colonies of the threatened or endangered species of piping plover, least tern and roseate tern, at Clapboard Island, The Nubbin, Jenny Island and Pond Island.
Raptor populations on Casco Bay islands and shorelines include osprey, with 86 nesting pairs observed in a 2011 survey, and 14 more nests that were deemed potentially active. After 30 years of monitoring produced no evidence of bald eagles in Casco Bay, a nesting pair was spotted in Freeport in 1992, followed by bald eagle pairs in Brunswick and Harpswell in 1994 and 1995. As of 2018, fifteen bald eagle pairs were observed in Casco Bay communities, nine of them in Harpswell.
A 1992 study determined found that while contaminants like polycyclic aromatic hydrocarbons, polychlorinated biphenyls and trace metals were detectable throughout Casco Bay, they were "generally far below levels suspected of evoking a toxic biological response."
History Native American population and arrival of European settlers
At the time of European contact in the 16th century, an Abenaki people called the Almouchiquois inhabited the region of present-day Casco Bay, including a group called the Aucocisco in the vicinity of the Presumpscot River in Casco Bay's western section; and the Pejepscot who lived along the bay's eastern section and in the Androscoggin River valley to the north.
Some Casco Bay islands have archaeological evidence of Native American visits and camps extending back 4,000 years, including shell middens and harpoon points.
It is uncertain whether early European explorers Giovanni da Verrazzano, John Cabot, Estêvão Gomes, or Bartholomew Gosnold entered Casco Bay. It is believed that Martin Pring made landfall in Casco Bay as part of a 1603 expedition, with Samuel de Champlain and Pierre Dugua de Mons exploring it in 1605 from a base in Nova Scotia. In establishing the Popham Colony settlement near the mouth of the Kennebec River, George Popham landed in Casco Bay in 1607 while exploring the wider region. After Henry Hudson's ship Half Moon was damaged in 1609 while attempting to discover a northwest passage to India, Hudson landed in Casco Bay for repairs.
In 1616, John Smith published a map of New England that included a depiction of Casco Bay based on his exploration of the region two years earlier.
Contact with Europeans exposed Wabanaki peoples to new diseases, with epidemics striking starting in 1616 that produced high mortality rates. By one estimate, just 25% of the Wabanaki survived epidemics that broke out through 1619.
On August 10, 1622, King James I of England awarded a land patent to Ferdinando Gorges and John Mason for coastal lands and interiors extending from the Merrimack River to the Kennebec. Gorges and Mason eventually split the patent, with Gorges getting land patent rights north of the Piscataqua River.