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Where is Casco Bay?

A lighthouse at the mouth of Casco Bay stands atop Halfway Rock, so named because it is almost exactly halfway between the boundaries of Casco Bay—Two Lights in Cape Elizabeth and Cape Small in Phippsburg.

Toward the southern end of Casco Bay, Maine’s most famous lighthouse—Portland Head Light—welcomes mariners into Portland Harbor. Imagine these lighthouses with signs that read, “Welcome to Casco Bay! Come on in, but wipe your feet. We are all working to keep Casco Bay clean!”

Casco Bay encompasses 14 coastal communities, including two of Maine’s largest cities, Portland and South Portland, and two of Maine’s newest towns, Long Island and Chebeague Island. Casco Bay is both a working waterfront—a port of call for cruise ships, oil tankers, and container ships—and a scenic postcard of historic forts, stalwart lighthouses, secluded anchorages, and many islands.

The Calendar Islands dot the Bay, so named because it is said there is an island for every day of the year. Actually, there are 785 islands and exposed ledges in the Bay.

The Casco Bay watershed covers nearly 1,000 square miles. Streams, rivers, snow melt, and rain runoff from 42 communities, from Bethel to the coast, flow to Casco Bay.

Several rivers, including the Fore, Presumpscot, Harraseeket, Royal, and Cousins, empty directly into Casco Bay. Even though it is not in the Casco Bay watershed, some of the flow of the Kennebec River circulates into eastern Casco Bay. We have detected the influence of fresh water from the Kennebec at Halfway Rock, nearly nine nautical miles from where the river meets the ocean.

Photograph by Kevin Morris

Beyond Halfway Rock is the Gulf of Maine, bounded by Massachusetts, New Hampshire, Maine, New Brunswick, and Nova Scotia. The shallow waters of Georges Bank and Browns Bank form its eastern rim, setting the Gulf of Maine apart from the rest of the Atlantic Ocean.

Casco Bay contributes nutrients and marine life to the Gulf of Maine. Likewise, the temperature, salinity, and acidity of Casco Bay are influenced by the Gulf.

What makes Casco Bay a great place to live—for sea creatures

Casco Bay, as an estuary, is where rivers and tides converge. Rivers add nutrients and tides deliver cold, oxygen-rich seawater. Winds, tides and currents mix nutrients throughout the water column, supporting vast quantities of plankton, which form the base of the food chain for everything else. The Bay provides relatively protected habitat for 850 species of marine life and 150 kinds of water birds that feed, breed, and raise their young here.*

Harbor seals are the most commonly seen marine mammals in Casco Bay. Boaters may also see finback, pilot, minke, and humpback whales, porpoises, Atlantic white sided dolphins, and the occasional gray seal.

Casco Bay supports economically important sea life such as soft-shell clams, quahogs, sandworms, bloodworms, crabs, lobsters, winter flounder, pollock, herring, and shad. Bluefin tuna, striped bass, Atlantic mackerel, and bluefish visit the Bay in summer. Fishermen report as the water warms, some mid-Atlantic species are showing up in Casco Bay.

From spring through late fall, thousands of lobster buoys dot the Bay, evidence that it is our most important fishery. Lobstering in Casco Bay supports many harvesters, as well as bait dealers, processors, and seafood merchants housed along its wharves.

Increasingly, the waters of Casco Bay have attracted the attention of aquaculture businesses. These ventures include shellfish and seaweed farms.

The Clean Water Act says: Casco Bay Is a Special Place

Casco Bay is the only waterbody in Maine specifically referenced in the Clean Water Act. Casco Bay is one of 28 estuaries in the nation designated as “an estuary of national significance” under the Clean Water Act “to collect, characterize, and assess data on toxics, nutrients, and natural resources within the estuarine zone to identify the cause of environmental problems…and…assess trends in water quality, natural resources, and uses of the estuary.” (Section 320)

Why is Casco Bay worth protecting?

Photograph by Kevin Morris • Aerial support provided by LightHawk

Casco Bay enriches our quality of life and drives our economy.

While it only encompasses 3% of the landmass of Maine, nearly one in five Mainers lives in the Casco Bay watershed. According to the Maine Office of Tourism, 10 million tourists a year visit Maine, and the Portland region, including Casco Bay, is their top destination. Portland is one of the busiest ports on the East Coast, making it vulnerable to oil spills and cruise ship pollution.

Casco Bay provides economic opportunity, supporting approximately 18,500 jobs, from fishing and aquaculture, to shipping and tourism. The Bay contributes an estimated $704 million of direct economic activity annually, measured as Gross Regional Product.** 

While ecologically and economically important, the Bay is also a source of inspiration and renewal for residents and visitors alike. Casco Bay’s waters, coves, and hundreds of islands provide endless recreational opportunities, from birding and angling to boating and beachcombing.

If you are like many Friends of the Bay, you live, work, and play in this region because of the influence Casco Bay has on our quality of life.

What are the biggest challenges to Casco Bay?

Thanks to advocacy under the Clean Water Act, we have decreased toxic pollution and bacteria flowing into Casco Bay. The Bay now faces increasing change caused by human activity and a myriad of threats, including nitrogen pollution, ocean and coastal acidification, climate chaos, and stormwater pollution. As these challenges grow, government resources to tackle these threats continue to decline. More and more work and responsibilities are falling to local organizations such as ours.

The Bay faces many challenges, including polluted runoff, overflows from sewage pipes enroute to sewage treatment plants, boater sewage, and the threat of oil spills. The dredging of shipping lanes and areas around the working waterfront often means dealing with where to put polluted sediments that can harm marine life. Plastic debris find its way into the ocean and the ocean food chain. Perhaps most daunting are the impacts of climate change and the changing chemistry of the Bay itself.

How is climate change impacting Casco Bay?

Climate change is impacting Casco Bay. Rising sea levels, warming water temperature, and ocean acidity all threaten the health of our coastal waters. You can learn more about the local impact of climate change here.

How is Friends of Casco Bay tackling these threats?

Photograph by Kevin Morris

It is a privilege to live, work, and play on Casco Bay. With that privilege, comes great responsibility. We remind our members, supporters, and community that the Bay belongs to all of us, and its stewardship is the responsibility of all.

How are we tackling these threats? On several fronts.

We use science. Through monitoring, assessing, and measuring, we are documenting changes we are observing in the chemistry of Casco Bay. By projecting potential impacts of human and natural changes, our goal is to help prevent the degradation that others coastal waters have experienced before it is too late to save our Bay.

We use advocacy. Our advocacy compels people to factor in the health of Casco Bay in actions and decisions that affect our coastal waters. Our science often provides the evidence that legislators and policy makers need to push for changes. We comment on proposed wastewater discharge permits, serve on advisory committees to help draft local ordinances, and work to block bad bills and to pass protective laws at the state level.

We use community engagement. We educate residents and regulators about the threats and provide information on how we all can work to change practices and policies in order to improve and protect the Bay. We send out calls to action to our supporters about pending issues that affect Casco Bay. Their response is often crucial in changing votes by legislators.

How well do you know Casco Bay?

Casco Bay in Numbers

229 = Square miles of water that Casco Bay covers
578 = Miles of shoreline around the rim of the Bay
785 = Islands and exposed ledges in the Bay
3 = Percent of the landmass of Maine the Casco Bay watershed encompasses
236,483 = Number of residents in the Casco Bay watershed, from Bethel to the Bay (2010 census)
1 in 5 = Number of Mainers living in the Casco Bay watershed

Casco Bay Extremes

Deepest spot: Lumbos Hole off Harpswell is 200 feet deep

Tallest structure along the shore: The Wyman Power Plant smokestack on Cousins Island, Yarmouth, is 400 feet tall.

Oldest lighthouse: Portland Head Light was originally commissioned by George Washington and was built in 1791. The tower is 72 feet tall; the light itself is 101 feet above the ocean surface.

Coldest air temperature: minus 39 degrees Fahrenheit (also -39⁰ Celsius)
According to the Portland Press Herald (January 4, 2014), the record for the coldest air temperature reading ever in Portland was set on February 26, 1943.

In our water quality testing data we found other extremes:

Coldest surface water temperature: 26.6⁰F (-3⁰C)
Recorded below the pier at Southern Maine Community College in early March 2003 and late February 2005

Coldest beach in Casco Bay: Willard Beach
The average temperature in August, when the water tends to be the warmest, is 61.7⁰F (16.5⁰C).
A tongue of cold ocean water from beyond Portland Head Light curves toward shore here.

Warmest water: 86⁰F (30⁰C)
Found in late summer in the Cousins River estuary and in New Meadows “Lake” (actually, it is not a lake at all, it is part of the Bay, dammed by a causeway)

Hardest place for a fish to breathe: New Meadows “Lake” in Brunswick
For much of the summer there is zero oxygen in the depths of this dammed embayment. As you know, almost all marine animals and plants need oxygen.

Easiest place for a fish to breathe: Mere Point Landing, Brunswick
We found 14.9 milligrams of oxygen per liter (mg/l) of water here, a very healthy level of dissolved oxygen. Fish become stressed when oxygen levels fall below about 5 mg/l.

Casco Bay Islands

Photograph by Kevin Morris • Aerial support provided by LightHawk

Largest inhabited island: Peaks Island, part of the City of Portland, has a year-round population of around 1,000, who are joined by 2,000-5,000 part-time residents and summer day-trippers. In the late 1800s, Peaks Island was known as the Coney Island of Maine, thanks to a boardwalk, theater, amusement park, and several hotels. Most of these structures are gone, but concrete observation towers remain, evidence of its use as a military outpost during World War II.

Island where you might see a polar bear (well in a museum): Eagle Island, summer home of Arctic explorer Admiral Robert E. Peary, who led the first successful expedition to the North Pole in 1908. (You also can find one at Bowdoin College, from which Peary graduated in 1877.)

Maine’s two newest towns are islands in Casco Bay: Long Island seceded from the city of Portland in 1993, and Chebeague Island, once part of Cumberland, became its own town on July 1, 2007.

Best nearly inaccessible attraction in Casco Bay: Fort Gorges is free and open to the public, if you can get there. A stairway to the pier deteriorated long ago; visitors either must beach a boat in the sandy shallows or scale the granite dock at high tide. There are changes afoot there thanks to the efforts of Friends of Fort Gorges.

What can I do to help protect this amazing place?

Casco Bay belongs to you! Help us help you take care of this amazing place.

Casco Bay Plan 2016-2021 (2015):  “CB has abundant maritime trades, a strong lobster fishery, and more than 800 documented marine species.”
** The Economic Contribution of Casco Bay, prepared for Casco Bay Estuary Partnership by Ryan D. Wallace, Rachel Bouvier, Laura M. Yeitz, and Charles S. Colgan, November 2017

Cover photo: Photograph by Kevin Morris • Aerial support provided by LightHawk

Read more about our work and Casco Bay:

Coastal Cleanup at Bug Light Park for International Coastal Cleanup Day

July 12, 2018

Join us at Bug Light Park for International Coastal Cleanup Day! When: Saturday, September 15, 2018 Where: Bug Light Park Questions? Email Sarah at slyman [at] cascobay [dot] org Do you want to help keep Casco Bay clean? Volunteer to help out at our public coastal cleanup! Trash is an unsightly blight… Read more

Spring Blooms in Casco Bay

June 26, 2018

What signs tell you that spring has arrived? Grass turning green? A robin in your yard? Ospreys returning to their nests? What about huge blooms of phytoplankton in Casco Bay? The chlorophyll fluorescence measurements in the graph above were recorded by our Continuous Monitoring Station, which has been in place… Read more

A New Way to Volunteer: Measure the Color of Casco Bay

June 7, 2018

From Homer’s “wine dark seas” to David Attenborough’s “Blue Planet,” the color of the ocean has held our fascination throughout the ages. People often consider blue water as a sign of a healthy ocean and dirty-brown water to indicate polluted water. Turns out, color is a valuable indicator of the… Read more

Our Warmest Time of Year is Getting Warmer

May 15, 2018

  According to Research Associate Mike Doan, Casco Bay’s critters are experiencing longer summers and warmer autumns. For 25 years, Friends of Casco Bay staff scientists monitored water quality every month of the year at three deepwater sentinel sites in the Bay, from surface to sea floor, aboard our research… Read more

Casco Bay begins in your backyard

April 27, 2018

Friends of Casco Bay has been a strong advocate for municipal ordinances to reduce the use of lawn care chemicals. As the grass turns green, we are celebrating a series of milestones in our effort to protect the Bay from pesticides and fertilizers: The City of South Portland Pesticide Use… Read more

Wild and Scenic Film Festival

April 1, 2018

We look forward to seeing you at our 11th Annual Wild & Scenic Film Festival!

Save the Date for 2018’s Film Fest: Saturday, November 3, 2018 Read more