Atlantic tomcod, Microgadus tomcod, is one of 12 species of fish found in Maine that spend part of their lives in freshwater and part in the sea (a lifestyle called diadromous). These small fish (9-12 inches) spend most of their life in estuaries, but migrate into streams to spawn in the winter. Overfishing, exposure to pollutants, and barriers that prevent tomcod from reaching their spawning areas have led to declines in tomcod populations and the commercial fishery closed in the 1950s. Today, their population is considered stable, but there is very little known about their current whereabouts or the numbers of tomcod in Maine’s rivers and streams. We need your help to conduct visual surveys of tomcod and the birds that eat them to identify the places that these fish are spawning in coastal rivers and streams, and begin to estimate how many tomcod are using our rivers and streams.
Atlantic tomcod are at home in the mouths of streams or estuaries because they are resistant to sudden changes in temperature and salinity: they may even be found in salt marshes! They are rarely found in water more than 20-feet deep or more than one mile offshore.
During the fall and winter, they move upstream into brackish or freshwater to spawn. Females deposit between 6,000-30,000 eggs that incubate for up to 30 days in water that is between 30 to 43ºF. The average adult Tomcod is 9-12 in and has a life expectancy of 4 years. Atlantic tomcod are not only found here in Maine: they range from Newfoundland and Labrador to as far south as Virginia! Tomcod eat a fairly wide range of prey, including small crustaceans and mollusks, polychaete worms, and fish larvae. They can also be cannibals, eating their own eggs and larvae! Tomcod are eaten by predators including Striped Bass, Bluefish, and several species of piscivorous (fish-eating) waterbirds.
You may have heard Tomcod called “tommy cod” for their resemblance to true cod, or “frost fish”, for the anti-freeze-like proteins in their blood that keeps them from freezing during their winter forays into streams.
