Atlantic Tomcod
Microgadus tomcod
Common name: Atlantic tomcod, winter cod, frostfish, tommycod
Scientific name: Microgadus tomcod
Classification: Members of the cod family, as their name suggests.
Range: Northwest Atlantic from Chesapeake Bay in Virginia to Southern Labrador in Canada. Tomcod are a bottom-dwelling species that prefers cold, coastal salt water, brackish estuaries and rivers connected to the sea. However, they can be found landlocked in several freshwater lakes.
Atlantic tomcod are at home in the mouths of streams or estuaries because they can tolerate sudden changes in temperature and salinity. In fact, all of their life stages are dependent on estuaries, be it for food, shelter, or passage to spawning grounds. They are rarely found in water more than 20 feet deep or more than one mile offshore.
Identification: Tomcod are small fish with a streamlined body shape that tapers near the tail. Their small head has a rounded end with a small barbel under the chin. They have relatively small eyes and scales, and have a pale lateral line that runs down their bodies. Their coloration is typically dark brown on their back fading to tan or yellowish with dark mottling on their sides, and white on the belly. The mottling is also visible on their three dorsal and two anal fins. Their pelvic fins have small filamentous extensions.
Adults are typically 6 to 12 inches in length, with a maximum reported size of 14.9 inches. Most weigh less than 1 pound, topping out at around 1 and ¼ pounds.
The tomcod is distinguishable from cod by its elongate pelvic rays, rounded tail and dark mottling on its back and sides.
Life history: Atlantic Tomcod are anadromous, but they do not travel far between salt and freshwater. During the fall and winter, they move upstream into brackish or freshwater to spawn. Spawning occurs over gravelly bottoms and sand. Females deposit between 6,000-30,000 eggs that sink to the bottom in masses. The eggs incubate for 24 to 30 days in water that is between 30 to 43ºF. Newly hatched larvae swim to the surface within 24 hours of hatching to gulp surface air before their pneumatic duct closes
Juveniles generally spend their first spring and summer in fresh or slightly brackish water before moving out of the streams where they were hatched into deeper salt water estuaries. Juvenile tomcod are attracted to shallow eelgrass beds and salt marshes, which are prime nursery grounds for this species.
The average adult tomcod is 9-12 inches in length and has a life expectancy of 4 years.
Diet and behavior: Tomcod eat a fairly wide range of prey, but mostly small crustaceans, mollusks, polychaete worms, squids, and fish larvae, including that of smelt, sticklebacks, striped bass, alewives, shed, herring and sculpins. They can also be cannibals, eating their own eggs and larvae.
Tomcod are very tolerant of the near ice-cold waters of the Northern Atlantic. They produce antifreeze-like proteins called TLSPs (trypsin-like serine protease) in their blood that keeps them from freezing during winter, hence the nickname “frost fish”. These proteins have a "sticky" end which binds to the pieces of developing ice crystals and prevents the blood from freezing.
Ecological importance: Tomcod are eaten by predators including Striped Bass, Bluefish, and several species of piscivorous (fish-eating) waterbirds.
Cultural importance: Tomcod bones have been found in archaeological digs through Wabanaki waste mounds along the coast of Maine, so presumably they were a part of their diet.
Threats: Overfishing, exposure to pollutants, and barriers that prevent tomcod from reaching their spawning areas have led to population decline. Today, their numbers are considered stable, but there is very little known about their current whereabouts or the numbers of tomcod in Maine’s rivers and streams. The Downeast Salmon Federation was a partner on a recently completed study that used citizen science to find out more about where and when tomcod are found in Maine's coastal streams. Data are still being processed, but we hope to be able to share the results of the study with the public soon.
Fun fact: Between 1947 and 1976, General Electric released more than a million pounds of PCBs into the Hudson River. PCBs can kill fish and seabirds and have been linked to cancer and other serious health problems in humans. PCBs were banned in 1979, but the toxins have remained at high levels in the Hudson because they settle into the sediments on the bottom of the river and don’t break down. A 200-mile stretch of the river is now the nation’s largest Superfund site, and it is also the site of rapid evolution in the local population of Atlantic tomcod. In 2011, scientists discovered that over the course of 60 years, the tomcod in this area have developed a genetic mutation that makes them resistant to toxic effects of these chemicals. This is a particularly rapid example of evolution via natural selection, where the fish with the mutation survived the adverse conditions and passed it down to their offspring. Unfortunately, while this is good news for the Hudson River tomcod, any predators that eat them will be exposed to the PCBs that are present in their tissues.
Restoration efforts: Like with all diadromous fish in Maine, the best way to restore populations is to remove barriers to spawning habitat, like dams. DSF has been involved with a number of these projects since its founding, and most recently completed the removal of a dam on a stream called Smelt Brook in Sullivan. The dam had been in place for 50 years, and soon after its removal tomcod were observed moving up into the stream to spawn. Overall, their population is presumed to be relatively stable, though the actual population size is unknown.
Fishery: Tomcod are a good tasting fish, despite being quite small in size. In the 1800’s, Atlantic tomcod were commercially important. They were harvested heavily in the Boston area, and were called “London trout.” However, by the 1950’s their numbers had declined too much to support continued commercial harvest, and the fishery was closed. Today, the Atlantic tomcod is a popular winter and early spring species often caught incidentally in the recreational smelt fishery. They can readily be caught by fishing with bait (such as sandworms) on the bottom.
Sources:
https://fishbase.mnhn.fr/summary/316
https://www.wildlife.nh.gov/atlantic-tomcod
https://portal.ct.gov/deep/fishing/freshwater/freshwater-fishes-of-connecticut/atlantic-tomcod
https://investigate.gmri.org/project/tomcod_survey
https://wellsreserve.org/visit/trails-beach/flora-fauna/fishes-of-the-wells-reserve/atlantic-tomcod
https://gulfofme.com/all-sea-life/atlantic-tomcod-microgadus-tomcod
https://www.fishermensvoice.com/archives/201403TommyCod.html
https://cybrary.friendsofmerrymeetingbay.org/fgom/Microgadus_tomcod.htm
https://evolution.berkeley.edu/evo-news/toxic-river-means-rapid-evolution-for-one-fish-species/
https://atlanticsalmonrestoration.org/resources/fact-sheets/atlantic-tomcod-microgadus-tomcod