Tomcod spawning time

In the Mi’kmaw language, January is “Punamuiku’s,” or Tomcod spawning time.

In times past, this small fish (in Mi’kmaw: Punamu) was a nutritious staple when other menu items were scarce. Early Mi’kmaq caught the Tomcod with lines inserted through holes in the ice. This resource was beneficial for the women and children to sustain themselves in times when the men would leave the community for the winter hunt. These days, the men return at night in their trucks and ATVs, while supermarkets take up the slack.

Tomcod is also known as frostfish, Atlantic Tomcod, or winter cod, and it is found in North American coastal waters from the St. Lawrence Seaway, St. Lawrence River, and northern Newfoundland, south to Virginia. The Tomcod is not the same species as the famous Atlantic cod that used to dominate the offshore Atlantic waters.

The Latin name of the tomcod (Microgadus tomcod) is a clue to its not-so-close relationship to the Atlantic cod (Gadus morhua) because they do not even share a genus (ie: the first word in the Latin genus and species combination). The tomcod does not grow as large or live as deep as the Atlantic cod. Whereas the Atlantic cod is deemed a “threatened species” by the International Union for the Conservation of Nature, the tomcod is considered to be of “least concern.”

Across the Atlantic region of Canada and the United States, adult Tomcod move from deeper coastal waters into sheltered estuaries in November and December after the first hard frost strikes the region. This is why they have been dubbed frost fish. When ice covers the estuary in January, a spawning frenzy begins. The hungry, carnivorous Tomcod will show an interest in any bait during this lovefest and provide endless excitement to those fishermen who choose to set up shop on the ice.

There are several attributes that make the Bras d’Or Lake estuary an ideal spawning and nursery habitat for Tomcod. It provides a mosaic of salty, brackish and fresh water pockets, extensive seagrass beds, and an abundance of food such as small shrimp, worms and fish.

The mosaic of fresh, brackish and salt water habitats is ideal in that the tomcod releases eggs and sperm in relatively fresh waters but the larvae develop optimally in saltier waters, a short drift away. In fact, some Tomcod are commonly found to move further up rivers and spawn in Nova Scotia’s fresh water lakes.

In the Bras d’Or estuary, the amorous adults spawn in areas of lower salinity such as the mouths of the Skye, Middle and Baddeck Rivers (in salinities up to half the strength of seawater). The average female will lay about 20,000 eggs in the fresh or slightly brackish water which, after being fertilized by a male, will hatch in about 25 or so days. The fertilized eggs are large (about 1.5 millimeters in diameter) and they sink to the bottom after spawning, adhering in masses to bottom substrates such as gravel and seagrass.

When the eggs hatch, the developing larvae move to the saltier parts of the estuary (over half seawater strength) throughout the summer, and then will move to coastal waters in the autumn. The spent adults leave their fertilized egg masses behind and return to coastal waters in late January or early February.

The second attribute of the Bras d’Or estuary that contributes to its’ status as excellent tomcod breeding habitat is the extensive, healthy seagrass meadows that cover the bottom. There is only one species of seagrass in the Bras d’Or estuary, Zostera marina. On a neighbouring coast, scientists from the Maine Department of Marine Resources found that Zostera meadows were critical nursery habitat for Tomcod.

It isn’t only the Tomcod that benefit from the protected, productive Zostera nursery habitats. These field studies in Maine showed that an impressive list of fish species including that other cod, the famous Atlantic variety, depend on these meadows. Other species that were shown to rely on the Zostera as nursery habitat included smelt, herring and winter flounder.

I hope you have a new appreciation for the humble seagrass as valuable nursery habitat for many species that we value and some of the prey that sustain them. We are keeping track of the health of Zostera meadows during Bras d’Or Watch each summer.

Dr. Annamarie Hatcher is a consulting ecologist and a board member of the Bras d’Or Lake Biosphere Reserve Association. For more information about the Bras d’Or Lake Biosphere Reserve Association, please visit http://blbra.ca/. For information about Bras d’Or Watch, visit our Facebook page. Annamarie would like to thank Dianne Chisholm from the Mi’kmaw Resource Centre for her assistance.