Fisheries and Oceans Canada

Fisheries and Oceans Canada

June 09, 2008 11:59 ET

Fisheries and Oceans Canada/Nearshore Habitats: Sanctuary for Juvenile Cod

OTTAWA, ONTARIO--(Marketwire - June 9, 2008) - A large, mature female Atlantic cod can produce up to 30 million eggs during a single spawning season. This is a staggering number, but it is just barely enough to beat the equally staggering odds stacked up against any of the eggs surviving past a few days. The vast majority of them are just so much food for other species (including older cod). In their first few weeks, the mortality rate can be an incredible 99% per day! The odds get better for those making it to the larva stage, with only five to ten per cent gobbled up every day. The meagre handful that survives the 60 to 70 days necessary to become juvenile cod continues to see their odds increase. And it is becoming clear that how much those odds grow in their favour is very much dependent upon the type of habitat they settle into for the next 60 to 90 days of their life.

When Newfoundland and Labrador's waters were teeming with Atlantic cod, there wasn't any pressing need to know about all the factors involved in guaranteeing that each generation of cod would produce sufficient replacements to sustain a robust fishery. The bounty was seemingly endless. It's a completely different story now. The collapse of the fishery in the early 1990s imposed a stark new reality on the industry and the people whose livelihoods depend upon it, and it is now crucial to understand every aspect of the cod's life cycle and what factors influence or inhibit its ability to survive to adulthood. While the effects of fishing upon the adult cod are fairly well documented, those of the environment upon a young cod's survivability are not. This is a new area of research - and it has to be built from the ground up, because next to nothing is known.

Bob Gregory, a scientist at the Northwest Atlantic Fisheries Centre, part of Fisheries and Oceans Canada, in St. John's, Newfoundland, has been working, since the mid-1990s, to increase our knowledge about juvenile cod. One of the most important results of his work is that we now have a very clear picture of the type of habitat that these post-larval juvenile cod need to settle into in order to survive and thrive, and it has turned out, in many ways, to be quite a revelation.

His research has shown that the ideal cod nurseries are not everywhere, as was previously assumed, but in very shallow waters within a stone's throw of the shore. It has also defined the gold standard for habitats to be eelgrass beds, where a juvenile cod's chances of survival are 17,000 times higher, as opposed to more barren habitats.

The implications of this are profound. Eelgrass grows where freshwater enters the sea, which is precisely where humans tend to put their communities. The beds are located close to shore in water less than ten metres deep, thus placing these prime sanctuaries for juvenile cod under pressure from human activities and infrastructure. Another emerging threat to eelgrass beds is the green crab, an invasive species that has been moving north along Canada's east coast since the 1950s. It is just beginning to turn up in Newfoundland, and, in fact, was reported for the first time in the summer of 2007 at North Harbour in Placentia Bay. The green crab can uproot entire beds of eelgrass by its burrowing behaviour. Given the importance of the eelgrass beds to the survivability of juvenile cod, conservation and protection measures for them are going to be essential in the fight to increase cod populations. In fact, Gregory's program is providing habitat managers with the science they need to protect these critical nurseries for juvenile cod when reviewing plans for new developments along the shoreline. Through the research team's interaction with fishers, it is also instilling a need for community-driven stewardship.

Gregory's work involves tracking the distribution of the cod eggs and larvae, and then studying the survivors once they have settled into their shallow water sanctuaries. It also involves mapping eelgrass beds around the coast and is looking at understanding the food chain as it applies to juvenile cod. The bottom line is that it is now undeniably clear that these first 60 to 90 days as a juvenile, although a very short time span en route to becoming an adult, have an absolutely critical role in the survival of the fish stock. At this post-larval stage, the most important job of a juvenile cod is to avoid being eaten, while at the same time lessening predator pressure by growing bigger. As Gregory explains it, "a juvenile cod can go hungry for a day, but it must successfully avoid predators every moment - there is no second chance."

Much of the fieldwork for this research has been carried out with students from Memorial University in Newman Sound, adjacent to Terra Nova National Park. Over the course of the research program, close to 30 students have worked with Gregory and have focused their studies in this realm, creating a critical mass of researchers. Additionally, an ecotourism company, Coastal Connections Ltd., is incorporating the research into their educational programming, with the result that they not only collect data, but, by doing so, are raising public awareness about juvenile cod in the ecosystem.

In the numbers game where stock assessment is the basis for setting quotas for fish harvesting, Gregory's research is a powerful new tool. His annual report to stock assessment managers includes population numbers of the current crop of juvenile cod, and these have proven to be an extremely accurate preview of what's coming down the pike in terms of adult cod a few years later. As Gregory sums it up, this research has taken "a bit more fog out of the crystal ball." Encouragingly, 2007 has been the best year, in terms of juvenile cod surviving those critical early days, and it is going to be very interesting to see how this plays out for future cod stocks.

Photo 1 is available at the following address:

Caption: Age 0 juvenile cod. (Photo by David Cote)

Photo 2 is available at the following address:

Caption: In its first few months as a juvenile cod, the survival rate increases by 17,000 times if the cod settles into an eelgrass bed. (Photo by Corey Morris)

Photo 3 is available at the following address:

Caption: Piecing together habitat preference and population numbers of juvenile cod involves lots of measuring and counting. Beach seines, deployed at 13 sampling sites in Newman Sound, are one of the research tools used. (Photo by David Cote)

Contact Information

  • Fisheries and Oceans Canada, Ottawa
    Phil Jenkins
    Media Relations