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The Role of Closed Areas in Maintaining Cod Health

Aug 21, 2012
Summer 2012

This is the final article in a three-part series examining key elements in cod life history. The previous two articles covered recent research into cod's stock structure and food. This final article explores recent findings concerning the role that closed areas play in the health of cod in the Gulf of Maine and Georges Bank.

The recent dire news about the health of both Gulf of Maine and Georges Bank cod has raised startling questions about what we know about this iconic species. The near-total collapse of the Canadian cod fishery two decades ago offers a grim picture of what can happen when managers do not fully appreciate the dynamics of a species' life history.

Migrants Versus Residents
Cod in the Gulf of Maine and on Georges Bank are likely divided between two basic life history strategies: migratory and resident. Migrant cod travel throughout the region, crossing management and international boundaries. Resident cod, on the other hand, stay within a small area. Migrant cod tend to be lean and sleek, designed to swim quickly and easily. Resident cod tend to be rounder and more robust - a body shape to match their lifestyle.

Graham Sherwood, GMRI's Demersal Ecologist, speculates that in recent years the fishery has targeted migratory fish over resident fish for a couple of reasons. Migratory fish tend to move along specific pathways and spawn in highly predictable locations. Fishermen have focused their effort in response. Meanwhile, areas closed to commercial fishing have allowed resident populations to flourish. Thus, the general effect of the current management system has been to favor resident cod, especially those residing in closed areas, over their migratory counterparts.

Health Differences
Sherwood studied the differences between cod sampled inside the region's closed areas and those outside. He and his colleagues have found that cod inside closed areas tend to be healthier. Fish inside closed areas were generally fatter, larger, more productive, and feed on larger prey. The farther from the coast, the more this tended to be true, which may reflect the impact of recreational fishing within the closed areas closer to shore (e.g., western Gulf of Maine and on Cashes Ledge). 

Older Fish
Cod sampled within closed areas tend to be much older than those found outside. In fact, they found eight times more fish over five years old inside those areas. Again, this was especially true for more remote areas, such as Closed Area I in the Great South Channel and Closed Area II on Georges Bank. The crucial implication is that older, larger fish are highly productive, producing more and healthier young than smaller, younger fish.

Genetic Difference?
It is not clear if there is a genetic difference between resident and migrant fish populations. If there is, rebuilding migratory stocks will likely not lead to rebuilt resident stocks along the coast. If, however, migrant stocks can become resident under the right conditions, then they could recolonize areas where local stocks have been depleted. 

Difficult Choices
As the region struggles to respond to increasingly dire stock assessments and the resulting cuts in catch limits, managers search for alternatives to maintain the economic viability of the groundfish fleet. One option under consideration is opening much of the year-round closed areas so fishermen can harvest more of their allocations of healthy stocks, such as haddock. This is a logical move since closed areas were introduced largely to control effort and the fishery is now under an output control. But Sherwood's research suggests that closed areas play a larger role in maintaining the long-term health of fish stocks.