Fisheries: Maximum Sustainable Yield, Quotas and the Collapse of the Grand Banks Cod

The yield curve. A fish population grows fastest at intermediate size. When it is small, few adults breed, so few young are added; when it is near the carrying capacity K, competition for food slows growth. Plotting annual growth against population size gives a hump-shaped curve whose peak, at roughly K/2, is the maximum sustainable yield: the largest catch that can be taken every year while the population stays the same size, because the catch exactly equals the growth. Harvest below the growth and the stock rises; harvest above it and the stock falls — and because a smaller stock grows more slowly, the shortfall widens and the decline accelerates. That asymmetry is why overfishing collapses rather than declines gently.

Why MSY is hard to hit. Fish cannot be counted; stock size is estimated from catch rates and models, and the estimates are uncertain and often optimistic. Short-term profit pushes every fleet to take a little more. The ocean is a common-pool resource: fish left by one boat are taken by another, so no one gains by restraint. Governments subsidise fuel and boats, making it profitable to fish stocks that would otherwise not pay. And the ecosystem shifts: when a top predator is fished out, its prey or competitors expand and may eat the juveniles, so the stock does not return even when fishing stops. The safe policy is to harvest below MSY with a precautionary margin, because estimation errors only run one way — towards fewer fish.

The Grand Banks cod. Off Newfoundland, cod had been fished for 400 years at roughly 250 000 tonnes a year by small boats. In the 1960s foreign factory trawlers with bottom nets and freezers pushed the catch to 800 000 tonnes in 1968. Canada declared a 200-nautical-mile exclusive economic zone in 1977 and expelled the foreign fleet — then subsidised its own large trawlers. Through the 1980s quotas were set from models that overestimated the stock; fishers reported smaller fish and scientists warned, but quotas stayed high for jobs and votes. In 1992 the stock was found to be about 1% of its former size and a total moratorium closed the fishery overnight, ending 35 000 jobs. Thirty years later the cod have not recovered: shrimp and crab have taken their place and prey on juvenile cod. The lesson is that exceeding MSY does not merely reduce a stock; it can reorganise the ecosystem into a state that no longer supports it.

The toolkit. Total allowable catch and quotas, closed seasons during spawning, minimum mesh size so juveniles escape, marine protected areas where stocks rebuild and spill over, gear restrictions banning bottom trawling, and — most effective and least popular — cutting the subsidies that make overfishing profitable. Individual transferable quotas give each boat a property right in the catch and so an incentive to conserve. Every tool needs enforcement, which is why the incentive structure decides how much enforcement is needed.