Targets, Thresholds, and Control Rules -- The Next Big Lobster Management Debate

Fishery management plans and the conservation measures they contain are generally based on fishing mortality rate reference points. These reference points specify the amount of fishing that has been estimated to be necessary to keep the spawning stock biomass at various levels of productivity.

Fishing mortality rates are usually called F rates in fishery management jargon.

F_msy for example, is the fishing mortality rate that will keep a stock at a level that can produce the maximum sustainable yield (MSY).

F_10%epr is the fishing mortality rate that will keep the spawning stock biomass at a level that will produce 10% of the eggs that a non-fished population would produce. This is the fishing mortality rate that matches the definition of overfishing for the lobster resource.

F_MEY is the fishing mortality rate that will keep the stock at a level that will produce the maximum economic yield (MEY).

These reference points are illustrated on the classic fishery production function, or yield curve, shown above:

The recruitment overfishing reference point can be confusing because it is seldom an absolute point that is known with certainty. The only way that the recruitment overfishing point could be known for sure would be to drive a fishery to collapse repeatedly, in order to see how much fishing pressure it could really stand. Because no one wants to do such an experiment with a valuable fishery, the recruitment overfishing reference point is based on the best judgement of knowledgeable fishery scientists. It is important to realize that the definition of overfishing reference point is set at a level that is expected to have a minimal risk of stock collapse actually occurring. So fishing pressure may exceed the reference point indefinitely without the stock actually collapsing. It is similar to having a cholesterol level of 250 when the acceptable level is 210. You may live for years and get struck by lightning before your high cholesterol causes a heart attack. But the accumulated knowledge of multitudes of medical researchers says that anything over 210 creates a high risk.

Another confusing issue arises because many fisheries (like the lobster fishery) already have fishing mortality rates that are higher than those that create an unacceptably high risk of recruitment failure, even though it hasn't happened yet. In that case, the first step in conserving the fishery is to bring fishing mortality rates below the recruitment overfishing level. In the lobster fishery that is the 10%epr level. Because everyone's attention is focussed on the difficulty of achieving the 10%epr level, that reference point becomes a target in people's minds.

A definition of overfishing based on the risk of recruitment overfishing, however, is actually intended to be a threshold. There are many reasons why the fishery should be maintained at some level to the left of the 10%epr level on the yield curve, rather than right at the 10%epr level.

Two possible target reference points are F_msy and F_mey. For a stock to produce either maximum sustainable yield, or maximum economic yield, the fishing mortality rate must generally be reduced to a level significantly lower than the recruitment overfishing reference point.

As can be seen in the yield curve diagram, the yield from a fishery can usually be increased substantially by reducing effort from the recruitment overfishing level. And to obtain the greatest profits, or economic rent, from a fishery, the fishing effort must usually be reduced even further.

The most recent amendments to the Sustainable Fisheries Act made it clear that federally managed fisheries are to be managed in a way that maintains the stock biomass at a level that is capable of producing maximum sustainable yield on a continuing basis. This means that definitions of overfishing that are based on the risk of recruitment overfishing, but that do not keep a fishery at MSY levels are no longer acceptable in federal fishery management plans. This is a more conservative or precautionary approach to fishery management, and it is likely to become the standard for all fishery management plans, not just those developed by the federal system.

This means that a reference point like F_10%epr, which is now considered a target by most people, is likely to become a threshold as a more conservative target is established. For example, if you weigh 220 pounds, you may establish a target weight of 200 pounds by the end of the year, but your ideal weight may be 180, which is your ultimate target.

In the lobster fishery, the first objective is to reduce the fishing mortality rate to a level that will reduce the risk of recruitment overfishing. The next goal may be similar to that expressed in the Sustainable Fisheries Act -- to maintain a stock biomass that can produce the maximum yield from the fishery on a continuing basis. And ultimately, the goal may be to maximize the economic returns from the fishery by maintaining a biomass level that will provide the highest total revenue compared to the total costs of harvesting the catch.