Explanation of Fishing Pressure
"Fishing at a level and in a manner that it is reasonably believed can be sustained indefinitely, assuming proper responses to changes in the ecosystem," is a good goal, but what exactly should it mean to fishery managers? To answer this question, there are a few fisheries management concepts we need to understand.
The first such concept is maximum sustainable yield. A good definition is given by the Organization for Economic Cooperation and Development: "Maximum sustainable yield refers to the maximum use that a renewable resource can sustain without impairing its renewability through natural growth or replenishment" (OECD, 2001). For a fishery, maximum sustainable yield is the maximum amount of fish that can be taken in a particular time period without depleting the fishery.
The second concept is fishing death rate, which we will define as the total death rate of fish from fishing-related causes. These causes include being taken (legally or illegally) in a fishery and being unintentionally killed by fishing activity.
In an ideal world, biologists would study a fish stock and calculate its maximum sustainable yield. Then all that would be necessary to sustain the fishery would be to prevent fishers from taking more than this maximum sustainable yield.
But real-world managers must deal with the uncertainties in all of the data they use to make their decisions. Because of imperfect information about fish stocks, maximum sustainable yield cannot be precisely determined. Nor can the death rate associated with fishing be perfectly measured: Not all fishing is documented and it is also difficult to account for fish that die because of fishing-inflicted injuries, from being caught in lost fishing equipment, and other causes.
Ultimately, all we can hope for is to find some sort of probability distributions for the maximum sustainable yield and the fishing death rate.
We then propose the following guide for making management decisions: Management actions should be made with the intent of reducing to five percent (Dallal, 2007) or less the probability of the actual fishing death rate being greater than the actual maximum sustainable yield.
But many overfished stocks are not close to the population levels at which they could provide maximum sustainable yield. Clearly, these populations should be rebuilt. We therefore suggest that for populations below the level at which they could provide maximum sustainable yield, fishing should be limited so that they can reach the population associated with maximum sustainable yield in a specific period of time. The length of this rebuilding period should vary depending on the characteristics of the fishery in question.
In addition, real fish stocks are parts of an ecosystem. The interactions between the species being fished and the other species present must also be considered when making management decisions. We will not further discuss this issue here, except to note that this aspect of the problem is highly complex.
We recognize that this analysis has been done largely in terms of a single-stock approach to fisheries management. Information and comment regarding an ecosystem-based management approach is here.