In the world of fisheries management, the terms overfished, fully fished and overfishing are used often, but the actual meaning of the terms and where they are used is highly variable and leads to confusion. In this brief blog I want to clarify some of the issues.
First, understanding a little basic population dynamics is necessary. The long term harvest from a fish population depends on how hard it is harvested. If you don’t harvest at all, or harvest a very small fraction of the population, you obtain very little yield. If you harvest too high a fraction, you may drive the population extinct or have little long term yield as you don’t have a very large population for long. So in simple theory, there is a harvest rate (proportion of the population harvested each year) that maximizes your long term yield. This yield is called “Maximum Sustainable Yield” or MSY, and the fraction harvested each year is often called Fmsy (short for fishing mortality rate that produces MSY).
So in a very technical sense, any harvest rate greater than Fmsy is called “overfishing,” and conversely any harvest rate less than Fmsy is “underfishing” in that the long term yield will be less than the maximum possible. A common mistake is to equate “overfishing” with “not sustainable.” Populations can and have been sustainably harvested for centuries at rates higher than Fmsy. The consequence has been that the long term yield has been less than it could have been had harvest rates been lower, but if the harvest rate is only slightly above Fmsy, the lost yield may be very little.
If one harvests at the MSY rate, the average population size that results is called Bmsy, that is the biomass that produces MSY. If fishing mortality is higher than Fmsy, then the stock abundance will, on average, be below Bmsy. Thus Fmsy and Bmsy can be considered targets if we want to maximize food from a fishery.