CC.16 Maximum Sustainable Yield
ESSENTIAL TO KNOW
- Fishing initially reduces the size of a fish stock. However, because the reduction results in greater food availability for the fishes remaining, the rate of production normally increases.
- The additional biomass produced represents an excess over that needed to maintain the population. This excess can be harvested safely.
- As fishing increases, stock size is reduced to a critical level known as the “maximum sustainable yield,” at which the production and reproduction rates of the population are just sufficient to balance the removal rate due to predation and fishing. If fishing yield is increased and continued beyond the maximum sustainable yield, the population can no longer sustain itself and collapses.
- Maximum sustainable yield is difficult to establish because fish stocks vary as a result of year-to-year climate-induced changes and changes caused by other factors, such as diseases.
- Maximum sustainable yield also depends on the age structure of the population and on the degree of age selectivity in fishing methods used. Harvesting older fishes tends to increase the sustainable yield because the remaining younger fishes are faster-growing, but it also tends to reduce the breeding population because the younger fishes are sexually immature.
- Maximum sustainable yield is usually established by using one year’s data for stock size and reproductive success to project the survival of adults and young into the next year. Unexpected events, such as disease outbreaks, can render such estimates inaccurate and inadequately protective.
- Most fisheries are managed at a yield 20% to 40% below the estimated maximum sustainable yield to allow a safety margin. This safety margin may not always be adequate, but any safety factor means that fishes that could be harvested will not be.