Definition of MSY

Maximum sustainable yield (MSY) is a theoretical concept used extensively in fisheries science and management. In fisheries, MSY is defined as the maximum catch (yield) in numbers or mass that can be removed from a population over an indefinite period. When a population is depleted to below its environmental carrying capacity, the increase in biomass due to reproduction and growth exceeds the reduction in biomass (abundance of fish in weight) due to natural mortality, and the population will rebuild from its depleted state toward its carrying capacity. The concept of

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40 000 35 000 30 000 25 000 20 000 15 000 10 000 5000 0

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Catch

Figure 1 Catch and MSY for (a) yellowfin tuna in the eastern Pacific Ocean and (b) orange roughy in the spawning box and northeast flats of the East Chatham Rise, New Zealand. The low catches of yellowfin tuna in the early part of the time period are due to poor environmental conditions and younger average age of yellowfin in the catch (lower yield per recruit). The high catches of yellowfin tuna in the latter part of the time period are due to favorable environmental conditions. The high catches of orange roughy in the early part of the time series are due to fishing down the standing biomass and overfishing due to an over-estimation of MSY used for management advice. The low orange roughy catches in the latter part of the time series are due to the stock being overfished and management limiting catches to rebuild the stock. (a) Data from http://www.iattc.org/PDFFiles2/ FisheryStatusReport4ENG.pdf (accessed April 2008). (b) Data from http://services.fish.govt.nz/fishresourcespublic/ Plenary2006/ORH_3B_06.pdf (accessed April 2008).

MSY relies on this surplus production to provide sustainable yields. Under deterministic equilibrium assumptions, if the surplus production is harvested the population will remain at the same abundance level. MSY is therefore achieved at the abundance level that produces the greatest surplus production. It should be noted that unsustainable yields greater than MSY may occur as the population is fished down to the depletion level that supports MSY. The concepts involved in determining optimal yields or rotational harvest in forest management and harvest levels for wildlife populations are similar to the MSY concepts in fisheries (Figure 1).

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