fig. 4.13 > The maximum sustainable yield (MSY) is the maximum catch that can be taken from a species’ stock over an indefinite period without jeopardizing that stock’s productivity. The maximum sustainable yield (MSY) is achieved at a certain level of biomass (BMSY). This differs in size from fish stock to fish stock. At BMSY the annual production of new biomass is at its maximum – firstly because the fish grow particularly well and increase their weight, and secondly because more eggs and larvae survive to develop into fish.
Above or below BMSY, the stock is less productive. At about 200 000 tonnes biomass, for example, the stock provides only 15 000 tonnes of new biomass per year. This is because there are more fish in the stock to compete for food, and they each put on less weight. Also, more eggs and juvenile fish are cannibalized. A stock of only 50 000 tonnes biomass experiences a similar level of biomass growth. Although this smaller stock contains fewer spawners, the total achieved from the increase in weight of the individual fish (as a result of reduced competition for food) and the biomass of the offspring (which have a greater chance of survival within a smaller stock) is the same as for a large stock.
It is interesting that sustainable fishing is also possible with larger or smaller sized stocks than the BMSY, but the annual fish yield is lower.