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Maximum Sustainable Yield (MSY)
The central objective of the Maximum Sustainable Yield (MSY) concept is to sustainably harvest a broad range of fish stocks and ultimately avoid over-exploitation. There are a couple of fundamental ideas behind MSY. The first is that every fish stock can be fished, providing a yield. The next is that this can be done sustainably. Finally, there is a level of fishing intensity which provides the biggest catch while maintaining a sustainable stock. The measure of fishing intensity in this case is known as fishing mortality (F) while the highest catch is the 'maximum sustainable yield' - put all these together and you get FMSY.
FMSY is a target level of fishing and is now the primary indicator used by the International Council for the Exploration of the Sea (ICES) when assessing its stocks, as well as by other Regional Fisheries Management Organisations (RFMOs) worldwide, who are responsible for regulating fisheries on the high seas.
Is MSY suitable for sharks?
FMSY as a concept can be applied to every fish stock. Practically, however, there are a couple of problems when applying FMSY to elasmobranchs. Firstly, it is very difficult to age sharks, skates and rays. Knowing the age distribution of a stock is one of the most useful pieces of information in stock assessment, and therefore in calculating FMSY. This is usually done by looking at bony structures, such as the otoliths or ear-stones, under a microscope. As elasmobranchs are cartilaginous, and therefore don’t have these bony structures, we are usually missing a piece of information that is available to those who work with bony fish. Therefore, instead of having a direct measure of FMSY we usually have to use proxies. These can be other variations of F such as changes in maximum length (Lmax).
The other problem we have is the current state of some shark stocks. Compared to the historic biomass, some of these are now at very low levels. In these cases it will take a very long time for stocks to recover to a level within safe biological limits. There is a zero Total Allowable Catch (TAC) for Spiny Dogfish (Squalus acanthias), to allow the stock to rebuild. However, the current level of the stock is around 10% of what it was during the first half of the 20th century. For stocks such as these, any measure of FMSY is not really appropriate in the short-term, until they rebuild to a healthy level. When they are within safe biological limits, a sustainable yield could in theory be achieved. But when a stock is far outside safe biological limits, MSY is not an option.
The concept of FMSY can be applied to any stock, be it bony fish or elasmobranch. The practical aspects may not be as simple to apply to sharks, skates and rays but the concept remains.
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