28th September 2012
Throughout the reform of the Common Fisheries Policy (CFP) the term Maximum Sustainable Yield (MSY) has appeared in different forms. MSY represents our best hope of saving EU fish stocks from overexploitation, depletion, and even collapse. If the CFP reform allows us to successfully achieve MSY we will soon start getting more fish, higher profits, and better careers in a fishing industry that has a future. However, for something that is in essence quite simple (MSY is the most fish you can take from a stock indefinitely), it can get confusing pretty quickly.
Fish stocks (populations of single species, or occasionally species groups, that are caught in a particular geographic region), grow in the same way as most other groups of animals; the more mature fish within them that are reproducing, the faster the stock grows. This growth rate increases until competition for resources (such as food) causes it to slow down. It is this peak – where the growth rate is at its highest – that fisheries managers refer to as MSY. At this point there is a surplus of fish which can be harvested by fishing. In theory, as long as the harvest of fish at MSY does not exceed the surplus amount being generated by reproduction, the fishing can continue at this level indefinitely. However, in reality, changes in the environment can threaten this surplus amount of fish (we’ll come back to this “precautionary approach” in Part 3 of the post).
MSY is a "biological reference point" which is determined based on scientific stock assessment data, and used to estimate:
- the total amount of biomass (or size) a stock will have when it is at MSY (BMSY),
- the biomass of mature fish (which are able to reproduce) when a stock is at MSY (SSBMSY),
- the fishing mortality that will maintain a stock at MSY (FMSY).
These all rely on environmental conditions – which can have a strong impact on the growth rate and/or natural mortality of the stock – stay the same (this assumption will not always be correct; we’ll come back to that)
- BMSY refers to the biomass (B) level (or size) of a fish stock when it is estimated to be capable of producing MSY. BMSY is based on the particular “life history traits” of the stock such as the age at which the fish starts to reproduce, how many offspring they tend to have, and the stock’s level of natural mortality (what proportion of them die due to causes other than fishing). Because the life history traits of many fish are highly sensitive to changes in environmental conditions (temperature, salinity, etc.), BMSY can only be accurate within a certain range of environmental conditions. Generally, if biomass is below BMSY (measured in tonnes) the stock is classified as “overfished”.
- SSBMSY is the biomass of the stock’s fish that have reached a size or age where at least half of them can reproduce – hence spawning stock biomass (SSB) – when the stock is at MSY. The values of SSBMSY and BMSY will differ for the same stock, as juvenile fish will be included in B estimates, but not SSB. Fishing can shorten or simplify the natural age structure of the population by selectively removing the larger, reproductive individuals, so stock assessments often rely on SSB as opposed to just B as this gives a better indication of the expected growth of the stock (due to reproduction and survival of young fish). As with BMSY, SSBMSY will only be accurate for certain (usually current) environmental conditions, and could increase or decrease in response to changes in the ecosystem. As with BMSY, if stock are below SSBMSY (measured in tonnes) then there are not enough fish in the stock to replenish it to MSY and the stock would be classified as overfished.
- FMSY is the level of fishing mortality (F) – that is, the death of fish as a result of fishing – that theoretically would maintain a healthy stock at the biomass level capable of producing MSY indefinitely (if environmental conditions don’t change, see Part 3). Knowing the growth rate of the stock (the “recruitment”) and the level of natural mortality for the stock are central to producing accurate and sustainable fishing mortality estimates. However, if stock biomass becomes low enough that recruitment is impaired (the growth rate slows), then fishing at FMSY is very unlikely to allow the stock to recover to SSBMSY and the stock is at risk of further depletion. If fishing mortality is higher than FMSY, then fish are being removed at higher rates than would keep a stock at SSBMSY and “overfishing” is occurring.
Although MSY is in theory the largest catch of fish we can take every year indefinitely, this would rely on environmental conditions staying the same forever. This is like assuming when you leave your house in the morning that you will stay dry because it will always be sunny, whereas in reality, at some point it will rain! Calculating the amount of fish in a stock can only ever be an estimate, because we can't count all the fish in the sea – they’re mostly invisible and they move! Just as weather forecasters can only make predictions based on the evidence available and sometimes they get it wrong, fisheries managers should also be cautious because of the unknowns and uncertainty of stock assessments.
We can account for these uncertainties and/or environmental changes by taking a "precautionary approach”, which in our weather analogy is a bit like taking an umbrella in case the weather does change and it rains. In fisheries, this means we should keep stocks above MSY levels and fish less than the maximal MSY fishing level to avoid accidentally taking more fish than is sustainable. This could happen for example when the growth rate of the stock has decreased due to environmental changes, or if we overestimate how much fish is in the stock, or if we underestimate how much fish we are taking out. In fact we are legally bound to take this precautionary approach under the Treaty on the Functioning of the European Union, and in the international law that governs our seas (UNCLOS).
Think of a fish stock as a savings account at a bank that gives you 10% interest per year. If you withdraw less than 10% of your savings per year, the amount in your account will increase. If you withdraw more, it will decrease. If you regularly withdraw more than the interest your savings are generating, you will eventually empty your account. And, if your savings account was really low and you wanted it to increase back to a safe level, you would need to take out less than the interest it was generating.
In this example, the interest is equivalent to the fish stock’s growth rate, and the withdrawal is equivalent to the fishing mortality rate. So if total mortality (from fishing and natural causes) regularly exceeds growth, the stock will decline. When stocks are depleted, we need to make sure we are fishing at a low enough fishing mortality that the growth rate of the stock will be higher than the mortality rate – thus leading to an increase in fish. Once a stock is at MSY, we must not let fishing mortality exceed its MSY level, and preferably lower to be precautionary and allow for uncertainty.
So now we know what MSY is; does it matter when we achieve it? Actually, it does. With international commitments to reach MSY by 2015, where possible (agreed on at the World Summit on Sustainable Development in Johannesburg in 2002 and again during this year's Rio+20 conference, where the EU agreed to urgently take the measures needed to reach MSY, by 2015, in the shortest time possible), and over half of Europe's fish stocks overfished (57% are currently below MSY), the race is on! Thankfully our scientists are already giving advice that should enable our stocks (that we have data for) to achieve MSY by 2015: so there are few difficulties there – as long as we are actually fishing at levels that will achieve this. Even better, there are no environmental obstacles to stop us fishing at these levels from tomorrow. However we are lacking data on 54% of our stocks (including most stocks in the Mediterranean), so it will take longer to reach MSY because stock assessment scientists will need time to collect data and adjust their model of the stock. However, a European law called the Marine Strategy Framework Directive binds us to achieving MSY by 2020 for all stocks – so we need to get a move on.
Currently, the following reform documents have been published:
Hopefully, the Commission, Council, and Parliament will find common ground and agree on strong wording and binding timeframes for achieving sustainable fishing levels. This will be essential for the future of the fishing sector, for the sustainability of European seafood, and for ensuring our marine environment is healthy and resilient