MSY

Last changed: 05 March 2025

MSY, or Maximum Sustainable Yield, is a key concept in the EU's Common Fisheries Policy. For many years, the EU has aimed to achieve the optimal utilization of all fish stocks. This means fishing as much as possible without jeopardizing the long-term health and sustainability of the stocks. In other words, fishing should be conducted in line with the MSY principle.

The concept of Maximum Sustainable Yield (MSY) originated in the first half of the 20th century and is primarily a theoretical framework designed to maximize long-term catch without jeopardizing the recovery of fish stocks.

The concept is based on the principle that population growth is density-dependent: when the number of fish in a stock decreases due to fishing, the growth rate of the remaining fish increases. As long as a sufficient number of spawning-age fish remain, recovery of the stock is ensured.

To ensure that the Spawning Stock Biomass (SSB) is adequate, minimum thresholds are established. These thresholds have two reference points: the biomass limit (Blim) and the precautionary biomass limit (Bpa). Biomass refers to the total weight of all the fish that reproduce within a stock.

The biomass limit (Blim) represents the lowest point to which a stock can decline without jeopardizing its ability to recover. Below this level, the stock is considered so small that its capacity to produce young fish is significantly reduced, putting the entire stock’s future at risk. In the worst-case scenario, overfishing could drive the stock to a point where it cannot recover.

Stock estimates are always subject to uncertainty, and this also applies to the determination of the biomass limit (Blim). To account for this natural uncertainty and provide a safety margin, a precautionary biomass limit (Bpa) is set above the biomass limit.

Below is an example of how a diagram illustrating the development of a spawning stock's size over time might appear, with cod in the eastern Baltic Sea as an example.

MSY-lekbiomassa-torsk-västra.2022.PNG

Minimum levels are also established for fishing mortality (F), which refers to the amount of fish caught and killed through fishing, with corresponding reference values. If a stock is fished above the level of Flim, there is a high probability that the spawning biomass will be significantly reduced, putting the recruitment of young fish at risk. Additionally, a precautionary limit for fishing pressure, called Fpa, is also set.

Below is an example of a diagram illustrating the development of fishing pressure over time. In this case, cod from the eastern Baltic Sea is used as an example.

MSY-fisketryck-torsk-västra-2022.PNG

Facts:

Abbreviations

MSY = Maximum Sustainable Yield: The maximum amount of fish that can be fished from a stock while leaving enough fish for the stock to develop sustainably.

SSB = spawning biomass: The amount of fish that are sexually mature and can produce juveniles

B = Biomass: The total weight of a fish stock

Recruitment = The influx of new fish

MSY Btrigger = If the biomass reaches this level, ICES must recommend measures to restore the spawning biomass to a level that is sustainable in the long term.

Bmsy = The biomass of a stock cannot be as high as you like. Bmsy is the biomass that gives maximum sustainable yield.

Blim = If the biomass of the stock falls below this level, there is a high risk of reduced recruitment.

Bpa = Precautionary level to avoid Blim. As long as the biomass of the stock is above Bpa, the risk of reduced recruitment is considered low.

F = fishing mortality: The proportion of the stock that is killed by fishing.

Fmsy = The fishing pressure that can produce maximum sustainable yield, MSY, in the long term. Fishing at this level is considered sustainable.

Flim = If a stock is fished above this level, the stock's biomass will be greatly reduced and fall below Blim.

Fpa = Precautionary level to avoid falling below Flim. If fishing pressure reaches Fpa, management measures must be taken to protect the stock.

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Contact

Mikaela Bergenius, Researcher
Department of Aquatic Resources, Institute of Marine Research, SLU 
mikaela.bergenius@slu.se, +46 10 478 41 15