Definition
Maximum sustainable yield for fishing
The Maximum Sustainable Yield (MSY) is the highest possible annual catch that can be harvested indefinitely without depleting a fish population.
- Maximum Sustainable Yield (MSY) is the largest long-term average catch or yield that can be taken from a natural population under constant environmental conditions without reducing its capacity to replenish.
- It is a biological reference point used in managing renewable resources such as fish stocks, forests, and game animals.
- MSY represents a balance between population growth and harvesting rate, ensuring sustainability over an indefinite period.
Analogy
- Think of MSY like a savings account.
- If you only withdraw the interest (growth) each year, the balance (population) remains stable; withdraw too much, and the account (ecosystem) declines.
Note
MSY aims to maintain populations at a size that allows maximum growth rate, typically around half the carrying capacity (K/2).
Relationship Between Sustainable Yield (SY) and MSY
- Sustainable Yield (SY) is the rate of increase in natural capital that can be exploited without diminishing the original stock.
- MSY is the highest possible sustainable yield, the maximum point before overexploitation begins.
SY = (Annual growth and recruitment) − (Annual death and emigration)
Example
In a fish population, if 1,000 new fish are born each year and 200 die naturally, the sustainable yield is 800 fish per year. Catching more than that would reduce the population.
How MSY Works in Fisheries
- Fisheries managers aim to harvest the same number of fish as are naturally added through growth and reproduction.
- This maintains the fish population at a stable equilibrium, avoiding collapse.
Note
- If harvest = population growth rate → fish stock remains stable
- If harvest > growth rate → overfishing and decline occur
- If harvest < growth rate → underutilization (potential yield wasted)
Example
- In a fish population where 10,000 new fish are born each year, catching 10,000 fish annually maintains a stable population.
- This is the MSY level.
Principle Behind MSY: Logistic Growth Model
- The population growth rate of renewable resources (like fish) follows an S-shaped (logistic) curve:
- Growth is slow when the population is small (few individuals to reproduce).
- Growth accelerates as the population increases.
- Growth slows again near the carrying capacity (K) due to limited food, space, and resources.
- The maximum growth rate occurs when the population is about half of its carrying capacity (½K).
- This is the point of MSY, where the addition rate equals the harvest rate.
Note
- At ½K, the population reproduces fastest.
- Harvesting at this point yields the maximum long-term production without population collapse.
Management Strategies for Sustainable Yields
- Setting Quotas Below MSY: Using a Precautionary Approach to avoid collapse during poor recruitment years.
- Seasonal Closures and No-Take Zones: Allow fish to breed and replenish populations.
- Gear Restrictions: Reducing bycatch and habitat damage (e.g., banning bottom trawls in sensitive areas).
- Monitoring and Adaptive Management: Adjusting quotas annually based on population surveys and satellite data.
- Community-Based Management: Empowering local fishers to manage resources sustainably ensures compliance and stewardship.
Self review
- Define sustainable yield and maximum sustainable yield, and explain how they differ.
- Describe how MSY is determined using the logistic population growth model.
- Explain why the population growth rate is highest at half the carrying capacity.
