4.3.6—Maximum sustainable yield (MSY)

Maximum sustainable yield is at the peak of the repeatable yield curve; effort beyond it can lower both stock and future catch.

As effort rises, catch first increases, then declines when breeding stock and recruitment are damaged. A real quota should normally sit below the theoretical peak because estimates are uncertain.

If modelled yield peaks at 800 tonnes at effort 60, a fleet catching 900 at effort 90 is not more successful—it is above the sustainable peak.

Over-effort reduces replenishment, so each extra boat shrinks the future stock.

The largest historical catch is not MSY; MSY is a repeatable rate under stated conditions.