LltCLEO -

Great question - I'll try.

First we'll consider MSH (maximum sustained harvest) and MSY (maximum sustained yield) to be the same - there are slight differences. MSH escapement point would be that point that would produce the most "harvestable" fish. That is to that point where the diference between number of returning adults from a given escapement is the largest.

An example: let's say on river X MSH escapement is 5,000 fish and they produce on the average return of 8,000 fish. The "harvestable surplus" would 3,000 fish. For the same river an escapement of 6,000 might produce 8,500 fish or a difference of 2,500. The same with lower escapements - say 4,000 may produce only 6,500 returnees or a difference of 2,500. Thus the maximum surplus on the average occurs at an escapement of 5,000. The further away from the MSH escapement point one moves the smaller the difference between the number of spawners and the adults they produce.

The other point I referred to was carry capacity. This is exactly what it sounds like. The maximum capacity of the system. It is possible to have escapements higher than carrying capacity (due to better than average survival conditions) but those escapements on the average will produce less fish than the parent escapements. Retruning to the river above if escapements over say 9,000 on the average produce less returns than the parent year and those under 9,000 on the average produce more returns than the parent year then 9,000 is the average carrying capacity.

One note - these points are dynamic and change with survial conditions. That's why they are usually qualified as "MSH under average conditions". If the habitat is a system is degraded then both the MSH and carrying capacity points are reduced.

Hope than helps.

Tight lines
Smalma