When a species approaches this limit, starvation and disease cause an increasing
mortality rate and decreasing fertility. Finally, the same number of animals will
die as is born. If there is widespread consensus that stocks should be kept at such
a level, there will be nothing to harvest.
If the stock is reduced, the mortality rate will sink, fertility increase and each year,
more animals will be born than die. It is possible to stabilize the stock at such a
reduced level and harvest the annual surplus instead of allowing the stock to
grow.
Some species provide the biggest possible surplus when the stock is at 50% of the
ecosystem's capacity for that particular species. Other species provide the biggest
surplus when at a somewhat higher level in order to the ecosystem's capacity.