Under
natural conditions, populations of animals and fish tend to
vary in size from one year to the next. Population changes
are affected by reproduction, harvest, natural mortality,
and a variety of environmental conditions. Even after its
size has been reduced by multiple circumstances, a sustained
population can meet the needs of the next generation.
The abundance
of a population, often described as the population density,
can be affected by both density-dependent responses and density-independent
factors. As a population increases within a given habitat,
the death rate increases, and the birth rate decreases as
the population responds to the available food and resources
to survive. In contrast, natural events like fires, ocean
currents, and storms often reduce the density of a population,
independent of the availability of food, space, and resources.
Favorable
conditions for most animals and plants can result in temporary
overpopulation, which the available habitat cannot support
for an extended length of time. In these instances, populations
respond through lower birth rates and higher death rates.
When numbers are reduced or increased as a result of density-independent
influences, these phenomena tend to direct the population
back to what is called the carrying capacity, where birth
rates and death rates balance out to maintain numbers at the
maximum level the environment can support.
Population
numbers are generally not permanently reduced when a natural
disaster strikes. All species have the capacity to increase
their numbers in response to these events. When a cold front
kills large numbers of speckled trout in coastal marshes,
the following year there are fewer trout but more food and
space per individual fish. As a result, reproductive success
increases and over the next several generations the population
moves back toward the numbers present before the freeze.
Unfortunately,
most environments which support animals and fish are never
constant long enough to allow very stable populations. Population
numbers are always shifting up or down in density-dependent
response to the increases or decreases brought on by density-independent
conditions such as adverse weather or changes in available
habitat.
This
principle is easy for sportsmen to observe: when fish or deer
are thinned out, their rate of productivity increases. Animals
tend to eat better, have more offspring, and live longer.
When the degree to which they are cropped back can be monitored
and controlled, the natural response to increase in numbers
can be used to produce a sustainable surplus without endangering
the long-term survival of the population. Although throughout
the world, populations of fish and wildlife support subsistence,
commercial and recreational harvest year after year utilize
an understanding of the density-dependent responses and density-independent
factors affecting a specific population in a given year.
Populations
of game and fish can be manipulated to provide a surplus under
most conditions. The key is in determining how much surplus
is available without affecting future population numbers and
harvests.
Some species
are adapted to produce many thousands of offspring, grow rapidly,
and mature at an early age. Under these circumstances, very
high natural mortality rates are unavoidable, and these types
of fish and animals are usually adapted to highly variable
environments. These species are available for high rates of
harvest or exploitation with a wide safety margin in terms
of long-term survival.
Other
species are less adapted to change, and rely on a stable environment
to ensure the survival of relatively fewer offspring. These
species may grow relatively slowly and mature much later in
life. These populations are much less suited to sustain long-term
exploitation without careful monitoring and control of the
harvest.
When people
harvest fish or other animals, they do so with the expectation
that they can harvest again, in the future. Through study,
reports, and monitoring, scientists and government regulators
work together to control the harvest for future use by considering
the density-dependent responses and density-independent factors
of a fishery during a particular period. They calculate a
maximum number of fish or animals in a population that can
be removed without having a negative effect on its density,
known as maximum sustainable yield. Two quantities are used
to determine this –– production and yield. Production
is the combined weight of the species in the habitat over
a given period of time. Yield is the amount of that production
that is harvested and put to use. Although these two quantities,
especially the production figure, are inexact, they do provide
some measure of the fishery resource to be used as a guideline
for sustaining it for future harvests.
Download:
sustainable_yields.pdf
(112KB)
