EXERCISE 18
Enumeration of Fish or Other
Aquatic Animals
There are various ways of estimating the size of
a natural population of large, mobile animals in
a freshwater habitat. Obviously, the most
accurate method would involve catching or, in
some way, counting the entire population.
However, this approach is usually either
impossible to do in practice or, at least,
destructive to the natural populations. Instead,
a mark-and-recapture procedure frequently is
used to obtain a statistical estimate of the size
of the natural population.
The mark-and-recapture technique is based
on the premise that recognizable (marked)
organisms released to the population will be
recaught in numbers proportional to their
abundance in that population. The size of the
natural population can be estimated from the
proportion of marked to unmarked organisms
in random samples obtained from the entire
population.
The basis for this technique apparently was
first published in 1662 by John Graunt in an
article on human demography in London
(Ricker, 1975). Later, Petersen (1896) applied the
method to fish populations, and others have used
it in various ways since, e.g., the Lincoln (1930)
index for birds. According to this procedure:
~
SM
N= R
where N = estimate of total number in the
population; S = total number of organisms in a
sample from the population; M = total number
of marked organisms in the population; and
R = number of marked organisms in the sample.
However, certain basic assumptions must be
made for the mark-and-recapture technique to
be valid:
1. There can be no difference in mortality or
emigration between marked and unmarked
orgamsms.
2. Tags or other marks must remain recognizable and must not be lost. All marks on
recaptures must be reported.
3. There must not be a difference in catchability
between marked and unmarked organisms.
4. Marked organisms must be mixed randomly
within the entire population.
5. There can be no unknown recruitment or
immigration to the population.
There are many modifications of the basic
Petersen relationship [cf., Ricker (1975)], but
only the two commonly used for fish or other
aquatic vertebrates will be presented here.
247
Enumeration of Fish or Other
Aquatic Animals
There are various ways of estimating the size of
a natural population of large, mobile animals in
a freshwater habitat. Obviously, the most
accurate method would involve catching or, in
some way, counting the entire population.
However, this approach is usually either
impossible to do in practice or, at least,
destructive to the natural populations. Instead,
a mark-and-recapture procedure frequently is
used to obtain a statistical estimate of the size
of the natural population.
The mark-and-recapture technique is based
on the premise that recognizable (marked)
organisms released to the population will be
recaught in numbers proportional to their
abundance in that population. The size of the
natural population can be estimated from the
proportion of marked to unmarked organisms
in random samples obtained from the entire
population.
The basis for this technique apparently was
first published in 1662 by John Graunt in an
article on human demography in London
(Ricker, 1975). Later, Petersen (1896) applied the
method to fish populations, and others have used
it in various ways since, e.g., the Lincoln (1930)
index for birds. According to this procedure:
~
SM
N= R
where N = estimate of total number in the
population; S = total number of organisms in a
sample from the population; M = total number
of marked organisms in the population; and
R = number of marked organisms in the sample.
However, certain basic assumptions must be
made for the mark-and-recapture technique to
be valid:
1. There can be no difference in mortality or
emigration between marked and unmarked
orgamsms.
2. Tags or other marks must remain recognizable and must not be lost. All marks on
recaptures must be reported.
3. There must not be a difference in catchability
between marked and unmarked organisms.
4. Marked organisms must be mixed randomly
within the entire population.
5. There can be no unknown recruitment or
immigration to the population.
There are many modifications of the basic
Petersen relationship [cf., Ricker (1975)], but
only the two commonly used for fish or other
aquatic vertebrates will be presented here.
247