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Population estimation Introduction Estimates of population size play a vital
role in many fisheries management decisions. The numbers of fish in a stock are
used to identify influences of environmental factors, human exploitation, and
ultimately to identify the effectiveness of management strategies. (Van Den
Avyle & Hayward 1999)
The three most common methods of population estimation among biologist are
sample plots, mark and recapture, and removal. Counts on sample plots is based
on the principle that an estimate of population size can be obtained by
determining the average density of animals per unit area and multiplying its
value by the total area covered by the population. This procedure is done by
setting up a pre-determined number of circular, square, or rectangular plots.
These plots should be randomly placed and should not over lap.
This method is used when all members of the target population can be counted
with reasonable certainty(Van Der Avyle & Hayward 1999). The formula used with
this method is: Where A is the size of the study area, a is the size of the
plot, and n is the average number of animals counted per sample plot (Van Der
Avyle & Hayward 1999). The mark and recapture method is simply preformed by
collecting fish, marking them, releasing them, and at a later time collecting
fish from the same area and examining them for marks.
This is based on the principle that the number of marked fish in the second
sample is proportional to the total number of fish in the population. This is
called the Peterson method and the equation is as follows: Where M is the number
of fish initially marked and released, C is the number of fish collected, and R
is the number of recaptures (Van Der Avyle & Hayward 1999). The Peterson index
can give biased estimates when the numbers of fish sampled are low; so several
modifications have been made to correct this.
One being Bailey's modification which is used when sampling during the
recapture period is conducted with replacement. The Chapman method is used if
replacement is not taking place. The differences in these three methods would be
of little significance if the recapture number exceeds 7 (Van Der Avyle &
Hayward). All three of these variations are based on the assumptions that: 1.
Marked fish don't lose their marks. 2. Marked fish are not over looked in the
recapture sample. 3. Marked and unmarked fish are equally vulnerable to
recapture. 4. Marked and unmarked fish have equal mortality. 5. Following
release marked and unmarked fish become randomly mixed. 6. There are no
additions to the population during the study. If any of these conditions are not
met over estimation will most likely occur (Van Der Avyle & Hayward 1999). The
third and final method of population estimation is the removal method. This is
based on the idea that the number of fish caught per unit of effort will
progressively decline as members of the population are removed. The population
can be estimated from data on fishing effort and catch rates.
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