Population estimation Essay

This essay has a total of 1240 words and 7 pages.

population estimation



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. The assumptions with this method are: 1. All members of
the target population are equally vulnerable to capture. 2. Vulnerability to capture is
constant over time. 3. There are no additions to the population or losses other than the
removal itself (Van Der Avyle & Hayward). The Leslie and DeLury methods are used in cases
when sampling effort my vary among periods. They are used on large populations where the
probability of catching an individual fish is low (Kohler 138). The Leslie method assumes
that the number of fish caught per unit effort is proportional to the number of fish
present at the beginning of the interval. The DeLury method differs in that the population
estimate is based on total effort rather than cumulative data. The Zippin method is used
where the catchability is high and equal effort is expended in each sample period. This is
most commonly used in small mountain streams in conjunction with electrofishing as the
removal method.

Materials & Methods
On September 13, 2000 we went to Mull Creek. We set up two stop nets spanning 75m one
upstream and one downstream to keep the population contained. We made three runs with
electrofishing gear to remove the fish. Once removed we anesthetized the fish using clove
oil. Then we measured their length in cm and weight in grams with slide scale, and clipped
their adipose fin and released them. We returned on September 20, 2000 and used the same
method recaptured the fish. This time taking their weight, length, and examining for
marks.
Continues for 4 more pages >>




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