9458

• UCREFRP Catalog Number: 9458
• Author: National Park Service.
• Title: Workshop on Population Estimates for Colorado Pikeminnow and Humpback Chub.
• USFW Year: 2001.
• USFW - Doc Type: Fort Collins.
• Copyright Material: NO

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MODELS USED:

Petersen Population Estimate

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The following information was excerpted from Van Den Avyle (1993).

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The simplest mark-recapture technique of population estimation requires
one sample period in which fish are collected, marked, and released, and
another period in which fish are collected and examined for marks. This
method is the Petersen index (alternatively known as the Lincoln index), which
is based on the assumption that the proportion of marked fish in the second
sample estimates the proportion of marked fish in the total population.

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The Petersen population estimation formula is:

^
N=

MC
R

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where M is the number of fish initially marked and released, C is the number
of fish collected and examined for marks in the second period, and R is the
number of recaptures found in C. This estimate applies to the population
present during the first sample period, not the recapture period.

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Adjustments for small sample sizes

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The Petersen index can give biased estimates of population size when the
number of fish sampled is low, but several modifications of the Petersen
equation have been proposed to help correct this bias.

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Bailey's modification to the Petersen population estimation formula is:

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^ M(C+1)
N=
(R + 1)

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This is used in cases where sampling during the recapture period is conducted
"with replacement," meaning that each fish is returned to (replaced in) the
population after it is examined for marks and thus is eligible to be included
in the sample again.

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