8097

• UCREFRP Catalog Number: 8097
• Author: American Fisheries Society.
• Title: Proceedings of the 18th Annual Meeting, Colorado - Wyoming Chapter, American Fisheries Society.
• USFW Year: 1983.
• USFW - Doc Type: March 2-3, 1983.
• Copyright Material: NO

CHANGES IN CATCHABILITY RELATED TO
MULTIPLE ELECTROSHOCK

STEVE YUNDT

WYOMING GAME AND FISH DEPARTMENT
LARAMIE, WYOMING 82070

ABSTRACT

Electrofishing mark/recapture population estimates were
shown to be affected by the length of time between the mark
and recapture runs. On six different Wyoming streams, popula-
tion estimates made with 0 to 3 days (within week) between mark
and recapture runs averaged 75.1 and 54.0% for rainbow (Salmo
gairdneri) and brown trout (Salmo trutta), respectively, of
population estimates made with 6 to 10 days (between week)
between mark and recapture runs. Movement information collected
from the Big Horn River, Wyoming, suggested that movement of
marked rainbow trout out of the study section was not respon-
sible for the difference between within week versus between week
estimates. Rather, the difference seemed to be due to a change
in the probability of capture following exposure to electro-
shock.

INTRODUCTION

Accurate estimates of fish population are of paramount importance to
fishery management because management cannot be effective if the limits of
fish populations remain unknown or if estimates of fish stocks are largely
inaccurate. There are many methods used to estimate fish populations and
there are associated assumptions that must be met to have confidence in the
estimates. The dilemma in modeling mark/recapture population estimation
methods is whether (to what extent) the assumptions are violated.
The hypergeometric probability distribution is a commonly used method
to model mark/recapture populations estimates. Associated with this model
are the common assumptions that: 1) the population size is constant during
the course of the experiment, 2) All animals have the same probability of
being captured in the first sample, 3) Marking does not affect the catch-
ability of the animal, 4) The second sample is a simple random sample, 5)
Animals do not lose their marks between the two samples, and 6) All marks
are reported on recovery in the second sample (Seber, 1973). These assump-
tions are more fully discussed by Seber (1973), Ricker (1975), and Cormack
(1979) .
This model is the same model that is used to estimate the number of
beans in an urn where a sample of beans is drawn from a population of beans
containing a known number of marked beans and an unknown number of unmarked
beans. The question often encountered in animal mark/recapture studies is
whether or not animal populations in the wild behave enough like beans in

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