Authors
Report Reference
#08-07
Publication Date
4 Augsut
2008
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Procedures
for Estimating Tag Loss Rate and Spawning Escapement in a Mark-Recapture
Study of Metolius River Kokanee Oncorhynchus nerka
Abstract |
| Mark-Recapture/Resight
procedures were used to estimate the 2007 spawning escapement of kokanee
Oncorhynchus nerka migrating from Lake Billy Chinook into
the Metolius River, Jefferson County, Oregon. An initial analysis
of data, which included an adjustment for a presumed 25% tag loss
rate (without confidence limits), provided an escapement estimate
of 101,854 ± 11,151 (±10.9%; 95% confidence interval).
Here we present an expanded analysis of the data to look for effects
of the survey protocols, and we have re-estimated abundance, incorporating
the additional uncertainty associated with estimation of tag loss
rate ased on recapture (resight) counts of double-tagged fish. The
resight procedures in project involved subdivision of the spawning
area in the Metolius into five sections, each of which was surveyed
once in each of three different weeks. A two-way analysis of variance
of these data indicated no significant effect on the proportion of
marked fish among observed spawners (overall average = 2.42%) for
either survey section or survey week. Among the 3,298 fish tagged
in 2007, 491 received a second tag. A al of 59 of these double-tagged
fish were resighted - 35 with both tags and 24 with a single tag.
A binomial likelihood analysis of these data indicated a tag loss
rate of 6% ± 9.9%. Incorporation of the uncertainty associated
with tag loss estimation into a hypergeometric likelihood procedure
to calculate escapement yielded an estimate of 106,630 ± 16,393
(± 15.4%). Several scenarios based on the 200 ta were run using
these likelihood procedures, to illustrate the effects on precision
of estimate from an increase or decrease in the total number of tagged
fish, in the number of double-tagged fish, and in the total number
of fish observed during the surveys, as well from an increase or decrease
in observed tag loss rate. The most dramatic improvement in precision
of the abundance estimate per unit change in 2007 tagging/survey effort
could have been obtained from increasing the proportion of double-tagged
ish - to increase certainty of the tag-loss estimate. We propose adoption
of the bi al-hypergeometric likelihood estimator procedures to incorporate
tag loss rate and its ncertainty into a Mark-Recapture/Resight analysis
for estimation of abundance, and provide suggestions for modifications
of the Metolius kokanee tagging and survey procedures for spawning
escapement estimation in future years. |
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