9711

• UCREFRP Catalog Number: 9711
• Author: Coggins, L.G.
• Title: Abundance trends and status of the Little Colorado River population of humpback chub
• USFW Year: 2008.
• USFW - Doc Type: an update considering 1989-2006 data.
• Copyright Material: NO

Abundance Trends and Status of the Little Colorado River
Population of Humpback Chub: An Update Considering
1989-2006 Data

By Lewis G. Coggins, Jr.

Executive Summary

In 1967, the humpback chub (Gila cypha) (HBC) was
added to the federal list of endangered species and is today
protected under the Endangered Species Act of 1973. Only six
populations of humpback chub are currently known to exist,
five in the Colorado River Basin above Lees Ferry, Arizona,
and one in Grand Canyon, Arizona. The majority of Grand
Canyon humpback chub are found in the Little Colorado
River (LCR)-the largest tributary to the Colorado River in
Grand Canyon-and the Colorado River near its confluence
with the Little Colorado River. Monitoring and research of the
Grand Canyon humpback chub population is overseen by the
U.S. Geological Survey's (USGS) Grand Cmyon Monitoring
and Research Center (GCMRC) under the auspices of the Glen
Canyon Dam Adaptive Management Program (GCDAMP), a
Federal initiative to protect and improve resources downstream
of Glen Canyon Dam.
This report provides updated information on the status
and trends of the LCR population in light of new information
and refined assessment methodology. An earlier assessment
of the LCR population (Coggins and others, 2006a) used data
collected dming 1989-2002; the assessment provided here
includes that data and additional data collected through 2006.
Catch-rate indices, closed population mark-recapture model
abundance estimates, results from the Oliginal age-stmctured
mark recapture (ASMR) model (Coggins and others, 2006b),
and a newly refined ASMR model are presented. This report
also seeks to (I) formally evaluate alternative stock assess-
ment models using Pearson residual analyses and information
theoretic procedures, (2) use mark-recapture data to estimate
the relationship between HBC age and length, (3) translate
uncertainty in the assignment of individual fish age to result-
ing estimates of recmitment and abundance from the ASMR
model, and (4) evaluate past and present stock assessments
considering the available data sources and analyses, recogniz-
ing the limitations inherent in both.
A major task of this study was to improve the overall
methodology used to conduct HBC stock assessment by
addressing concems identified in an independent review
conducted in 2003 (Kitchell and others, 2003). The review

report identified that the current technique of assigning age
to individual tish based on length was a potential source of
bias in ASMR estimates of abundance and recruitment, and
called for a more complete examination of this potential error
source. Additionally, the review suggested that further work
to develop procedures to better arbitrate among alternative
assessment models (e.g., ASMR 1-3) would be beneficial.
To address the tirst of the concerns identified by the
independent review, this study uses mark-recapture data to
develop a temperature-dependent growth model to character-
ize the relationship between HBC age and length. This model
attempts to account for temperature diHerences resulting from
both ontogenetic habitat shifts between the Little Colorado and
the mainstem Colorado Rivers as well as seasonal variation
in water temperature within the LCR. The resulting growth
model is then used to characterize the enol' in assigning age to
individual fish based on length. Results presented in this study
suggest that ageing error does not result in large bias in either
abundance or recruitment estimates from the ASMR model.
However, incorporating ageing error into the assessment does
result in less precise estimates, particularly for recruitment.
To address the second concern brought forward in the
review repOlt related to model selection procedures, this study
arbitrated among the competing models by both examining
model fit using Pearson residual analyses and considering
information theoretic measures. Although adult abundance
estimates and trend varied little among all models considered,
these procedures identitied ASMR 3 as the model whose
underlying assumptions were most consistent with the data.
Because ASMR 3 is also the most complex model. with a
structure that allows for complex patterns in capture probabil-
ity across ages and through time, further examination of model
results suggest a decline in sampling efficacy for middle-
aged fish since approximately 2001. Although the cause of
this shift in sampling efficacy is still unknown, it is possible
that changes in the timing of LCR sampling events or subtle
changes in sampling gear may be, at least, partly responsible
for this tindi ng.
MonitOling data and assessment model results reported
herein continue to support the hypothesis that the adult
(age-4+) component of the LCR population experienced