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

2 Abundance Trends and the Status of the Little Colorado River Population of Humpback Chub 1989-2006

approximately a 40%-50(;(; decline between 1989 and 2001.
More recently, the population appears to have increased,
reaching between 5,300 and 6,800 individuals in 2006. This
increase in adult fish abundance since 200 1 appears to be a
result of increased recruitment beginning in the mid- to late-
1990s and continuing through at least 2002.
Inclusion of ageing error in the assessment procedures
has resulted in less precise estimates of adult abundance and
recruitment. These results suggest that experimental man-
agement actions that result in large changes in recruitment
are much more likely to be detected than actions resulting
in small changes in recruitment. Therefore, if ASMR results
continue to be used as the primary measure of HBC recruit-
ment variation, experimental management actions designed to
induce large changes in HBC recruitment should be prefen-ed
to those likely to induce only small changes. Adherence to this
recommendation will help guard against failing to recognize
beneficial management policies simply because the magnitude
of the response was not sufficient to be detected by the cun-ent
stock assessment program.

Introduction

The humpback chub is a focal resource of the Glen
Canyon Dam Adaptive Management Program. a federally
authorized initiative to protect and mitigate adverse impacts
to resources downstream of Glen Canyon Dam. The focus on
BBC is primarily a result of its unique ecological role as one
of the few remaining endemic aquatic species within Grand
Canyon and its endangered listing status lmder the Endangered
Species Act of 1973 (Gloss and Coggins, 20(5). The purpose
of this report is to provide updated infonnation on the status
and trends of the Little Colorado River population of BBC in
light of new information and retined assessment methodol-
ogy. Such information constitutes the cornerstone of the BBC
monitoring program within the GCDAMP and is also poten-
tially useful to evaluate the recovery goals for this species
as specitied by the U.S. Fish and Wildlife Service (USFWS;
U.S. Fish and Wildlife Service, 2002). The U.S. Geological
Survey's Grand Canyon Monitoring and Research Center has
responsibility for the scientific monitoring .md research efforts
for the GCDAMP. including the preparation of reports such as
this one.
The unique life-history attributes of BBC and the large
vmiety of sampling and monitoring programs ongoing since
the 1980s (Coggins and others. 2006a) prompted the develop-
ment of a new type of age-structured open population capture-
recapture model called the age-structured mark recapture
model (Coggins and others, 2006b). This model was s.ubse-
quently used in combination with other capture-recapture and
index-based assessments to provide a comprehensive assess-
ment of the LCR population of HBC (Coggins and others,
2006a). The ASMR approach has been subjected to a series
of independent peer evaluations. both a" part of the GCDAMP

(Kitcbell and others, 2003; Otis and Wickham, U.S. Geologi-
cal Survey. written commun.. 20(6) and peer-review processes
required to publish journal articles and USGS products. These
reviews all provide suppOli for the ASMR as an appropri-
ate modeling approach to evaluate trends in BBC population
size and recruitment patterns. The ASMR model has under-
gone continuous retinement since the publication of the last
assessment to improve the model's ability to provide insight
into BBC population dynamics and responses to manage-
ment actions. Presented herein are improvements to the model
that include the development of a fonnal model comparison
approach using Pearson residuals and information theory to
evaluate model tit. Comparison of the three main formulations
of ASMR, detailed below and in Coggins and others (2006b),
allows more precise examination of model iit and shows how
incorrect structural assumptions may bia" model output.
A central problem in conducting HBC stock assess-
ment is the assignment of age to individual fish. Though this
problem is ubiquitous in fish assessment programs (Coggins
and Quinn. 1998; Sampson and Yin, 1998), it is particu-
larly ditlicult when working with endangered fish and when
determination of age is only possible by killing the fish. In the
case of humpback chub, an endangered species. determining
the age of an individual requires killing the animal. To avoid
this problem, individual fish ages must be assigned based on
fish lengths and assuming some relationship between these
two metrics. In past BBC assessments (Coggins and others,
2006a), it was assumed that the age-length relationship was
adequately described (U.S. Fish and Wildlife Service, 2002);
however. this age-length relationship is based on an exu'emely
small sample size (n ;:::; 57) and is therefore suspect. Addition-
ally, when assigning individual age based on this relationship,
it was originally assumed that fish could be aged without error,
clearly not a valid assumption. To alleviate these shortcom"
ings, a new method is presented here for estimating the rela-
tionship between fish age and length using capture-recapture
data. The uncertainty in therelationship between fish age and
length is used to evaluate how uncertainty in the determina-
tion of age translates to uncertainty in abundance and recruit-
ment estimates from ASMR using Monte Carlo simulations.
These analyses otfer insight into the HBC assessment and
other monitoring programs for aquatic and terrestrial species
where capture-recapture methodologies serve as the core of
the assessment approach and precise trends in recruitment and
mortality are difticult to quantify because of uncertainty in
assigning age.
Retrospective analyses were employed to evaluate the
performance of past model predictions of trends in adult BBC
abundance and recruitment. These retrospective analyses
consider only a subset of the available data and are structured
to demonstrate how information collected annually is used to
"update-' estimates from previous years. Mortality and capture
probability are potentially confounded in open population
models (Williams and others, 200 I). As an example, if an
animal is not captured in a sample year is it because the ani-
mal died, or is it simply because the animal was not captured?