9724

• UCREFRP Catalog Number: 9724
• Author: Coggins, L.G., W.E. Pine, C.J. Walters, D.R. VanHaverbeke, D. Ward and H.C. Johnstone.
• Title: Abundance trends and status of the Little Colorado River population of humpback chub.
• USFW Year: 2006.
• USFW - Doc Type: North American Journal of Fisheries Management
• Copyright Material: YES

234

COGGINS ET AL.

and Grand canyons. However, only the aggregation
near the confluence of the LCR and Colorado River
(hereafter referred to as the LCR population) is known
to successfully reproduce. Because of abiotic and biotic
changes in the Colorado River after the construction of
Glen Canyon Dam in 1963, the LCR population relies
on the LCR as its primary spawning and juvenile
rearing habitat (Gorman and Stone 1999).
Humpback chub demonstrate potadromous spawn-
ing migrations between the Colorado River and the
LCR (Gorman and Stone 1999). Recaptures of tagged
animals suggest that the geographic extent of this
population is the lower 15 km of the LCR and the Little
Colorado River inflow reach of the Colorado River
(LCR inflow reach, defIned as approximately 9 km
upstream and 11 km downstream of the confluence;
Valdez and Ryel 1995). The spawning migration
creates both difficulties and opportunities for monitor-
ing the population. Typically, adult humpback chub
stage near the mouth of the LCR in March and April,
ascend into the LCR in April and May, and return to
the LCR inflow reach over a protracted time period
from June to September (Valdez and Ryel 1995;
Gorman and Stone 1999). Though the existence of
a spawning migration between the LCR inflow reach
and the LCR is accepted, a number of uncertainties
remain. The most important of these are the size or age
at which fish begin to exhibit migratory behavior and
whether a proportion of the adult population fails to
migrate every year (i.e., skip spawn). Douglas and
Marsh (1996) suggested that two populations exist: one
resident population in the LCR and one that migrates
between the LCR and LCR inflow reach. However,
Gorman and Stone (1999) suggested that the majority
of adult humpback chub larger than 300 nun total
length (TL) live in the LCR inflow reach except during
the spawning migration.
Because of ongoing management disputes related to
water use issues within the Colorado River basin and
the potential effect of these management actions on
humpback chub population viability, a monitoring
program coordinated by the U.S. Geological Survey,
Grand Canyon Monitoring and Research Center
(GCMRC), was developed to track changes in the
abundance and recruitment of the LCR population.
Information from this program is also potentially useful
in evaluating the ESA recovery goals for humpback
chub, which specify capture-recapture studies to
estimate absolute population size and trend (USFWS
2002). The GCMRC program includes analyses of
historical data, ongoing sampling, and external peer
review (Kitchell et al. 2003). Here we describe
a synthesis of data to characterize the dynamics of
the population. We assessed humpback chub popula-

tion status and trend using catch rate indices and
capture-recapture-based abundance estimates from
open and closed models. While each of these methods
requires various assumptions, the results of the
synthesis are clear: the LCR humpback chub popula-
tion has steadily declined since at least 1989. We
present these results and suggest strengths and
weaknesses in the various approaches as related to
stock assessment of the LCR population.

Methods

Field methods.-Repeated sampling for long-term
population trends in native fIsh abundance began in
1987 with the initiation of standardized hoop net
sampling in the lower LCR. During 1991-1995,
intensive sampling was conducted both in the Colorado
River and the LCR in conjunction with an environ-
mental impact study of the operation of Glen Canyon
Dam (USBOR 1995). Sampling in both the Colorado
River and the LCR has continued from 1996 to the
present, but at reduced intensity. Although sampling
effort and research groups have varied across the study
period, the methodologies and sampling personnel
have remained fairly consistent (Table 1). Fish were
predominately collected using hoop nets (0.5-1.0 m in
diameter, U~5.0 m long, 6~nun mesh, single or double
1O-cm throat) and trammel nets (7.6--45.7 m long, 1.8
m deep, 1.3-3.8-cm inner mesh and 30-em outer mesh)
in the LCR (Douglas and Marsh 1996; Gorman and
Stone 1999) and hoop nets, trammel nets, and pulsed
DC electrofIshing (Coeffelt Mark XXII CPS) in the
Colorado River (Valdez and Ryel 1995).
Index-based assessments.-We analyzed two long-
term catch rate data sets for trends in the abundance of
the LCR population. The first data set consists of the
hoop net catch rate in the lower LCR, which was
collected annually for 20-30 consecutive days in April
and May during the years 1987-1999 and 2002-2003.
Hoop nets were deployed at 13 standardized locations
in the LCR within 1,200 m of the confluence with the
Colorado River. The nets were fIshed daily for about
24 h throughout the monitoring period. The second
data set consists of the monthly trammel net catch rate
in the LCR inflow reach. In various months between
1990 and 2003, trammel nets were deployed during
crepuscular and night periods. Sample locations were
chosen at slow-water and current separation points. We
use the results of these two analyses as gross indices of
long-term population trends with which to compare the
estimated population sizes from the capture-recapture
models described below.
Tagging-based assessments.-Between 1989 and
2002, over 14,500 unique humpback chub (TL >
150 nun) were measured and implanted with passive