9713

• UCREFRP Catalog Number: 9713
• Author: VanHaverbeke, D.R.
• Title: Stock assessment and fisheries monitoring activities in the Little Colorado River within Grand Canyon during 2004.
• USFW Year: 2005.
• USFW - Doc Type: U.S. Fish and Wildlife Service
• Copyright Material: NO

DISCUSSION

Spring Abundance Estimate

A length stratified Chapman Petersen estimate of 2,334 (SE = 411) HBC ~
150 mm is given as the estimate for spring 2004. Because significant differences
were found in the length frequency distributions between the marked [M] and
captured [C] populations, and between the marked [M] and recaptured [R]
populations, it was considered necessary to stratify the data by length. The
length frequency analyses provides indication that there was potentially some
migratory activity occurring in the LCR between the mark and recapture events.
This means that the assumption of population closure was possibly violated.

Since 2001, the spring abundance estimates of HBC ~ 150 have increased
and again declined. Abundance estimates increased from 2,082 in spring of
2001 to 2,666 in spring 2002, and to 3,419 in spring of 2003. This apparent
increase may have been because of survivorship and recruitment from the 2001
(and possibly the 2000) age-O cohorts. For instance, the 2001 age-O cohort
(mode -80 mm) can be strongly seen in fall 2001 (Figure 24). The cohort can
then be tracked into spring 2002 (mode -115 mm) and into fall 2002 (mode -155
mm). By spring 2003 this cohort is well into the 150-200 mm size category and
may reflect the high abundance of HBC ;;:= 150 mm during spring 2003 (Figure
11). In spring of 2004, we detected a decrease in the abundance of HBC ~ 150
mm relative to spring 2003 (Figure 11). This decrease may be the result of low
recruitment from the 2002 age-O cohort, thought to be poor. Very few age-O fish
were captured during fall 2002 (Figure 24). This void can be tracked through
spring and fall 2003 and shows up as a low spot in the length frequency
histogram for fish roughly in the 155-180 mm category during spring 2004. In
other words, the passing through of a strong cohort or two may have caused the
increase in abundance in spring 2002 and 2003, while the entering of a weak
cohort may have caused the decline in spring 2004. Finally, all spring estimates
obtained between 2001 through 2004 have been lower (although not all have
been significantly lower) than the spring 1992 estimates provided by Douglas and
Marsh (1996).

Also of interest are the abundance estimates of HBC ~ 200 mm. In addition of
a criterion for no significant decline, the Recovery Goals for HBC call for a
minimum viable population of 2,1 00 HBC ~ 200 mm in Grand Canyon (USFWS
2002). The spring 2004 estimate for HBC ~ 200 mm in the LCR falls at 1,816
(SE = 397). It is noteworthy that all four spring estimates provided from 2001 to
2004 fall below 2,100 fish. However, as mentioned in the introduction section,
our annual closed LCR estimates by themselves are not intended to provide an
estimate of the overall LCR population, because some portion of HBC will be in
the mainstem during our activities and will not be captured in the estimate. Our
annual data are being incorporated into open population models (i.e., Jolly-Seber
in Program Mark, and ASMR) in order to estimate the entire LCR population.

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