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

category in fall of 2004 (Figure 24) and the higher fall 2004 abundance estimate
for HBC ;:: 150 mm appears to verify this (Figure 22).

To generate the fall 2004 estimate of 796 (SE = 184) HBC ~ 200 mm, the
length stratified Chapman-Petersen estimate for fish > 200 mm was corrected to
include fish ~ 200 mm. This procedure was performed because it was preferable
to truncating the data at 200 mm and obtaining an independent length stratified
estimate (Seber 1982). It should be mentioned that for an additional exercise,
the data were truncated and a length stratified estimate was obtained that was
similar (although higher by 55 fish and containing only 3 recaptures within one of
the strata). It should be made clear that the low fall estimates for HBC ~ 200 mm
obtained over the past four years (Table 16) are expected to be lower than the
spring estimates (Table 9), since a portion of HBC migrate out of the LCR after
the spring spawning event (Gorman and Stone 1999). Nevertheless, both the fall
and spring abundance estimates do provide trend data indicating that the
numbers of these larger fish are low.

No estimate was provided this year for the abundance of HBC between 100 to
149 mm. This is because PIT tagging of HBC <150 mm was discontinued in
2004 because of concerns about mortality and potential tag loss in this size class
of small fish. This estimate was also not possible to make in fall 2003 because
only one fish was recaptured out of only 26 marked (Van Haverbeke 2004). This
complication in fall 2003 (Le., lack of captures and failure to achieve an
abundance estimate for this size class of fish) may have been largely caused by
the poor 2002 age-O cohort, as Figure 24 clearly shows a lack of fish in the 100-
150 mm size category during fall 2003. In fall 2002, a length stratified Chapman
Petersen estimate of 2,033 fish (SE = 284) was given for the fall abundance of
HBC from 100 to 149 mm (Van Haverbeke 2003). This success in obtaining an
abundance estimate in fall 2002 may have been a result of the large 2001 age-O
cohort (Le., sufficient fish were available for marking and recapture in this size
category during fall 2002; Figure 24).

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