|
<br />1370
<br />
<br />Yampa River
<br />
<br />"I
<br />(;'40
<br />~ )0
<br />
<br />!:~ ........ bh,..,..
<br />
<br />2000
<br />
<br />,~ '\~ ~ ~ _*'>, 0'0, 0'0,
<br />, ,cS' ~cS'~ #' "" "cS' #
<br />
<br />;I ..,o,r5'
<br />%'"
<br />
<br />501 JOOI
<br />~40 IlL
<br />~ 30
<br />
<br />!:~ ......" ,~..... ..
<br />
<br />~ ~o, r:::-o, ':l~ _*0, q.o, iJ'o, "lfJ ~'" '?',r5'
<br />"',cS'~cS'#"""cS'#,,,,,%cS' ~
<br />
<br />"I
<br />~ 40
<br />g 30
<br />
<br />!:~ ....... ,," ...,... ..
<br />....0, ~ b?I.;ft>C\ ~iJ'o, ~""#
<br />'" cS" cS'~ ~ ~ ~'j '" ~'& "
<br />~ ~ ~ ~ ~ ~ ~ ~
<br />
<br />"I
<br />",40
<br />g 30
<br />~ 20
<br />~ 10
<br />o
<br />
<br />I.
<br />. . .
<br />
<br />~ ~ ~ # ~ ~ ~ ~ ~ #
<br />~ ,cS" ~&~ ,cS' ,cS' ,,&" # ,&'.& ~
<br />Colorado pikeminnow TL (mm)
<br />
<br />BESTGEN ET AL,
<br />
<br />White River
<br />
<br />2000
<br />
<br />:~L .JL
<br />
<br />~~ ~o, ~o, y-.o, ~o, .,~ iJ'o,
<br />,&~&,&JS',,&#
<br />
<br />~~e ~#
<br />.
<br />
<br />2001
<br />
<br />OWl
<br />100
<br />g 80
<br />g 60
<br />[ 40
<br />"' 20
<br />o ...... ....d....,I'lI..... . ..
<br />
<br />f' ~...o, ",""';)'0., ~iJ'-o, ~ ~~\:J
<br />~ ,,~,~ ~>S''Y ...,>Sf") ~ 'JrSJ"3 # ,\~';I 't,rSIct3 ..,
<br />
<br />2002
<br />
<br />:~l
<br />~ 80
<br />~ 60
<br />e 40
<br />~ 2~ ........ .b,lll,lll....
<br />~ ",0, ~ _~/~ ~ rJ'o, T}) ~ #'
<br />" &' cS'~ ~' ~'", &' ~. "
<br />'\ '\. "J ":l \j-' ~ "I:l
<br />
<br />2003
<br />
<br />^ :::1
<br />g 80
<br />~ 60
<br />! ig .... I.,...L.....
<br />
<br /><:::J"'o, ~o, ;f' ')~ ,t>-o, '-"J~ )!-q ';It.-o, ~o, ..,#
<br />,&~"',"'JS',,&#,"',cS'
<br />
<br />Colorado pikcrninnow TL (rom)
<br />
<br />middle Green River
<br />
<br />
<br />~l
<br />250
<br />
<br />t"...d."
<br />
<br />,~@ ~l'qr)!.o, ~iJ'q,\l>.q..}--q'?l~
<br />I::! ....rSf.... ~r5'y -,rSJ ~ <-;,rSJ<-J #' '\& "brS' ..,
<br />
<br />2001
<br />
<br />.OOl
<br />250
<br />g 200
<br />~ 150
<br />
<br />!I:! ...... .,..d..,., ..
<br />
<br />
<br />r;}'o, ~o, "'@ ~ _*0, "'~ '<5'0, '\~ ~q '?l~
<br />,,<5'" '\.rSJ~ ...,rSJ"'J ~ <-;,<Sf' 'c""r:s '\& 't,tSJ "1
<br />
<br />2002
<br />
<br />:j
<br />i7 200
<br />~ 150
<br />! 100
<br />50
<br />o ..
<br />t-"} ~Cj
<br />'" &'
<br />"
<br />
<br />2002
<br />
<br />..
<br />. . .
<br />
<br />q-o, ~q ':Jt.ct iJ'-o, "..",0, .J'-o, #'
<br />",<S" ~' <-;,rSf b~' ",rSJ 't,rS' "1
<br />
<br />2003
<br />
<br />2003
<br />
<br />~l
<br />250
<br />g 200
<br />g 150
<br />f l~~ ......... ~I... . . .
<br />~ ~ ~ #~~ ~ ~ # $ #
<br />~ ....&':'> ,\-<Sfn; ,,'\fi # '-J'\fi # ,,'\fi q,<Sf -1
<br />
<br />Colorado pikeminnow TL (nun)
<br />
<br />FIGURE 9.-Length frequency histograms for Colorado pikeminnow captured in various reaches within the Green River basin
<br />during abundance estimation sampling, 2000--2003.
<br />
<br />abundance estimates suggest that this was a significant
<br />and biologically important decline. Furthermore, a 48%
<br />reduction occurred over 2000--2003, when we added
<br />lower Green and Desolation-Gray Canyon reach
<br />estimates from 2001 (as a measure of their 2000
<br />abundance) to the estimates from other reaches in
<br />2000. Reduced survival of the recruit and adult life
<br />stages and especially, reduced abundance of recruits
<br />from the year-classes produced in the 1990s, are
<br />presumably responsible for these declines. The condi-
<br />tion of Colorado pikeminnow in 2000-2003 also
<br />declined relative to that in 1991-1999. Below we
<br />discuss the abundance estimation model, model
<br />assumptions, implications of the estimates of demo-
<br />graphic parameters, and potential reasons for the
<br />decline of Colorado pikeminnow in the Green River
<br />basin during the study period.
<br />
<br />Model Selection and Assumptions
<br />
<br />We explored a series of models before selecting the
<br />Huggins robust-design, multistratum (reach) model as
<br />the most realistic one for estimation. The multistratum
<br />aspect of the estimating model was important because
<br />
<br />of the sampling design and because differences in the
<br />size structure of Colorado pikeminnow among reaches
<br />affected capture probabilities and ultimately the
<br />abundance estimates. Differences in capture probabil-
<br />ities across reaches or time might also be expected
<br />because of differences among reaches with respect to
<br />geomorphology (canyon versus valley segments),
<br />habitat conditions at different flow levels, sampling
<br />crews, and the type of sampling craft (raft versus boat
<br />electrofishing).
<br />The assumption of demographic closure was met, in
<br />part, because within-year sampling was limited to a
<br />relatively short period in spring prior to the movement
<br />of Colorado pikeminnow to spawning areas or other
<br />localities that were not sampled. Closure was also
<br />assumed because the study area was large and
<br />emigration to or immigration from the only physically
<br />connected population in the Colorado River was
<br />thought unlikely based on the tendency of these fish
<br />to occupy small and stable home ranges at that time of
<br />year (Tyus 1990; Irving and Modde 2000). The closure
<br />assumption was also supported by the low number of
<br />Colorado pikeminnow moving between river reaches
<br />
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