Population Status and Trends of Colorado Pikeminnow of the 

Upper Colorado River, 1991-2005 

 

 

 

July 2009 

U.S. Fish and Wildlife Service 
Population Status and Trends of Colorado Pikeminnow of 

the Upper Colorado River, 1991–2005 

Final Report

July 2009

Prepared by 

Douglas B. Osmundson1 and Gary C. White2

1

Colorado River Fishery Project, U. S. Fish and Wildlife Service, 764 Horizon Drive, Building B, 

Grand Junction, Colorado 81506–3946, USA 

2

Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins 

80523, USA 

Recovery Implementation Program 

Project No. 127 

U. S. FISH AND WILDLIFE SERVICE
Disclaimer: 

Mention of trade names or commercial products does not constitute endorsements or 

recommendations by the U. S. Fish and Wildlife Service. 

To obtain copies of this report: 

Contact: U. S. Fish and Wildlife Service, Colorado River Fishery Project, 764 Horizon Drive, 

Bldg. B, Grand Junction, CO 81506-3946. Phone: 970-245-9319 or FAX: 970-245-6933 or 

e-mail the lead author at: doug_osmundson@fws.gov

Suggested citation: 

Osmundson, D. B., and G. C. White. 2009. Population status and trends of Colorado pikeminnow of the upper 

Colorado River, 1991-2005. Final Report. U. S. Fish and Wildlife Service, Grand Junction, Colorado. 
TABLE OF CONTENTS 

LIST OF TABLES.............................................................................................................................................III

LIST OF FIGURES.............................................................................................................................................V

ACKNOWLEDGEMENTS........................................................................................................................... VIII

EXECUTIVE SUMMARY...............................................................................................................................IX

INTRODUCTION................................................................................................................................................1

METHODS............................................................................................................................................................2

STUDY AREA .....................................................................................................................................................2

ANALYSES..........................................................................................................................................................6

Survival rate and abundance estimation.......................................................................................................6

Electrofishing catch per effort. ...................................................................................................................10

Length frequency.........................................................................................................................................10

Relative year-class strength........................................................................................................................11

Temporal variation in median length..........................................................................................................11

Body condition ............................................................................................................................................11

Inter-system movements..............................................................................................................................12

RESULTS............................................................................................................................................................13

FISH CAPTURES...............................................................................................................................................13

MODEL SELECTION.........................................................................................................................................14

CAPTURE PROBABILITY..................................................................................................................................17

POPULATION SIZE...........................................................................................................................................21

POPULATION REPLACEMENT .........................................................................................................................24

ABILITY TO DETECT CHANGE IN ABUNDANCE..............................................................................................24

TRANSITION PROBABILITIES ..........................................................................................................................27

ELECTROFISHING CATCH-PER-EFFORT ........................................................................................................31

LENGTH FREQUENCY......................................................................................................................................36

Temporal variation in median length..........................................................................................................41

Years 2003–2005 ........................................................................................................................................42

RELATIVE YEAR-CLASS STRENGTH ...............................................................................................................45

YEAR-CLASS STRENGTH AT AGE-5 IN RELATION TO STRENGTH AT AGE-0.................................................48

BODY CONDITION ...........................................................................................................................................49

Differences among length classe.................................................................................................................50

Differences among periods. ........................................................................................................................50

Differences among year. .............................................................................................................................50

Relations with abundance. ..........................................................................................................................53

MOVEMENTS INTO AND OUT OF THE GREEN RIVER SYSTEM.......................................................................54

CAPTURES OF STOCKED COLORADO PIKEMINNOW......................................................................................58

DISCUSSION......................................................................................................................................................60

ABUNDANCE AND SURVIVAL ESTIMATES.......................................................................................................60

TRANSITION PROBABILITY .............................................................................................................................63

ELECTROFISHING CATCH PER EFFORT .........................................................................................................64

LENGTH FREQUENCY......................................................................................................................................65

RELATIVE YEAR-CLASS STRENGTH................................................................................................................67

YEAR-CLASS STRENGTH AND LATER RECRUITMENT....................................................................................67

BODY CONDITION AND CARRYING CAPACITY...............................................................................................70

ASSUMPTIONS..................................................................................................................................................72

i
TABLE OF CONTENTS (continued) 

INTER-SYSTEM MOVEMENT ...........................................................................................................................73

STOCKED FISH ................................................................................................................................................77

SUMMARY AND CONCLUSIONS.................................................................................................................78

RECOMMENDATIONS ...................................................................................................................................80

LITERATURE CITED......................................................................................................................................82

APPENDIX .........................................................................................................................................................86

 

ii
LIST OF TABLES 

1. Total number of Colorado pikeminnow > 250 mm TL captured in each 

 sampling pass and year in the Colorado River study area, Colorado and 

 Utah, 1991-2005...........................................................................13 

2. Estimates for the von Bertalanffy growth curve for Colorado pikeminnow 

in the Colorado River study area, 1991-2005...........................................14 

3. Model selection results of the robust design multi-state model for 

Colorado pikeminnow in the upper Colorado River....................................15 

4. Annual (1991–2005) transition probabilities for Colorado pikeminnow 

500 mm TL moving from one study reach to the other as estimated by the 

top ranked model in Table 3...............................................................29 

5. Total lengths of Colorado pikeminnow before and after movement from 

the lower reach to the upper reach of the Colorado River study area................30 

6. Qualitative estimates of Colorado pikeminnow year-class strength based 

on length-frequency histograms from the Colorado River lower-reach 

study area, 1991–1994, 1998–2000, and 2003–2005.................................47 

 

7. Total number of Colorado pikeminnow captures in upper basin rivers 

since use of PIT tags began, 1990-2005.................................................55 

8. Colorado pikeminnow stocking information for the Colorado and 

Gunnison rivers, 2003 and 2004.........................................................59 

9. Estimates of Colorado pikeminnow population size in the Colorado 

River study area (combined reaches) for fish > 450 mm long using 

two estimators: CAPTURE(M0 + Mt) and MARK(Huggins)..........................61 

 

Appendix 

I. Estimated mean length and mean annual growth increments by age for Colorado 

pikeminnow in the Colorado River for ages 0-7...................................86 

II. Abundance estimates ( N ) for all Colorado pikeminnow ˆ > 250 mm 

TL in the lower and upper Colorado River study reaches, and for the 

reaches combined, with lower and upper confidence intervals (CI) and standard 

error (SE).................................................................................87 

iii
LIST OF TABLES (continued) 

III. Abundance estimates ( N ) for all Colorado pikeminnow ˆ > 450 mm 

TL in the lower and upper Colorado River study reaches, and for 

the reaches combined, with lower and upper confidence intervals 

(CI) and standard error (SE)..........................................................88 

IV. Abundance estimates ( N ) for all Colorado pikeminnow ˆ > 500 mm 

TL in the lower and upper Colorado River study reaches, and for the 

reaches combined, with lower and upper confidence intervals (CI) 

and standard error (SE)................................................................89 

V. Abundance estimates ( N ) for all Colorado pikeminnow 400–449 mm ˆ

TL in the lower and upper Colorado River study reaches, and for the 

reaches combined, with lower and upper confidence intervals (CI) 

and standard error (SE)................................................................90 

VI. Results of the z-test to detect change in two abundance estimates...............91 

VII. Documented movements of Colorado pikeminnow (captures and 

 recaptures of PIT tagged individuals) between rivers of the 

 Colorado River sub-basin (mainstem Colorado and Gunnison 

 rivers) and rivers of the Green River sub-basin (Green, Yampa, 

 and Duschene), 1991–2005..........................................................92 

 

iv
LIST OF FIGURES 

1. Map of the upper and lower reaches of the Colorado River study area...............3 

2. Capture probability for pass 1 in 2005 (arbitrarily chosen as an example 

to demonstrate the relationship) as a function of fish length and study 

reach..........................................................................................17 

3. Capture probability by pass in the lower (top) and upper (bottom) 

reaches for a 500-mm-long fish..........................................................18 

4. Annual survival rate (S) of Colorado pikeminnow as a function of fish 

total length and reach based on a model with constant annual survival, 

1991–2005...................................................................................19 

5. Annual survival rate (S) of Colorado pikeminnow > 500 mm TL by reach 

(upper: U; lower: L) for the entire period 1991–2005, for the three multiyear periods, and also for the two reaches combined (U-L) during each 

of the multi-year periods...................................................................20 

6. Abundance estimates of Colorado pikeminnow of three sizes 

classes: > 250 mm TL; > 450 mm TL; > 500 mm TL in the upper 

Colorado River study area (reaches combined), 1992–2005.........................22 

7. Abundance estimates of Colorado pikeminnow > 250 mm TL in 

the upper (top) and lower (bottom) Colorado River study reaches, 

1991–2005...................................................................................23 

8. Annual abundance estimates of Colorado pikeminnow 400–449 mm 

TL in the lower, upper and combined reaches, 1991–2005...........................25 

9. Annual net gain or loss of Colorado pikeminnow > 450 mm TL in 

the summed-reach population based on the estimated number of fish 

400–499 mm TL present each year minus the estimated number of 

deaths of fish > 450 mm TL...............................................................26 

10. Relations between coefficient of variation (100 x SE/ N ) and percent ˆ

change in population abundance required for a change to be detected 

at α = 0.05 and at α = 0.10................................................................27 

v
LIST OF FIGURES (continued) 

11. Transition (movement from the lower reach to the upper reach of 

the Colorado River study area) probabilities of Colorado 

pikeminnow as a function of total length (mm) as estimated 

using a quadratic spline model with cutpoints at 400 mm............................30 

12. Electrofishing catch rates (fish per hour) of Colorado pikeminnow in 

the upper reach (top), lower reach (middle), and whole Colorado 

River study area (lower) during sampling passes of 2003............................32 

 

13. Electrofishing catch rates (fish per hour) of Colorado pikeminnow in 

the upper reach (top), lower reach (middle), and whole Colorado 

River study area (lower) during sampling passes of 2004............................33 

14. Electrofishing catch rates (fish per hour) of Colorado pikeminnow in 

the upper reach (top), lower reach (middle), and whole Colorado 

River study area (lower) during sampling passes of 2005............................34 

15. Interagency Standardized Monitoring Program (ISMP) electrofishing 

mean catch rates (fish/hr) of Colorado pikeminnow in the upper (top), 

lower (middle), and combined (bottom) ISMP reaches of the 

 Colorado River study area, 1986–2005.................................................35 

16. Linear regression of electrofishing capture rates (fish/hr) of Colorado 

pikeminnow in the Colorado River study area and year of capture..................36 

17. Length frequencies of Colorado pikeminnow captured in the lower 

Colorado River study reach, 1991–1994................................................38 

18. Length frequencies of Colorado pikeminnow captured in the lower 

Colorado River study reach, 1998–2000................................................39 

19. Length frequencies of Colorado pikeminnow captured in the upper 

Colorado River study reach, 1991–1994................................................40 

20. Length frequencies of Colorado pikeminnow captured in the upper 

Colorado River study reach, 1998–2000................................................41 

21. Mean and median lengths of Colorado pikeminnow captured in the 

upper Colorado River study reach, 1991–2005........................................42 

22. Length frequencies of Colorado pikeminnow captured in the lower 

Colorado River study reach, 2003–2005................................................43 

vi
LIST OF FIGURES (continued) 

23. Length frequencies of Colorado pikeminnow captured in the upper 

Colorado River study reach, 2003–2005................................................44 

24. Length frequency of Colorado pikeminnow captured in the lower and 

upper Colorado River study reaches (data combined), 2003–2005....................46 

25. Interagency Standardized Monitoring Program (ISMP) catch-perunit-effort of young-of-the-year Colorado pikeminnow in the 

Colorado River, 1982–1999...............................................................49 

26. Mean relative body condition (Kn) of Colorado pikeminnow in the 

upper and lower reaches of the Colorado River study area during 

three sampling periods, 1991–1994, 1998–2000, and 2003–2005..................51 

 

27. Mean relative body condition (Kn) of Colorado pikeminnow 

500–599 mm TL by year in the upper and lower reaches of the 

Colorado River study area................................................................52 

28. Length frequencies of stocked and wild Colorado pikeminnow 

captured from the whole Colorado River study area in 2005........................60 

29. Difference in Colorado pikeminnow length frequencies between the 

2005 sample of fish captured from the upper and lower Colorado 

River study reaches (grey bars) and the estimated actual population 

when the frequencies of sampled fish were adjusted by length-specific 

probabilities of capture (black bars).....................................................66 

30. Estimated main-channel annual thermal units (ATU) for Colorado 

pikeminnow growth at rk 83.7 on the Colorado River, downstream 

of Moab, Utah, 1986–2005...............................................................69 

31. Lines fitted to the annual abundance point estimates for Colorado 

pikeminnow in the upper and lower reaches of the Colorado River 

study area....................................................................................71 

 

vii
ACKNOWLEDGEMENTS 

We thank the many individuals that provided assistance during various phases of this study. 

Primary help in the field during the 2003–2005 effort was provided by Tony Bonaquista, Bob 

Burdick, Jamie Carillo, Matt Carol, Travis Francis, Tara Hicks, Christine Hinds, Chris 

Lyons, Chuck McAda, Logan Poisson, Amber Richens, Dale Ryden, Andrew Scott, Rick 

Smaniotto, Bill Snodgrass, and Kirstie Yaeger. We thank Bob Burdick for sharing Colorado 

pikeminnow capture records from his smallmouth bass removal project and Chuck McAda, 

Cheryl Harris, Angela Kantola and Tom Czapla for providing administrative support. David 

Speas, Kevin Bestgen, Dave Propst and Tom Czapla provided thoughtful, critical reviews of 

an earlier draft. Finally, we thank Ken Burnham, Ron Ryel, and the many other individuals 

that contributed to this project during the earlier two multi-year efforts. 

This research was funded by the Upper Colorado River Endangered Fish Recovery Program. 

The Recovery Program is a joint effort of the U. S. Fish and Wildlife Service, U. S. Bureau 

of Reclamation, U. S. National Park Service, Western Area Power Administration, states of 

Colorado, Utah, and Wyoming, Upper Basin water users, environmental organizations, and 

the Colorado River Energy Distributers Association. 

viii
EXECUTIVE SUMMARY 

 Mark-recapture studies from 1991 through 2005 were used to assess population 

trends of Colorado pikeminnow Ptychocheilus lucius in the upper Colorado River. Three 

multi-year data collection efforts were made: 1991–1994, 1998–2000, and 2003–2005. 

Primary objectives included capturing and marking Colorado pikeminnow > 250 mm long 

from throughout the study area, developing estimates of population abundance and survival 

rate and assessing trends in recruitment. Although results of the first two study periods have 

been provided in previous reports, data from these earlier efforts were reanalyzed using new 

techniques. Here, a synthesis is provided of the updated information from the earlier studies 

and the results from the most recent study period. 

 The 287-km-long study area was divided into two reaches: (1) the lower reach, 

extending from the confluence of the Colorado and Green rivers in Canyonlands National 

Park, Utah, upstream to Cottonwood Wash at the base of Westwater Canyon, and (2) the 

upper reach, extending from Westwater Wash upstream to the Price Stubb Diversion Dam at 

Palisade, Colorado. The upper reach also included the lowermost 3.5 km of the Gunnison 

River downstream of the Redlands Diversion Dam at Grand Junction, Colorado. The 20-km 

Westwater Canyon, separating the 106-km upper reach from the 181-km lower reach, was 

excluded from study because few Colorado pikeminnow are thought to reside there and 

because it is difficult to sample. During spring runoff of the first two multi-year sampling 

periods, the upper reach was sampled three times; the lower reach, two times. Backwater 

trammel netting was the primary means of sampling, supplemented with shoreline boat 

electrofishing. In most cases, one two-person crew did all the sampling. In the most recent 

sampling effort, four two-person crews worked concurrently: two in the upper reach; two in 

the lower reach. Most of the recent sampling was done with electrofishing. Depending on 

the duration of runoff, three to five passes were made through both reaches each year. In all 

three sampling periods, captured Colorado pikeminnow were measured, weighed, PIT-tagged 

and released. 

 The Huggins estimator within the robust design multi-state data type of Program 

MARK was used to generate abundance and survival estimates from mark-recapture data. 

ix
The Huggins estimator of population size was used to incorporate the individual covariate, 

length, as a predictor of capture probability. Annual survival rates were estimated between 

primary occasions (years) in the robust design multi-state model. Covariates used to predict 

survival included year, reach, and fish length. Parameters were estimated for each reach 

separately and these estimates were combined to produce population-wide values. 

 Nine combined-reach annual abundance estimates and one earlier upper-reach 

estimate indicated the Colorado River population increased substantially in number during 

the study period. Point estimates of individuals > 450 mm long in the most recent year, 

2005, were 412 in the lower reach (2.3 fish/km) and 477 in the upper reach (4.5 fish/km), for 

a river-wide total of 889. Although no abundance estimate was available for the lower reach 

in 1991, the length frequency histogram of fish captured there that year suggested there were 

very few fish > 450 mm TL present; hence the river-wide number of fish this size may not 

have been much greater than the estimate provided for the upper reach alone (i.e., around 200 

individuals). Given that the 2005, river-wide, abundance estimate of fish > 450 mm TL was 

over 800, the sub-population of fish this size might have quadrupled since 1991. Summed 

reach-specific abundance estimates generated with the Huggins model were about 14% 

higher on average than the summed estimates generated with Model Mo (upper reach) and Mt

(lower reach) from program CAPTURE. Probability of capture varied among years and was 

generally lowest during the most recent three-year period despite the additional effort 

expended. Capture probability also varied by reach and fish length: for any given length, 

lower-reach fish had higher capture probabilities than upper-reach fish, and in both reaches, 

fish approximately 750 mm in length had capture probabilities 3–4 times that of fish 250 mm 

TL. Survival also varied by reach and fish length: fish in the upper reach had significantly 

higher survival rates than fish in the lower reach, and survival was maximized at fish lengths 

of 450–650 mm. Rates for smaller fish were not substantially lower. However, mortality 

rate was notably higher for very large fish (> 850 mm TL). Overall annual survival rates 

(combined-reach estimates) for fish > 500 mm TL appeared to decline over time from 88% 

(1991–1994) to 86% (1998–2000) to 80% (2003–2005), but differences were not statistically 

significant. 

 Annual recruitment (number of fish 400–449 mm TL) appeared to exceed the 

estimated number of annual mortalities of fish > 450 mm TL in six of the nine years for 

x
which estimates were available. When annual gains and losses were summed there was an 

estimated net gain of 332 fish > 450 mm TL. Lapses in sampling between the three multiyear efforts precluded estimating the total net gain for the period 1992–2005. 

 Precision of abundance estimates as measured by the coefficient of variation (CV) 

was lowest during the first multi-year effort (mean CV of 24%) and higher during the second 

and third multi-year efforts (mean CVs of 14% and 15%, respectively). Precision of 

estimates affects the ability to detect change in population abundance over time. Assuming 

precision of future efforts is similar to our most recent effort, the population would have to 

decline by 30% (between any two years used for comparison) for the difference to be 

statistically significant at α = 0.05, and 24% at α = 0.10. 

 Electrofishing catch rates (mean number of Colorado pikeminnow captured per hour), 

used in the past (Interagency Standardized Monitoring Program) as a means to detect trends 

in population abundance and as a consistency check for mark-recapture estimates, were 

found to not track trends in mark-recapture abundance estimates. A line fitted through the 

mean annual catch rates from 1986 to 2005 indicated no upward or downward trend. 

Although there was a substantial rise in catch rates from the late 1980s through the 1990s, 

recent catch rates were low again. This result probably stems from the generally low 

probability of capture experienced during the recent multi-year sampling period, but may 

also be related to differences in sampling protocol between the two studies and perhaps to 

changes in electrofishing gear. When the recent low capture rates were excluded, an upward 

trend was indicated by the ISMP catch rate results, consistent with that indicated by the 

annual abundance estimates. 

 Length-frequency histograms revealed a very strong year class (1996) of age-0 

Colorado pikeminnow, previously documented from fall seine surveys, virtually disappeared 

before becoming large enough to be susceptible to electrofishing or trammel net sampling. 

Additionally, 1998 year-class individuals, scarce during fall seine surveys, later recruited to 

the adult population in relatively high numbers making it one of the strongest year-classes to 

recruit on record. These observations cast doubt on earlier conclusions that year-class 

strength in the year of origin is a good predictor of later recruitment strength. Our 

interpretation of the annual length-frequency histograms from both reaches was that in the 14 

years from 1985 to 2000 there were seven very weak year-classes (based on relative 

xi
abundance of age-5 fish, 326-453 mm long) and only two very strong ones. However, the 

resulting recruitment rate was evidently an improvement over prior periods because adult 

abundance was very low at the start of our study. 

 Previous suggestions that the population had reached carrying capacity (based on a 

significant decline in relative body condition between 1994 and 1998) were not borne out by 

more recent results. The capacity of the system to support Colorado pikeminnow in good 

condition may have only been temporarily lowered. After the period 1998-2000 relative 

condition returned to levels similar to or higher than those observed in the first multi-year 

period. We found no relation between annual abundance and mean body condition of fish 

500-599 mm TL; additionally, mean body condition in 1992 and 2005 was essentially the 

same even though abundance had significantly increased. Hence, if one uses a significant 

decline in body condition accompanying a significant increase in abundance as a signal that 

carrying capacity has been reached, we would have to conclude that this has not yet occurred. 

 The Recovery Program’s database of PIT-tagged Colorado pikeminnow captured 

from throughout the Colorado and Green river sub-basins revealed that 29 individuals, or 

1.4% of the total number of fish recaptured at least once, moved between the Colorado River 

and Green River systems between 1990 and 2005. This level of movement suggests enough 

gene flow to keep the two populations from differentiating genetically over time but not 

enough exchange of individuals for one population to affect the demographics of the other 

population. 

 Efforts to speed re-establishment of Colorado pikeminnow in reaches upstream of the 

diversion dams on the Colorado and Gunnison rivers through fish stocking in 2003 and 2004 

were evidently unsuccessful. A high rate of long-distance downstream dispersal was 

documented during our study while other studies failed to recover any of the hatchery-raised 

fish in the reaches in which they were stocked. 

 Mark-recapture studies of fish in large rivers are labor-intensive and estimates of 

abundance and survival often have less-than-desirable levels of precision. They nevertheless 

appear to be the most reliable method for monitoring the status of Colorado pikeminnow 

populations. We recommend the current sampling regime be continued.

xii
INTRODUCTION 

 Colorado pikeminnow Ptychocheilus lucius Girard once ranged throughout warmwater reaches of the Colorado River Basin, from the Wyoming border south to the Gulf of 

California. Today, the species is restricted to upper basin reaches, upstream of Glen Canyon 

Dam, and is federally classified as an endangered species (USFWS 2000). The largest 

population occurs in the Green River sub-basin, and includes fish inhabiting the mainstem 

Green River and two primary tributaries, the White and Yampa rivers, and also in some 

smaller tributaries such as the Duchesne, Price and San Rafael rivers. Bestgen et al. (2007) 

recently provided abundance estimates from the first mark-recapture-based study (2000–

2003) of the Green River population. Their point estimates ranged from 2,142 to 3,304 

adults, depending on year. A few wild individuals may still persist in the San Juan River, a 

Colorado River tributary that today flows directly into Lake Powell. That population was 

essentially extirpated during the 1990s but has been recently augmented with hatcheryproduced individuals (Ryden 2003). The mainstem Colorado River upstream of Lake Powell 

and upstream of the Green River confluence (Figure 1) hosts the second largest extant 

population. The status of this population is the focus of this report. 

 Estimating abundance of Colorado pikeminnow in the mainstem Colorado River subbasin began in 1991. Results from an initial four-year, mark-recapture, field effort (1991–

1994), were provided by Osmundson and Burnham (1998); these included annual abundance 

estimates and an estimate of annual adult survival rate averaged over the four-year period. A 

second field effort spanned 1998–2000; annual abundance estimates from this work were 

presented by Osmundson (2002). These studies also provided information on other 

important Colorado pikeminnow life history attributes including dispersal patterns 

(Osmundson et al. 1998), mean length-at-age, age-at-first reproduction and sex ratio 

(Osmundson et al. 1997, Osmundson 2006). A third multi-year, mark-recapture field effort 

was conducted during 2003–2005. This report presents these recent results and provides a 

synthesis of results for the entire 1991–2005 period. 
METHODS 

Study Area 

Sampling was conducted throughout those portions of the upper mainstem Colorado 

River currently inhabited by Colorado pikeminnow (Figure 1). Colorado River locations are 

described herein as river kilometers (rk) from the Green River confluence and were 

converted from river miles mapped by Belknap and Belknap (1974) and the Colorado 

Division of Wildlife (CDOW). The study area extended from the Green River confluence (rk 

0.0) upstream to the limit of the fish’s range at Palisade, Colorado, where the Price Stubb 

Dam (rk 303) blocked further upstream fish movement. For this study, the study area was 

partitioned into two major reaches, lower (rk 0–181) and upper (rk 201–303). The 20-rklong Westwater Canyon, separating the two reaches, was not sampled because of logistic 

difficulties and because past studies indicated low Colorado pikeminnow occurrence (Valdez 

et al. 1982). 

The upper reach also included the lowermost 3.5 km of the Gunnison River 

downstream of the Redlands Diversion Dam. In 1996, Colorado pikeminnow gained access 

to the Gunnison River upstream of the dam following the completion of a fish ladder there. 

Hence, upstream dispersal past the diversion, blocked during the first study period (1991–

1994), became possible prior to the start of the second study period (1998). However, all fish 

moving upstream through the ladder were first captured in a fish trap, sorted and identified 

before release. Hence, any Colorado pikeminnow tagged during our study that later moved 

upstream of the fish ladder were accounted for. Upstream movements through the ladder 

occurred primarily in July or August (one in early-September), either after the annual markrecapture sampling was completed or during the last sampling effort of the year. Hence, this 

emigration from the study area did not violate the assumption of geographic closure for the 

within-year population estimates. Some of the individuals that used the ladder were later 

found using it a second or third time or were recaptured downstream of the dam in the lower 

Gunnison River or in the Colorado River, indicating they had passed down over the dam 

sometime after they first ascended it (Burdick 2001). Hence, use of the ladder did not 

necessarily mean an individual fish had been permanently removed from the study area; 

however, those that passed upstream and were never again detected downstream were 

2
 

Figure 1. Map of the upper and lower reaches of the Colorado River study area. Downstream 

boundary of lower reach is river kilometer (rk) 0.0 and upstream boundary is the lower end 

of Westwater Canyon. Downstream boundary of upper reach is the upper end of Westwater 

Canyon and upstream ends are the fish barrier at rk 303 on the Colorado River and the 

Redlands fish ladder (rk 5) on the Gunnison River. Grand Junction is abbreviated G.J. 

assumed removed from the study area for survival estimate purposes. No attempt was made 

during this study to estimate the number of Colorado pikeminnow in the Gunnison River 

upstream of the dam. On the Colorado River mainstem, upstream dispersal remained 

blocked at the Price Stubb Dam (rk 303) during all three study periods. 

3
Fish Capture and Marking

 Capture methods during the 2003–2005 effort essentially followed those of the earlier 

two multi-year efforts, previously described in the aforementioned reports. Those procedures 

common to all three multi-year efforts are again briefly described here followed by a 

description of sampling variations unique to the recent 2003–2005 effort. 

A combination of trammel-netting and electrofishing was used to capture Colorado 

pikeminnow > 250 mm long during mid-April to mid-June. Trammel nets (1.8 m deep with 

a 2.5-cm-bar mesh inner panel and a 25-cm-bar-mesh outer wall) were used to capture fish 

from backwaters throughout the entire study area. Subadults and adults congregate in lowvelocity, backwater habitats during spring when main-channel flow increases from snowmelt 

runoff (Osmundson and Kaeding 1989). Electrofishing was used to capture fish from 

shorelines in reaches where, or at times when, backwaters were few. 

 Fish were actively entrapped in nets by the ‘scare and snare’ method (Osmundson and 

Burnham 1998). One net was placed at the mouth of each backwater and, if the backwater 

was large, additional nets set inside the backwater. The total number of nets set (1–5) 

increased with backwater size. A 4.3-m-long motorized aluminum johnboat used for net 

setting was then driven rapidly between the set nets in an effort to scare the fish toward the 

backwater mouth and thereby become entangled. As soon as the last net was set, those set 

first were checked for fish. When it was obvious that a Colorado pikeminnow hit a net and 

was entangled (tail or head seen above water at the top of a net), it was removed before other 

nets were set or checked for fish. As the nets were checked, ensnared Colorado pikeminnow 

were placed in a live well until all fish were removed from all nets. Fish were anesthetized 

with ms-222 (tricane methanesulfonate), measured for maximum total length (TL: Anderson 

and Gutreuter 1983), weighed with an electronic balance (to the nearest gram) and 

electronically scanned for a passive integrated transponder (PIT) tag. If a PIT tag was not 

found, one was implanted in the body cavity using a hypodermic needle inserted 2–5 mm 

posterior to the base of the left pelvic fin. Fish were released after recovery from the 

anesthetic. 

During the first two multi-year efforts, three sampling passes through the upper study 

reach and two passes through the lower study reach were made each spring, except in the 

4
lower reach in 1991, when only one pass was made. During these years, the first pass 

commenced in mid-April after runoff had begun. During 2003–2005, the number of passes 

varied by year (described below) and sampling began in early April prior to runoff so there 

would be sufficient time for additional passes. The goal was to complete sampling prior to 

the onset of spawning migrations. With each pass, every backwater deep enough to allow 

entry by the boat (> 0.5 m) was netted. When electrofishing was employed, both shorelines 

were sampled in a downstream direction with a 5-m-long, hard-bottomed, electrofishing boat. 

Each boat had one netter stationed on the bow with a long-handled dip net. In reaches 

containing rapids, a 5-m-long, inflatable raft outfitted for electrofishing was used. Each craft 

was equipped with a Coffelt VVP-15 during the first two multi-year efforts; and a SmithRoot GPP on some boats during the 2003–2005 effort. Both units produced pulsed DC. 

During the two earlier multi-year efforts, capture data for portions of some passes 

were supplemented with capture records obtained from unrelated studies conducted by the 

CDOW, U.S. Fish and Wildlife Service (USFWS), and the Utah Division of Wildlife 

Resources (UDWR). During 2003–2005, capture records were supplemented with data from 

other USFWS studies. Data from other studies were collected using similar boatelectrofishing methods. 

Because the variance associated with the annual abundance estimates was high during 

the first two multi-year efforts, one objective for the 2003–2005 effort was to capture and 

recapture more fish per year to reduce variance. The goal was to increase the number of 

passes per year and increase sampling effort per pass. Also, the unequal number of passes in 

the two reaches (three in the upper; two in the lower) during the first two multi-year efforts 

meant that length frequencies had to be calculated for each reach separately (more effort in 

one reach would result in over representation of sizes specific to that reach). Therefore, an 

additional goal for the 2003–2005 effort was to have an equal number of passes in the two 

reaches so that one length-frequency could be developed each year that would reflect the 

whole-river population. 

Limited runoff in the upper Colorado River basin prevented adequate flooding of 

backwaters in the study area during 2003 and 2004; consequently, electrofishing shorelines 

replaced trammel-netting as the primary capture technique during these years. In 2005, 

increased runoff allowed more backwater netting. When conditions allowed netting, each 

5
daily sub-reach was sampled with one netting boat that moved from backwater to backwater 

and one electrofishing boat that sampled habitat on either shoreline, depending on where the 

operator perceived the best habitat to be. To sample the whole study area in a relatively short 

period, two crews worked in the upper reach while two other crews worked in the lower 

reach. Each pass generally took nine days to complete in the upper reach and 11 days in the 

lower reach. 

The number of passes made through the study area varied annually. In 2003, four 

sampling passes were made through both the upper and lower reaches. In 2004, a rapid 

decline in water levels in June resulted in an early initiation of Colorado pikeminnow 

spawning activities and sampling ceased after three passes. Because of a low rate of withinyear recaptures that year, third-pass capture data in the upper reach were supplemented with 

post-spawning July capture data collected during an unrelated study (non-native fish 

removal). In 2005, five passes were completed in the lower reach and four in the upper 

reach. To provide a fifth pass for the upper reach, data were assembled from Colorado 

pikeminnow captures made in July during the non-native fish removal project. 

Analyses 

 Survival rate and abundance estimation. — A capture history matrix was developed 

with each row representing a unique fish (identified by PIT-tag number) captured between 

1991 and 2005; columns represented sequential sampling passes. The length at capture and 

the reach the fish was captured in was entered in each column for each pass in which the fish 

was encountered. Rows were grouped by reach in which the fish was first encountered 

(initial captures in the lower-reach followed by initial upper-reach captures). Thus, the 

completed matrix, with new captures listed in chronological order, indicated not only the 

history of captures of each fish by primary (year) and secondary occasion (within-year pass), 

but also the capture length and reach through time. These data were then used as input to 

Program MARK (White and Burnham 1999). Those individuals that were last detected 

moving upstream of the Redlands Fish Ladder, were designated as ’removed’ and zeros in 

subsequent passes of the capture history matrix were therefore ignored in the likelihood 

calculation so that mortality rate would not be overestimated. 

6
To use length as a covariate, lengths for each captured fish were needed for each year 

of the study. However, because individual fish were not captured in each sampling year, 

their lengths in years when not captured had to be estimated by interpolation or extrapolation. 

To do so, three models were fitted to the measured lengths: von Bertalanffy, logistic, and 

Richards (1959). For fish that were captured more than once within a year, the mean of the 

measured lengths was used for that year. The von Bertalanffy model provided the best fit of 

the three models based on the smallest mean squared error, so it was used to 

interpolate/extrapolate missing lengths. To fit each of the three models, a difference equation 

was assumed, following generally the procedures of White and Brisbin (1980). For the von 

Bertalanffy model: 

= ++ − iii ∞ − + LLLkttL ii)()( 11 , 

where is the length at year i , is the actual year of the observation, k is the von 

Bertalanffy growth coefficient, and is the asymptotic length. To estimate the two 

parameters, the equation was implemented recursively, with 

Li i t

L∞

+1 − tt ii = 1. So, to predict a 

length for 1998 from a length in 1994, for example, the equation was first applied with the 

observed length from 1994 to predict a 1995 length. The predicted 1995 length was then 

used to predict a 1996 length, and this process repeated until the 1998 length was predicted. 

The model was thus used to produce individual covariate values of length for each year. 

Using these lengths, an input file for Program MARK was created. 

 The robust design multi-state data type was fit to the encounter histories with two 

states: lower and upper reaches. Primary occasions were years, and secondary occasions 

within years were passes. Data on two fish, captured in both the lower and upper reaches 

within the same year, had to be changed to be compatible with this model, which assumes no 

transitions (movements between reaches) within a primary occasion. In both cases, the fish 

were captured first in the lower reach, so the second capture (in the upper reach) was 

changed to a lower-reach capture. 

 Annual survival rates (S) were estimated between primary occasions in the robust 

design multi-state model, following Bestgen et al. (2007). Covariates used to predict survival 

included year, reach and fish length. Transition probabilities from lower to upper reach 

7
(ψLU) and upper to lower reach (ψUL) were computed for intervals between primary occasions 

as a function of year, reach, and fish length. 

 Population estimates were generated with the Huggins (1989, 1991) estimator, with p

= c, i.e., initial capture probability each year was assumed equal to the recapture probability 

on subsequent passes with no change in capture probability as a result of capture. The 

Huggins estimator was used because the individual covariate length was a predictor of 

capture probability. To model the temporal variation in p within and between primary 

occasions, we also considered models of p that included flow, water temperature, and number 

of boat days for each pass. Flow was the mean discharge (cfs) of the days during which 

sampling took place, as measured at the USGS gauge at the Utah-Colorado state line. 

Because sampling days for each pass were not always synchronized between reaches, mean 

discharge was calculated separately for each reach. Mean temperatures were calculated the 

same way. Because trammel-netting and electrofishing were not comparable in terms of 

effort, we used boat days as a measure of effort, albeit a rather coarse one. 

 Estimates were constructed by using model averaging with model weights from the 

combined analyses to obtain estimates for four size classes of Colorado pikeminnow: > 250 

mm TL (essentially all sampled fish), 400–449 mm TL (Recovery Goal length criterion used 

to define fish about to recruit; USFWS 2002), > 450 mm TL (Recovery Goal length criterion 

used to define adults; USFWS 2002), and > 500 mm TL (length criterion for adults assuming 

a minimum adult length of 476 mm for most males and 525 mm for most females; 

Osmundson 2006). Confidence intervals for were computed using the lognormal 

transformation of the estimated number of animals never seen ( ), with the number of 

animals seen ( ) added into the confidence interval. The formulae for the lower and 

upper boundaries are LCI = and UCI = , where 

Nˆ

0

ˆ

f

Mt +1

0 1 / ˆ + MCf t + 0 1 ˆ +× MCf t+

⎪

⎭

⎪

⎬

⎫

⎪

⎩

⎪

⎨

⎧

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +×

2

0

ˆ

) ˆ ( 1log96.1exp fNSE C e . 

Annual abundance was calculated for each of the two reaches, and these estimates were 

summed to provide annual population estimates for the entire study area. However, no 

8
summed estimate is provided for the first year, 1991, because no lower-reach estimate was 

available for that year (only one sampling pass was made). Variance around these summed 

estimates was calculated by the delta method (Seber 1982) with covariances included in the 

estimate. Coefficient of Variation (CV: 100 x SE/ ) was also calculated and used as a 

measure of estimate precision. An accepted precision standard is a CV of 20% or less 

(Pollock et al. 1990). 

Nˆ

 To evaluate whether the population increased or decreased during the study period we 

used the overlap or non-overlap of 95% confidence intervals as evidence of statistically 

significant differences among annual, combined-reach, abundance estimates (Schenker and 

Gentleman 2001). We also used the variance components module of MARK to fit a trend 

line to the estimates, a technique that takes the variance around the point estimates into 

account. In addition, shrinkage estimation was used to produce a set of shrunk estimates, 

estimates that have the sampling variance of the maximum likelihood estimates removed to 

produce estimates closer to the trend line. A Wald chi-square statistic was then used to test 

for a slope different than zero. 

 Recovery Goal criteria for downlisting Colorado pikeminnow include the requirement 

that mean annual recruitment to the adult population balances or exceeds the number or rate 

of adult annual mortality (USFWS 2002); i.e., that the population be self-sustaining. To 

make this evaluation, length criteria were set forth in the Recovery Goal document defining 

adults as all individuals > 450 mm TL and subadults about to recruit as all individuals 400–

449 mm TL. Toward this end, we attempted to ascertain the frequency and magnitude of 

annual net gains and losses of individuals > 450 mm TL by estimating annual abundance of 

individuals 400–449 mm TL and subtracting the estimated number of deaths ) of fish ˆ 1( ˆ − SN

> 450 mm TL (based on the survival rate estimated for the applicable three-year period). 

 Recovery Goal criteria also require that for downlisting to occur, populations must 

not significantly decline during the five-year monitoring period (USFWS 2002). The 

monitoring period is set to begin the year in which the Green River and Colorado River 

populations equal or exceed the adult population size targets. A simple method of evaluating 

decline is to compute a z-statistic from two estimates ( and ) and their associated 

standard errors [ and ]: 

1 Nˆ 2 Nˆ

) ˆ (SE N1 ) ˆ (SE N2

9
) ˆ (SE) ˆ (SE

ˆˆ

1 2

21

NN

NN

z

+

− = . 

Although the CV will vary from year to year, some idea of the magnitude of the decline 

required for it to be significant can be obtained by computing the z-test with various artificial 

point estimates and associated CV levels and then observe how the P values vary. For 

simplicity and illustrative purposes, the CV used was the same for both abundance estimates, 

and the SE of each estimate was determined by and the CV. Using CVs typical of those 

encountered during the study period we calculated the percent decline required for the 

decline to be significant at α = 0.05 and α = 0.10. For these exercises we set to 1,000 

individuals and to various values less than a 1000. 

Nˆ

1 Nˆ

2 Nˆ

 Electrofishing catch per effort. — Annual relative abundance of Colorado 

pikeminnow, as measured by catch rate (mean number captured per hour of electrofishing), 

was monitored from 1986 to 2000 by CDOW and UDWR as part of the Interagency 

Standardized Monitoring Program (ISMP). Because this program was discontinued, 

electrofishing catch rates during 2003–2005 (this study) were computed to extend the longterm catch rate results and to see whether ISMP sampling reaches provided a good 

representation of river-wide catch rates. Means were calculated for each pass by reach and 

for the entire study area (reaches combined), and for all passes combined by reach and for the 

entire study area. Separate means were also calculated for just those sub-reaches regularly 

sampled during ISMP (see McAda 2002). Calculations of catch rates for 2003–2005 

included all Colorado pikeminnow large enough to be PIT tagged (> 200 mm TL). However, 

calculations of mean catch rates used for comparison with earlier ISMP results included all 

sizes of fish captured, including those < 200 mm TL, because these sizes were included in the 

catch rates reported by McAda (2002). 

Length frequency. — For this study, aspects of the length data were best viewed when 

fish were partitioned by capture reach. This was because: 1) Colorado pikeminnow ageclasses (and therefore length-classes) were distributed throughout the study area differently 

(i.e., older and larger individuals occur predominately in upstream reaches; younger 

individuals, in downstream reaches), and 2) sampling effort was unequal during the first two 

multi-year efforts (i.e., three passes in the upper reach, two in the lower) and fish from the 

10
lower reach would therefore be under represented in a pooled sample. Hence, for these 

years, length frequencies from each reach are presented separately. For 2003–2005, there 

was an equal number of passes in the two reaches each year; length frequencies for these 

years were therefore displayed both separately by reach and as a combined-reach data set. 

Relative year-class strength. — We assessed the relative strength of Colorado 

pikeminnow year-classes at age-5 by the appearance and relative abundance in the lower 

reach of individuals with lengths corresponding to this age (see Appendix Table I and 

Osmundson et al. 1997). As Colorado pikeminnow grow beyond age-5 it becomes 

progressively more difficult to assign age to an individual based on its length, and capturing 

individuals younger than age-5 appears limited by the gear types used for this study. Even 

using relative abundance of age-5 fish as an index to year-class strength had its problems. 

The incomplete record from the 2–3 year gap between multi-year sampling efforts made 

some year classes difficult to assess. No scale aging was done after 1992, and year classes 

could only be assigned based on length. Among-year variation in growing conditions (water 

temperatures, food availability) may result in fish lengths that don’t match the predicted 

average for a particular age-class. Also, the overlap in lengths among adjacent year classes 

can make it difficult to assign a year of origin to a group of fish of a given length range (i.e., 

the pulse of fish in 1991 appeared to be from one year-class but scale analysis indicated it 

consisted of three). Hence, judging year-class strength from length-frequency histograms is a 

less-than-exact science and the qualitative results that we present should be viewed as best 

estimates only. 

Temporal variation in median length. — Tracking average length of adults in the 

upper reach provided another means of evaluating the relative strength of recruitment 

through time. We used the median length as an index for tracking changes in average 

Colorado pikeminnow size because the mean can be unduly influenced by the capture of a 

few large fish. 

Body condition. — Relative condition was calculated for each Colorado pikeminnow 

for which there were length and accurate weight measurements (those weighed with an 

electronic balance). Relative condition accounts for allometric growth and therefore allows 

condition comparisons among size-classes (Le Cren 1951). Relative condition (Kn) is the 

observed mass (Mo) of a given fish divided by the expected mass for a fish of its length:

11
100 0 ×=

e

n M

M K

 

The expected mass (Me) is calculated using constants derived from mass-length regressions: 

log10Me = ((log10 length) slope) + y-intercept 

The constants for these month-specific mass-length regressions were previously derived from 

Colorado pikeminnow captured from the Colorado River during 1991–1994 and provided in 

Osmundson et al. (1998). Relative condition of each individual was calculated using the 

constants specific to the month during which the fish was captured. Mean Kn was then 

compared between upper and lower reaches within 100-mm length-classes and among 

length-classes within reaches. To simplify monitoring relative body condition through time, 

the mean Kn of one length class (500–599 mm TL) was used as an index for making amongyear comparisons. This length-class was well suited for this because it occurred in both 

reaches in all years, sample sizes were relatively large, and because mean Kn of these fish 

significantly differed in the two reaches in all three sampling periods. 

To examine whether reduced body condition might be related to increases in 

population abundance (i.e., a density-dependent response), we regressed our annual upperreach abundance point estimates of fish > 450 mm TL (independent variable) against the 

annual, upper-reach mean Kn of fish 500–599 mm TL (dependent variable). Regressing 

against the annual mean Kn of all fish > 450 mm TL was not done because Kn varied with 

length and any annual changes in mean Kn could simply be related to differences in 

proportions of various length-classes making up the annual sample. Hence, by keeping 

length constant (using just 500–599 mm long fish) abundance effects on mean Kn were better 

isolated. 

Inter-system movements. — Movement of marked Colorado pikeminnow between the 

Colorado and Green River systems was enumerated through visual inspection of the 

Colorado pikeminnow PIT-tag database maintained by the Upper Basin Recovery Program. 

An inter-system movement was identified when an individual fish captured in one system 

was later recaptured in the other system. 

12
RESULTS 

Fish Captures 

 There were 1,258 unique Colorado pikeminnow captured during the overall study 

period (i.e., all three multi-year sampling periods combined). Twenty-five passes were 

completed in the lower reach and 33 in the upper reach. Numbers of captures per pass per 

reach ranged from 11 to 73 (Table 1). There were only four fish captured in the first year 

that were recaptured in the last year of study (14 years apart), and these provided some 

interesting long-term growth information: two first captured in the lower reach and later 

recaptured in the upper reach, grew from 401 to 763 mm and 461 to 793 mm TL; two others, 

captured in the upper reach on both occasions, grew from 590 to 835 mm and 612 to 705 mm 

TL. Other growth rate information has been previously reported and is not summarized here 

(see Osmundson 2006). 

Table 1. Total number of Colorado pikeminnow > 250 mm TL captured in each sampling 

pass and year in the Colorado River study area, Colorado and Utah, 1991-2005. Totals 

include recaptures of the same fish caught in previous passes of the same year (parentheses). 

Captures are partitioned by upper and lower reach (see text) because abundance estimates 

were reach-specific. 

________________________________________________________________________ 

 Lower reach passes Upper reach passes 

Year 1 2 3 4 5 1 2 3 4 5 

1991 37 - - - - 23 17 (4) 25 (2) - - 

1992 18 15 (1) - - - 21 24 (2) 23 (2) - - 

1993 51 41 (4) - - - 31 31 (6) 33 (11) - - 

1994 47 22 (3) - - - 28 37 (3) 38 (6) - - 

1998 31 56 (6) - - - 47 73 (8) 55 (16) - - 

1999 38 24 (2) - - - 52 65 (8) 54 (18) - - 

2000 31 19 (1) - - - 51 52 (8) 28 (6) - - 

2003 11 16 (0) 44 (0) 40 (2) - 11 15 (0) 16 (2) 11 (1) - 

2004 28 36 (1) 27 (0) - - 19 16 (2) 48 (8) - - 

2005 26 50 (3) 46 (7) 36 (6) 34 (5) 22 30 (4) 26 (4) 46 (5) 38 (9) 

13
14

LU UL ψ

Model Selection 

 Parameters for the von Bertalanffy growth curve were estimated, with an asymptotic 

size of 865 mm TL (Table 2). These values were used to predict fish lengths for unobserved 

fish in the robust-design multi-state model. Model selection results for the robust-design 

multi-state analysis are shown in Table 3. 

The minimum AICc (Akaike’s Information Criterion; Akaike 1973) model 

{S(reach+length^2) ψ(reach*t+reach*length) p(reach*primary*t+length^2) = c DM} 

included a reach effect on survival (S) and fish total length (TL) as a quadratic model (β0 + 

β1TL + , β2TL2) but no time effect. Hence, survival is assumed constant across years, but is a 

function of fish total length that is identical for both reaches. The reach + time-specific 

model with length (i.e., year-specific survival, model {S(reach+t+length^2) 

ψ(reach*t+length) p(reach*primary*t+length) = c DM}) was not supported compared to the 

reach-only model with length, with ΔAICc = 14.14 units greater (Table 3). For the minimum 

AICc model, transitions ψ and between the two reaches (movements from lower to 

upper reach and from upper to lower reach) were time- and length-specific, and when models 

with constant transitions were considered, all were completely unsupported with ΔAICc > 10. 

Initial capture probabilities (p) were reach- and time-specific for both primary and secondary 

occasions. All competitive models for p included linear or quadratic models of length. 

Recapture probabilities within years were assumed identical to initial capture probabilities in 

all models considered, i.e., p = c. None of the models of p that included flow, water 

temperature, and number of boat days for each pass explained the temporal variation in p

within and between primary occasions estimated with the minimum AICc model (Table 3). 

No other models of p were considered because of the differences in p within and between 

primary sessions. 

 

Table 2. Estimates for the von Bertalanffy growth curve for Colorado pikeminnow in the 

Colorado River study area, 1991-2005. 

Parameter Estimate Standard error 95% CI (lower) 95% CI (upper) 

K 0.0666 0.00478 0.0572 0.076 

L∞ 864.6 19.0188 827.2 901.9 
Table 3. Model selection results of the robust design multi-state model for Colorado pikeminnow in the upper Colorad

 (reach), transition or movement rates between reaches (ψ), probability of capture (p), and fish total length (length). O in these models include primary and secondary occasion time effects, river flow (CFS), water temperature, and numb of effort to predict capture probability. Parameters modeled with length^2 include both a linear and quadratic term fo probabilities (c) were assumed equal to initial capture probabilities (p). The third and fourth ranked models denoted  models that were developed in response to the transition analysis (see section on transitions). _______________________________________________________________________________________________

Delta AICc M

Model AICc AICc Weights Lik

{S(reach+length^2) ψ(reach*t+reach*length) p(reach*primary*t+length^2)=c DM} 8306.75 0.00 0.36 {S(reach+length^2) ψ(reach*t+reach*length) p(reach*primary*t+length)=c DM} 8306.88 0.13 0.33 {ADDED {S(reach+length^2) ψ(reach*t+reach*length^2 + quad spline) p(reach*primary*t+length^2)=c DM} 8308.72 1.97 0.13 {ADDED {S(reach+length^2) ψ(reach*t+reach*length linear spline) p(reach*primary*t+length^2)=c DM} 8308.86 2.10 0.12 {S(reach+length^2) ψ(reach*t) p(reach*primary*t+length)=c DM} 8313.56 6.81 0.01 {S(reach+length^2) ψ(reach*t) p(reach*primary*t+length^2)=c DM} 8313.61 6.85 0.01 {S(reach+length^2) ψ(reach*t+reach*length^2) p(reach*primary*t+length)=c DM} 8313.80 7.05 0.01 {S(reach+length^2) ψ(reach*t+length^2) p(reach*primary*t+length)=c DM} 8315.17 8.42 0.01 {S(reach+length^2) ψ(reach*t) p(reach*primary*t)=c DM} 8315.20 8.45 0.01 {S(reach+GE(length,500)) ψ(reach*t+length) p(reach*primary*t+length^2)=c DM} 8316.84 10.09 0.00 {S(reach+3 Periods+GE(length,450)) ψ(reach*t+length) p(reach*primary*t+length^2)=c DM} 8317.92 11.17 0.00 {S(reach*length^2) ψ(reach*t) p(reach*primary*t)=c DM} 8318.24 11.48 0.00 {S(reach+3 Periods+GE(length,500)) ψ(reach*t+length) p(reach*primary*t+length^2)=c DM} 8320.33 13.57 0.00 {S(reach+t+length^2) ψ(reach*t+length) p(reach*primary*t+length)=c DM} 8320.89 14.14 0.00 {S(reach) ψ(reach*t) p(reach*primary*t)=c DM} 8321.21 14.45 0.00 {S(reach+GE(length,500)) ψ(reach*t+length) p(reach*primary*t+length)=c DM} 8322.33 15.58 0.00 {S(reach+length) ψ(reach*t) p(reach*primary*t)=c DM} 8323.33 16.58 0.00 {S(3 Periods+GE(length,450)) ψ(reach*t+length) p(reach*primary*t+length^2)=c DM} 8324.58 17.83 0.00 {S(reach+3 Periods+GE(length,500)) ψ(reach*t+length) p(reach*primary*t+length)=c DM} 8325.72 18.97 0.00 {S(3 Periods+GE(length,500)) ψ(reach*t+length) p(reach*primary*t+length^2)=c DM} 8327.32 20.57 0.00 {S(3 Periods+GE(length,500)) ψ(reach*t+length) p(reach*primary*t+length)=c DM} 8329.94 23.19 0.00 

15
Table 3 continued. 

Delta AICc M

Model AICc AICc Weights Lik

{S(reach*t) ψ(reach*t) p(reach*primary*t)=c logit} 8330.61 23.86 0.00 {S(.)ψ(reach*t) p(reach*primary*t)=c} 8332.94 26.19 0.00 {S(reach) ψ(reach) p(reach*primary*t)=c} 8336.84 30.09 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach*mean CFS)=c DM} 8395.32 88.57 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach*mean CFS+length)=c DM} 8396.43 89.68 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach*median CFS)=c DM} 8405.85 99.10 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach*median CFS+length)=c DM} 8407.08 100.33 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach+U temperature)=c DM} 8413.89 107.13 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach)=c DM} 8415.13 108.38 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach*boat days)=c DM} 8415.90 109.14 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach+length)=c DM} 8416.65 109.90 0.00 {S(reach+length^2) ψ(reach*t+length) p(reach*boat days+length)=c DM} 8417.61 110.85 0.00 

16
Capture Probability 

 Using pass 1 of 2005 as the arbitrary baseline to demonstrate the relationship for all 

passes in all years, capture probabilities increased with fish size up to a length of 750 mm 

and then declined, and capture probabilities were generally higher in the lower reach than in 

the upper reach (Figure 2). In both reaches, the probabilities of capturing the largest 

individuals were greater than the probabilities of capturing individuals < 500 mm TL. When 

the whole study period was considered and fish length held constant (500 mm TL), capture 

probabilities were highly variable between reaches and within and among primary sessions 

(Figure 3). Capture probabilities were especially low in 1992 in the lower reach and in 2003 

and 2004 in both the lower and upper reaches. Capture probability was artificially high in 

pass three in the upper reach in 2004 (see Figure 3) as a result of additional sampling effort in 

July (i.e., capture effort, not considered in calculating capture probability, was substantially 

increased during the third pass). 

250 350 450 550 650 750 850 950

Total length (mm)

0

0.02

0.04

0.06

0.08

C

a

pture

pro

b

a

bility (p-h

at)

Lower reach

Upper reach

Figure 2. Capture probability for pass 1 in 2005 (arbitrarily chosen as an example to 

demonstrate the relationship) as a function of fish length and study reach. 

17
1991

1992

1993

1994

1998

1999

2000

2003

2004

2005

0

0.05

0.1

0.15

0.2

1991

1992

1993

1994

1998

1999

2000

2003

2004

2005

0

0.05

0.1

0.15

0.2

Lower reach

Upper reach

C

a

pture

pro

b

a

bility

Figure 3. Capture probability by pass in the lower (top) and upper (bottom) reaches for a 

500-mm-long fish. All capture probabilities included the quadratic length relationship shown 

in Figure 2, so that the maximum capture probability would be for fish approximately 750 

mm. 

18
Survival Rate

The effect of reach and a quadratic model of length on survival indicated that annual 

survival rate was consistently highest in the upper reach, and that in both reaches, 

survival rate increased with fish size up to a length of 550 mm, after which survival declined 

with additional length. The relationship was generally dome-shaped in both reaches, with a 

maximum plateau of 80% annual survival for fish 490–620 mm TL in the lower reach and 

89% for fish 450–650 mm TL in the upper reach (Figure 4). Because the effect of length on 

survival was modeled as an identical additive quadratic relationship for both reaches, the 

model forces a constant difference in survival on the logit scale between the two reaches. 

Annual survival rates in the two reaches were most similar for Colorado pikeminnow 550–

600 mm TL (differing by 9%), and were most dissimilar for those about 970 mm TL: 

(differing by 18%). These differences are a result of the constant difference between reaches 

on the logit scale. However, differences for the very large fish were based on extrapolation 

as none were actually captured in the lower reach during the entire study period. These 

estimated survival rates take into account fish moving from one reach to the other 

 

250 350 450 550 650 750 850 950

Total length (mm)

0

0.2

0.4

0.6

0.8

1

A

n

n

u

al s

urviv

al rate (S)

Lower reach

Upper reach

 Figure 4. Annual survival rate (S) of Colorado pikeminnow as a function of fish total length 

and reach based on a model with constant annual survival, 1991–2005. 

19
(transition probabilities), and hence, reflect actual differences in survival to the extent 

possible. 

 For fish > 500 mm TL, annual survival rate was significantly higher in the upper 

reach (88.4%) than in the lower reach (79.8%) when survival was calculated for the entire 

1991–2005 period with no length effect for fish > 500 mm TL (Figure 5). Point estimates 

suggested a decline in annual survival rate over the three multi-year study periods both in the 

lower and upper reaches and when the two reaches were combined; however, differences 

among periods were not statistically significant. When analyzed as one river-wide 

population, annual survival of Colorado pikeminnow > 500 mm long was estimated as 88.2% 

(95% CI = 85–91%) during 1991–1994, 85.9% (95% CI = 81–89%) during 1998–2000, and 

80.4% (95% CI = 66–90%) during 2003–2005. 

 

L-1991-2005

U-1991-2005

L-1991-1994

L-1998-2000

L-2003-2005

U-1991-1994

U-1998-2000

U-2003-2005

U-L-1991-1994

U-L-1998-2000

U-L-2003-2005

Reach and time period

0.4

0.5

0.6

0.7

0.8

0.9

1

A

n

n

u

al s

urviv

al rate

S( )

Figure 5. Annual survival rate (S) of Colorado pikeminnow > 500 mm TL by reach (upper: 

U; lower: L) for the entire period 1991–2005, for the three multi-year periods, and also for 

the two reaches combined (U-L) during each of the multi-year periods. 

20
Population Size 

Annual abundance estimates for the four length groups of Colorado pikeminnow in 

the two reaches, and for the two reaches combined are provided in Appendix tables I–IV. As 

previously noted, no summed estimate is provided for 1991 because no lower-reach estimate 

was available for that year. Abundance of 400–449 mm-long Colorado pikeminnow (those 

about to recruit) is reported in the Population Replacement section below. 

 For Colorado pikeminnow > 250 mm TL, abundance point estimates for the lower 

reach ranged from 402 (1998) to 1,192 (2003) and for the upper reach, from 217 (1991) to 

484 (2005); summed estimates ranged from 772 (1992) to 1,516 (2003). For individuals >

450 mm TL, abundance estimates ranged from 160 (1992) to 492 (1993) in the lower reach 

and 202 (1991) to 477 (2005) in the upper reach; summed estimates ranged from 440 (1992) 

to 889 (2005). For those fish > 500 mm TL, estimates ranged from 75 (1992) to 297 (2003) 

in the lower reach and from 175 (1993) to 399 (2005) in the upper reach; summed estimates 

ranged from 334 (1992) to 656 (2005). 

 Regardless of the length group, summed point estimates were lowest in 1992 and 

highest in 2005. The narrowest confidence intervals were for fish > 500 mm TL. However, 

for all length groups, differences among annual estimates were not statistically significant as 

evidenced by overlapping confidence intervals (Figure 6). Precision of annual estimates was 

generally higher (lower CVs) for the upper reach than for the lower reach (Appendix tables IIII). For all three length groups described above, six of nine annual CVs for the summedreach estimates were < 20% and the most precise estimates were in 2005 (CV range of 9.4-

10.4). 

Slopes of lines fitted to the ‘shrunk’ estimates derived from the variance components 

trend analysis indicated the adult population significantly increased during the 1992–2005 

study period. For fish > 450 mm TL, maximum likelihood population estimates indicated a 

positive trend over time (slope: 12.26/year; SE: 4.12) that significantly differed from zero 

(Wald chi-square: 8.8; P = 0.003), as was the case for fish > 500 mm TL (slope: 10.29/year; 

SE: 3.36; Wald chi-square: 9.4; P = 0.002). Slopes reported are estimated increases of fish 

per year. 

21
 

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

500

1000

1500

2000

2500

CPM > 250 mm TL

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

200

400

600

800

1000

1200

1400

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

200

400

600

800

1000

1200

1400

P

o

p

ulatio

n

a

b

u

n

d

a

n

c

e

CPM > 450 mm TL

CPM > 500 mm TL

Figure 6. Abundance estimates of Colorado pikeminnow of three sizes classes: > 250 mm 

TL; > 450 mm TL; > 500 mm TL in the upper Colorado River study area (reaches 

combined), 1992–2005. Annual population abundance estimates shown were derived by 

summing separate estimates for the lower and upper reaches (see Appendix Tables II, III and 

IV for numbers). 

22
Temporal changes in abundance of all fish (> 250 mm TL) did not appear to be 

synchronized between the two reaches as evidenced by annual changes in point estimates 

(Figure 7). Explanations for why this might be can be found in sections below that describe 

differences in length frequency between the two reaches and movement of fish between the 

reaches. 

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

200

400

600

800

P

o

p

ulatio

n

a

b

u

n

d

a

n

c

e

Upper reach 

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

500

1000

1500

2000

2500

Lower reach

Figure 7. Abundance estimates of Colorado pikeminnow > 250 mm TL in the upper (top) 

and lower (bottom) Colorado River study reaches, 1991–2005 (see Appendix Tables I, II and 

III for numbers). 

23
Population Replacement 

 Abundance estimates of fish 400–449 mm TL (Recovery Goals criterion for fish 

about to recruit) in the two reaches indicated the bulk of these fish were in the lower reach 

(Figure 8). There, annual estimates ranged from 23 to 248; in the upper reach, 1 to 12. 

Because this length group is a fairly small subset of the total population, captures and 

recaptures were limited, resulting in wide confidence intervals around , large standard 

errors, and CVs greater than the recommended 20% (Appendix Table IV). Estimates from 

the upper reach were especially imprecise. 

Nˆ

 Despite this imprecision, the combined-reach abundance estimates along with 

mortality rate estimates provided a means to assess (if only in a general way) whether 

recruitment equaled or exceeded adult mortality. For years 1992–1994, we used an adult 

mortality rate of 12.2%; for years 1998–2000, 14.7%; for years 2003–2005, 16.2%. Results 

indicated a gain of fish > 450 mm TL in six of the nine years studied, ranging from 1 to 143 

additional individuals per year (Figure 9). Annual losses ranged from 35 to 119 individuals 

per year. The estimated net gain for the nine years studied was 332 fish > 450 mm TL. 

Because estimates were not available for 1995–1997, 2001, and 2002, total gain or loss for 

the 14-year period could not be estimated. 

Ability to Detect Change in Abundance 

 Using CVs typical of those encountered during the study, we calculated the percent 

decline that would be required for the decline to be significant at α = 0.05 and α = 0.10. The 

overall mean CV of the nine annual combined-reach estimates for fish > 450 mm TL was 

18.0%. During the last three-year effort (2003–2005) considerable sampling effort was 

expended, including the addition of post-spawning captures from another study in two of the 

three years. This improved the mean CV over that of the first three-year effort, but not the 

second three-year effort: mean CV for the first three years was 25%; for the second three 

years, 14.3%; for the last three years, 15.2%. 

24
 1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

150

300

450

600

750

900

Whole study area

0

150

300

450

600

750

900

0

150

300

450

600

750

900

P

o

p

ulatio

n

a

b

u

n

d

a

n

c

e

Upper reach

Lower reach

Figure 8. Annual abundance estimates of Colorado pikeminnow 400–449 mm TL in the 

lower, upper and combined reaches, 1991–2005. 

25
1992

1993

1994

1998

1999

2000

2003

2004

2005

Net

Year

0

100

200

300

400

-100

-200

N

et g

ain

or lo

s

s (a

d

ults)

 

Figure 9. Annual net gain or loss of Colorado pikeminnow > 450 mm TL in the summedreach population based on the estimated number of fish 400–499 mm TL present each year 

minus the estimated number of deaths of fish > 450 mm TL. 

With a CV of 18%, would have to decline by 35.2% for it to be significant at α = 

0.05, and 28.3% at α = 0.10 (Figure 10 and Appendix Table V). With a CV of 15%, 

would have to decline by 30.1% for it to be significant at α = 0.05, and 24.1% at α = 0.10. 

For our earliest mean CV value of 25%, would have to decline by 46.6% for it to be 

significant at α = 0.05, and 37.7% at α = 0.10. Finally, the smallest CV, 9.4% was in 2005, 

the year we completed five passes in each reach, with the fifth upper-reach pass consisting of 

post-spawning captures. For two annual abundance estimates with this CV, would have 

to decline by 19.8% for it to be significant at α = 0.05, and 15.7% at α = 0.10. 

1 Nˆ

1 Nˆ

1 Nˆ

1 Nˆ

 

26
0 5 10 15 20 25 30

Coefficient of Variation (%)

0

10

20

30

40

50

60

C

h

a

n

g

e in

a

b

u

n

d

a

n

c

e

%( )

P = 0.10

P = 0.05

Figure 10. Relations between coefficient of variation (100 x SE/ ) and percent change in 

population abundance required for a change to be detected at α = 0.05 and at α = 0.10. 

Nˆ

Transition Probabilities 

 As previously reported, most between-reach movements by Colorado pikeminnow in 

the Colorado River have been in an upstream direction, i.e., from the lower to the upper reach 

(Osmundson et al. 1998). Overall, we documented 48 movements from the lower to the 

upper reach and seven movements from the upper to the lower reach. Unless a fish was 

captured in one reach in one year and recaptured in the other reach the following year, the 

year in which the movement was made or the approximate size the fish was when it moved 

could not be identified. However, the probability that a fish would make a between-reach 

movement sometime between primary sampling periods can be estimated with the multi-state 

model and is termed a transition probability (psi, ψ). Because transition probabilities were 

found to vary with fish size, we used a length of 500 mm TL as a standard to make amongyear comparisons. Based on the probabilities calculated, there appeared to be no between27
reach movements in two of the one-year intervals, a net upstream movement in at least five 

of the one-year intervals, and a net downstream movement in at least two of the one-year 

intervals (Table 4). 

 For lower-reach Colorado pikeminnow, there was a fairly high probability of 

movement to the upper reach between 1992 and 1993 (24%) and between 1993 and 1994 

(23%). After 1994, there was a three-year hiatus in sampling and so transition probability 

could only be estimated for the entire period 1994 to 1998. During this interval, there was a 

59% probability of movement to the upper reach. Assuming these movements were spread 

equally over these years, the average annual probability was 20% (Table 4). However, much 

of the movement may have occurred early in the four-year interval because from 1998 to 

1999 probability of movement to the upper reach had dropped to 0% and was low during 

1999 to 2000 (5%). Transition probability was 0% during the non-sampling interval between 

2000 and 2003, and only 6% from 2003 to 2004. Then, from 2004 to 2005, the probability of 

a lower-reach, 500-mm-long fish moving to the upper reach jumped to 30%. 

 For upper-reach Colorado pikeminnow, there was a 0% probability of movement to 

the lower reach in all years from 1991 to 1999. From 1999 to 2000 there was a 16% 

transition probability. During the subsequent non-sampling interval between 2000 and 2003, 

transition probability was 25%, or an annual average of 9%. Again, much of this movement 

may have occurred early in the interval because during the two subsequent annual intervals 

(2003 to 2004 and 2004 to 2005) probabilities of movement to the lower reach were again 

0%. 

 When the top model (minimum AICc) for generating survival and abundance 

estimates was used for assessing the relationship between length and transition probability, 

the resulting relationship was not supported by empirical evidence. The model indicated that 

the smallest Colorado pikeminnow had the greatest probability of moving from the lower to 

the upper reach and this probability declined with increased length. However, length 

frequency histograms of Colorado pikeminnow captured from the upper reach (see length 

frequency section) indicated there were essentially no fish in the upper reach smaller than 

about 400 mm TL. In addition, of 10 cases in which the recapture in the upper reach 

occurred one year after the initial capture in the lower reach, the smallest individual when 

28
Table 4. Annual (1991–2005) transition probabilities for Colorado pikeminnow 500 mm TL 

moving from one study reach to the other as estimated by the top ranked model in Table 3. 

Movement 

Start year End Year From lower to 

upper reach 

From upper 

to lower 

reach 

Net movement 

to upper reach 

1991 1992 0.0000 0.0000 0.0000 

1992 1993 0.2431 0.0000 0.2431 

1993 1994 0.2320 0.0000 0.2320 

1994 1995 0.19901 0.00001 0.19901

1995 1996 0.19901 0.00001 0.19901

1996 1997 0.19901 0.00001 0.19901

1997 1998 0.19901 0.00001 0.19901

1998 1999 0.0000 0.0000 0.0000 

1999 2000 0.0461 0.1580 -0.1119 

2000 2001 0.00002 0.09002 -0.09002

2001 2002 0.00002 0.09002 -0.09002

2002 2003 0.00002 0.09002 -0.09002

2003 2004 0.0563 0.0000 0.0563 

2004 2005 0.3046 0.0000 0.3046 

1

 Average per year calculated from single value for period 1994-1998; no capture data available for these 

individual un-sampled years; annual estimates for these years might be higher or lower than average value 

provided if capture data were available. 

2

 Average per year calculated from single value for period 1998-2000. 

captured in the lower reach (before having moved) was 402 mm TL (Table 5), suggesting 

that few smaller individuals made the lower-to-upper reach transition. Two additional post 

hoc models were developed to provide a more biologically realistic relation between length 

and transition, or at least one more consistent with the empirical data (Figure 11). Both 

linear and quadratic spline models were fitted to the transition probabilities, with cutpoints at 

400 mm for ψLU and 500 mm for ψUL. For ψLU, the additional cutpoint variable is the total 

29
Table 5. Total lengths of Colorado pikeminnow before and after movement from the lower 

reach to the upper reach of the Colorado River study area. Only those fish moving between 

reaches based on capture-recapture in consecutive years are included; rk = river kilometers 

from the confluence; GU = Gunnison River. 

Lower reach capture Upper reach capture 

Fish ID 

number Year 

Location 

(rk) 

Length 

(mm) Year 

Location 

(rk) 

Length 

(mm) 

129 1992 81.5 438 1993 175.2 478 

186 1992 98.9 421 1993 154.3 449 

238 1993 58.2 523 1994 147.1 540 

323 1993 26.5 456 1994 GU-1.1 466 

837 2003 43.1 402 2004 135.5 445 

851 2004 49.9 411 2005 150.7 459 

990 2004 72.7 435 2005 159.6 487 

991 2004 66.4 472 2005 183.0 495 

993 2004 67.4 451 2005 169.8 474 

 1004 2004 39.6 477 2005 162.8 511 

250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000

Total length

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

p

si (lo

w

er ot u

p

p

er)

1992-1993

1993-1994

1994-1998

1999-2000

2003-2004

2004-2005

 

Figure 11. Transition (movement from the lower reach to the upper reach of the Colorado 

River study area) probabilities of Colorado pikeminnow as a function of total length (mm) as 

estimated using a quadratic spline model with cutpoints at 400 mm. Only intervals in which 

transitions occurred are shown. The dotted line indicates probabilities of transition during 

the four-year interval of 1994-1998 when sampling was curtailed for three years; other lines 

(solid) indicate probabilities of transition during one-year intervals. 

30
 length minus 400, but with all negative values taken as zero. For ψUL, the additional 

cutpoint variable is the total length minus 500, with all negative values taken as zero. The 

linear spline model was identical to the minimum AICc model of Table 3, but with these two 

cutpoint variables added. The quadratic spline model included a quadratic model of length 

for both ψLU and ψUL plus the squared values of the two cutpoint variables. Because of the 

small number of fish that made transitions and the four additional parameters in the quadratic 

spline compared to the minimum AICc model, this quadratic spline model {S(reach+length^2) 

psi(reach*t+reach*length^2 + quad spline) p(reach*primary*t+length^2)=c DM} did not improve 

AICc of the current top model and thus was not used for estimating abundance or survival. 

Electrofishing Catch-per-Effort 

 Mean catch rates were relatively low during 2003–2005. There was some variation 

among passes within years, but most differences were not significant (Figures 12–14). The 

mean pass catch rate in both reaches was < 0.5 fish/hr except in 2005 when the upper-reach, 

pass-4, catch rate was 0.8 fish/hr. There was no consistent trend of catch rates either 

increasing or decreasing within years as spring sampling progressed. 

 Mean catch rates for the entire study area (passes and reaches combined) 

progressively increased from 2003 to 2005 (Figures 12–14, bottom panels). The mean catch 

rate in 2005 (0.43 fish/hr) was significantly greater than the mean in 2003 (0.21 fish/hr). 

Mean catch rate in the lower reach (0.33 fish/hr) was almost identical to the mean catch rate 

in the upper reach (0.32 fish/hr) when data from all three years were pooled by reach (not 

shown). 

 Mean catch rates within ISMP river segments were very similar to those within the 

larger study area (Figures 12–14, top, middle, bottom). Only in 2004 did catch rates in the 

ISMP upper-reach segment appear to differ from the entire upper reach (lower catch rates in 

all passes), but differences were not significant (Figure 13, top). When data from the whole 

study area (upper and lower reaches) were pooled, ISMP results closely matched the larger 

data set. 

 As reported by McAda (2002), mean ISMP catch rate (ISMP river segments 

combined) fluctuated around 0.25 fish/hr during 1986–1990 and increased to about 1.0 

31
 

12345

0

0.5

1

1.5

2

2003 Low er reach

all

1 2 3 4 5a

0

0.5

1

1.5

2

2003 Upper reach

Fis

h

c

a

p

ut re

d

H/ r E

L

ll

12345

Pass

0

0.5

1

1.5

2

2003 Whole Study Area

all

Figure 12. Electrofishing catch rates (fish per hour) of Colorado pikeminnow in the upper 

reach (top), lower reach (middle), and whole Colorado River study area (lower) during 

sampling passes of 2003. For each pass, catch rates of entire shorelines (left bar) and catch 

rates of shorelines from ISMP sub-reaches only (right bar) are displayed. Also shown are 

catch rates when captures and effort from all passes were combined (all). 

fish/hr during 1992–1998. It then declined to 0.62 in 1999 and 0.34 fish/hr in 2000 (Figure 

15-bottom). Mean catch rate in five of the seven years during 1992–1998 was significantly 

higher than in all five years of 1986–1990. Mean catch rate in 2000 was significantly lower 

than the mean of 2000. Confidence intervals around the 2003–2005 means were narrower 

than previous ISMP means because the multiple passes provided larger annual sample sizes. 

 

32
 

12345

0

0.5

1

1.5

2

2004 Low er reach

all

1 2 3 4 5a

0

0.5

1

1.5

2

2004 Upper reach

Fis

h

c

a

p

ut re

d

H/ r E

L

ll

12345

Pass

0

0.5

1

1.5

2

2004 Whole Study Area

all

Figure 13. Electrofishing catch rates (fish per hour) of Colorado pikeminnow in the upper 

reach (top), lower reach (middle), and whole Colorado River study area (lower) during 

sampling passes of 2004. Explanation provided in Figure 12 caption. 

When mean annual catch rates of 1986–2005 (river segments combined) were regressed with 

year (no values for 2001 and 2002), no temporal trend was evident (r

2

 = 0.04; P = 0.43), i.e., 

there was no lasting increase in mean catch rates from beginning to end of the period even 

though rates clearly increased during the middle of the period (Figure 16-top). Electrofishing 

catch rates may, however, have been biased low during the 2003-2005 sampling efforts (see 

Discussion); when these mean annual catch rates were removed from the regression analysis 

(Figure 16-bottom) a significant upward trend was indicated (r

2

 = 0.33; P = 0.02). 

33
 

12345

0

0.5

1

1.5

2

2005 Low er reach

all

1 2 3 4 5a

0

0.5

1

1.5

2

2005 Upper reach

Fis

h

c

a

p

ut re

d

H/ r E

L

ll

12345

Pass

0

0.5

1

1.5

2

2005 Whole Study Area

all

Figure 14. Electrofishing catch rates (fish per hour) of Colorado pikeminnow in the upper 

reach (top), lower reach (middle), and whole Colorado River study area (lower) during 

sampling passes of 2005. Explanation provided in Fig. 12 caption. In 2005, catch-per-effort 

was not documented during pass 5 in the upper reach (captures were from an unrelated 

nonnative fish removal program). 

 

34
 

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

0

0.5

1

1.5

2

2.5

ISMP Upper Reach

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

0

0.5

1

1.5

2

2.5

ISMP Lower Reach

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

0

0.5

1

1.5

2

2.5

ISMP River

M

e

an c

a

pture

s/hr E

L

Figure 15. Interagency Standardized Monitoring Program (ISMP) electrofishing mean catch 

rates (fish/hr) of Colorado pikeminnow in the upper (top), lower (middle), and combined 

(bottom) ISMP reaches of the Colorado River study area, 1986–2005. For years 1986–2000, 

one pass was made along both shorelines of each sampling segment within each reach 

(McAda 2002); for years 2003–2005 (this study), only capture rates within the ISMP 

sampling segments are shown, and rates reflect the combined results of 3–5 electrofishing 

passes through each segment per year. Error bars represent the 95% confidence interval about 

the mean. 

35
 

$

$

$

$$$

$

$

$

$

$ $

$ $

$

$

$

$

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

0

0.2

0.4

0.6

0.8

1

1.2

$

$ $

$$$

$

$

$

$

$ $

$ $

$

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year

0

0.2

0.4

0.6

0.8

1

1.2

M

e

a

n

C

P

E (c

a

pture

s

p

er h

o

ur)

Figure 16. Linear regression of electrofishing capture rates (fish/hr) of Colorado 

pikeminnow in the Colorado River study area and year of capture. Top graph includes 1986–

2005 results (see Figure 15-bottom; no results for 2001 and 2002); bottom graph includes 

only 1986-2000 results. 

Length Frequency 

Although abundance estimates and capture rates provide insight into intermediateterm population trends, the high variance associated with these estimates limits 

understanding of short-term population dynamics. Examination of length-frequency 

histograms can be useful in providing additional information including recruitment history. 

Years 1991–1994 & 1998–2000. — In the first year of study, 1991, almost all 

Colorado pikeminnow captured in the lower reach were juveniles 310–400 mm TL, and most 

36
of these were 340–380 mm TL (Figure 17). As reported by Osmundson and Burnham 

(1998), scale analysis indicated this pulse of young fish primarily consisted of three yearclasses, 1985, 1986 and 1987, with the 1986 year-class occurring most frequently in samples. 

These cohorts dominated the lower reach subpopulation during 1991–1994. Length data 

indicated fish younger than the 1985–1987 year-classes were beginning to appear by 1994, 

but there was no obvious spike in abundance of any one length-class that might suggest a 

strong year-class. In 1991, there was a scarcity of fish > 500 mm TL in the lower reach: only 

one was captured (560 mm), or 3% of the lower-reach sample (n = 37). A similarly small 

sample in 1992 (n = 32) also yielded few individuals > 500 mm TL (19% of captures). By 

1993 and 1994, many fish from the 1985–1987 year-classes had reached this size: in 1993, 

40% of the lower reach sub-population was > 500 mm TL; in 1994, 56%. 

During 1998–1999, juvenile-sized fish again comprised much of the lower-reach 

subpopulation, but their length-frequency distributions were not strongly bell-shaped as in 

1991–1994 (Figure 18). However, in 1998, individuals between 330 and 450 mm TL were 

relatively abundant (45% of the subpopulation). Judging from their size, many of these fish 

were likely age-5 (see Osmundson et al. 1998), suggesting that 1993 may have produced a 

relatively strong year class. Individuals > 500 mm TL made up 32% of the subpopulation in 

1998 and 28% in 1999. By 2000, many of the juveniles present in the two previous years had 

grown to > 500 mm TL, and with few new juveniles, these larger fish made up 65% of the 

lower-reach sample. The relatively low number of individuals 250–449 mm TL (ages 4 and 

5) suggests that 1995 and 1996 were weak year classes. 

In the upper reach, fish < 460 mm TL comprised 0–5% of the sampled population 

during 1991–1994 and 1998–2000 (Figures 19 & 20), indicating that few fish in the upper 

reach were reared there and that when individuals moved to the upper reach from the lower 

reach, they evidently did not do so until they were in the mid-400 mm range or longer 

(discussed later). 

 

37
 

250

260270

280

290

300310

320

330

340350

360

370380

390

400

410

420430

440450

460

470

480

490500

510

520

530540

550

560

570580

590

600610

620

630

640

650

660670

680

690

700710

720

730740

750

760

770780

790

800810

820

830

840850

860

870

880

890900

910

920

930940

950

960970

980

990

0

5

10

15

20

25

250

260

270

280

290

300

310

320

330

340

350

360

370

380

390

400

410

420

430

440

450

460

470

480

490

500

510

520

530

540

550

560

570

580

590

600

610

620

630

640

650

660

670

680

690

700

710

720

730

740

750

760

770

780

790

800

810

820

830

840

850

860

870

880

890

900

910

920

930

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Figure 17. Length frequencies of Colorado pikeminnow captured in the lower Colorado 

River study reach, 1991–1994. 

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Figure 18. Length frequencies of Colorado pikeminnow captured in the lower Colorado 

River study reach, 1998–2000. 

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Figure 19. Length frequencies of Colorado pikeminnow captured in the upper Colorado 

River study reach, 1991–1994. 

 

 

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Figure 20. Length frequencies of Colorado pikeminnow captured in the upper Colorado 

River study reach, 1998–2000. 

Temporal variation in median length -- A constant median length from year to year 

would be expected if recruitment and adult mortality were consistent and balanced each year. 

However, as the length-frequency data from the lower reach indicates, Colorado pikeminnow 

recruitment often comes in pulses, with only some years producing strong year-classes. A 

drop in the median length of adults in the upper reach, as observed in the early 1990s (Figure 

21), results from an infusion of young recruits to the adult population, i.e., numbers of small 

adults entering the population are great enough to offset the effect that growth of older adults 

has on the median length. By 1998, the median length had increased and was essentially 

back to where it had been in 1991, suggesting that the infusion of small adults had dropped 

off during the intervening (non-sampled) years of 1995–1997. This increase in median 

length continued through 2000, indicating that fish growth was having a greater effect on the 

41
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

550

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Figure 21. Mean and median lengths of Colorado pikeminnow captured in the upper 

Colorado River study reach, 1991–2005. N = sample size. 

median than did the infusion of young adults. It should be noted that upper-reach abundance 

could theoretically remain constant or even increase during periods of low recruitment in the 

lower reach if adults from the lower reach continued to emigrate upstream at a rate equal to 

or higher than that of upper-reach adult mortality. In fact, upper-reach abundance appeared 

to increase between 1994 and 1998 and then leveled off or declined while the median length 

increased, suggesting that the strong cohorts of the mid-1980s continued to disperse upstream 

during the non-sampled years of 1995–1997 but then upstream dispersal declined. This 

would be consistent with the transition results previously described. The increase in median 

length in the upper reach at this time might be explained in part by the growth of fish prior to 

dispersal to the upper reach. 

 Years 2003–2005. — In 2003 there was a large group of young fish captured from the 

lower reach (310–429 mm TL), much like in 1991 (Figure 22). This group made up 65% of 

the lower-reach subpopulation. Many of these were 370–419 mm long and, based on size, 

were probably age-5 (i.e., the 1998 year class). Or, this group was made up of more than one 

year-class, with overlapping lengths (i.e., 1997, 1998 and 1999 year classes), as was 

evidently the case in 1991. However, no scale aging was done to determine cohort overlap. 

There were very few fish younger than these in the 2003 and 2004 samples. By 2005 many 

of these were > 500 mm TL. Also in 2005, some fish 330–399 mm TL had appeared and, 

based on size, were probably age-5 (2000 year class). Individuals > 500 mm TL made up 

42
 

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Figure 22. Length frequencies of Colorado pikeminnow captured in the lower Colorado 

River study reach, 2003–2005. 

 

29% of the 2003 lower-reach sample and 35% of the 2004 sample. By 2005, many 

individuals of the strong cohort(s) had grown and fish > 500 mm TL made up 53% of the 

sample. Also, there were more large adults captured in the lower reach during 2003–2005 

than in the early 1990s: during 1991–1994, individuals > 650 mm TL made up 0–2% of the 

sample; during 1998–2000, 0–8%; during 2003–2005, 5–12%. 

 In the upper reach, there were few young adults captured in 2003: only 4% of the 

sample was < 550 mm and none were < 500 mm long (Figure 23). Hence, evidence of recent 

recruitment or dispersal to the upper reach was minimal. In 2004, 7% of the sample was < 

550 mm long, and half of these were < 500 mm TL. These may have represented some of 

43
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Figure 23. Length frequencies of Colorado pikeminnow captured in the upper Colorado 

River study reach, 2003–2005. 

the first migrants dispersing upstream from the large group of young fish observed in the 

lower reach in 2003. By 2005, fish < 550 mm TL comprised 25% of the upper reach sample. 

 The mean and median lengths of upper-reach fish in 2003 (mean: 664; median: 633) 

were higher than in 2000, and by 2004, were considerably higher (mean: 697; median: 693), 

continuing the trend seen in 1998–2000 (Figure 21). Again, individuals of the pulse of fish 

first observed in the early 1990s continued to grow and were now large adults. This, coupled 

with low numbers of young individuals immigrating from the lower reach would account for 

increased average lengths. However, this trend was reversed in 2005 when substantial 

44
numbers of sub-adults and young adults appeared again in the upper reach (presumably the 

1998 year class) causing the mean and median lengths to decline. The percentages of 

relatively large individuals (> 650 mm TL) within the upper-reach adult sub-population were 

similar during the first two sampling periods (1991–1994: 25–35%; 1998–2000: 24–36%), 

but markedly increased during the recent 2003–2005 period (47–66%), further illustrating the 

continued effect that growth by the relatively large year-classes produced in 1985–1987 had 

on the upper-reach adult population. The percentage of adults captured that were very large 

and old (> 800 mm TL) varied substantially among years: 0–14% during 1991–1994, 3–5% 

during 1998–2000, and 8–10% during 2003–2005. Curiously, the greatest variability 

occurred in consecutive years: 14% in 1991, 0% in 1992, and 10% in 1993. 

 The effect of the new strong cohort(s) on the overall population was most evident 

when lengths of all fish captured from both reaches were pooled (Figure 24). In 2003, fish 

310–439 mm TL were 44% of fish captured. In 2004, fish presumably from the same 

cohort(s), then 360–499 mm TL, made up 39% of fish captured. By 2005, the length range 

of this group had become more difficult to distinguish, but appeared to be 410–549 mm TL 

(see Figure 24). Fish within this length range made up 41% of all captured fish. 

Relative Year-class Strength 

 The appearance and annual relative abundance of age-5 Colorado pikeminnow in the 

lower reach indicated there may have only been two strong year-classes with origins between 

1985 and 2000. As noted above, capture rates and length frequencies provided evidence of a 

strong pulse of young fish in the lower reach during 1991–1992. Scale aging indicated there 

was an especially strong year-class produced in 1986, but the year classes of 1985 and 1987 

also contributed to this group. The 1988 year class, however, appeared to be very weak 

based on the relative rarity of age-5 fish in 1993. By 1994, young fish again appeared in the 

lower reach whose lengths suggested ages 5 and 4. These were not particularly abundant 

suggesting 1989 and 1990 year-classes were relatively weak, but nonetheless stronger than 

that of 1988 (Table 6). 

 After a three-year hiatus, it was difficult to identify year-classes from the 1998, 

lower-reach, length-frequency histogram. There was, however, a distinct group of fish with 

45
 

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Figure 24. Length frequency of Colorado pikeminnow captured in the lower and upper 

Colorado River study reaches (data combined), 2003–2005. 

lengths (520–609 mm TL) consistent with what we might expect from fish hatched from 

1985 to 1987 (age-11 through age-13). If so, this group would represent the remainder of the 

large pulse of fish first observed in 1991. For fish younger than this, there was a small gap 

that likely reflected the very weak year-class of 1988. After this gap there was a continuous 

block of fish ranging in length from 330 to 510 mm TL. There were no major spikes or gaps 

46
Table 6. Qualitative estimates of Colorado pikeminnow year-class strength based on lengthfrequency histograms from the Colorado River lower-reach study area, 1991–1994, 1998–

2000, and 2003–2005. 

________________________________________________________________________ 

Year Low Moderate Strong Year Low Moderate Strong 

1985 x 1993 x 

1986 x 1994 x 

1987 x 1995 x 

1988 x 1996 x 

1989 x 1997 x 

1990 x 1998 x 

1991 x 1999 x 

1992 x 2000 x 

________________________________________________________________________ 

within this group suggesting a series of weak-to-moderately strong year-classes from perhaps 

1989 through 1993. The 1999 histogram indicated a higher abundance of fish with ages 

estimated at 6–8 years, or the 1991, 1992 and 1993 year classes, and lends some support to 

the above observation that 1989 and 1990 may have produced relatively weak year-classes. 

There were also a few fish captured in 1999 that appeared to be age 4 and/or age 5; in either 

case, their low numbers in the captured sample suggests that 1994 was also a relatively weak 

year-class. In 2000, the almost complete absence of fish captured with lengths indicating 

ages 4 and 5 suggests that 1995 and 1996 were very weak year-classes. After 2000, there 

was a two-year hiatus in sampling. 

 When sampling recommenced in 2003, a large pulse of what appeared to be age-5 

fish was evident in the lower reach. The smaller of these may have been age-4. However, 

we assumed most to be age-5 and their abundance suggested that 1998 was a strong yearclass. There was a very low number of fish 436–509 mm long, suggesting that ages 6-, 7- 

and 8-year fish represented weak year-classes (1997, 1996 and 1995). The 2004 histogram 

indicated a much smaller number of age-5 fish; hence, the 1999 year-class was of low to 

moderate strength. Finally, in 2005 there appeared a new, distinct length-class that 

corresponded to sizes expected of age-5 fish, i.e., the 2000 year class. Their relative 

abundance in the sample suggested a year class of moderate strength. 

47
Year-class Strength at Age-5 in Relation to Strength at Age-0 

 A fish year-class, strong at age-5, or when later recruiting to the adult population, is 

so because young were produced in high numbers in the year of origin, or because survival 

during the juvenile phase was especially high, or perhaps for both reasons. Understanding 

how environmental factors affect production of young and survival of juveniles, and 

ultimately recruitment level, is prerequisite to devising management strategies aimed at 

increasing the size of small populations. As a first step, it would be useful to know at what 

stage a cohort exhibits an increase or decrease in relative abundance. Annual sampling of 

drifting larvae during the reproductive period of Colorado pikeminnow might allow 

identification of years of high production of young; however, in the mainstem Colorado 

River, such sampling has been limited. Seining age-0 fish from backwaters during fall 

(September-October), when young are 2–4 months old, was systematically conducted 

annually for 19 years (1982–2000) as part of the ISMP. Catch rates from this fall young-ofthe-year (YOY) dataset provide the only available index of age-0 relative abundance against 

which the strength of an age-5 cohort can later be compared. 

 The strong pulse of juveniles 310–410 mm TL (ages 4–6 years) first noted in the 

lower reach in 1991 corresponded to year-classes that previously exhibited relatively high 

abundance as larvae or as fall YOY. The catch rate of Colorado pikeminnow larvae was 

highest in 1986 during a 1986–1994 annual dip-net sampling program of upper-reach 

shorelines and backwaters. Upper-reach, seining catch rates of fall YOY in backwaters 

during the same period were also highest in 1986, followed by 1987 (Osmundson and 

Burnham 1998). Perhaps more relevant are the results of the river-wide ISMP fall sampling 

of YOY mentioned above, as summarized by McAda et al. (1994) and McAda (unpublished 

data). These results indicate that both 1986 and 1985 were years of relatively high catch 

rates of 2–4-month-old Colorado pikeminnow (Figure 25). 

 This linkage between high catch rates of young fish in their first year and high catch 

rates of juveniles 4–6 years later suggests that factors that limit numbers of age-5 juveniles 

may occur in the first year of life, and that relative abundance of age-0 fish may be a good 

predictor of later abundance of the same year-class at age-5 or when later recruiting to the 

48
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1995

1996

1997

1998

1999

Year

0

0.5

1

1.5

2

2.5

3

Fis

h 1/ 0

m2 s

e

ni e

d

Figure 25. Interagency Standardized Monitoring Program (ISMP) catch-per-unit-effort of 

young-of-the-year Colorado pikeminnow in the Colorado River, 1982–1999 (Data from 

McAda et al. 1994 and C. McAda, unpublished data). 

adult population. However, more recent data presented here do not support this earlier 

observation. The results from ISMP indicate that after 1985–1986, the next year with 

exceptionally high catch rates of fall YOY was 1996. Based on the annual length-frequency 

histograms of juveniles and adults captured during this study, the 1996 year-class did not 

later result in high numbers of age-5 juveniles. In fact, 1996 was identified above (previous 

section) as a year-class with one of the lowest levels of abundance at age-5 years (Table 6). 

 

Body Condition 

 A pattern of declining body condition with increased fish length in the lower reach 

and increasing body condition with increased fish length in the upper reach was earlier 

reported for fish captured during the 1991–1994 period (Osmundson et al. 1998). 

Additionally, a river-wide decline in Colorado pikeminnow body condition between the 

1991–1994 and 1998–2000 sampling efforts was noted by Osmundson (2002). Here the 

2003–2005 data is examined to see if the aforementioned pattern and earlier river-wide 

decline was still evident. 

49
Differences among length classes. — Mean relative body condition (Kn) of lowerreach fish during 1998–2000 decreased with increased fish length as it had during 1991–

1994. This phenomenon was again evident in 2003-2005: mean Kn differed significantly 

between 300–399 mm and 400–499 mm length-classes, between 400–499 mm and 500–599 

mm length-classes, and between 500–599 mm and 600–699 mm length-classes (Figure 26-

bottom). 

 In the upper reach, body condition increased with fish size in all three sampling 

periods (Figure 26-top). Mean Kn was not always significantly different among lengthclasses but upward trends were generally similar. During 1991–1994 and 2003–2005, mean 

Kn appeared to decline after fish reached 800 mm TL; however, differences between fish 

700–799 mm TL and fish 800–899 long were not significant. 

 Differences among periods. — Mean Kn of almost all 100-mm length-classes in the 

lower reach declined significantly between the first (1991–1994) and second sampling 

periods (1998–2000). However, by the third sampling period (2003–2005) mean Kn had 

increased and was again as high or higher than in the first sampling period (Figure 26-

bottom). Most of the significant differences among periods were for three length-classes: 

300–399 mm, 400–499 mm, and 500–599 mm TL; differences in mean Kn among periods for 

fish 200–299 mm and fish 600–699 mm TL were not significant. A similar pattern was 

observed in the upper reach: for three length-classes (500–599 mm, 600–699 mm, and 700–

799 mm TL), mean Kn significantly declined between the first and second periods, followed 

by a significant increase in mean Kn by the third sampling period (Figure 26-top). However, 

differences in mean Kn among periods for two 100-mm length classes (400–499 mm and 

800–899 mm TL) were small and not significant. 

Differences among years. — The above analyses and accompanying graphs focus on 

differences in Kn among length-classes within sampling periods (within reaches), and also 

within length-classes among sampling periods (within reaches). To simplify monitoring 

relative body condition through time, the mean Kn of one length class 500–599 was used as 

an index for making among-year comparisons. 

50
 

65

70

75

80

85

90

95

100

105

110

115

120

1991-1994

1998-2000

2003-2005

200-299 400-499 600-699 800-899

300-399 500-599 700-799

Upper reach

65

70

75

80

85

90

95

100

105

110

115

120

1991-1994

1998-2000

2003-2005

200-299 400-499 600-699 800-899

300-399 500-599 700-799

Length-class (mm TL)

R

elativ

e

c

o

n

ditio

n

K( n)

Lower reach

Figure 26. Mean relative body condition (Kn) of Colorado pikeminnow in the upper and 

lower reaches of the Colorado River study area during three sampling periods, 1991–1994, 

1998–2000, and 2003–2005. Means are for seven 100-mm length classes. Data from all 

years within each multi-year period were pooled before calculating means. Upper and lower 

bars represent 95% confidence intervals. 

 In the lower reach, mean Kn of fish 500–599 mm TL was similar among years within 

the first sampling period (Figure 27-bottom). However, mean Kn then apparently declined 

sometime during the subsequent three non-sampling years. When fish were again sampled in 

1998, their condition was the lowest of any sampled year. During the next two years, mean 

Kn progressively increased; by 2000 it was significantly higher than in 1998. This upward 

51
 

 

75

80

85

90

95

100

105

110

115

120

125

130

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

75

80

85

90

95

100

105

110

115

120

125

130

R

elativ

e

c

ondition ( Kn)

Upper Reach

Lower Reach

Year

Figure 27. Mean relative body condition (Kn) of Colorado pikeminnow 500–599 mm TL by 

year in the upper and lower reaches of the Colorado River study area. Dashed horizontal lines 

at Kn = 100 represent the average relative body condition for the population calculated from 

all fish captured during 1991-1994. 

trend may have continued through the next two non-sampling years (2001 and 2002) because 

by 2003 mean Kn was at the highest level of any sampling year. However, mean Kn

significantly declined again by 2004. Finally, in 2005, it had again significantly increased 

and was similar to levels observed during 1991–1994. 

 In the upper reach, mean Kn of fish 500–599 mm TL was also similar among the first 

four years of sampling (Figure 27-top). Between 1994 and 1998 it significantly declined and 

remained reduced through 2000. However, when fish were again sampled in 2003, mean Kn

was significantly higher. During 2003 and 2004 mean Kn was at the highest levels observed 

in the 15-yr period. By 2005, it was significantly lower than in 2003 but was similar to 

1992–1994 levels. 

52
There was some consistency in body condition dynamics between the upper and 

lower reaches for the 500–599 mm length-class. For instance, in both reaches mean Kn was 

fairly stable during 1991–1994, it decreased between 1994 and 1998, and it increased 

between 2000 and 2003. By 2005, mean Kn in both sub-reaches had returned to levels very 

similar to those during the first sampling period. Fish of the two reaches did, however, 

exhibit some dissimilarity in the direction of year-to-year changes. For instance, while body 

condition remained fairly stable in the upper reach during 1998–2000, it steadily increased in 

the lower reach. Also, mean Kn significantly decreased from 2003 to 2004 in the lower reach 

while it may have increased (not significantly) in the upper reach. Finally, mean Kn

significantly increased between 2004 and 2005 in the lower reach while it significantly 

decreased in the upper reach. 

Relations with abundance -- No relationship was found (r

2

 = 0.15; P = 0.27) between 

our annual upper-reach abundance point estimates of fish > 450 mm TL and the annual mean 

Kn of fish 500–599 mm TL from the upper reach. Considering the variation in annual 

abundance point estimates, another option was to simply look for a positive or negative slope 

in mean Kn over time given that we earlier demonstrated a significant positive slope in annual 

abundance estimates. There was no slope for either upper-reach fish (r

2

 = 0.06; P = 0.48) or 

lower-reach fish 500–599 m TL (r

2

 = 0.06; P = 0.54), indicating neither an upward or 

downward trend in mean relative body condition over the study period. 

Finally, we compared mean Kn of Colorado pikeminnow in 1992 with those in 2005, 

given that the combined-reach, abundance, point estimates of fish > 500 mm TL increased 

from 342 to 663. Although the confidence intervals in Figure 27 tell the story, a two-sample 

t-test confirmed that mean Kn of fish 500–599 mm TL in the upper reach in 2005 (96.3) was 

not significantly different from the 1992 mean of 97.7 (P = 0.63). 

Comparing annual means of all fish caught (all sizes) in the upper reach also indicated no 

significant difference in body condition between the two years (1992 mean Kn = 100.8; 2005 

mean Kn = 99.6; P = 0.44). In the lower reach, small sample size (n = 4) of fish 500–599 mm 

TL in 1992 precluded using statistical tests to make comparisons; however, for fish 400–499 

mm long, mean Kn in 2005 (99.4) was significantly lower (P = 0.01) than in 1992 (104.3). 

Despite this, it would be difficult to characterize the 2005 fish as being in low or poor 

condition given that mean Kn was still very close to the overall population mean (100.0) 

53
calculated from fish of all lengths from both reaches for years 1991–1994. When mean Kn of 

all fish caught in the lower reach was compared between years, there was also a significant 

reduction (1992 mean Kn = 101.6; 2005 mean Kn = 96.4; P = 0.02). This difference can be 

attributed in part to the much greater proportion of relatively large fish (> 600 mm TL) in the 

2005 sample (18%) compared to the 1992 sample (6%), which typically have low condition 

in the lower reach (see Figure 26). 

Movements Into and Out of the Green River System 

 Although the Colorado River study area was closed to emigration at its upstream end 

(blocked by the Price-Stubb Diversion Dam), and movement up the Gunnison River was 

monitored at the Redlands fish ladder and trap (rk 5), it was open to un-monitored 

immigration and emigration downstream at the Green River confluence. Movement of 

Colorado pikeminnow between the Colorado River system (Colorado and Gunnison rivers) 

and the Green River system (Green, San Rafael, White, Price, Duschene, Little Snake and 

Yampa rivers) has been surmised (Gilpin 1993) but, until now, undocumented. Although the 

two systems are treated as separate populations for management purposes, the level of 

connectivity between the two systems (i.e., inter-system fish movement) is relevant to our 

understanding of demographics and gene flow. The level of exchange of individuals between 

the two groups affects whether the groups function as biologically separate populations. 

 Some insight into the frequency of inter-system movements can be gleaned from 

capture-recapture data collected throughout the upper basin. Although some PIT tags were 

used in 1990, widespread use did not begin until the following year, 1991. Between 1990 

and 2005, there was a total of 10,519 captures reported in the upper-basin (including 

recaptures) of Colorado pikeminnow that were PIT-tagged at the time of capture or 

previously (Table 7). These included 2,839 captures (1,546 different fish) in the Colorado 

River system and 7,680 captures (5,524 different fish) in the Green River system. To discern 

whether a fish made an inter-system movement, at least two captures of a fish must be made. 

By the end of 2005, there was a total of 2,117 PIT-tagged fish in the upper basin database 

with multi-capture histories (two or more captures, excluding those recaptures that occurred 

in the same day): 586 individuals first captured and tagged in the Colorado system and 

54
Table 7. Total number of Colorado pikeminnow captures in upper basin rivers since use of 

PIT tags began, 1990-2005. Values do not represent number of different fish captured, rather 

the number of captures, including recaptures of the same fish. Fish captured more than once 

on the same day are counted as only one capture. PIT tags were used in 1990 in the Colorado 

River but not in other rivers. Captures in other rivers in 1990, without use of PIT tags, are not 

shown. Captures recorded for the Gunnison River include fish above and below the Redlands 

Diversion Dam (rk 53.0). 

_______________________________________________________________________

Year CO1 GU2 GR4 WH5 YA6 DU7 PR8 SR9 LS10 TOTAL

1990 23 0 0 0 0 0 0 0 0 23

1991 118 3 80 22 66 0 0 0 0 289

1992 132 4 123 19 53 0 0 0 0 331

1993 209 11 113 71 42 7 0 0 0 453

1994 208 41 192 34 19 0 0 0 0 494

1995 115 20 429 38 21 0 1 0 3 627

1996 120 16 288 42 42 2 6 0 0 516

1997 132 4 307 59 23 3 11 0 0 539

1998 357 36 479 43 52 3 1 6 0 977

1999 264 15 344 71 61 24 2 0 0 781

2000 194 11 864 317 140 23 0 0 0 1,549

2001 39 3 952 238 236 0 0 0 0 1,468

2002 0 7 505 184 49 0 0 0 0 745

2003 185 7 388 120 63 0 0 0 0 763

2004 192 10 143 0 72 0 0 0 0 417

2005 357 6 155 0 29 0 0 0 0 547

Total 2,645 194 5,362 1,258 968 62 21 6 3 10,519

1

 Colorado River 6 Yampa River 2

 Gunnison River 7 Duchesne River 3

 Dolores River 8 Price River 4

 Green River 9 San Rafael River 5 White River 10 Little Snake River 

1,531 individuals similarly captured in the Green River system. Hence, 38% of the different 

Colorado-River-captured fish were recaptured at least once and 28% of the Green-Rivercaptured fish were recaptured. 

 During 1990–2005, there were 33 documented inter-system movements (Appendix 

Table VI). Seven of these capture-recapture events occurred only one year apart; another 

eight such events were two years apart. Only once did the recapture occur in the same year 

as the preceding capture (one month apart). The greatest elapsed time between captures was 

nine years. Some of the fish may have been sub- or young adults when the movement 

55
occurred, but not all. At least 17 fish were > 500 mm TL when last captured before moving 

to the other river system. Two fish were > 700 mm TL before they moved. 

 Rather than moving short distances into the adjacent river from locations near the 

confluence of the Green and Colorado rivers, many of these fish moved relatively long 

distances. Seven fish moved over 644 km (400 miles) between captures, and one fish moved 

from the Gunnison River to the Green River and then back to the Colorado River for a total 

of 1,588 km (987 miles) in a three-year period. Thirteen of 17 (76%) fish that moved from 

the Colorado system were tagged and last captured in the Colorado River within 113 km (70 

miles) of the confluence, whereas only four of 13 (25%) fish that moved out of the Green 

River were tagged and last captured within 113 km of the confluence. Ten of the 30 different 

fish that made inter-system movements first did so from locations at least 161 km (100 miles) 

upstream of the confluence. 

All fish that moved to the Colorado River system were previously caught in the Green 

River mainstem (not in a tributary). Most (71%) fish that moved from the Colorado River 

system were next captured in the Green River mainstem, but not all; some were next caught 

in tributaries: one in the Duschene River; three in the Yampa River; one in the White River. 

One fish that moved from the Green River was next caught in the Gunnison River; all others 

were caught in the mainstem Colorado River. 

 Of the 2,117 unique, upper-basin Colorado pikeminnow that were captured two or 

more times, 1.42% had made at least one inter-system movement. Seventeen individuals 

captured in the Colorado River system moved to the Green River system and 13 fish captured 

in the Green River system moved to the Colorado River system. Three of these fish moved 

to the other system but later returned; hence, six of the 33 inter-system movements were 

made by three fish. Two of these fish first moved from the Colorado to the Green River 

system and later returned; one first moved from the Green to the Colorado River system and 

later returned. 

 From a numerical standpoint, slightly more fish moved from the Colorado River 

system to the Green River system than those that moved in the opposite direction (17 fish 

moved to the Green versus 13 fish moved to the Colorado); however, on a percentage basis, 

more of the Colorado River population appeared to be made up of immigrants. Of the 1,546 

unique Colorado pikeminnow captured in the Colorado River system, 0.84% had previously 

56
been tagged in the Green River system, and of the 5,524 captured in the Green River system, 

0.31% had previously been tagged in the Colorado River system. The higher percentage of 

immigrants in the Colorado River system than in the Green River system should not be 

surprising given that the number of individuals tagged in the Green River system that could 

move and be detected in the Colorado system was much higher than the number tagged in the 

Colorado River system. Conclusions regarding net movement are difficult to make from 

these data because comparisons do not take into account unknown differences in sampling 

effort in each river system (i.e., relative percentages of each population sampled each year, 

relative percentages of each population that were tagged, relative survival rates of tagged 

fish, etc.). 

A higher percentage of Colorado-River-tagged fish emigrated to the Green River 

system than Green-River-tagged fish emigrated to the Colorado River; however, from a 

numerical standpoint, Colorado River emigration may have been roughly balanced by 

immigration. Of 586 fish initially tagged in the Colorado River system and recaptured at 

least once, 2.9% were recaptured in the Green River system, and of 1,531 fish initially tagged 

in the Green River system and recaptured at least once, 0.9% were recaptured in the 

Colorado River system. At least two of the fish that moved to the Green River and one fish 

that moved to the Colorado River later returned to the river they were tagged in. Taking this 

into account, an estimated 2.6% of Colorado-River-tagged fish emigrated to the Green River 

system compared to an estimated 0.8% of Green-River-tagged fish that emigrated to the 

Colorado River system. Because the long-term recapture rate has been higher in the 

Colorado River system than in the Green River system (38% in the Colorado River system; 

28% in the Green River system), Green-River-tagged fish may have a higher chance of being 

detected in the Colorado River once they have moved there compared to detection of those 

Colorado River fish that moved to the Green River. Hence, the above statistics may 

underestimate the difference in percentages of those multi-captured fish that moved between 

systems. Clearly, of the two populations, a greater percentage of the Colorado River group is 

lost to the neighboring system. The more important question is whether these losses to the 

Colorado River population from emigration were balanced by immigration. Biases 

associated with unequal sampling regimes do not allow strong inferences to be drawn from 

the above numbers. However, based on the 15 fish that we know emigrated (and presumably 

57
did not return) and the 12 fish that we know immigrated (and presumably stayed), we can 

speculate that if there was a net movement out of the Colorado River system, it was probably 

relatively small. 

 

Captures of Stocked Colorado Pikeminnow 

 Some hatchery-reared Colorado pikeminnow stocked in the Gunnison and Colorado 

rivers by the FWS and CDOW in 2003 and 2004 (Table 8) were captured during the 

sampling efforts for this study in both 2004 and 2005. Only 12 fish were stocked in spring 

2003 (April) that could have been captured during spring 2003 sampling; none of these were. 

Others stocked that year were stocked in October and November, after the 2003 spring 

sampling. None of these were later captured in 2004 or 2005. Two hatchery-reared fish 

were captured in 2004; these had been stocked May 18 in 2004 near Rifle, Colorado (RM 

240.7). One had traveled 168 miles downstream (to RM 72.7) in nine days, the other, 184 

miles (296 km) downstream (to RM 57.1) in eight days. Another 22 hatchery-reared 

Colorado pikeminnow were captured from Grand Valley canals in 2004 (Nov. 17–19) during 

unrelated fish salvage efforts when canals were drained in late fall. All had been stocked at 

Rifle: five stocked on May 18; the remainder, on September 15. All were returned to the 

river downstream of the diversions. 

 In 2005, 45 stocked fish were captured in the study area, five of which were captured 

twice. All had been stocked in 2004: 31 from the Gunnison River stocking; nine from the 

May 18 Colorado River stocking. None was from the September 15 stocking, and none was 

from the group salvaged from canals the previous fall. Nine of the 45 fish were captured in 

the Grand Valley and 36 were captured in the lower reach. Five of the Grand-Valleycaptured-fish were from the Gunnison River stocking and all were caught downstream of the 

Gunnison River confluence. Only one stocked fish was caught in the Colorado River 

upstream of the Gunnison River confluence and it had been stocked upstream in the Colorado 

River. Of the 36 fish caught in the lower reach, 23 were caught downstream of Moab, Utah 

(RM 64), including five within 8 km (5 miles) of the Green River confluence. 

 All 45 stocked fish captured in 2005 had been in the river approximately one year and 

their mean length at capture was 280 mm (range: 224–320 mm). The mean growth 

58
Table 8. Colorado pikeminnow stocking information for the Colorado and Gunnison rivers, 

2003 and 2004. Abbreviations: RM = river mile; FWS = U.S. Fish and Wildlife Service; 

CDOW = Colorado Division of Wildlife. 

Stocking 

Date 

Agency River RM 

location 

Number 

stocked 

Mean length 

(mm) 

Length 

range (mm) 

2003

Apr 14 FWS Colorado 167.7 12 120 100–140 

Oct 10 FWS Gunnison 57.1 1,048 242 116–311 

Nov 06 FWS Colorado 216.6 1,001 222 152–350 

Total 2,069

2004

May 18 CDOW Colorado 240.7 1,164 184 134–292 

Jun 01 CDOW Gunnison 57.0 1,200 217 142–270 

Sep 15 CDOW Colorado 240.7 651 204 150–235 

Total 3,015

 

increment while in the river was 64 mm (range: 31–114 mm). Though stocked in 2004, these 

fish were part of the 2002 year-class and were therefore age-3 when captured. Their size 

corresponded to wild fish between age-3 and age-4 years. Colorado pikeminnow of this size 

were rare in the wild population in 2005. Hence, stocked fish dominated what would have 

been the 2001 and 2002 year-classes, based on the length-frequency histogram of captured 

fish (Figure 28). Model Mo of Program CAPTURE, run separately for these fish, provided a 

combined-reach estimate of 190 individuals (95% CI = 100–426) inhabiting the study area 

during the 2005 spring sampling period (164 in the lower reach; 25 in the upper reach), or 

approximately 3.7% of the number stocked. Model Mo + length changed the result only 

slightly: 192 individuals (95% CI = 100–440). These estimates are probably low because of 

individual heterogeneity not accounted for with the Mo model; however, Pledger mixture 

models were not useful because of the low detection probabilities and low number of capture 

occasions. In contrast, wild fish 280 mm TL in the lower reach had an estimated annual 

survival rate of 67% in 2005. The number of stocked fish retained in the Colorado and 

Gunnison rivers upstream of the study area is unknown, but subsequent sampling efforts in 

both areas have turned up none (USFWS unpublished data). Four stocked fish were later 

59
 

220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550

560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 820 830 840 850 860 870 880

890 900 910

0

5

10

15

20

Wild

Stocked

2005

Length-class (10 mm intervals)

Fre

q

u

e

n

c

y (n

u

m

b

er)

Figure 28. Length frequencies of stocked and wild Colorado pikeminnow captured from the 

whole Colorado River study area in 2005. 

captured in 2008 in the Colorado River study area based on an initial scan of recent capture 

records (USFWS unpublished data). 

DISCUSSION 

Abundance and Survival Estimates 

 Abundance estimates generated with the Huggins estimator were generally higher 

than those previously reported and generated from Program CAPTURE (Table 9). The mean 

difference between annual estimates was 14% and can be attributed to length affecting 

capture probability. The Huggins estimator used length as a covariate in the estimation 

process and thus better accounts for individual heterogeneity whereas CAPTURE did not 

incorporate information from length to model individual differences. 

 

60
Table 9. Estimates of Colorado pikeminnow population size in the Colorado River study area 

(combined reaches) for fish > 450 mm long using two estimators: CAPTURE(M0 + Mt) and 

MARK(Huggins). The 1991–1994 and 1998–2000 CAPTURE estimates are from 

Osmundson (2002); the 2003–2005 estimates are from annual reports. The CAPTURE 

estimate for 2000 differs from that presented in the 2002 report (719) because it was later 

found that one recapture had been counted twice. 

________________________________________________________________________ 

Year Population Size 

CAPTURE/M0 + Mt

Population Size 

MARK/Huggins 

Percent change 

1992 382 440 +15.2 

1993 476 705 +48.1 

1994 652 687 +5.4 

1998 583 583 0.0 

1999 512 589 +15.0 

2000 690 773 +12.0 

2003 784 661 –15.7 

2004 481 688 +43.0 

2005 870 889 +2.2 

Mean +14.0 

 

Combined-reach abundance estimates of Colorado pikeminnow > 450 mm TL 

(Recovery Goal adult length criterion) exhibited a positive and significant slope during the 

13-year study period (1992–2005). For individuals > 450 mm TL, the most recent (2005) 

estimate suggests a population size of 889 (CI = 746–1,075). 

 The pattern of capture probability as a function of fish length in the two study reaches 

of the Colorado River system was notably different from that in the Green River system. In 

the Green River study area, distributions of capture probabilities were bell-shaped with 

probabilities being equally low for the smallest and largest fish, and capture probabilities 

were highest for fish 500–600 mm long (Bestgen et al. 2007). In the Colorado River, 

probabilities of capturing the largest individuals were greater than the probabilities of 

capturing individuals less than 500 mm TL and the highest capture probability was for fish 

about 750 mm TL. 

61
 Survival rate as a function of fish length followed a similar pattern to that found in the 

Green River in 2000–2003 in that survival increased with fish length up to a length of about 

600 mm and then declined with larger size. However, there was one notable difference: 

survival rate of fish > 800 mm TL was essentially zero in the Green River system (Bestgen et 

al. 2007) whereas in the Colorado River system, survival rate in the upper reach was 80% for 

fish 850 mm TL and declined to 67% for fish 960 mm long (965 mm was the largest fish 

captured during the study). Survival rate for individuals > 500 mm TL exhibited an apparent 

decline during the overall study period, from 88% in the first period to 80% during the latest 

period, but overlapping confidence intervals prevented us from concluding this decline was 

real. Bestgen et al. (2007) reported survival rate in 2000–2003 as 65% in the Green River 

system but included all sizes of fish in the analysis; hence, our estimates for fish > 500 mm 

TL could not be directly compared. 

 When the estimated number of deaths of fish > 450 mm TL was subtracted from 

abundance estimates of Colorado pikeminnow 400–449 mm TL in the concurrent year, a gain 

was indicated in six of the nine years, with a summed net gain of 332 individuals > 450 mm 

TL. Although these estimates are imprecise, and gains and losses for some years during the 

14-year period could not be estimated, the requirement of the Recovery Goals that mean 

annual recruitment equal or exceed mean annual adult mortality appears to have been met. 

These results also support the notion that the adult population increased in abundance during 

the study period. 

 To meet the Recovery Goal requirement that the adult population not decline 

significantly over a specified monitoring period, we found that the abundance estimate of the 

last year of such a period would have to be 30% less than that of the first year to be 

significantly different at α = 0.05, assuming a mean CV similar to that obtained during the 

most recent three-year sampling period (15%). Hence, for a five-year monitoring period, a 

decline would have to be substantial for it to be statistically significant. With the CV we 

obtained in 2005 (9.4%), abundance would have to decline by 20% to be significant (α = 

0.05). This was our most precise estimate (lowest CV) and required five passes, including 

supplemental post-spawning captures. It probably represents the best sampling effort we can 

hope to achieve. Assuming a CV of 15%, a 30% decline could occur fairly rapidly if there 

62
was a complete lack of recruitment. At a survival rate of 85%, an adult population of 1,000 

could decline to 723 in two years (28% decline) and to 614 in three years (39% decline). 

Transition probability 

 Transition probability estimates are useful in determining whether dispersal to the 

upper reach is a continual, steady process or whether it occurs in pulses. Also, the timing and 

magnitude of movement in both directions helps shed light on within-reach population 

dynamics. 

High upstream transition probabilities noted in the early to mid 1990s and from 2004 

to 2005 are consistent with observations of pulses of young fish detected in the lower reach 

in both 1991 and in 2003. As these fish grew, many moved upstream. An increase in the 

upper-reach abundance point estimate in 1998 compared to that in 1994 is consistent with the 

positive net upstream transition probabilities during that interval (Figure 7 and Table 4). 

Upstream movements of this first pulse of young fish had evidently almost ceased by 1998 

(Psi = 0.0), perhaps indicating the pool of fish inclined to move had become depleted. 

Additionally, the decline in upper-reach point estimates from 1998 through 2004 was 

consistent with the zero, low, and negative net upstream probabilities estimated for those 

years. Finally, a notable increase in upper-reach abundance in 2005 was consistent with the 

high net upstream transition probability (30%) estimated for the 2004–2005 period. 

Although all of these increases and decreases in annual abundance point estimates were not 

statistically significant, they did fit what we might expect given the net transition 

probabilities. 

In addition to perhaps being related to lower-reach population dynamics, the lack of 

upstream movement from 1998 to 1999 and the negative net upstream movement (more fish 

moving downstream than upstream) from 1999 to 2000 (and during the subsequent 1-3 

annual intervals) might also be consistent with the relative body condition results we found 

for upper-reach Colorado pikeminnow during this period. Mean Kn in the upper reach was 

significantly lower in 1998 than it had been when fish were last sampled in 1994, and it 

remained low through at least 2000. Net downstream movements during and after 1999, 

heretofore not observed during the prior eight years, might have been related to individuals 

63
seeking better feeding conditions than they were experiencing in the upper reach at that time. 

The change in direction of net movement in 2003 (no additional downstream movements) 

coincided with a significant improvement in mean Kn. Though speculative, this causal 

explanation of downstream movements deserves consideration and perhaps future hypothesis 

testing. 

Electrofishing Catch per Effort 

 Although ISMP sampling reaches provided a good representation of river-wide catch 

rates, the reliability of electrofishing catch rates as an index to relative abundance of 

Colorado pikeminnow remains unclear. The mark-recapture estimates indicated a clear 

upward trend during the period 1992–2005; yet no such trend was evident from 

electrofishing catch rates during the same period. Population monitoring using catch rates as 

an index to relative abundance assumes that probability of capture is mostly uniform across 

years and rates of capture are therefore expected to be proportional to abundance in a 

consistent manner. In the past, inconsistencies between trends in population estimates and 

trends in catch rates raised doubt about the accuracy and reliability of the mark-recapture 

abundance estimates in some years (Osmundson 2002). However, results here indicate high 

variability in annual capture probability, violating one of the key assumptions of catch-pereffort indices. Because capture probability at time of sampling is estimated in markrecapture studies and is taken into account when calculating abundance, estimates so derived 

should be considered more reliable than catch-rate results for discerning population trends. 

In contrast to our results, Bestgen et al. (2007) found good agreement between catchrate and abundance estimation trends when analyzing Colorado pikeminnow monitoring 

results from the Green River system. Our findings from the Colorado River are not 

conclusive because of two confounding issues that we were unable to resolve. One was that 

the protocol for electrofishing shorelines for this study was somewhat different from that 

used during ISMP and may have negatively biased mean CPE results. In our study, when an 

electrofishing boat was accompanied by a trammel-netting boat on a given day, the 

electrofishing crew was instructed to avoid sampling backwaters so that fish were not scared 

away prior to arrival of the netting boat. In contrast, during ISMP, electrofishers were 

64
allowed to shock backwaters and this may have allowed them to catch more fish. Hence, the 

data sets are not entirely comparable. When the 2003-2005 catch rate data from our study 

were removed from the long-term catch rate data set, a significant upward trend was 

indicated. Additionally, early ISMP electrofishing utilized VVP (variable voltage pulsator) 

units to produce pulsed DC. Recent studies, including ours, have used VVP units on some 

boats and GPP units on others. Difference in capture efficiencies of these two unit types is 

unknown but could potentially bias early-versus- recent catch rate comparisons. 

A second issue was that the probabilities of capture calculated here were based on a 

combination of electrofishing and trammel-netting captures. Hence, the level of temporal 

variability in capture probability for electrofishing alone was not measured and thus could 

have been more or less than that indicated by the combined-gear results. If variability in 

capture probability for electrofishing was in fact low, then violation of this key assumption 

may not have been important. However, investigators in the Green River system, who relied 

almost exclusively on electrofishing for mark-recapture sampling, also found high amongyear variability in capture probability (Bestgen et al. 2007), suggesting this may indeed be a 

serious shortcoming of catch-rate based trend analysis. 

 

Length Frequency 

 

 Length-frequencies of captured Colorado pikeminnow were useful in identifying 

strong and weak year-classes; however, because probability of capture varies with fish size, it 

should be noted that frequencies of various length-classes from capture data do not 

necessarily provide an accurate representation of relative abundance of length-classes in the 

population. This effect poses less of a problem in the upper reach because most fish there are 

of adult size. In the lower reach, length frequencies are more biased because young (smaller) 

fish have a lower probability of capture, resulting in an over- representation of larger fish in a 

sample. To illustrate the overall effect, we applied correction factors to the observed 

frequencies of each length-class in the samples from 2005, with the correction factor based 

on the relation between probability of capture and fish size. Although the basic shapes of the 

histograms were retained, the lower-reach distribution revealed that fish < 480 mm TL were 

under-represented in the sample and fish > 490 mm TL were over-represented (Figure 29). 

65
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Figure 29. Difference in Colorado pikeminnow length frequencies between the 2005 sample 

of fish captured from the upper and lower Colorado River study reaches (grey bars) and the 

estimated actual population when the frequencies of sampled fish were adjusted by lengthspecific probabilities of capture (black bars). The correction factor applied to the number of 

captures in each length class was p*, the probability of capturing a fish one or more times 

during the 5 passes, derived from the beta estimates from the top model such that p* = 1 − (1 

− p1)(1 − p2)(1 − p3)(1 − p4)(1 − p5). Length class labels represent the lower end of a 10-mm 

group, i.e., one labeled 300 includes fish 300–309 mm TL. 

 

The estimated actual frequency of the smallest fish (250–259 mm TL) was twice that found 

in the sample. In the upper reach, fish in the captured sample < 590 mm TL were underrepresented and fish > 600 mm TL were over-represented. However, the purposes for which 

we used length frequencies in this study (identification of strong year-classes), should not be 

affected by these biases. 

66
Relative year-class Strength 

 Our best estimates of year-class strength based on relative abundance of age-5 fish in 

ten annual length-frequency distributions suggested there were only two strong year-classes 

between 1985 and 2000. These two years, along with seven years of moderately strong yearclasses, together fueled a significant increase in the adult population. This was despite seven 

of the 16 years (44%) having weak year-classes. Based on a comparison of 1992 and 2005 

combined-reach, abundance, point estimates, the adult population may have doubled in size 

during the study period, regardless of whether the > 450 mm TL or > 500 mm TL length 

criterion is used for defining adults. 

Although no abundance estimate was available for the lower reach in 1991, the length 

frequency histogram of that year indicates there were very few individuals > 450 mm TL 

present and even fewer individuals > 500 mm TL. A rough estimate of lower-reach adults in 

1991 can be obtained by applying the 1991 percent composition of these length-classes to the 

1992 lower-reach abundance estimate of fish > 250 mm TL. When such an estimate is added 

to the 1991 upper-reach estimate, about 218 adults are indicated for the river-wide, 1991 

population. If true, the number of Colorado pikeminnow > 450 mm TL may have only been 

about 25% of that estimated for 2005; for those > 500 mm TL, only 28%. Hence, Colorado 

pikeminnow adults, numbering around 200 in 1991, exhibited a marked increase in 

abundance during the 14 years of our study despite a 44% rate of weak age-5 year-classes. 

Strong year-classes of 1986 and 1998 were evidently critical contributors to this population 

increase. One implication of this is that year-classes leading up to 1985 must have been 

especially poor for a long period to have allowed the population to drift down to the small 

number observed in 1991. The extreme scarcity of adults in the lower reach in 1991 might 

be explained by a long series of prior weak year-classes in combination with dispersal to the 

upper reach of those few fish that did successfully rear in the lower reach during prior years. 

Year-class Strength and Later Recruitment 

 The apparent loss of the initially strong (based on high fall YOY catch rates) 1996 

cohort was unexpected and therefore deserves a closer look. For reasons previously given, 

67
aging fish based on their length is not an exact science. In fact, scale aging, upon which 

estimates of mean length by age of young fish are based, is not infallible either. For instance, 

if a given fish lays down an annulus during the first year when it is assumed that it did not 

(see Hawkins 1992), a scale-aged fish may in fact be a year younger than estimated. If this 

occurred, the strong year-class identified from scale-aging fish captured in 1991 and 1992 

(see Osmundson et al. 1997), should have been the 1987 year-class instead of the reported 

1986 year-class, and mean lengths by age would have been underestimated. The ramification 

of this would be that a pulse of age-4 fish (the 1996 year-class) with a mean length of 376 

mm instead of 315 mm should have appeared in the length-frequency histogram of 2000. 

However, no new pulse of any size appeared that year. Thus, the absence of the initially 

abundant 1996 year-class cannot be ascribed to a faulty assumption regarding the missing 

first annulus in scales. 

In addition to the apparent disappearance of the initially-strong 1996 year class, the 

large pulse of what we believed to be age-5 Colorado pikeminnow noted in the lower reach 

in 2003 (the first like it since 1991) also exhibited no linkage to the relative abundance of its 

cohort in the estimated year of origin (i.e., catch rate of fall YOY in 1998 was relatively 

low). This lack of linkage would be difficult to explain by length-based aging being off by 

one year because catch rates of YOY of the adjacent two year-classes were also very low, 

i.e., the 1997 catch rate of YOY was lower than in 1998 and the 1999 YOY catch rate was 

the lowest on record (Figure 25). Hence, if the strong pulse observed in 2003 was the 1996 

year-class, as might be expected, these fish would had to have been age-7 rather than the 

estimated age-5, and the estimates of length-at-age used here would be off by two years. 

Another approach to this question would be to start off with the assumption that strong 

cohorts in the year of origin will remain strong cohorts until they recruit to the adult 

population; growth during the intervening years would be evaluated accordingly without any 

reliance on scale aging at all. Doing this, we would interpret the strong pulse of young fish 

first detected in 1991 to consist primarily of 1985 (age-6) and 1986 (age-5) year-classes 

because these were not only equally abundant but also significantly more abundant in the 

years of origin than any year-class from 1982 through 1991 (McAda and Kaeding 1989, 

McAda and Ryel 1999). The majority of the fish captured in the lower reach in 1991 were 

310-400 mm TL and lengths of the two age-classes would have therefore overlapped 

68
considerably. If the more recent pulse of young fish, mostly 340-420 mm TL, first detected 

in 2003 was in fact the 1996 year-class, they would have been age-7. For fish of the 1991 

and 2003 pulses to have such similar lengths, growth of the 1985 and 1986 year-classes 

would had to have been rapid whereas growth of the 1996 year-class would had to have been 

very slow. If this occurred, it must have happened after the initial year of growth. The mean 

of mean lengths of fall YOY from ISMP collections (1982-1996; n = 15 years) was 35.5 mm 

(from McAda and Ryel 1999), and the mean lengths for 1985 and 1986, both 27.9 mm, were 

therefore less than average. Mean length in fall 1996, 39.6 mm, was higher than average. 

After YOY leave backwater habitats at approximately age-1, temperatures of the main 

channel likely become more relevant to juvenile Colorado pikeminnow growth rate. 

Estimated annual thermal units for Colorado pikeminnow growth (ATU; after Osmundson et 

al. 1998) in the main channel summed to 476 thermal units during 1986-1990 and 411 

thermal units during 1987-1990 (see Figure 30). This could have resulted in above-average 

growth, especially for the 1986 year-class. However, the 1996 year-class would have 

1986

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Figure 30. Estimated main-channel annual thermal units (ATU) for Colorado pikeminnow 

growth at rk 83.7 on the Colorado River, downstream of Moab, Utah, 1986–2005. See 

Osmundson (1999) for methodology in determining ATU. Horizontal line marks the 20-year 

mean (92.9 ATU). 

69
experienced a total of 564 thermal units during 1997-2002. Based on fall sizes in the year of 

origin as well as subsequent main-channel thermal conditions, the 1996 cohort at age-7 

should have been notably larger than the 1985 and 1986 cohorts at age-6 and age-5, 

respectively. Hence, even using this approach, it is difficult to link the 2003 pulse of young 

fish to the 1996 year-class. 

Body Condition and Carrying Capacity 

 Recovery goals for Colorado pikeminnow state that downlisting may occur 

(presuming other conditions are met) when the Colorado River population reaches 700 

individuals > 450 mm TL (USFWS 2002), and is based on the idea that that is the maximum 

number of healthy adults the system can currently support, i.e., carrying capacity is reached. 

The idea was originally suggested in a report that outlined the preliminary results of the 1998 

population monitoring wherein it was noted that a significant reduction in mean relative body 

condition coincided with an apparent increase in Colorado pikeminnow abundance (see 

Osmundson 1999). The reasoning behind the suggested link being that Colorado 

pikeminnow numbers may have exceeded the supply of available prey, and as a consequence, 

individuals were losing weight. A spike in use of the Redlands fish ladder at the same time 

(evidence of emigration) lent support to this idea. With more years of abundance and body 

condition results now available, we are in a better position to evaluate the 700-adult-carryingcapacity hypothesis. 

Mean body condition of lower-reach fish displayed some reduction between the first 

and last years of study whereas body condition of upper-reach fish did not. The reduction in 

the lower reach was significant but not large. There, relief exists for Colorado pikeminnow if 

the system becomes crowded, i.e., they can move to the upper reach. Upper-reach fish are 

free to similarly move to the lower reach, though evidence suggests comparatively few have 

done so. However, they can and have moved upstream to the Gunnison River as evidenced 

by Redlands fish ladder trap data. Although there were relatively high numbers of Colorado 

pikeminnow using this ladder in 1997 and 1998 (18 and 21), use was low in 2005 (4), the 

year when estimated abundance was highest. 

70
Osmundson et al. (1998) suggested that scarcity of appropriate-sized prey in the 

lower reach prompts many adults to move upstream and eventually take up residence in the 

upper reach (see Osmundson et al. 2002 for discussion of abiotic factors that influence fish 

assemblage distribution). If the capacity of the lower reach to support adult pikeminnow had 

been reached, we might expect abundance there to have leveled off within some range, with 

further additions of adults simply having moved to the upper reach (or perhaps to the Green 

River) where we would expect abundance to rise until the capacity there was reached. 

However, lines fitted to the annual point estimates of individuals > 500 mm TL in the lower 

and upper reaches essentially parallel one another, suggesting adult abundance increased in 

both reaches at about the same rate (Figure 31). Additionally, we found no relation between 

abundance and relative body condition in either reach. In the upper reach, there was no 

difference in mean body condition between fish in 1992 and those in 2005. Given that 2005 

was the year with the highest abundance estimate in the upper reach, it would be difficult to 

make the case that carrying capacity was reached. Similarly, in the lower reach, body 

condition was highest in 2003, the year with the highest abundance point estimate. Why 

 

'

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Figure 31. Lines fitted to the annual abundance point estimates for Colorado pikeminnow in 

the upper and lower reaches of the Colorado River study area. 

71
body condition in the lower reach was at its lowest point in 1998 and in the upper reach 

during 1998–2000 remains unknown. A temporary river-wide reduction in productivity or 

forage abundance might provide a causal explanation, but we have no supporting evidence 

that this occurred. 

Assumptions 

The robust design multi-state model employed here that produced separate annual 

estimates for the two study reaches, assumes demographic closure within each reach during 

the annual sampling period. This assumption appears to have largely been met, with two 

exceptions worth noting here. One entailed between-reach movements, and the other, 

movements to and from the Green River system. 

Of the 55 documented movements between reaches, two occurred during an annual 

sampling period: one in 2004 and one in 2005. In both cases, the first capture was in the 

lower reach and the second in the upper reach. Also in both cases, the second capture was at 

the Redlands Fish Ladder trap in the Gunnison River after the estimated spawning period. 

As mentioned earlier, third-pass data in 2004 were supplemented with captures made during 

July after the standard April-June sampling was over. Similarly in 2005, fifth-pass data for 

the upper reach consisted entirely of captures made during the smallmouth bass removal 

project in July. Hence, the assumption of closure within reaches appears to have been 

violated only during these two times and these violations occurred only when the standard 

sampling period was extended into or beyond the spawning period. 

The effects on our abundance and survival estimates from Green River Colorado 

pikeminnow having entered our study area, as well our marked individuals having left our 

study area when they moved into the Green River, are difficult to assess because the actual 

number immigrating and emigrating could not be estimated. For annual abundance 

estimation, the effects are probably negligible because immigration and emigration are only 

relevant if they occur during the annual sampling period. Of the 33 inter-system movements 

we documented over a 14-year period, only one was known to have occurred during an 

annual sampling period (Appendix Table VI). Based on the absence of documented 

movements between the upper and lower reaches of the Colorado River study area during 

72
April-June periods, it is reasonable to assume that inter-system movements might similarly 

occur mostly during times of the year other than the April-June period. 

Survival estimates, on the other hand, are assessed over years rather than over months 

and would therefore be affected by movements that occur outside the annual sampling 

period. Because survival is estimated from capture histories of marked fish, such estimates 

would be unaffected by new unmarked fish having entered our study area from the Green 

River. However, the model cannot differentiate between mortality and emigration, so if 

marked Colorado River individuals left the study area during a survival estimation interval, 

the survival estimate would be biased low. There were 15 marked fish that we know 

emigrated from our study area to the Green River over a 14-year period (about one per year), 

so we might assume the resulting bias to our survival estimates was very low; however, we 

do not know the level of non-detection in the Green River. Biologically, the effects on the 

Colorado River population of emigration and mortality are the same. Because both result in 

losses of fish, our inability to tease the sources of loss apart is perhaps not that critical. 

Unlike estimates of survival, which include emigration as part of mortality, our 

estimates of recruitment, based on abundance estimates of individuals 400-450 mm TL, 

would not include immigrants from the Green River fish if the individuals that immigrated 

fell outside of the 400-450 mm length-class. Of the 12 Green River Colorado pikeminnow 

that we know entered and, we assumed, stayed in the Colorado River, at least nine were 

larger than 450 mm TL when they immigrated. We can therefore conclude that our estimates 

of annual additions to the adult population are biased low to some unknown degree. 

Other assumptions inherent in our methods, such as the availability of all individuals 

to capture, minimal loss of marks (PIT tags), similarity of capture and recapture probabilities, 

and others, were well covered by Bestgen et al. (2007). The rationale provided by those 

authors for how such assumptions were met can be applied here because field methodologies 

and many of the analyses used by them and us were similar or identical. 

Inter-system Movement 

Evidence presented here of Colorado pikeminnow movement between the Colorado 

and Green river systems requires cautious interpretation. Although movement between the 

73
systems suggests gene flow between the populations, it does not provide direct evidence of it. 

Some possible scenarios would allow gene flow while others might not. We explore some of 

these scenarios below. 

Because a fish is first captured in a given river, it does not necessarily mean that that 

river is the fish’s place of origin. Additionally, young fish may move down river, either as 

drifting larvae or as swimming YOY, past the confluence and spend their first years in 

Cataract Canyon or even in Lake Powell. As they develop, upstream dispersal will bring 

them back to the confluence of the Green and Colorado rivers. If we assume that these 

young fish have an innate sense to detect their river of origin and have fidelity to it, we might 

also assume that most proceed up their natal river. Even in such a case, there is likely some 

percentage of these young fish that, for whatever reason, proceed up the other river, such as 

Pacific salmon that ‘stray’ and spawn in a river other than their river or stream of origin 

(Quinn et al. 1991). It may be several years before an individual senses something is amiss. 

If these fish have fidelity to their natal river or spawning location when it comes time to 

spawn, as has been suggested (Tyus 1985), these errant fish may not seek their natal river 

until they have matured and undertake their first spawning migration. In such a scenario, an 

individual fish may be caught and tagged in one sub-basin, but finds the natal sub-basin prior 

to spawning for the first time (no gene flow). However, two of the 14 fish displaying interriver movements did so relatively late in life (one was > 615 mm TL, the other > 752 mm), 

suggesting that if this was their first of such movements, they may have been spawning in the 

‘wrong’ river for some time (gene flow). Again assuming strong spawning fidelity to the 

natal river, it is also possible these errant fish are captured in the ‘wrong’ river when they are 

there temporarily to forage, returning to spawn in the ‘correct’ river some time after being 

captured (no gene flow). Another possibility is that there is no fidelity to the river of origin 

and young fish returning upstream move into the Green and Colorado rivers randomly (high 

gene flow). Similarly, some small percentage of older fish might make exploratory 

downstream movements, discover the other river, move up it, and for whatever reason take 

up residence and spawn there (gene flow). 

 Those few sub-adult or adult fish that make substantial movements downstream one 

river and upstream the neighboring river may represent a small segment of the population 

that does not establish a strong fidelity to a home range and can be considered ‘wanderers’, 

74
perhaps foraging or spawning in more than one river during their lives. The most definitive 

evidence of gene flow would be the capture of a given fish on the spawning grounds of one 

river system in one year and on spawning grounds within the other river system in another 

year. So far, this has not occurred, and it may remain unlikely given that sampling is usually 

conducted prior to the spawning period. Even if the vast majority of fish of each population 

possesses a strong affinity for its natal river, such wanderers may provide a small but 

important level of gene flow between the populations. In addition, such wanderers may 

provide an important function over evolutionary time by re-colonizing rivers where 

populations have died out for one reason or another, such that the two rivers function as an 

abbreviated metapopulation. 

 The metapopulation concept probably applies to the groups of Colorado pikeminnow 

in the two sub-basins. For the most part, the fish of each sub-basin are separated in space, 

even though a direct connection for migration is continuously provided at the confluence; 

hence, the populations are not entirely spatially discreet, an assumption of the original 

metapopulation concept (Levins 1969). However, more recent use of the term and concept 

stresses the importance of discreet local breeding populations connected by migration, such 

that recolonization is possible (effecting long-term meta-population dynamics) but that the 

exchange rate of individuals is so low that migration has no real effect on local short-term 

population dynamics (Hanski and Simberloff 1997). The two sub-basin groups appear to fit 

these criteria based on the low exchange rate of PIT-tagged individuals documented here. 

Behavior may keep the respective populations relatively discreet in space even though a 

physical conduit for continuous inter-system migration exists. One such possibility might be 

an innate disposition or affinity for the waters of the natal system. Once a fish enters one of 

the two rivers when young, there might also be a natural disposition toward upstream 

migration that serves to keep the fish in that river. Later establishment of a home foraging 

range, and fidelity to it, might strengthen this effect. 

 An alternative scenario might be that upstream dispersal at the confluence is random. 

If so, mixing of young would likely be significant and the metapopulation concept would 

therefore not apply and the two groups might then be considered one population. Hence, 

application of the metapopulation concept here must entail the assumption that larvae that 

drift downstream of the confluence possess a strong fidelity to the natal system and this 

75
mechanism directs upstream traffic at the confluence, and for the most part, successfully 

returns fish to their respective system. To date, this has not been demonstrated or 

documented in any way. However, if such a mechanism was absent, and upstream migration 

at the confluence was random, years of high reproductive success in one river resulting in 

high numbers of larvae drifting downstream of the confluence, would later benefit both rivers 

when the growing fish returned upstream. Hence, dynamics of the receiving population 

would show some correlation to those of the donor population, i.e., years of high or low 

relative abundance of age-5 fish in the two systems would be synchronized to some extent. 

To date, this has not been the case: demographic dynamics of Colorado pikeminnow in the 

two systems appear to behave independently. For instance, Bestgen et al. (2007) reported a 

complete absence of recruit-sized (400-450 mm TL) Colorado pikeminnow in the Green 

River system in 2003 whereas our point estimate of fish this size in the Colorado River 

system was higher in 2003 than in any other year studied. Hence, there might be a 

mechanism that keeps fish on course to the natal system when they encounter mixing waters 

at the confluence. In summary, even though there is probably enough errant movements into 

the adjoining river to keep the two groups from differentiating genetically over time, the two 

groups are demographically isolated for the most part and the two systems can therefore be 

viewed as functioning as an abbreviated metapopulation. 

At least one other scenario is possible: the number of young fish that return upstream 

to the confluence is so small that even though they enter the two rivers randomly, 

demographics of the receiving population are not materially affected. If such a situation 

exists, it could be a recent phenomenon related to increased mortality of young when they 

enter the predator-rich waters of Lake Powell. Historically, mixing of young downstream of 

the confluence followed by random upstream movements may have allowed more linkage 

between demographics of the two populations and the metapopulation concept may not have 

applied. Although all such scenarios are entirely speculative, it is hoped this discussion 

encourages future hypothesis testing. 

76
Stocked Fish 

 Colorado pikeminnow stocked in the Colorado River had low survival and/or 

retention (4%) during their first year at large. Based on the distribution of captures, it 

appeared that most, if not all, fish dispersed downstream after being stocked, with an 

unknown number entrained in Grand Valley irrigation canals. About half of those caught 

from the river were found downstream of Moab, Utah, suggesting that many of those that did 

not die may have moved out of our study area and perhaps into Lake Powell. However, the 

four that were caught in 2008 indicate some limited long-term survival of this group. One of 

the primary objectives of stocking these fish was to speed recovery in the unpopulated 

reaches upstream of the diversion dams (Nesler 1998, Nesler et al. 2003). This objective was 

evidently not met. None of these Colorado pikeminnow has been found in the upstream 

reaches of the Colorado River where they were stocked despite extensive sampling there by 

electrofishing crews searching for smallmouth bass (B. Burdick personal communication). 

Future monitoring of fish traps at the Grand Valley Diversion Dam on the Colorado River 

and the Redlands Diversion Dam on the Gunnison River will reveal whether any of the 

surviving stocked fish attempt to return to their stocking reach. 

 In light of the fact that the wild population in the Colorado River is steadily 

increasing in abundance through natural reproduction and recruitment and because fish 

ladders have been completed and diversions screened, we see no reason to continue attempts 

to stock hatchery-reared Colorado pikeminnow in upstream reaches. Individuals have 

demonstrated their ability and willingness to ascend fish ladders and colonize upstream 

reaches if suitable habitat exists. Re-establishment objectives might be better served with 

habitat improvement efforts such as predator control, flow management and temperature 

augmentation. 

 

77
SUMMARY AND CONCLUSIONS 

1) The Colorado River population of Colorado pikeminnow increased in abundance since 

the start of the 1992–2005 mark-recapture monitoring program. Annual point 

estimates, with associated wide confidence intervals, tend to vary from year to year 

making it difficult to have confidence in any one number. However, we found a 

significant positive trend in the annual abundance estimates for Colorado pikeminnow 

> 450 mm TL. The combined-reach point estimate for fish > 450 mm TL in 2005 was 

889. 

2) The Huggins estimator from Program MARK produced combined–reach abundance 

point estimates about 14% higher and with narrower confidence intervals than estimates 

produced earlier with models Mo and Mt from Program CAPTURE. The minimum 

AICc model included a reach effect on survival and fish total length as a quadratic 

model, but no time effect. 

3) Capture probability was higher in the lower reach than in the upper reach and varied by 

fish size, with fish 750 mm TL being 3–4 times more susceptible to capture than the 

smallest fish (250 mm TL). 

4) Capture probability was generally lower during the most recent 3-year study period 

than during the two earlier 3-year study periods, despite additional effort. 

5) Survival was higher in the upper reach than in the lower reach and varied with fish size, 

with the highest rates for fish about 450–650 mm TL (about 80% in the lower reach; 

89% in the upper reach). The lowest survival rates were for the very largest (and 

oldest) fish (> 850 mm TL) suggesting that mortality increased with old age. However, 

for fish 850 mm TL, annual survival was still 80% in the upper reach. 

6) Combined-reach, annual, survival rates appeared to decline during the three study 

periods, from 88% (1991–1994) to 86% (1998–2000) to 80% (2003–2005) for fish >

500 mm TL, but differences among periods were not significant. This trend, if real, has 

serious implications for the future viability of the population. However, conclusions 

should not be drawn until future assessments indicate a significant decline in adult 

survival has indeed occurred. 

78
7) Self-sustainability of the population was evidenced not only by a significant positive 

slope in annual adult abundance estimates, but also by annual abundance estimates of 

sub-adults (those 400–449 mm TL) about to recruit that indicated recruitment exceeded 

estimated adult mortality in six of the nine years for which data were available. 

Although a net gain of 332 adults was estimated for the nine years studied, lapses in 

sampling years between multi-year efforts precluded estimating the total net gain for 

the period 1992–2005. Also, precision of estimates for this size group was low. Hence, 

although the direction of results (a net gain) is supportive of the general upward trend 

indicated for adult abundance, little confidence can be placed in the size of the gain 

estimated from this analysis. 

8) Our ability to detect a significant decline in abundance from one point estimate to the 

next is limited by the precision of our estimates. Assuming future adult abundance 

estimates have mean precision similar to that of our estimates during the most recent 

two multi-year sampling periods (CV = 15%), declines of 30% over a specified interval 

would be required to be significant at α = 0.05 and 24% at α = 0.10. 

9) ISMP electrofishing catch rates combined with those from this study, failed to track the 

upward trend in abundance that we detected using mark-recapture techniques. 

However, the comparison between the two techniques was inconclusive because 

electrofishing protocols in recent years deviated from those used in earlier ISMP 

monitoring. Nevertheless, catch rate metrics may be susceptible to biases associated 

with temporal variation in capture probability, a problem not shared by mark-recapture 

abundance estimation techniques. 

10) Results of efforts to link pulses of 300-400-mm-long Colorado pikeminnow to 

individual year-classes suggested that high catch rates of young-of-the-year in 1996 did 

not later result in strong recruitment to the adult population. If true, relative abundance 

of fall YOY may not always be a good predictor of later recruitment. Environmental 

factors that influence survival of juvenile age-classes may be just as important in 

determining recruitment levels as factors that influence levels of larval production and 

survival of early life stages. 

11) Body condition, an indirect indicator of forage availability and perhaps other factors, 

significantly declined between 1994 and 1998 but significantly increased again between 

79
2000 and 2003. Earlier interpretations of lowered body condition signaling that 

carrying capacity had been reached were not supported by additional years of data. We 

found no correlation between levels of abundance and mean body condition. 

12) Some Colorado pikeminnow moved between the Colorado River and Green River 

systems. Thirty-three inter-system movements between 1990 and 2005 were 

documented from capture-recapture records obtained from the Recovery Program’s PIT 

tag database, representing 1.42% of the 2,117 unique Colorado pikeminnow that were 

captured two or more times. If it is assumed that many of these migrants spawn in a 

river other than their river of origin, genetic exchange may be substantial. However, 

net movements in each direction appear relatively balanced and occur at a low enough 

frequency that the demographics of one system have a negligible effect on the 

demographics of the neighboring system. Hence, the interaction of the two populations 

fit those of a metapopulation. 

13) Juvenile Colorado pikeminnow (100–350 mm TL) stocked into reaches upstream of the 

study area in both the Colorado and Gunnison rivers in 2003 and 2004 dispersed 

downstream into the study area and probably beyond. Forty-five fish stocked in 2004 

were found in 2005. A mark-recapture estimate of the total number in the study area at 

that time was 192, or 3.7% of the total number stocked (5,084). Most stocked fish were 

found in the lower reach, some near the confluence with the Green River. No fish from 

the 2003 stocking were found. No stocked fish have since been found in the upstream 

reaches in which they were stocked, but four were found within our study area in 2008, 

indicating some survival and retention within the Colorado River. 

RECOMMENDATIONS 

1) We recommend that mark-recapture studies be continued in the upper Colorado River as 

the primary means of assessing trends of the Colorado pikeminnow population. 

Electrofishing catch-rate indices do not take temporal variation in capture probability into 

account and thereby may provide misleading results. 

80
2) We recommend that the current regimen of sampling in three consecutive years followed 

by two years of no sampling be continued. A reasonable goal is to conduct four sampling 

passes per year. It does not appear possible to fit five complete passes in prior to the 

Colorado pikeminnow spawning period. Also, increasing current levels of effort per pass 

is probably not practical.

3) We recommend that boat launches be installed at the upper and lower ends of the reach 

between Government Highline and Price-Stubb dams. With the Price-Stubb fish ladder 

now in place, the closure assumption will be violated if marked fish move into this reach 

and are not sampled. Currently, there is no boat access that allows sampling of this reach.

4) We recommend that if numeric recovery goals for Colorado pikeminnow in the Colorado 

River are to remain set by carrying capacity of the system, a better assessment is needed 

of what constitutes carrying capacity there. If persistent declines in mean relative body 

condition are to be used as an index, the capacity of the study area, not including habitat 

upstream of the Price-Stubb and Redlands dams, was not reached during this study when 

estimates of individuals > 450 mm TL totaled 889. 

5) We recommend that the consequences of using program CAPTURE models versus more 

complex models within program MARK be evaluated, perhaps by the population 

estimation ad-hoc group. Although Colorado pikeminnow monitoring in the Colorado 

and Green river systems now employ models within MARK, the benefits of applying 

these models to humpback chub monitoring should be assessed.

6) We recommend that mark-recapture abundance estimation be employed for monitoring 

progress of razorback sucker population reestablishment in the Colorado River. 

Razorback sucker capture data is already collected concurrently with Colorado 

pikeminnow sampling and additional field effort would not be required. Funding would 

be required for a biostatistician versed in abundance estimation models and a principal 

investigator to manage the data, work with the biostatistician, and write a final report.

7) We recommend that no additional Colorado pikeminnow be stocked in the upper 

Colorado or Gunnison rivers. 

8) We recommend that a feasibility study be initiated to determine whether a mark-recapture 

population estimate can be developed for the Gunnison River. Past efforts have had 

difficulty catching a sufficient number of individuals over a short period to make such an 

81
estimate practical. Now that the Redlands fish ladder has been in place for 13 years, it 

might be worthwhile to see how many Colorado pikeminnow have taken up residence 

upstream of the diversion. A pilot study, perhaps consisting of one field season, might be 

accomplished in a year of non-sampling in the mainstem Colorado River. 

9) We recommend annual larval production be monitored in the Colorado River using drift 

net sampling as is currently done in the Green River system. Annual YOY monitoring in 

the lower reach has already been reinitiated. Understanding factors affecting larval 

production may aid our understanding of YOY abundance and factors that ultimately 

affect recruitment. 

10) We recommend initiating a study that develops a more reliable means of determining the 

year of origin of individuals 300-400 mm TL. Understanding environmental factors 

responsible for variation in recruitment strength begins with the ability to link recruitsized fish to a particular year-class so that year-to-year abundance and survival of a given 

cohort can be tracked through time. Increasing the frequency of years with strong 

recruitment is the key to recovering this population. Effective management actions cannot 

be developed toward this end without understanding factors that affect recruitment 

strength. 

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and supplement to the Colorado Squawfish Recovery Plan. U. S. Fish and Wildlife 

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85
APPENDIX 

Appendix Table I. Estimated mean length and mean annual growth increments by age for 

Colorado pikeminnow in the Colorado River for ages 0-7. Mean length for Age 0 value is 

from Snyder (1981). Mean length at Age 1 is from measured lengths of fish seined near RM 

54 on June 28, 1989 and assumed to be 1-yr old. Mean lengths of Ages 2-7 are from 

measurements of fish aged using scales. Insufficient captures of Age 2 fish precluded 

estimates of growth increments between ages 1 and 2, and 2 and 3. Growth increments for 

fish between ages 7 and 8 were not calculated because presumptive Age 8 fish could not be 

reliably aged. Table and caption from Osmundson et al. (1997). 

 

Growth Annual growth 

Age Total length (mm) period increment (mm) 

(years) N Mean Range SD (age) Mean SD 

0 8 7.7 7.0-8.5 0.5 0-1 63.5 13.6 

1 73 71.2 50-103 13.6 1-2 

2 1 181.0 2-3 

3 3 232.7 190-259 37.3 3-4 82.0 56.0 

4 6 314.7 267-374 41.8 4-5 61.5 53.4 

5 19 376.2 326-453 33.3 5-6 47.9 45.3 

6 10 424.1 375-472 30.6 6-7 32.2 36.6 

7 7 456.3 430-479 20.0

 

86
Appendix Table II. Abundance estimates ( N ) for all Colorado pikeminnow ˆ > 250 mm TL 

in the lower and upper Colorado River study reaches, and for the reaches combined, with 

lower and upper confidence intervals (CI) and standard error (SE). is the number of 

unique individuals captured. CV is the coefficient of variation (100 x SE/ ). 

Mt+1

Nˆ

_______________________________________________________________________ 

Year Nˆ Lower CI Upper CI SE Mt+1 CV

 Lower reach 

1992 480.1 151.4 1,714.2 340.4 32 70.9

1993 590.1 314.1 1,203.4 213.2 88 36.1

1994 467.6 265.1 876.0 148.5 66 31.8

1998 402.2 233.2 765.2 128.2 86 31.9

1999 416.1 249.0 731.0 118.2 60 28.4

2000 487.9 275.4 901.5 152.9 51 31.3

2003 1,192.2 683.0 2,155.3 360.8 112 30.3

2004 687.1 445.1 1,093.6 161.1 89 23.4

2005 535.8 408.4 730.2 80.6 166 15.0

Upper reach

1991 217.3 122.7 452.5 77.7 59 35.8

1992 292.4 187.3 487.2 73.7 64 25.2

1993 223.7 167.4 315.6 36.9 78 16.5

1994 370.1 273.5 518.7 61.4 94 16.6

1998 425.5 343.1 543.3 50.4 151 11.8

1999 394.8 323.3 495.0 43.3 145 11.0

2000 377.2 297.8 491.4 48.7 117 12.9

2003 324.7 215.6 505.7 72.1 50 22.2

2004 304.8 221.3 435.1 53.5 72 17.5

2005 483.8 376.1 640.6 66.5 140 13.7

combined

1992 772.5 357.5 1,814.5 348.3 96 45.1

1993 813.9 511.0 1,393.4 213.8 166 26.3

1994 837.7 596.1 1,213.1 154.4 160 18.4

1998 827.7 614.3 1,161.8 136.9 237 16.5

1999 810.9 609.7 1,112.2 126.1 205 15.6

2000 865.1 617.2 1,249.8 158.3 168 18.3

2003 1,516.9 967.0 2,442.3 366.3 162 24.1

2004 991.9 721.4 1,392.9 168.6 161 17.0

2005 1,019.7 852.9 1,237.3 97.4 306 9.5

87
Appendix table III. Abundance estimates ( N ) for all Colorado pikeminnow ˆ > 450 mm TL 

in the lower and upper Colorado River study reaches, and for the reaches combined, with 

lower and upper confidence intervals (CI) and standard error (SE). is the number of 

unique individuals captured. CV is the coefficient of variation (100 x SE/ ). 

Mt+1

Nˆ

________________________________________________________________________ 

 

Year Nˆ Lower CI Upper CI SE Mt+1 CV

 Lower reach 

1992 159.8 48.1 617.4 121.7 12 76.1

1993 491.7 261.0 1,008.8 179.1 75 36.4

1994 317.2 179.5 599.0 101.8 48 32.1

1998 173.6 99.8 341.6 57.8 42 33.3

1999 205.7 119.6 376.2 62.5 32 30.4

2000 400.4 225.1 745.5 126.9 44 31.7

2003 336.6 182.1 657.8 115.1 39 34.2

2004 388.2 245.3 638.0 97.1 54 25.0

2005 412.1 313.1 565.8 63.2 134 15.3

Upper reach 

1991 202.3 114.6 421.2 72.2 56 35.7

1992 280.2 179.5 467.2 70.7 62 25.2

1993 212.9 159.2 300.7 35.2 75 16.5

1994 370.2 273.6 518.8 61.4 94 16.6

1998 409.6 330.5 522.7 48.4 147 11.8

1999 383.8 314.4 481.0 42.0 141 11.0

2000 372.7 294.3 485.6 48.2 116 12.9

2003 324.8 215.6 505.8 72.1 50 22.2

2004 299.3 217.3 427.7 52.6 72 17.6

2005 477.1 371.5 630.6 65.2 138 13.7

Combined 

1992 440.0 250.8 831.9 140.7 74 32.0

1993 704.6 448.3 1,181.3 180.0 150 25.5

1994 687.4 508.1 954.5 112.1 142 16.3

1998 583.1 461.9 758.3 74.6 189 12.8

1999 589.4 466.4 764.1 75.0 173 12.7

2000 773.1 562.3 1,094.6 133.4 160 17.3

2003 661.4 452.4 990.4 134.4 89 20.3

2004 687.6 510.8 945.6 109.3 126 15.9

2005 889.2 746.2 1,075.4 83.4 272 9.4

 

88
Appendix table IV. Abundance estimates ( N ) for all Colorado pikeminnow ˆ > 500 mm TL 

in the lower and upper Colorado River study reaches, and for the reaches combined, with 

lower and upper confidence intervals (CI) and standard error (SE). is the number of 

unique individuals captured. CV is the coefficient of variation (100 x SE/ ). 

Mt+1

Nˆ

________________________________________________________________________ 

 

Year Nˆ Lower CI Upper CI SE Mt+1 CV

 Lower reach 

1992 75.4 19.9 354.1 68.3 6 90.5

1993 227.8 115.2 500.1 91.1 36 40.0

1994 239.9 133.2 464.8 80.1 37 33.4

1998 111.4 61.1 238.2 41.6 28 37.4

1999 103.7 54.0 220.1 39.5 17 38.1

2000 290.4 159.6 556.5 96.4 33 33.2

2003 297.4 157.0 599.5 106.6 35 35.8

2004 197.3 112.3 368.7 62.3 29 31.6

2005 257.2 188.3 372.9 45.8 87 17.8

Upper reach 

1991 184.7 104.6 384.6 65.9 51 35.7

1992 258.5 165.4 432.4 65.5 58 25.4

1993 175.1 130.3 249.8 29.7 63 17.0

1994 312.7 230.2 440.6 52.6 80 16.8

1998 393.1 317.0 502.3 46.7 142 11.9

1999 351.5 287.5 441.9 38.9 131 11.1

2000 356.8 281.3 465.4 46.3 110 13.0

2003 324.8 215.6 505.8 72.1 50 22.2

2004 283.6 205.5 406.0 50.1 68 17.7

2005 398.5 309.8 528.3 54.9 118 13.8

Combined 

1992 334.0 202.0 592.0 95.2 64 28.5

1993 402.9 266.2 651.1 94.8 99 23.5

1994 552.6 407.3 770.7 91.2 117 16.5

1998 504.5 402.7 650.9 62.5 170 12.4

1999 455.3 363.4 586.2 56.1 148 12.3

2000 647.2 479.6 898.2 105.0 143 16.2

2003 622.2 424.7 934.5 127.3 85 20.5

2004 480.9 353.5 671.4 79.8 97 16.6

2005 655.7 540.3 810.9 68.4 205 10.4

89
Appendix Table V. Abundance estimates ( ) for all Colorado pikeminnow 400–449 mm 

TL in the lower and upper Colorado River study reaches, and for the reaches combined, with 

lower and upper confidence intervals (CI) and standard error (SE). is the number of 

unique individuals captured. CV is the coefficient of variation (100 x SE/ ). 

Nˆ

Mt+1

Nˆ

________________________________________________________________________ 

 

Year Nˆ Lower CI Upper CI SE Mt+1 CV

 Lower reach 

1992 230.1 70.0 856.6 169.8 15 73.8

1993 75.5 27.1 261.6 50.8 10 67.3

1994 48.0 15.9 184.5 35.7 6 74.5

1998 101.9 52.6 228.3 41.2 21 40.4

1999 132.4 70.0 269.7 47.9 18 36.2

2000 56.6 18.2 206.6 40.7 5 71.8

2003 248.5 119.7 549.2 102.2 23 41.1

2004 232.1 132.3 426.5 71.8 27 31.0

2005 23.2 7.5 202.1 33.0 6 142.0

Upper reach 

1991 10.1 2.8 84.3 14.1 2 140.3

1992 6.9 1.3 114.9 17.5 1 252.7

1993 11.6 4.0 79.9 13.6 3 116.9

1994 1.2 0.0 129.5 20.7 0 1,721.8

1998 12.1 3.5 162.4 24.8 3 204.8

1999 8.3 3.2 160.4 23.1 3 277.0

2000 5.8 1.2 153.1 22.2 1 382.6

2003 1.1 0.0 124.3 20.2 0 1,851.3

2004 6.5 1.3 122.8 18.4 1 281.5

2005 1.6 1.0 134.9 27.3 1 1,726.8

Combined 

1992 237.0 73.2 869.7 172.4 16 72.7

1993 87.1 31.4 311.9 60.2 13 69.1

1994 49.2 12.6 287.3 52.8 6 107.4

1998 114.1 53.2 302.1 56.4 24 49.4

1999 140.7 68.4 323.2 59.9 21 42.5

2000 62.4 16.7 302.6 57.8 6 92.5

2003 249.6 113.1 593.0 112.8 23 45.2

2004 238.6 130.2 461.8 80.4 28 33.7

2005 24.8 7.8 401.4 59.5 7 239.8

90
Appendix Table VI. Results of the z-test to detect change in two abundance estimates. 

One‐Sided (N2 < N1)

CV N1 SE(N1) N2 SE(N2) z P % decline

0.25 1000 250 1000 250 0.0000 0.5000 0

0.25 1000 250 950 237.5 0.1450 0.4424 5

0.25 1000 250 900 225 0.2973 0.3831 10

0.25 1000 250 850 212.5 0.4572 0.3238 15

0.25 1000 250 800 200 0.6247 0.2661 20

0.25 1000 250 750 187.5 0.8000 0.2119 25

0.25 1000 250 700 175 0.9831 0.1628 30

0.25 1000 250 650 162.5 1.1738 0.1202 35

0.25 1000 250 623 155.75 1.2799 0.1003 37.7

0.25 1000 250 600 150 1.3720 0.0850 40

0.25 1000 250 550 137.5 1.5772 0.0574 45

0.25 1000 250 534 133.5 1.6443 0.0501 46.6

0.18 1000 180 1000 180 0.0000 0.5000 0

0.18 1000 180 950 171 0.2014 0.4202 5

0.18 1000 180 900 162 0.4129 0.3398 10

0.18 1000 180 850 153 0.6349 0.2627 15

0.18 1000 180 800 144 0.8676 0.1928 20

0.18 1000 180 750 135 1.1111 0.1333 25

0.18 1000 180 717 129.06 1.2777 0.1007 28.3

0.18 1000 180 700 126 1.3654 0.0861 30

0.18 1000 180 650 117 1.6303 0.0515 35

0.18 1000 180 648 116.64 1.6411 0.0504 35.2

0.18 1000 180 600 108 1.9055 0.0284 40

0.18 1000 180 550 99 2.1905 0.0142 45

0.15 1000 150 1000 150 0.0000 0.5000 0

0.15 1000 150 950 142.5 0.2417 0.4045 5

0.15 1000 150 900 135 0.4955 0.3101 10

0.15 1000 150 850 127.5 0.7619 0.2230 15

0.15 1000 150 800 120 1.0412 0.1489 20

0.15 1000 150 759 113.85 1.2798 0.1003 24.1

0.15 1000 150 750 112.5 1.3333 0.0912 25

0.15 1000 150 700 105 1.6385 0.0507 30

0.15 1000 150 699 104.85 1.6447 0.0500 30.1

0.15 1000 150 650 97.5 1.9564 0.0252 35

0.15 1000 150 600 90 2.2866 0.0111 40

0.15 1000 150 550 82.5 2.6286 0.0043 45

0.094 1000 94 1000 94 0.0000 0.5000 0

0.094 1000 94 950 89.3 0.3856 0.3499 5

0.094 1000 94 900 84.6 0.7907 0.2145 10

0.094 1000 94 850 79.9 1.2159 0.1120 15

0.094 1000 94 843 79.242 1.2770 0.1008 15.7

0.094 1000 94 802 75.388 1.6432 0.0502 19.8

0.094 1000 94 800 75.2 1.6614 0.0483 20

0.094 1000 94 750 70.5 2.1277 0.0167 25

0.094 1000 94 700 65.8 2.6146 0.0045 30

0.094 1000 94 650 61.1 3.1219 0.0009 35

0.094 1000 94 600 56.4 3.6489 0.0001 40

0.094 1000 94 550 51.7 4.1947 0.0000 45

91
Appendix Table VII. Documented movements of Colorado pikeminnow (captures and 

recaptures of PIT tagged individuals) between rivers of the Colorado River sub-basin 

(mainstem Colorado and Gunnison rivers) and rivers of the Green River sub-basin (Green, 

Yampa, and Duschene), 1991–2005. 

_____________________________________________________________________________________ 

River 11 Rmi 12 Date 13 TL 14 River 25 Rmi 26 Date 27 TL 28 Total 

miles9

Total 

Years10

Colorado 58.3 1991 

05-23 

381 Yampa 73.0 1999 

05-11 

652 476 8 

Colorado 54.1 1993 

04-09 

500 Green 160.0 1997 

07-18 

576 214 4 

Colorado 21.7 1993 

05-21 

490 Yampa 94.8 1996 

05-15 

602 461 3 

Colorado 90.2 1994 

06-22 

358 Green 254.8 2000 

05–31 

526 345 6 

Colorado 62.0 1995 

05-05 

550 Green 112.5 2002 

05-06 

696 174.5 7 

Colorado 53.3 1997 

05-12 

615 Green 184.6 1998 

07-15 

619 238 1 

Colorado 133.4 1998 

05-08 

752 Green 90.1 2000 

05-18 

800 224 2 

Colorado 26.5 1998 

05-12 

421 Duschene 2.0 1999 

06-10 

466 277 1 

Colorado 58.2 1998 

05-28 

420 Green 269.9 2002 

06-07 

620 328.1 4 

Colorado 43.8 1998 

06-01 

369 Green 182.2 2001 

04-19 

501 226 3 

Colorado 67.7 1999 

05-06 

454 Yampa 41.6 2000 

06-19 

490 454 1 

Colorado 26.5 1999 

05-26 

347 Green 89.0 2001 

05-22 

490 115.5 2 

Colorado 16.5 1999 

05-27 

450 Green 52.2 2002 

04-25 

545 68.7 3 

Colorado 43.9 1999 

06-08 

429 White 101.5 2001 

04-16 

480 391.6 2 

Colorado 58.2 2000 

05-10 

470 Green 50.0 2002 

05-12 

562 108.2 2 

Coloradoa 167.9 2000 

03-09 

557 Green 30.7 2001 

05-28 

570 198.6 1 

Greena 30.7 2001 

05-28 

570 Colorado 168.2 2003 

05-07 

592 198.9 2 

Gunnisonb 25.3 2000 

08-03 

531 Green 261.8 2001 

05-23 

550 457.1 1 

Greenb 261.8 2001 

05-23 

550 Colorado 168.2 2003 

05-07 

589 430 2 

Green 51.5 1991 

05-08 

330 Colorado 183.3 2000 

05-01 

587 235 9 

92
93

Appendix Table VII (continued).

River 11 Rmi 12 Date 13 TL 14 River 25 Rmi 26 Date 27 TL 28 Total 

miles9

Total 

Years10

Greenc 52.5 1994 

05-10 

458 Gunnison 3.0 1996 

08-15 

579 226 2 

Gunnisonc 3.0 1996 

08-15 

579 Green 0.5 2001 

03-21 

694 173.5 5 

Green 254.0 1995 

05-10 

519 Colorado 174.4 1999 

06-16 

597 428 4 

Green 174.0 1995 

07-27 

445 Colorado 98.7 2000 

05-05 

585 273 5 

Green 261.8 1996 

05-01 

576 Colorado 56.7 2004 

05-26 

720 318.5 8 

Green 255.8 1996 

06-10 

567 Colorado 34.8 2000 

05-15 

596 291 4 

Green 279.5 1998 

04-15 

625 Colorado 151.2 2003 

07–31 

643 430.7 5 

Green 114.9 1998 

05-05 

540 Colorado 26.5 1999 

05-26 

542 141 1 

Green 41.0 1999 

05-12 

462 Colorado 10.9 2003 

04-10 

611 51.9 4 

Green 252.8 2000 

04-26 

612 Colorado 26.5 2000 

05–31 

617 279.3 0.1 

Green 80.4 2001 

05-23 

721 Colorado 67.0 2004 

04-14 

773 147.4 3 

Green 12.0 2002 

04-17 

301 Colorado 51.3 2003 

06-05 

427 63.3 1 

Green 103.7 2003 

04-23 

565 Colorado 175.5 2005 

04-21 

606 279.2 2 

 1

River 1 – river fish was last captured in prior to moving to River 2 

2

River 2 – river fish moved to after last capture in River 1 

3

RMI 1 – river mile location (measured from mouth of respective river) of last capture in River 1 

4

RMI 2 -- river mile location (measured from mouth of respective river) of first capture in River 2 

5

Date 1 – date of last capture in River 1 

6

Date 2 – date of first capture in River 2 

7

TL 1 – length (mm) of fish at last capture in River 1 

8

TL 2 – length (mm) of fish at first capture in River 2 

9

Total miles – distance traveled between last capture in River 1 and first capture in River 2 10Total years – years (approximate) between last capture in River 1 and first capture in River 2 a

Fish-a that made two separate inter-river movements 

b

Fish-b that made two separate inter-river movements 

c

Fish-c that made two separate inter-river movements 
Cover Photos:

Top: The Colorado River near rk 55 (rm 34); looking upstream towards Shafer Canyon and Dead Horse 

Point, Utah, 2008. Photo by D. B. Osmundson © 

Bottom: Young Colorado pikeminnow Ptychocheilus lucius. Photo by J. Ferreira 

Colorado River Fishery Project 

764 Horizon Drive, Bldg. B 

Grand Junction, CO 81506 

970/245-9319 

doug_osmundson@fws.gov

U.S. Fish and Wildlife Service 

http://www.fws.gov

July 2009