U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the U.S. Fish and Wildlife Service

Colorado River Fish Monitoring in Grand Canyon, 

Arizona: 2002–14 Humpback Chub Aggregations

By William R. Persons, David R. Van Haverbeke, and Michael J. Dodrill

Open-File Report 2016–1177
Cover. Photograph of a humpback chub lying on a hoop net. Photograph courtesy of David R. Van 

Haverbeke, U.S. Fish and Wildlife Service.
Prepared in cooperation with the U.S. Fish and Wildlife Service

Colorado River Fish Monitoring in Grand Canyon, 

Arizona: 2002–14 Humpback Chub Aggregations

By William R. Persons, David R. Van Haverbeke, and Michael J. Dodrill 

Open-File Report 2016–1177

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary 

U.S. Geological Survey 

Suzette M. Kimball, Director 

U.S. Geological Survey, Reston, Virginia: 2017

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Suggested citation:

Persons, W.R., Van Haverbeke, D.R., and Dodrill, M.J., 2017, Colorado River fish monitoring in Grand 

Canyon, Arizona; 2002–14 humpback chub aggregations: U.S. Geological Survey Open-File Report

2016–1177, 43 p., https://doi.org/10.3133/ofr20161177. 

ISSN 2331-1258 (online)
iii

Contents

Abstract ......................................................................................................................................................... 1

Introduction.................................................................................................................................................... 1

Purpose and Scope ................................................................................................................................... 2

Study Area ................................................................................................................................................. 3

Flow and Temperature Regimes................................................................................................................ 3

Previous Investigations .............................................................................................................................. 4

Methods of Investigation................................................................................................................................ 4

Data Compilation ....................................................................................................................................... 5

Aggregation Boundaries............................................................................................................................. 5

Fish Sampling ............................................................................................................................................ 5

Data Collection and Fish Handling............................................................................................................. 6

Data Analysis and Summaries ................................................................................................................... 6

Results and Discussion ................................................................................................................................. 8

Species Composition, Distribution, and Relative Abundance..................................................................... 8

30-Mile Aggregation................................................................................................................................... 9

Little Colorado River Inflow Aggregation.................................................................................................. 10

Lava Chuar to Hance Aggregation........................................................................................................... 11

Bright Angel Creek Inflow Aggregation .................................................................................................... 11

Shinumo Creek Inflow Aggregation.......................................................................................................... 12

Stephen Aisle Aggregation....................................................................................................................... 13

Middle Granite Gorge Aggregation .......................................................................................................... 13

Havasu Creek Inflow Aggregation............................................................................................................ 14

Pumpkin Spring Aggregation ................................................................................................................... 14

Humpback Chub Not Associated With Aggregations ............................................................................... 15

Summary and Conclusions.......................................................................................................................... 15

Acknowledgments........................................................................................................................................ 18

References Cited......................................................................................................................................... 19

Tables

1. Original and revised locations of nine main-stem humpback chub aggregations, including river

mile boundaries, estimates of adult abundance, and 95 percent confidence interval estimated

by Valdez and Ryel (1995) ............................................................................................................. 24

2. Sampling dates; number of days sampled; mean, minimum, and maximum 15-minute discharge

values; range in discharge in ft3/s; and mean water temperature (oC) during each sampling trip

measured at the U.S. Geological Survey Grand Canyon gage during humpback chub aggregation

surveys, 2002–14. .......................................................................................................................... 25

3. Tag and recapture locations with river miles in parentheses for humpback chub recaptured 14+

days after tagging, 1991 to 2014 .................................................................................................... 26

4. Number of fish captured and hours fished by hoop net and trammel net at each aggregation (A–H)

and totaled for all sites (I–J), 1990–2014........................................................................................ 27

5. Alternative models of adult humpback chub catch.......................................................................... 33

6. Number and percent of humpback chub classified by sexual condition and secondary sexual

characteristics by location of capture with river miles in parentheses, 1977 to 2014...................... 34
iv

Figures

1. Map of study area identifying Glen Canyon Dam, the Little Colorado River, and humpback chub 

aggregation locations. .................................................................................................................... 35

2. Original and revised aggregation boundaries. Original from Valdez and Ryel (1995), revised from 

this study. ....................................................................................................................................... 36

3. Length-frequency distribution of humpback chub captured by hoop nets and trammel nets in the 

Colorado River between Lees Ferry and Lake Mead from 1980 to 2014........................................ 37

4. Predicted catch of adult humpback chub per 24 hours for hoop nets during three time periods..... 38

5. Predicted catch of adult humpback chub per 2 hours for trammel nets during three time periods.. 39

6. Pridicted catch of adult humpback chub per 24 hours comparing the terms “Location – NonAggregation” and “Location” from the two most highly supported models (table 5). The estimates 

are shown for a common gear (hoop nets) and time period (Period 3) to facilitate comparison ..... 40

7. Comparison of predicted catch rates for hoop nets (catch/24 hours) and trammel nets (catch/2 

hours), showing the effect of translocated fish at Shinumo Creek and Havasu Creek inflow 

aggregations................................................................................................................................... 41

8. Catch per unit effort (fish/hour) of humpback chub ≥300 millimeters total length (TL) captured by 

trammel net, and catch per unit effort (fish/hour) of humpback chub ≤100 mm TL captured by hoop 

net by river mile main-stem Colorado River, 1981 to 2014............................................................. 42

9. Frequency of humpback chub captures by size class at each aggregation on the Colorado River

between Lees Ferry and Lake Mead, 1990 to 2014 aggregation sampling trips............................. 43
v

Conversion Factors

[This report uses metric units for all measurements except for river flow, the standard measure of which is cubic feet per second 

(ft3/s), and river mile (RM), which is used to describe distances along the Colorado River downstream of Glen Canyon Dam]

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.
Colorado River Fish Monitoring in Grand Canyon, 

Arizona: 2002–14 Humpback Chub Aggregations

By William R. Persons, David R. Van Haverbeke, and Michael J. Dodrill

Abstract

The humpback chub (Gila cypha) is an endangered cyprinid species endemic to the 

Colorado River. The largest remaining population of the species spawns and rears in the Little 

Colorado River in Grand Canyon. Construction and operation of Glen Canyon Dam has altered the 

main-stem Colorado River in Glen and Grand Canyons. Cold, clear water releases from the dam 

result in a river that is generally unsuitable for successful humpback chub reproduction. During the 

early 1990s, nine locations within the main-stem Colorado River were identified as humpback chub 

aggregations—areas with a consistent and disjunct group of fish with no significant exchange of 

individuals with other aggregations. We monitored main-stem Colorado River aggregations of 

humpback chub in Grand Canyon during 2010 to 2014 and compared our results to previous 

investigations. Relative abundance, as described by catch per unit effort (fish per hour) of adult 

humpback chub at most main-stem aggregations, generally increased from the 1990s to 2014. In 

addition, distribution of humpback chub in the main-stem Colorado River has increased since the 

1990s. Movement of humpback chub between the Little Colorado River and other aggregations 

likely adds fish to those aggregations. There is clear evidence of reproduction near the 30-Mile 

aggregation, and reproduction at Middle Granite Gorge and downstream seems likely based on 

catches of gravid fish and captures of very young fish, especially during relatively warm water 

releases from Glen Canyon Dam, 2004 to 2011. Humpback chub relative abundance at Shinumo 

and Havasu Creek inflows increased following translocations of young humpback chub starting in 

2009. In light of this information, we modify the original nine aggregations, combining two 

previously separate aggregations and dropping two locations to form six distinct aggregations of 

humpback chub. Trends in humpback chub abundance at main-stem aggregations, relative to 

management actions (for example, translocations) or changing environmental conditions (for 

example, river warming), informs management of the species across a riverscape scale within the 

Colorado River.

Introduction

The humpback chub (Gila cypha) is an endangered cyprinid species endemic to the 

Colorado River Basin of western United States. The species was described by R. Miller (1946) from 

a specimen taken near the mouth of Bright Angel Creek, Grand Canyon National Park, Arizona; 

listed as endangered in 1967; and grandfathered into the Endangered Species Act of 1973. Since the 

closure of Glen Canyon Dam in 1963, three of eight native fish species have been extirpated in 

Grand Canyon, including Colorado pikeminnow (Ptychocheilus lucius), bonytail (Gila elegans), 

and roundtail chub (Gila robusta). A fourth, the razorback sucker (Xyrauchen texanus), was 
2

suspected to be extirpated (Suttkus and others, 1976; Minckley, 1991) but has recently been 

captured in western Grand Canyon (Kegerries and others, 2015; Rogowski and Wolters, 2014). 

Humpback chub is the last remaining native big-river cyprinid in Grand Canyon and belongs to a 

native fish community that includes flannelmouth sucker (Catostomus latipinnis), bluehead sucker 

(Catostomus discobolus), and speckled dace (Rhinichthys osculus). Grand Canyon is also occupied 

by about 20 species of nonnative fish (Valdez and Ryel, 1995).

Six populations of humpback chub remain, including five in the upper Colorado River Basin

upstream of Glen Canyon Dam and one in Grand Canyon (U.S. Fish and Wildlife Service, 2011). 

The Grand Canyon population consists of several main-stem aggregations and one known spawning 

aggregation around the Little Colorado River (LCR) inflow. Nine humpback chub aggregations 

were originally identified based on fish collected during 1990–93, and closed population abundance 

estimates were generated for six of those aggregations (table 1, fig. 1) (Valdez and Ryel, 1995). An 

aggregation was defined as “a consistent and disjunct group of fish with no significant exchange of 

individuals with other aggregations, as indicated by recapture of passive integrated transponder 

(PIT) tagged juveniles and adults and movement of radio-tagged adults” (Valdez and Ryel, 1995).

Humpback chub are obligate warm-water species with preferred spawning, hatching, and growth 

temperatures of 16 to 22 °C (Hamman, 1982). Depression of spring and summer water temperatures 

in the Colorado River following closure of Glen Canyon Dam in 1963 likely has precluded 

significant main-stem reproduction by humpback chub, owing to mortality during incubation 

(Hamman, 1982; Kaeding and Zimmerman, 1983; Marsh, 1985) and thermal shock of newly 

hatched larvae (Clarkson and Childs, 2000). The LCR aggregation of humpback chub has been 

studied extensively (Douglas and Marsh, 1996; Marsh and Douglas, 1997; Coggins and others, 

2006a,b; Van Haverbeke and others, 2013; Yackulic and others, 2014). Both closed- and openpopulation models are used to estimate abundance of chub in the LCR and near the confluence of 

the Colorado River (Coggins and Walters, 2009; Van Haverbeke and others, 2013; Yackulic and 

others, 2014). Reasons for use of areas outside of the LCR vicinity by adult humpback chub is 

unclear, but most aggregations are associated with seasonally warm tributary streams (LCR, Bright 

Angel Creek, Shinumo Creek, and Havasu Creek) or warm springs (30-Mile, Pumpkin Spring).

The LCR serves as the main spawning location for humpback chub, and this location is 

potentially at risk to catastrophic loss. For long-term conservation of the species, a second spawning 

population outside of the LCR is needed (U.S. Fish and Wildlife Service, 2002). Since 2009, the 

National Park Service stocked 840 and 1,102 humpback chub into Havasu and Shinumo Creeks,

respectively, in an effort to establish a second spawning population within Grand Canyon (Trammel 

and others, 2012). Main-stem aggregations are associated with Havasu and Shinumo Creeks, and it 

may be that stocked fish are contributing to these aggregations through passive or active dispersal 

from the tributaries. Information on the contribution of stocked fish to aggregations and on the 

status and trends of humpback chub is needed for managers to assess impacts of operations of Glen 

Canyon Dam on main-stem populations of humpback chub. This information may also be important 

in determining if recovery criteria for the humpback chub can be achieved in the Grand Canyon 

population.

Purpose and Scope 

The purpose of this report is to (1) present and summarize fish sampling results at humpback 

chub aggregations from 2010 through 2014, (2) compare these results to previous investigations 

conducted between 2002 and 2006 (Ackerman, 2008) and between 1990 and 1993 (Valdez and 

Ryel, 1995), and (3) summarize humpback chub data collected in the main-stem Colorado River 
3

from 1981 to 2014. Population variables evaluated for the study include humpback chub catch per 

unit effort (CPUE), size structure, and movement from PIT tag recapture information, as well as 

overall fish species composition. This report clarifies locations of aggregations and redefines 

aggregations based on updated capture and PIT tag recapture information. This report also provides 

information on humpback chub translocated to Shinumo and Havasu Creeks that have subsequently 

moved into the main-stem Colorado River.

Study Area

All locations are referred to in river miles (RM) downstream of Lees Ferry (Coconino 

County; north-central Arizona, RM 0), approximately 15 miles downstream of Glen Canyon Dam

(fig.1)1. Sampling was conducted between RM 0 and RM 259.7, from Lees Ferry to Quartermaster 

Canyon in the Lake Mead inflow. In general, the river varies in character from large eddy 

complexes in depositional areas to narrow, deeply incised sections composed of resistant rock types 

(Webb and others, 1989; Schmidt and others, 1998). Water quality is strongly influenced by 

hypolimnetic water discharged from Glen Canyon Dam at RM −15, near Page, Ariz. Water 

discharged from Glen Canyon Dam is typically clear (<5 nephelometric turbidity units) (Vernieu, 

2009), cold (8 to 11 o

C) (Stanford and Ward, 1991; Voichick and Wright, 2007), and has 

intermediate conductivity (700 to 900 microsiemens per centimeter [μS/cm]) (Vernieu, 2009).

However, inputs from side tributaries can dramatically alter turbidity and flows. Water temperatures 

can also vary from long-term averages. For example, during 2004–2005, and again in 2011, water 

temperatures from Glen Canyon Dam exceeded 14 oC in October and November at the Lees Ferry 

gage (station No. 09380000), and during 2003 to 2014, water temperatures regularly exceeded 12

oC at the Lees Ferry gage.

Different investigators studying the aggregations have used distinct river maps with river 

mile designations varying slightly between mapping methods. Earlier sampling projects used a 

variety of river maps (Carothers and Minckley, 1981; Kaeding and Zimmerman, 1983; Maddux and 

others, 1987); Valdez and Ryel (1995) used the Belknap and Belknap-Evans (1989) river map; and 

Ackerman (2008) used the Stevens (1990) river map and also obtained RM locations from 

orthorectified aerial photos developed by the U.S. Geological Survey’s (USGS) Grand Canyon 

Monitoring and Research Center (GCMRC). Since 2010, orthorectified aerial photos developed by 

GCMRC with matching river maps (Martin and Whitis, 2007) have been used, and all references to 

aggregation boundaries in this report refer to these designations.

Flow and Temperature Regimes

Discharge, measured every 15 minutes at the USGS Grand Canyon river gage (USGS gage 

09402500), ranged from 6,670 to 22,000 cubic feet per second (ft3

/s) during the surveys from 2002–

14 (table 2). Discharge from Glen Canyon Dam during some sampling trips was relatively constant 

(for example 2010, 2011), whereas during other years there were large fluctuations in discharge (for 

example, 2013). Effect of stage change on the ability of nets to catch fish is unknown, but in 

general, nets are easier to set and retrieve when river stage does not change. For example, hoop nets 

can be dewatered when river stage decreases, particularly along low angle shorelines; alternatively, 

trammel nets may foul when increasing discharge alters eddy velocities or flow patterns.

 1

The use of river mile has a historical precedent and provides a reproducible method for describing 

locations along the Colorado River below Glen Canyon Dam. Lees Ferry is the starting point, river mile 

0, with mileage measured for both upstream (–) and downstream (+).
4

Water released from Glen Canyon Dam is generally warmed by solar radiation as it moves 

downstream; maximum warming in summer is about 0.02 oC per kilometer (Wright and others, 

2008), so water temperature at the LCR confluence is usually about 2 oC warmer than Glen Canyon 

Dam releases. Mean water temperatures during sampling trips in the main-stem Colorado River just 

upstream of the LCR confluence ranged from 13.1 to 15.6 °C. Mean water temperatures at the same 

location were 15.6 and 15.3 °C in 2011 and 2014, respectively; these temperatures were warmer 

than other sampling trips, largely owing to increased water release temperatures from Glen Canyon 

Dam (table 2) (Wright and others, 2008).

Previous Investigations 

Studies of humpback chub in Grand Canyon began in the 1970s. Early efforts included 

morphological studies (Suttkus and others, 1976), life history summaries (Kaeding and 

Zimmerman, 1983), and early fish surveys (Carothers and Minckley, 1981; Maddux and others, 

1987). Catches of adult humpback chub outside of the immediate area of the LCR confluence

(approximately RM 62) were uncommon, probably because of limited logistical capabilities, and 

the distribution of humpback chub throughout Grand Canyon remained obscure. Kaeding and 

Zimmerman (1983) described a “bell shaped” distribution of humpback chub near the LCR inflow, 

and others reported sporadic captures throughout the Grand Canyon from about RM 19.5 to Spencer 

Creek at RM 246, including a few select tributaries summarized in Minckley (1996).

In the early 1990s, studies were conducted to better understand the population abundance 

and distribution patterns of humpback chub (Valdez and Ryel, 1995; Douglas and Marsh 1996; 

Arizona Game and Fish Department, 1996) in order to gather information for the Operation of Glen 

Canyon Dam Environmental Impact Statement (U.S. Department of the Interior, 1995). Valdez and 

Ryel (1995) identified nine aggregations of humpback chub in the main-stem Colorado River in 

Grand Canyon based on fish captured by electrofishing and netting during 1990–1993; these nine 

aggregations were 30-Mile, LCR inflow, Lava Chuar to Hance, Bright Angel Creek inflow, 

Shinumo Creek inflow, Stephen Aisle, Middle Granite Gorge, Havasu Creek inflow, and Pumpkin 

Spring (table 1). Of these, the LCR inflow aggregation has been sampled extensively, and 

population status and trend data have been regularly reported (Coggins and others, 2006a, 2006b; 

Coggins and Walters, 2009, Van Haverbeke and others, 2013). The LCR inflow supports the largest 

aggregation, and spawning is known to occur in the LCR (Kaeding and Zimmerman, 1983; Douglas 

and Marsh, 1996). Limited reproduction has also been documented from the 30-Mile aggregation 

(Valdez and Masslich, 1999; Andersen and others, 2010). Aggregations from the LCR to Havasu 

Creek are genetically relatively homogeneous, indicating gene flow between aggregations (Douglas 

and Douglas, 2010). Most recaptures of humpback chub occurred in the same main-stem river reach 

or tributary as original captures, and most fish were captured near the LCR (Paukert and others, 

2006).

Methods of Investigation

Humpback chub aggregations were sampled with trammel nets and hoop nets as part of 

USGS’s monitoring and research program during 2002–4 and 2006 (Ackerman, 2008). Methods 

and gear types used by Ackerman (2008) were also used in 2010–14, although sampling locations 

varied between trips. In addition, the method of baiting hoop nets changed after 2010 from using 

perforated PVC bait tubes that prevented fish from ingesting bait to using mesh bait bags that allow 

fish to consume small bait particles. 
5

Data Compilation

Humpback chub aggregations were sampled during 1990–93, 2002–4, and 2006 and 

documented in published and unpublished reports (Valdez and Ryel 1997; Valdez and Ryel, 1995; 

Ackerman, 2008). Data collected during previous investigations were transferred into a Microsoft 

Access database maintained by GCMRC. Data were imported into SPSS software (SPSS Inc., 

Chicago, Illinois) or program R (R Core Development Team, 2014) for analysis. For most analyses, 

data were filtered to exclude samples not collected using standard methods, and checked for errors. 

The entire database was searched for PIT-tagged fish for analyses of movement and distribution of 

humpback chub. Data from 38 main-stem sampling trips that used trammel and hoop nets at 

aggregations from 1990–93 were compared with more recent data from 2002–14 (11 trips).

Aggregation Boundaries

River mile designations of nine humpback chub aggregations described by Valdez and Ryel 

(1995) were modified slightly based on sampling conducted during 2002–14 to include adjoining 

locations where fish were sampled (table 1, fig. 2). We assigned catches to aggregations based both 

on the river mile location of catches in the GCMRC fish database and distribution of catches near 

those locations. In some cases, fish catches were assigned to river miles based on maps other than 

Belknap and Belknap-Evans (1989), so although fish may have been caught in the same location, 

they were recorded in the database at different river miles. For example, between 1981 and 2011 

there were 653 humpback chub recorded as being caught between RM 65.4 and 65.7, which is the 

area originally identified as Lava Chuar rapid, the boundary between the LCR aggregation and the 

Lava Chuar to Hance aggregation. In this instance, these fish were assigned to the LCR aggregation 

because the reach in the rapid and for a distance below the rapid is not fishable. In order to assign 

fish catches to particular aggregations, we modified the defined boundaries of aggregations based 

on where fish were reported to be captured in the GCMRC database (see table 1). Fish captured in 

Shinumo Creek downstream of the first waterfall barrier were assigned to the Shinumo Creek 

inflow aggregation, and fish captured in the mouth of Havasu Creek were assigned to the Havasu 

Creek inflow aggregation.

We also examined movement of fish between the LCR inflow aggregation and the Lava 

Chuar to Hance aggregation based on individual PIT-tagged fish recaptures. Because of the high 

rate of movement between the two aggregations, we merged the Lava Chuar to Hance aggregation 

into the LCR inflow aggregation. 

Fish Sampling

Monitoring the fish community of the Colorado River by boat electroshockers has provided 

information on the status and trends of most common nonnative and native fishes, but adult 

humpback chub are relatively invulnerable to nearshore electrofishing (Makinster and others, 2010). 

However, adult humpback chub have been effectively captured both in the main-stem Colorado 

River and the LCR by hoop nets and trammel nets (Gorman and others, 2005; Coggins and others, 

2006a; Ackerman, 2008; Van Haverbeke and others, 2013). The Colorado River upstream of the 

LCR is numerically dominated by the nonnative rainbow trout (Oncorhynchus mykiss) (Makinster 

and others, 2010; Makinster and others, 2011). The LCR is a spawning tributary for native fishes 

including flannelmouth sucker, bluehead sucker, and speckled dace (Kaeding and Zimmerman, 

1983; Valdez and Ryel, 1995; Douglas and Marsh, 1996; Coggins and others, 2006b), and these 

native fish are common in the Colorado River downstream from the LCR confluence. Rainbow 
6

trout relative abundance declines downstream of the LCR, whereas relative abundance of native 

suckers, common carp, and warm water species increases (Makinster and others, 2010).

Humpback chub aggregation surveys were conducted in June, July, or September, 2002–14, 

and nets were fished for 12 to 16 days (table 2). Two motorized boats with 50-horsepower, 4-stroke 

outboard motors (5.3-meter [m] aluminum hulled Osprey or 4.9-m inflatable Achilles boats) were 

used for netting operations. One boat operator and two fish handlers were employed per boat.

Trammel nets measured 22.9 m to 30.5 m × 1.8 m (length × width) with 2.54 cm and 20.5 

cm, mesh and panel mesh, respectively). Trammel nets were typically set off of debris fan points 

where eddy and main river currents converged, along cut bank and vegetated shoreline habitat 

inside of eddy and backwater complexes, and across the mouths of small coves. Trammel net 

locations were limited by water velocity, and nets were generally set in fairly slow moving or slack 

water. Trammel nets were initially set each day at approximately 1600 to 1800 hours and fished for 

three approximately 2-hour sets (mean soak time = 2.0 hours). Two-hour net sets were conducted to 

limit stress and injury to fish (Hunt and others, 2012). Each netting boat generally set 4 to 5 

trammel nets for three 2-hour sets for a total of 24 to 30 trammel net sets per night. Netters moved 

or discontinued netting at their discretion if sampling conditions were unsuitable for effective 

netting or if nets became entangled in debris. This was a similar sampling protocol to that employed 

by Valdez and Ryel (1995).

Hoop nets were 0.5 to 0.6 m in diameter, 1.0 m long, with 6-mm mesh and a single 10-cm 

throat. Nets were set in suitable locations, usually in areas of low velocity current. Nets were tied to 

shore and set at depths typically less than 3 m but deep enough to ensure that nets were not 

dewatered during fluctuating flows (Ackerman, 2008). Each boat set 10 to 25 baited hoop nets 

overnight (mean soak time = 19.3 hours). Hoop nets were baited with commercial fish food (AquaMax Carnivorous Fish Food, Purina Mills, Inc.) placed inside of mesh bags tied toward the cod end 

of the net, although during 2002–6, nets were baited with Aqua-Max placed inside of small 

perforated PVC scent tubes that prevented fish from eating the bait. Beginning in 2011, hoop nets 

were more heavily baited using mesh bags to attempt to increase capture probability. Hoop nets 

were used by Ackerman (2008) but not by Valdez and Ryel (1995).

Data Collection and Fish Handling

Total length (TL) in millimeters (mm) was measured for all fish collected, and fork length 

(FL) in mm was measured for humpback chub, flannelmouth sucker, and bluehead sucker. Fish 

were not weighed to reduce handling time. Sex and stage of maturity of captured fish were 

determined based on external morphological characteristics (for example, coloration, tubercles, 

swollen cloaca; Suttkus and others, 1976) and on manual extrusion of gametes. 

Humpback chub were implanted with PIT tags, according to standard protocols for handling 

fish in Grand Canyon (Persons and others, 2013). All PIT tag numbers were recorded on data sheets 

and stored in battery-powered PIT tag readers. Reader files were downloaded and archived to 

confirm accuracy of data sheets and databases. All PIT tags used were 134.2-kHz (kilohertz), full 

duplex, 12.5-mm-long (Biomark HPT 12) tags.

Data Analysis and Summaries

A variety of data were analyzed and summarized for this report. For comparisons of relative 

abundance between time periods, we used only trammel net and hoop net captures. Size 

composition was evaluated by examining length-frequency distributions and mean length of fish 

captured by trammel and hoop nets. Information about previously PIT-tagged fish is from the fish 
7

database maintained by GCMRC and includes fish captured by any gear that were tagged with 

external Floy or Carlin tags that may have later received a PIT tag (table 3). We used only 

recaptures for fish that were at large at least 14 days between capture and recapture (Paukert and 

others, 2006). Fish captures included in table 4 were from sampling trips identified in table 2 as well 

as from trips using hoop and trammel nets conducted during 1990–93. We used data from both of 

these gear types for the analysis and report the number of sexually mature humpback chub (table 6).

Adult humpback chub (>200 mm TL) caught by hoop and trammel nets were summarized 

by counts (for example, total catch) and total effort (for example, hours of either hoop or trammel 

net effort) by year and sample location (see below). Catch and effort were computed separately for 

hoop and trammel nets. We used generalized linear models to test hypotheses describing factors 

thought to affect catch of adult humpback chub in the main-stem Colorado River. The river was

divided into 5-mile bins from Lees Ferry to RM 260, resulting in 52 sections of river. Each 5-mile 

bin was located within a reach of river either previously described as an aggregation (for example,

RM 60–65 is located within the Little Colorado River inflow aggregation) or consisted of a reach of 

river between previously described aggregations (for example, RM 40–45 is located between the 

30-Mile aggregation and the Little Colorado River inflow aggregation). The catch within each 5-

mile bin was assigned to an aggregation (for example, the bin fell within river miles previously 

described as an aggregation) or a reach of river between aggregations (a non-aggregation location). 

This resulted in 17 discrete reaches of river (each potentially consisting of multiple 5-mile bins). 

For the generalized linear model fitting, these river reaches are described by a factor called 

“Location.” Three reaches of river were removed from the analysis, owing to the absence or scarcity 

of humpback chub captures in these locations, leaving 14 locations. The amount of effort (net 

hours) for both hoop nets and trammel nets was also tabulated for each reach of river as described 

above for catch.

To test whether catch of humpback chub was higher in known aggregations compared to 

other locations, we used a factor that indicated if a river location was an aggregation or nonaggregation (Aggregation). Additionally, we fit models that included a term that allowed catch 

estimates to vary at all aggregations, whereas catch at all non-aggregation locations was estimated 

with one common term (Location – Non-Aggregation). Models that included this term represent an 

intermediate condition between catch varying at all locations (Location) and catch varying only 

between aggregations and non-aggregations (Aggregation). In order to test whether catch was

higher near the confluence of the Little Colorado River, we used a factor specifying the reach of 

river closest to the LCR (LCR inflow). Humpback chub catch is also known to vary across years, 

and this was modeled by including the year captured as a covariate (Year). In order to account for 

differences in capture efficiency between gear types, a covariate specifying sampling gear (Gear)

was included in all models. Main-stem netting has occurred in three distinct time periods: Period 1 

(1991 to 1993), Period 2 (2002 to 2006), and Period 3 (2010 to 2013), and time period (Period) was 

included in addition to sampling year. Previous sampling indicated that the catch of humpback chub 

in the LCR inflow may exhibit a different response than other main-stem locations. This may be due 

to the influence of the LCR on catch of humpback chub in the main-stem Colorado River. 

Therefore, we included an interaction between the factor specifying the LCR inflow aggregation 

and time period. 

Generalized linear models are an extension of linear models (for example, regression, 

ANOVA), which allow for non-normal error structures, such as the Poisson distribution for 

modeling count data (Bolker, 2008). We fit generalized linear models with a negative binomial 

error distribution to model humpback chub catch (counts) and included the log of effort (hours) as 
8

an offset to account for varying levels of effort. This allowed us to essentially model the rate of fish 

captured per hour of effort. Additionally, using the negative binomial distribution allows us to 

account for over-dispersion in catch (for example, variation in the data beyond what would be 

expected from the Poisson distribution alone). The Poisson distribution assumes a variance equal to 

the mean, while using the negative binomial distribution allows for variance to be greater than the 

mean (for example, over-dispersion), a common attribute of ecological data (Lindén and 

Mäntyniemi, 2011). Models were fit with likelihood methods using the “glm.nb” function in the 

program R package “mass” (R version 3.0.2). Model fit was compared using Akaike information 

criterion corrected for small sample sizes (AICc) (Burnham and Anderson, 2002). We fit 19 models,

and the models with substantial support (0 – 2ΔAICc) were evaluated. 

In order to examine the influence of translocated fish in influencing catch of humpback chub 

in the main-stem Colorado River, we compared CPUE from two models differing only in the 

inclusion of translocated fish. All translocated fish were PIT tagged, and this allowed us to discern 

these fish from fish captured and tagged in the main-stem Colorado River. During Period 3, 

translocated fish were encountered in three locations during main-stem sampling with both hoop 

nets and trammel nets. We fit a negative binomial model with location and gear as effects to 

compare the catch with and without fish from translocations. 

Results and Discussion

Species Composition, Distribution, and Relative Abundance

In total, 15 species represented by 16,854 fish were captured at aggregations in more than 

107,000 hours of netting effort during 1990 to 2014 sampling trips (table 4I). Data include 

individuals captured more than once within the same trip or different trips as well as those captured 

only once. Flannelmouth sucker was the most numerically dominant species (40.1 percent of total 

individuals sampled), followed by rainbow trout (21.8 percent), humpback chub (18.1 percent), and 

speckled dace (10.7 percent). Native fish ranged from 42 percent to 87 percent of the annual catch 

and averaged approximately 67 percent of the catch from 1990 to 2014. Humpback chub made up

18.1 percent and 0.6 percent of the catch at aggregations and at locations other than aggregations,

respectively (tables 4I, 4J).

Trammel nets and hoop nets captured different size classes of humpback chub (fig. 3). Hoop 

nets tended to capture fish between 60 and 300 mm TL, and trammel nets primarily caught fish 

≥200 mm TL. Collectively, these two gear types provide a good cross section of sizes of humpback 

chub in the areas sampled.

We found the most support for a model of adult humpback chub catch that varies by time 

period, river location at previously described aggregations, and a term that groups adult humpback 

chub at non-aggregation locations together into one estimate (table 5). This model also includes an 

interaction with time period for the LCR inflow aggregation that allows estimates to vary between 

time periods compared to other main-stem locations (table 5). The most supported model included 

the term specifying a different catch rate at each aggregation location and a common rate at non–

aggregation locations. The difference between this model and the next most supported model 

including different catch rates at all locations was small (<1 ΔAICc). The difference between the 

two most highly supported models was whether the non-aggregation locations were grouped into a 

single estimate (Location – Non-Aggregation) or estimated separately (Location, see below).

Models that included a term specifying whether a location is an aggregation or non-aggregation 
9

(Aggregation, table 5) were not as highly supported (>19 ΔAICc), highlighting the variation in 

catch rate between aggregations and not only between aggregation and non-aggregation locations.

Modeled catch was higher in Period 3 compared to Periods 1 or 2 for both hoop nets (fig.4) 

and trammel nets (fig.5). Generally, the lowest catches occurred from 1991 to 1993, with increasing 

catch from 2002 to 2006, and the highest catch from 2010 to 2013. For many locations, the 

differences in catch are significantly higher from 2010 to 2013. This result may be in part due to 

changes in the sampling protocol between time periods, namely switching from un-baited to baited 

hoop nets during 2010 to 2013, although the trammel net catch rates show a similar trend between 

time periods. The switch to baited hoop nets during 2010 to 2013 could potentially alter the capture 

probability of the gear, leading to biased estimates during this time period. Unfortunately, we are 

not able to currently determine the effect of baiting on the CPUE indices, and this represents a 

fruitful area for future research. The LCR inflow exhibited a different temporal pattern with 

moderate levels of catch during Period 1, then lower catch during Period 2, and the highest levels of 

catch in Period 3. Catch rate at non-aggregation locations was low compared to previously 

described aggregations, especially during Periods 1 and 2, and has increased during Period 3 (figs. 

4, 5). Two models received substantial support (0 – 2ΔAICc) and differed only by the inclusion of

one term specifying whether non-aggregation locations are grouped into one estimate (Location –

Non-Aggregations, table 5) or estimated separately (Location). Estimates for a common gear (hoop 

nets) and time period (Period 3) are used to compare the two model structures, and these estimates 

show no significant difference between these effects for the two models (fig. 6, compare NonAggregations, above dashed line, with other estimates). Some estimates for non-aggregation 

locations (under the second most highly supported model) are unbounded, owing to the low or no 

catch of humpback chub occurring in these sections of river (fig. 6). 

Fish translocated into Shinumo and Havasu Creeks were recaptured in three sections of the 

main-stem Colorado River (table 3). In the two sections of river closest to these tributaries, the point 

estimates for modeled hoop net catch are 68 percent and 27 percent higher at Shinumo and Havasu 

Creeks, respectively, when including translocated fish (fig. 7). Trammel net catch is 65 percent

higher in the Shinumo Creek inflow and 22 percent higher in the Havasu Creek inflow when 

including translocated fish (fig.7). 

30-Mile Aggregation 

Humpback chub were reported near Lees Ferry by Holden (cited in Valdez and Ryel 1995), 

in the vicinity of Tiger Wash (RM 27) by Carothers and Minckley (1981), and near RM 25 by 

Kaeding and Zimmerman (1983). The 30-Mile aggregation was described by Valdez and Ryel 

(1995) based on netting and electrofishing captures of 26 adults between RM 30 and 31.3 during 8

surveys2. The aggregation is located near a series of warm springs associated with Fence Fault (see 

Huntoon, 1981). Most adults were captured in the proximity of the warm (21.5 °C) spring near RM 

31.5 (Spring 5 in Huntoon, 1981). Based on recapture of 6 adults, the population was estimated to 

be 52 fish in 1993 (Valdez and Ryel, 1995) (table 1). Evidence of reproduction was also reported 

from this spring, when about 100 postlarval fish were observed and 14 were captured and measured 

(18–31 mm TL) (Valdez and Masslich, 1999). Small humpback chub (<100 mm TL) that likely 

originated near RM 30 were commonly captured between RM 30 and the LCR (fig. 8B; Andersen 

 2

Valdez and Ryel (1995) generated population estimates based on fish captured by both 

electrofishing and netting, whereas table 4 includes only fish captured by netting, hence numbers in 

text and table 4 do not always agree.
10

and others, 2010). During 2013 and 2014, we sampled downstream of RM 31.3 and captured 105 

adult humpback chub between RM 34.2 and RM 35.7. This area, between Redwall Canyon and 36-

Mile rapid, had not been previously sampled by hoop nets or trammel nets, although it was sampled 

extensively by electrofishing (Valdez and Ryel, 1995; GCMRC, unpub. data, January 2016). It is 

unclear if this is an expansion of the 30-Mile aggregation or if there have been humpback chub in 

this area for decades. Based on catches in 2013 and 2014, we extended the lower boundary of this 

aggregation an additional 5 miles to RM 36.3. 

Of 162 unique humpback chub tagged in the 30-Mile aggregation, 44 were subsequently 

recaptured, and 43 of these fish were recaptured within the 30-Mile aggregation (table 3). Four 

individuals tagged in the LCR and seven individuals tagged in the LCR inflow aggregation were 

later captured at the 30-Mile aggregation, illustrating limited movement between the LCR and this 

aggregation. One individual tagged in the LCR (348 mm TL, March 1992) was recaptured twice at 

the 30-Mile aggregation (September 1995 and September 1996) and was also later recaptured in the 

LCR (May 2003 and April 2006). One humpback chub tagged in the LCR Aggregation (June 2005) 

was subsequently recaptured at 30-Mile (April 2007) and later near Little Redwall (RM 34.6, July 

2013). All other humpback chub tagged at 30-Mile have only been recaptured at the 30-Mile 

aggregation, indicating high apparent site fidelity. Between 1990 and 2014, 11 ripe male and 4 ripe 

female humpback chub were captured between RM 29.8 and 36.3 (table 6). In addition, ultrasound 

images taken during 2013 and 2014 revealed female humpback chub with well-developed eggs 

(Morgan Brizendine, University of Arizona, oral commun., January 2016).

Adult rainbow trout dominated the catch numerically and made up 70 percent of the catch at 

the 30-Mile aggregation from 1990 to 2014 (table 4A). Sampling the 30-Mile aggregation by hoop 

net and trammel net from 1990 to 2014 resulted in the collection of 187 humpback chub that ranged 

in size from 188 to 464 mm TL (mean = 320 mm) (fig. 8A). The 30-Mile aggregation meets the 

criteria of persistence and little exchange of individuals with other aggregations. In addition, capture 

of ripe adult fish showing secondary sexual characteristics (color and tubercles) (table 6) and 

capture of young-of-the-year (YOY) fish at the spring (Valdez and Masslich, 1999) and 

immediately downstream of RM 31.3 (Andersen and others, 2010) suggests local reproduction is 

occurring. This information, along with an increase in the trammel net and hoop net catch rates,

indicates that this aggregation of humpback chub is persistent and possibly increasing. 

Little Colorado River Inflow Aggregation

The LCR inflow aggregation is the largest aggregation known in the Colorado River within 

Grand Canyon National Park. Movement of humpback chub between the LCR and the main-stem 

Colorado River was reported by Kaeding and Zimmerman (1983). The LCR inflow aggregation was 

originally described based on capture of 1,558 adults between RM 57.0 (Malagosa Crest) and Lava 

Chuar rapid between 1990 and 1993 (Valdez and Ryel, 1995). They estimated 3,482 adult 

humpback chub in this area using a multiple pass mark-recapture method. Using an Age-Structured 

Mark-Recapture (ASMR) model, which includes captures of fish in the LCR, Coggins and Walters 

(2009) generated an estimate of 7,650 adult humpback chub. More recently, Yackulic and others 

(2014) estimated abundance of approximately 10,000 to 14,000 (95 percent confidence interval 

[CI]) adult humpback chub. Within a 2.8-km reach of the main-stem Colorado River near the 

confluence with the LCR, Finch and others (2015) estimated 615 to 2,801 juvenile fish/km and 94 

to 1,515 juvenile fish/km from open and closed population models, respectively. Mean annual hoop 

net and trammel net modeled CPUE of humpback chub ≥200 mm decreased at this aggregation 

from Period 1 to Period 2, then increased in Period 3 (figs. 4, 5).
11

There appears to be little dispersal of humpback chub from the LCR inflow aggregation 

other than with the nearby Lava Chuar to Hance aggregation. Almost all (99.9 percent) of the 

humpback chub recaptured in the LCR or the LCR inflow aggregation from 1990 to 2014 were 

previously tagged in the same area (table 3). Fish tagged in the LCR or the LCR inflow aggregation 

were recaptured in all other aggregations, at distances of about 30 to 90 miles from the LCR, with 

the exception of Pumpkin Spring (147 RM from the LCR; table 3). In addition, 37 humpback chub 

previously tagged at other aggregations were recaptured in the LCR or the LCR inflow aggregation. 

This included fish from all other aggregations except Pumpkin Spring. Between 1990 and 2014, 7

ripe females and 25 ripe males were captured between RM 57 and 77.2 (table 6).

Humpback chub were the most common species in the catch at the LCR inflow aggregation. 

From 1990 to 2014, 1,967 humpback chub were collected in the main-stem LCR inflow aggregation 

(table 4B). Humpback chub made up the highest proportion of the total catch at this site (39

percent), followed by flannelmouth sucker (28 percent) and rainbow trout (26 percent). Humpback 

chub ranged in size from 50 to 480 mm TL (mean = 311 mm) (fig. 8B). Small humpback chub were 

common in this reach (fig. 8B) because of YOY and juveniles that originated in the LCR (Valdez 

and Ryel, 1995; Yackulic and others, 2014; Finch and others 2015).

Based on exchanges of fish (see below), we expanded the original boundaries of the LCR 

inflow aggregation (RM 57 to 65.9) to include captures between RM 57 to just upstream of Hance 

rapid (RM 77.2). This includes the area previously defined as the Lava Chuar to Hance aggregation.

Lava Chuar to Hance Aggregation

Based on relatively high rates of movement of tagged fish between the original Lava Chuar 

to Hance aggregation and the original LCR inflow aggregation, we grouped the two areas into a 

single aggregation. The Lava Chuar to Hance aggregation was originally described by Valdez and 

Ryel (1995) based on the capture of 15 adults between the base of Lava Chuar rapid and Hance 

rapid. They recaptured only 3 fish and were unable to estimate the population of adults in this reach. 

Of the 351 unique humpback chub PIT tagged between Lava Chuar rapid and Hance rapid since 

1991, 47 were recaptured in the LCR, 18 in the LCR inflow aggregation, and 4 between Lava Chuar 

rapid and Hance rapid. In addition, 28 humpback chub tagged in the LCR were later recaptured 

between Lava Chuar rapid and Hance rapid. Many humpback chub captured in this area were 

smaller than 200 mm, suggesting emigration of young fish from the LCR. Humpback chub are not 

thought to spawn in this area, but likely migrate to the LCR to spawn. A single ripe female, 

however, was captured in this aggregation upstream of Tanner rapid in 2004. 

Bright Angel Creek Inflow Aggregation

The first humpback chub identified from the Grand Canyon were reported in the early 1940s 

from the vicinity of Bright Angel Creek, including the holotype used to describe the species (Miller,

1946). Valdez and Ryel (1995) defined this inflow aggregation from the base of 83-Mile rapid to 

just upstream of Salt Creek rapid associated with Bright Angel and Clear Creeks. They captured 

only eight adult humpback chub in this reach and were unable to generate a closed population 

estimate. Only 25 humpback chub that ranged in length from 54 to 357 mm TL (mean = 211 mm) 

were captured from 1990 to 2014 (table 4C, fig. 8C). Mean annual hoop net and trammel net 

CPUEs at this aggregation were generally low, and no temporal changes in CPUEs were apparent. 

Flannelmouth sucker and brown trout (Salmo trutta) dominated the catch at this aggregation (1990–

2014) (table 4C). Bright Angel Creek sustains reproducing populations of rainbow trout, brown 

trout, and flannelmouth sucker (Maddux and others, 1987; Valdez and Ryel, 1995; Weiss and 
12

others, 1998). A single ripe male humpback chub (TL = 324 mm) was captured in May 1992 at RM 

83.9 very near or at the Clear Creek inflow, and five tuberculate males were captured between 1991 

and 1993 (table 6).

Humpback chub originally captured in the Bright Angel Creek inflow aggregation were not

recaptured in the same location. Seven humpback chub tagged at the Bright Angel Creek inflow 

were subsequently recaptured, six at the LCR or LCR inflow and one at Middle Granite Gorge 

(table 3). Catch per unit effort at Bright Angel Creek inflow was low during all Periods (figs. 4, 5).

We conclude from this evidence that this group of fish is not persistent and have eliminated this 

aggregation from further consideration.

Shinumo Creek Inflow Aggregation

The Shinumo Creek inflow aggregation was originally defined from the base of Bass rapid 

to the top of Shinumo rapid, from RM 108.1–108.6. Based on captures of humpback chub in the 

GCMRC database, we redefined the aggregation as extending from RM 107.8 to 110.0 as in Martin 

and Whitis (2007). Valdez and Ryel (1995) attributed this aggregation to the presence of Shinumo 

Creek, a cool-water tributary. Researchers have historically pointed to Shinumo Creek as being a 

potential spawning tributary for humpback chub (Suttkus and Clemmer, 1977; Valdez and others, 

2000). An approximately 120-m-long section of the creek, which has been identified as a potential 

spawning area, is located downstream of a natural fish barrier. Humpback chub smaller than 100 

mm TL were collected between 1991 and 2014 from Shinumo Creek very near its confluence (fig. 

8D), but their source is unknown.

Valdez and Ryel (1995) generated a closed population estimate of 57 (31–149, 95 percent

CI) adult humpback chub based on capture of 27 adults and 6 recaptures. Between 2009 and 2013,

the U.S. Fish and Wildlife Service and the National Park Service moved 1,102 age 1 and age 2 

humpback chub from the LCR to Shinumo Creek as part of a translocation effort. Approximately 8

percent of humpback chub translocated to Shinumo Creek were subsequently captured in the 

Colorado River or in Shinumo Creek downstream of the barrier falls. Humpback chub showed 

relatively high apparent site fidelity to the Shinumo Creek inflow aggregation; 139 individuals 

tagged in this location (including translocated fish) were subsequently recaptured in the same 

aggregation (table 3). 

Although translocated humpback chub do show high apparent site fidelity, some move 

considerable distances, including returning to the LCR. One fish translocated to Shinumo Creek in 

2009 was captured at RM 128.2 (Middle Granite Gorge aggregation) in 2010, one fish translocated 

in 2009 was detected at a PIT-tag antenna in March 2012 in the LCR, and one fish translocated in 

2010 was detected at a PIT-tag antenna in the LCR in November 2013. 

Speckled dace (40 percent), flannelmouth sucker (29 percent), and humpback chub (11

percent) were the most common native species captured in this area (table 4D). Large numbers of 

speckled dace were captured by hoop net below the first falls in Shinumo Creek, whereas most 

humpback chub were captured in the main-stem Colorado River near Shinumo Creek.

The aggregation at Shinumo Creek appears to have increased since translocations began in 

2009, owing in large part to some of the 1,102 fish translocated to the creek upstream of the barrier 

falls moving downstream to the Colorado River. Modeled CPUE with trammel and hoop nets 

increased during Period 3, largely owing to capture of translocated humpback chub (fig. 7). Eight

ripe male humpback chub were collected in this aggregation, and several tuberculate and colored 

fish were also captured, suggesting that fish are in reproductive condition (table 6). A large debris 

flow in Shinumo Creek in 2015 scoured the stream channel and eliminated most of the fish from the 
13

system, including the translocated humpback chub (Brian Healy, Grand Canyon National Park, oral

commun., January 2016).

Stephen Aisle Aggregation

The Stephen Aisle aggregation was defined by Valdez and Ryel (1995) as distributed from 

below Garnet Canyon to Lower Blacktail Camp. Valdez and Ryel (1995) suggested that the 

occurrence of an aggregation in Stephen Aisle was because this reach of river represents one of the 

first stretches of river containing large eddy complexes and slower water velocities below the Inner 

Gorge, and it encompasses the vicinity of Elves Chasm Creek. The aggregation was defined based 

on capture of 7 juvenile and 17 adult humpback chub from 1990–93 (Valdez and Ryel, 1995). They 

were unable to estimate the abundance of humpback chub in this aggregation. During 2002 to 2014,

humpback chub in this aggregation ranged in size from 48 to 394 mm TL (mean = 239 mm) (fig. 

8E). Adult flannelmouth sucker made up the majority (64 percent) of the fish captured in Stephen 

Aisle during 1990–2014, followed by rainbow trout (11 percent) and humpback chub (9 percent) 

(table 4E). 

Since 1991, 98 humpback chub were PIT tagged in the Stephen Aisle aggregation, and only 

one was recaptured there. In addition, one humpback chub each was recaptured in the LCR, 

Shinumo Creek inflow, and Middle Granite Gorge aggregations (table 3). No ripe male or female 

humpback chub have been collected in this aggregation, although one tuberculate male and one 

male showing spawning colors were captured in 1991 and 1992, respectively (table 6). Modeled 

CPUE of humpback chub in trammel and hoop nets increased during Period 3 (figs. 4, 5).

Middle Granite Gorge Aggregation

More humpback chub were caught in Middle Granite Gorge than in any other aggregation 

except the LCR inflow (Table 4F). The Middle Granite Gorge aggregation, described by Valdez and 

Ryel (1995) from below Fossil rapid to Specter rapid, contained an estimated 98 adult humpback 

chub, based on mark-recapture studies from 1990 to 1993 (table 1). Most chub tagged in the Middle 

Granite Gorge aggregation were recaptured in Middle Granite Gorge. Two chub moved to Stephen 

Aisle, two fish moved to the Shinumo Creek inflow aggregation, eight to the LCR, and one to the 

LCR inflow aggregation (table 3). Of special note, 2 ripe male humpback chub were captured at this 

aggregation in September 2010 (215 and 236 mm TL) in a single baited hoop net with 47 other 

humpback chub (mean TL = 246, range 168 to 368 mm) on the downriver edge of a gravel debris 

fan located on river right at RM 128.2. The next highest catch of adults (≥200 mm) in a single 

baited hoop net in the main-stem Colorado River was 29 fish just upstream of the LCR confluence

during September 2014. Although no ripe female humpback chub have been captured in this 

aggregation, approximately 11 percent of the male chub captured were ripe, and tuberculate males 

and females were captured, suggesting that fish are in reproductive condition (table 6).

Flannelmouth sucker (46 percent), humpback chub (22 percent), and rainbow trout (14

percent) were the most common species captured by trammel nets and hoop nets in the Middle 

Granite Gorge aggregation (table 4F). Humpback chub were captured in all years the aggregation 

was sampled, and a relatively wide size range of chub was caught (range = 50 to 405 mm TL, mean 

= 248 mm) (fig. 8F). Modeled CPUE of humpback chub ≥200 mm TL in hoop nets and trammel 

nets increased at this aggregation during Period 3 (figs. 4, 5).
14

Havasu Creek Inflow Aggregation

Valdez and Ryel (1995) captured only seven adult humpback chub in this aggregation 

between 1990 and 1993 and were not able to generate a population estimate. They defined this 

aggregation as extending from about Last Chance Camp to the mouth of Havasu Creek, RM 156.2 

to 157.2 in Martin and Whittis (2007). We redefined the aggregation as extending from RM 155.8 

to 160 based on capture of 176 humpback chub from 1990–2014. Valdez and others (2000) 

identified Havasu Creek as a preferred tributary stream in Grand Canyon for establishment of a 

second spawning population of humpback chub. Humpback chub have been captured in the mouth 

of Havasu Creek since at least the late 1990s (Gorman and others, 2005). For example, 45

humpback chub were captured in the mouth of Havasu Creek during the months of June and

September in 1998 and 1999 (mean TL 185 mm, TL range 138–388 mm). Trammell and others

(2012) documented naturally occurring humpback chub approximately 3 miles upstream from the 

Colorado River in 2011. Fall spawning of flannelmouth sucker on gravel beds at the Havasu Creek 

outflow was documented by Douglas and Douglas (2000).

Flannelmouth sucker and speckled dace were the most abundant fish captured at this 

aggregation (table 4G), owing to high catches in the mouth of Havasu Creek, especially when 

flannelmouth sucker were ascending Havasu Creek presumably for spawning (Douglas and 

Douglas, 2000). Flannelmouth sucker made up 63 percent of the catch and humpback chub 6

percent of the catch. Humpback chub ranged in size from 123 to 440 mm TL (mean = 268 mm TL)

(fig. 8G). An increasing trend in CPUE across time periods for both hoop and trammel nets suggests 

increases in abundance at this aggregation. The increasing trend appears to be the result of an 

expansion of humpback chub downriver (below Havasu rapid) and from translocated fish (figs. 4–

6).

A total of 1,492 humpback chub, including 1,350 translocated individuals, were PIT tagged 

in Havasu Creek or in the Havasu Creek inflow aggregation (table 3). Eight chub moved from the 

Havasu inflow aggregation to the LCR or LCR inflow aggregation, and three chub also moved from 

the LCR to the Havasu Creek inflow aggregation (table 3). Seventy-three fish tagged in the Havasu 

Creek inflow aggregation were subsequently recaptured, and 64 of these were recaptured within the 

same aggregation. During June 2011, seven previously untagged humpback chub (mean = 257 mm 

TL) were captured in Havasu Creek just prior to a translocation release, and in 2013, two untagged 

juvenile chub (121 and 127 mm TL) were captured, suggesting that humpback chub spawned in 

Havasu Creek (Trammell and others, 2012; National Park Service, unpub. data January 2016). 

During 2013, 14 of 30 humpback chub captured in the Havasu Creek inflow aggregation had been 

previously translocated to Havasu Creek. Although no ripe humpback chub were captured at the 

Havasu Creek inflow aggregation, both colored and tuberculated males and females have been

collected, again suggesting that fish are in reproductive condition (table 6). A series of low falls 

occur in the lower end of Havasu Creek, but these are evidently not a barrier to movement, based on 

the recapture of fish in this stream.

Pumpkin Spring Aggregation

The Pumpkin Spring aggregation was described as a short stretch of river from the base of 

Little Bastard rapid (RM 212.5) to a short distance below Pumpkin Spring eddy. Based on capture 

of six adult humpback chub and two recaptures, five adult humpback chub were estimated to make 

up this aggregation. Valdez and Ryel (1995) and Ackerman (2008) sampled below the defined 

Pumpkin Spring aggregation area to about RM 214 but captured very few humpback chub. We 
15

extended the lower boundary of this aggregation to RM 216 (Three Springs Canyon) based on 

captures of 35 adult (≥200 mm TL) humpback chub between RM 213.2 and 216 from 2006 to 2014. 

Between 1990 and 2014, 92 humpback chub were captured, ranging in length from 42 to 

381 mm TL (mean = 173 mm TL) (fig. 8H). Relative abundance of humpback chub ≥200 mm TL 

increased during 2010–14, although there was very low hoop netting effort in this aggregation prior 

to 2010 (fig. 4, table 4H). Despite the low effort prior to 2010, the modeled CPUE shows an 

increase during the most recent time period (2010–13). Forty-one humpback chub have been 

implanted with PIT tags at this aggregation since 1991. Three of these fish have been recaptured 

within the aggregation (table 3), and none have been recaptured in other aggregations.

Flannelmouth sucker (56 percent) and speckled dace (30 percent) dominated the catch at the 

Pumpkin Spring aggregation (table 4H).

Humpback Chub Not Associated With Aggregations

The Colorado River through Marble and Grand Canyons has been sampled extensively with 

hoop nets, trammel nets, electrofishing, and seines since the late 1970s. We compiled humpback 

chub catch data to examine distribution of humpback chub in areas not associated with defined 

aggregations. During aggregation sampling trips, 3,046 humpback chub were captured at

aggregations during more than 107,000 net hours of effort. In contrast, only 102 were captured in 

areas not associated with revised aggregations during approximately 30,000 net hours of effort

(tables 4I, 4J). Our analysis of CPUE for both hoop and trammel net catches suggests that at some 

locations, CPUE of humpback chub not associated with aggregations may be similar to the CPUE at 

known aggregations. Continued sampling of both aggregations and non-aggregation locations will 

provide valuable information to discern the distribution and relative abundance of humpback chub. 

The capture of 105 adult chub near RM 34.5 in 2013 and 2014 illustrates the importance of periodic 

sampling outside of known aggregations, including tributaries; this new capture information has 

allowed us to extend the boundaries of several aggregations, including the 30-Mile aggregation.

Summary and Conclusions

Most Marble and Grand Canyon aggregations of humpback chub originally described by 

Valdez and Ryel (1995) appear to have increased in relative abundance since the 1990s. Based on 

trammel net and hoop net CPUE data, humpback chub numbers increased at most aggregations

compared to previous estimates. These increases were particularly visible in the post-2009 time 

period (Period 3). 

The LCR inflow aggregation displayed a pattern similar to the findings of Coggins and

Walters (2009) and Van Haverbeke and others (2013). That is, the LCR inflow aggregation 

experienced a decline in adult humpback chub abundance sometime between Periods 1 and 2 (1994 

to 2001). This decline was subsequently followed by a significant post-2006 increase in adult 

abundance. Interestingly, other main-stem aggregations did not show a decline between Periods 1 

and 2 but displayed uniformly depressed levels until Period 3.

Because many factors were in play, we cannot offer definitive answers as to why relative 

abundance of adult humpback chub increased during Period 3. However, the LCR experienced

relatively good production of age-0 chub during the 2003 to 2005 timeframe, and these fish 

appeared to recruit reasonably well into the larger size classes (Van Haverbeke and others, 2013). 

Main-stem water temperatures were unusually warm during 2003 to 2005 (Voichik and Wright,

2007), which potentially led to increased growth rates of juvenile chub (Clarkson and Childs, 2000) 
16

and presumably higher survival rates. The 2003 to 2006 time period was also accompanied by 

mechanical removal of salmonids in the LCR aggregation (Coggins and others, 2011; Yard and 

others, 2011), as well as a systemwide decline in trout abundance (Makinster and others, 2010). 

Because juvenile humpback chub may take several years to grow into adulthood in the main stem 

(Coggins and Pine, 2010), the opportunities afforded for increased growth and survival during 2002

to 2006 may have manifested as increases in adult humpback chub relative abundance during 2010

to 2014.

In addition, small humpback chub (<100 mm TL) have been commonly collected in areas 

not associated with aggregations (fig. 8B). During recent years, main-stem water temperature has 

frequently exceeded 16 °C at Middle Granite Gorge during summer months and has exceeded 16 °C 

at Pumpkin Spring during summer months approximately 50 percent of the time (Voichick and 

Wright, 2007; Wright and others, 2008). These conditions have likely allowed for greater survival 

of YOY chub and perhaps main-stem spawning by adults (Valdez and others, 2000). In 2014, for 

example, 66 likely YOY humpback chub (mean TL 55 mm, range TL 36–89 mm) were captured in 

western Grand Canyon between RM 213 and 244 as part of this project. Because only three likely 

YOY humpback chub were captured between the LCR (RM 61.5) and RM 212, this suggested that

local main-stem spawning had occurred in western Grand Canyon. Main-stem water temperatures 

were 16 to 20 oC from June to September in 2014 in western Grand Canyon, and fresh gravels

emitted from tributary fans may have functioned as spawning habitat.

Sampling humpback chub aggregations by trammel net and hoop net provides information 

on relative abundance, size distribution, and movement of humpback chub in areas of the main-stem 

Colorado River that otherwise might not be sampled. Sampling of aggregations also provides the 

opportunity to mark and recapture humpback chub with PIT tags systemwide. This has proven to be 

a critical tool in monitoring the status and trends of these disjunct groups of fish because results 

from this and previous studies show few of these fish move to the LCR inflow aggregation, thus are 

not included in population estimates generated using current methods (Van Haverbeke and others,

2013; Yackulic and others, 2014). Sampling Shinumo and Havasu Creek inflows also provides 

important information concerning the fate of humpback chub translocated to those tributaries. Data 

from these efforts have allowed us to quantify increases in humpback chub relative abundance at 

these aggregations and to determine that increases are due in part to translocation of approximately 

2,000 fish since 2009 and possibly to warmer river temperatures.

The 30-Mile aggregation is of special interest to managers because there is strong evidence 

of main-stem reproduction by humpback chub at this location (Valdez and Masslich, 1999; 

Andersen and others, 2010). Suttkus and others (1976) reported collecting young or small 

humpback chub at RM 44, and Carothers and Minckley (1981) reported humpback chub at Tiger 

Wash. In 1993 and 1994, age-0 humpback chub were captured in a backwater at RM 44.5

(Eminence Fault), and in 1994, age-1 humpback chub were captured in this same locality (Arizona 

Game and Fish Department, 1996). Andersen and others (2010) captured presumed age-0 and age-1 

humpback chub in 2006 and 2007 in backwaters between RM 30 and 56.5. These findings suggest 

some overwinter survival of fish thought to have originated near RM 30. Collection of adult 

humpback chub at previously unsampled areas near RM 35 suggests that the 30-Mile aggregation is 

larger than previously thought or has expanded in range, despite the high abundance of rainbow 

trout in Marble Canyon. Adult humpback chub were captured between RM 30 and 35 during each 

year sampled between 2000 and 2014, indicating high persistence of adult fish in this area. 

The LCR aggregation represents the majority of humpback chub found in the Colorado 

River downstream of Glen Canyon Dam. Based on movement of PIT-tagged humpback chub 
17

between the LCR inflow aggregation and the Lava Chuar to Hance aggregation and size of fish 

captured, we believe the Lava Chuar to Hance aggregation represents a geographic extension of the 

LCR aggregation and not a disjunct group of fish. By our broader definition this aggregation, it

would extend from RM 57 to 77.1, or from downstream of Kwagunt rapid to the top of Hance rapid.

Extremely low catches of humpback chub near the Bright Angel Creek inflow, as well as 

movement of fish from the Bright Angel Creek inflow aggregation to the LCR, suggest that this 

area does not currently support a disjunct aggregation of humpback chub. It is likely, however, that 

the area supported humpback chub in the past (see Miller, 1946) and perhaps could do so in the 

future. Electrofishing catch rates of brown trout have increased in the main-stem Colorado River 

near the confluence of Bright Angel Creek since 2006 (Makinster and others, 2010). Brown trout in 

the Colorado River are highly piscivorous (Yard and others, 2011), and it has been postulated that 

they limit humpback chub and other native fish near Bright Angel Creek. The National Park Service

has been experimentally removing rainbow trout and brown trout from Bright Angel Creek as a 

potential management tool to benefit native fishes (Omana-Smith and others, 2012). 

The Shinumo Creek inflow aggregation was originally defined as a very short reach (120 m) 

of the main-stem Colorado River. Recapture data from tagged humpback chub suggest expansion of 

the area of the aggregation upstream to RM 107.8 and downstream to RM 110 is warranted. This 

expansion is due in part to some of the humpback chub that have been translocated to Shinumo 

Creek since 2009, emigrating and dispersing into the main-stem Colorado River near the confluence 

of this tributary. 

The Stephen Aisle aggregation may represent a disjunct group of humpback chub or could

represent an area these fish simply traverse. It is rare to recapture humpback chub previously tagged 

in this aggregation. Because of few recaptures of fish tagged in this aggregation and the absence of 

gravid males or females, we are hesitant to define this location as a disjunct humpback chub 

aggregation. We do, however, recommend continued sampling in the area to better understand its 

use by humpback chub. Alternatively, the Stephen Aisle aggregation may be an extension of the 

Middle Granite Gorge aggregation that may become more evident as the numbers of tagged fish in 

these areas increase.

Catches near the Middle Granite Gorge aggregation were primarily in the same area 

described by Valdez and Ryel (1995), thus we do not propose to change the range of this 

aggregation. Over the duration of our study, humpback chub catches in the Middle Granite Gorge 

aggregation were second only to those in the LCR inflow aggregation. Humpback chub relative 

abundance at this aggregation increased during Period 3 (2010–13). In 2010, hoop net CPUE was 

strongly influenced by a single net with an unusually large catch of humpback chub (49 fish). Large 

catches of adult humpback chub in single nets are very uncommon except in the LCR during 

spawning season. It is possible that a gravid female humpback chub was present in this net and 

other fish were attracted to it by pheromones released by the gravid fish (Sorensen and Stacey, 

2004).

The original characterization of the Havasu Creek inflow aggregation was based on the 

capture of only a small number of adult humpback chub directly upstream of the Havasu inflow. 

Catches at this site remained low during monitoring efforts in the early to mid-2000s. 

Translocations of humpback chub into Havasu Creek by the National Park Service since 2009 have

resulted in dramatic increases in abundance in this adjoining aggregation. Initial captures of 

humpback chub and recaptures of previously tagged fish have consistently occurred over a broader 

range than the original definition of this aggregation, thus we recommend increasing its breadth

such that it extends from RM 155.8 to 160, or inclusive of a small reach of river downstream of the 
18

Havasu Creek inflow. As observed at the Shinumo Creek inflow aggregation, expansion here is due 

in part to some translocated humpback chub emigrating downstream and dispersing into the 

Colorado River. The survival and persistence of translocated humpback chub in both the Shinumo 

Creek and Havasu Creek inflow aggregations as well as increases in abundance at these sites

suggest translocations can play an important role in the maintenance and expansion of humpback 

chub populations in Grand Canyon. 

As with several other sites, we propose to enlarge the range of the Pumpkin Spring 

aggregation, based on catches of untagged and tagged humpback chub. The upstream extent of the 

aggregation would remain at RM 212.5, while the downstream boundary would be extended to RM 

216. 

Continued sampling of the aggregations is needed to provide critical information regarding 

the status and trends of this key component of the Grand Canyon humpback chub population. The 

intensity and frequency of future sampling should be determined by identifying what information is

needed by managers and decision makers and at what level of precision. If there is interest in

estimating capture probabilities to generate survival and population estimates within individual 

aggregations, then sampling should be structured to achieve that objective by scheduling more trips 

and attempting to generate population estimates using mark-recapture methodologies at

aggregations where catches might support an estimate (for example, 30-Mile, Middle Granite

Gorge, and Shinumo Creek inflow). Although thoroughly investigating capture probability (p) 

values for individual aggregations may be enticing to pursue, it may not provide reliable abundance 

estimates because of the low numbers of fish captured in the smaller aggregations. In addition, from 

a conservation standpoint, care should be taken to balance the level of effort required to answer 

questions of management interest with concerns of handling small populations of endangered 

species. If there is interest in determining if aggregations house humpback chub that are 

reproductively active and successfully producing offspring that are recruiting in the main-stem 

Colorado River, separate from the LCR population, efforts might be better directed at sampling 

when fish are likely to be gravid and in spawning condition (June and July; Valdez and Ryel 1995). 

Additionally, otolith microchemistry of young of the year chub offers a promising approach to 

discern whether fish are successfully spawning and recruiting in the main-stem Colorado River. 

Acknowledgments 

We thank Scott VanderKooi, Lew Coggins, Rich Valdez, and John Beeman for insightful 

comments on earlier versions of this manuscript. Data presented here were collected by a variety of

agencies and investigators, and we thank all those involved. We especially thank the field crews 

responsible for collecting the data upon which this report relies. We thank Tom Gushue for GIS 

assistance and Glenn Bennett for assistance with data compilation. 
19

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24

Table 1. Original and revised locations of nine main-stem humpback chub aggregations, including river 

mile boundaries, estimates of adult abundance (N), and 95 percent confidence interval estimated by Valdez 

and Ryel (1995). 

[Revised boundaries are indicated as bolded river miles and based on Martin and Whitis (2007)]

Aggregation Aggregation 

boundaries1

Redefined 

aggregation 

boundaries

N 95% confidence 

interval

30-Mile 29.8–31.3 29.8–36.3 52 24–136

Little Colorado River inflow 57.0–65.4 57.0–77.2 3,482 2,682–4,281

Lava Chuar to Hance 65.7–76.3 -- -- --

Bright Angel Creek inflow 83.8–92.2 -- -- --

Shinumo Creek inflow 108.1–108.6 107.8–110 57 31–149

Stephen Aisle 114.9–120.1 -- -- --

Middle Granite Gorge 126.1–129.0 125.0–129.7 98 74–153

Havasu Creek inflow 155.8–156.7 155.8–159.2 13 5–70

Pumpkin Spring 212.5–213.2 212.5–216.0 5 4–16

1

Defined by Valdez and Ryel (1995)
25

Table 2. Sampling dates; number of days sampled; mean, minimum, and maximum 15-minute discharge

values; range in discharge in ft3/s; and mean water temperature (oC) during each sampling trip measured at 

the U.S. Geological Survey Grand Canyon gage (http://waterdata.usgs.gov/az/nwis/uv?site_no=09402500) 

during humpback chub aggregation surveys, 2002–14.

Year Start date End date

Trip 

duration 

in days

Mean

discharge

Minimum

discharge

Maximum

discharge

Range in 

discharge

Mean

temperature 

at RM 61

2002 12–Sep–02 24–Sep–02 12 10,103 6,670 14,400 7,730 13.6

2003 14–Jun–03 30–Jun–03 16 14,787 10,500 19,100 8,600 14.0

2004 12–Jun–04 28–Jun–04 16 14,057 10,200 17,700 7,500 13.1

2006 3–Jun–06 19–Jun–06 16 14,103 10,613 17,634 7,021 13.4

2010 11–Sep–10 27–Sep–10 16 8,854 8,410 11,200 2,790 14.1

2011 9–Sep–11 24–Sep–11 15 17,001 16,200 19,300 3,100 15.6

2012 7–Sep–12 24–Sep–12 17 8,835 8,250 13,100 4,850 13.7

2013 20–Jul–13 5–Aug–13 16 15,353 10,400 22,000 11,600 13.6

2013 7–Sep–13 23–Sep–13 16 11,745 7,590 19,100 11,510 14.1

2014 19–Jul–14 3–Aug–14 15 13,754 10,600 17,300 6,700 14.6

2014 6–Sep–14 21–Sep–14 15 10,827 7,070 13,800 6,730 15.3
26

Table 3. Tag and recapture locations with river miles in parentheses for humpback chub recaptured 14+ days after tagging, 1991 to 2014. 

[Numbers in bold represent fish captured and recaptured in the same location (for example, apparent site fidelity). Numbers above the diagonal indicate 

downstream movement, whereas numbers below the diagonal indicate upstream movement. Includes fish translocated to Shinumo Creek and Havasu Creek. 

Total recaptures column includes fish recaptured more than once, but individual fish are only counted once within the same aggregation]

Location tagged

Number 

tagged

Location recaptured

Total

recaptures

Percentage 

of fish 

recaptured 

in same 

river reach 

as captured

30-Mile

(RM 

29.8–

31.3)

Little 

Colorado 

River 

inflow

(RM 57–

77.2)

Little 

Colorado 

River

Bright 

Angel 

Creek 

inflow

(RM 

83.8–

92.2)

Shinumo 

Creek 

inflow

(RM 

107.8–

110.0)

Stephen 

Aisle

(RM 

114.9–

121.0)

Middle 

Granite 

Gorge

(RM 

125.0–

129.0)

Havasu 

Creek 

inflow

(RM 

155.8–

160) 

Pumpkin 

Spring 

(RM 

212.5–

215.1) .

30-Mile (RM 29.8–

31.3) 162 43 0 1 0 0 0 0 0 0 44 97.7%

Little Colorado 

River inflow

(RM 57–77.2)

6,250 7 1,417 2,449 0 0 1 1 1 0 3,876 36.6%

Little Colorado 

River 48,941 4 1,815 21,413 1 1 1 1 3 0 23,239 92.1%

Bright Angel Creek 

inflow (RM 

83.8–92.2)

33 0 3 5 0 0 0 1 0 0 9 0.0%

Shinumo Creek 

inflow (RM 

107.8–110.0)

1,233 0 2 8 0 139 3 6 0 0 158 88.0%

Stephen Aisle (RM 

114.9–121.0) 98 0 0 1 0 1 1 1 0 0 4 25.0%

Middle Granite 

Gorge (RM 

125.0–129.0)

370 0 1 8 0 2 2 68 0 0 81 84.0%

Havasu Creek 

inflow (RM 

155.8–160)

1,492 0 3 5 0 0 0 1 64 0 73 87.7%

Pumpkin Spring 

(RM 212.5–

215.1)

41 0 0 0 0 0 0 0 0 3 3 100.0%

Total 58,620 54 3,241 23,890 1 143 8 79 68 3 27,487 
27

Table 4. Number of fish captured and hours fished by hoop net and trammel net at each aggregation (A–H) and totaled 

for all sites (I–J), 1990–2014. [River miles (RM) sampled are given for each aggregation in parenthesis. Species are listed in order of total abundance at each 

aggregation]

A. 30-Mile (29.8–36.3)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Rainbow trout 215 64 35 59 6 67 79 160 704 146 1,535 

Flannelmouth sucker 9 -- 5 5 4 22 16 18 51 319 449 

Humpback chub 19 5 3 3 -- 9 16 3 41 88 187 

Bluehead sucker 1 -- -- -- 1 -- -- 1 2 1 6 

Common carp 7 -- -- -- -- -- -- -- -- -- 7

Brown trout -- -- 1 -- -- -- -- -- 1 -- 2 

Total 251 69 44 67 11 98 111 182 799 554 2,186 

Hoop net hours 450 1,394 1,274 590 376 494 462 3,484 2,993 11,517 

Trammel net hours 232 45 117 110 53 56 55 43 193 73 977 

Total net hours 232 495 1,511 1,384 643 432 549 505 3,677 3,066 12,494 

B. Little Colorado River inflow (57–77.2)

Species 1990-93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Humpback chub 1,099 27 7 33 105 112 239 103 20 222 1,967 

Flannelmouth sucker 591 2 1 12 173 185 107 92 17 226 1,406 

Rainbow trout 1,040 18 9 8 19 81 87 19 9 22 1,312 

Bluehead sucker 109 17 -- -- 55 15 42 10 3 11 262 

Fathead minnow -- 2 -- -- 33 1 10 1 -- 1 48 

Brown trout 24 -- -- 1 1 -- -- -- -- -- 26 

Common carp 20 1 -- -- 1 -- 1 -- -- -- 23 

Channel catfish 19 -- -- -- -- -- -- -- -- -- 19 

Speckled dace 1 -- -- 1 11 -- -- 1 -- -- 14 

Black bullhead 1 -- -- 1 1 1 -- 2 -- -- 6 

Flannelmouth razorback hybrid 3 -- -- -- -- -- -- -- -- -- 3 

Plains killifish -- 2 -- -- -- -- -- -- -- -- 2 

Total 2,907 69 17 56 399 395 486 228 49 482 5,088 

Hoop net hours 2 1,225 603 590 2,873 1,237 2,009 2,509 2,053 2,230 15,331 

Trammel net hours 4,158 171 58 60 235 187 152 29 -- 27 5,077 

Total net hours 4,160 1,396 661 650 3,108 1,424 2,161 2,538 2,053 2,257 20,407 
28

C. Bright Angel Creek inflow (83.8–92.2)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 135 -- 4 -- 4 18 4 -- 7 99 271 

Brown trout 151 -- 19 5 1 4 2 -- -- -- 182 

Rainbow trout 75 -- 11 5 -- 11 1 1 6 -- 110 

Bluehead sucker 16 -- 1 1 3 20 4 -- -- -- 45 

Humpback chub 8 -- 2 2 6 1 2 -- 1 3 25 

Speckled dace -- -- -- -- 2 16 4 -- -- -- 22 

Common carp 2 -- 1 -- -- 2 -- -- -- -- 5 

Fathead minnow -- -- -- -- 2 -- -- -- -- -- 2 

Flannelmouth razorback 

hybrid 1 -- -- -- -- -- -- -- -- -- 1 

Channel catfish -- -- 1 -- -- -- -- -- -- -- 1 

Total 253 -- 35 13 14 54 13 1 7 3 393 

Hoop net hours 186 -- 569 292 628 376 570 34 1,115 543 4,314 

Trammel net hours 631 -- 57 19 56 59 20 -- 25 -- 867 

Total net hours 817 -- 626 311 684 435 590 34 1,141 543 5,181 

D. Shinumo Creek inflow (107.8–110.0)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Speckled dace -- -- 6 -- 3 264 -- 471 -- -- 744 

Flannelmouth sucker 32 -- 5 6 13 27 27 309 15 96 530 

Rainbow trout 112 1 47 11 -- 37 12 8 1 6 235 

Humpback chub 24 2 20 6 2 41 51 33 11 7 197 

Bluehead sucker 24 1 1 -- 3 14 9 16 2 -- 70 

Brown trout 14 -- 10 2 1 -- -- -- -- -- 27 

Fathead minnow -- -- -- -- -- 1 -- 18 -- -- 19 

Common carp 6 1 3 -- 2 1 1 3 -- 1 18 

Total 212 5 92 25 24 385 100 858 29 110 1,840 

Hoop net hours 278 609 2,195 1,436 628 1,042 1,532 3,509 252 861 12,341 

Trammel net hours 937 27 178 115 59 57 46 -- 22 -- 1,440 

Total net hours 1,215 635 2,373 1,551 687 1,099 1,578 3,509 274 861 13,781 
29

E. Stephen Aisle (114.9–121.0)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 17 -- -- 15 41 117 50 30 79 301 650 

Rainbow trout 37 2 -- 14 5 29 9 1 10 9 116 

Humpback chub 12 -- -- 5 11 19 11 3 11 15 87 

Bluehead sucker 4 5 -- 2 8 29 18 -- 1 -- 67 

Speckled dace -- -- -- -- 25 4 -- -- 2 1 32 

Common carp 11 1 -- 2 2 2 1 4 -- 3 26 

Fathead minnow -- -- -- -- 10 3 1 1 1 -- 16 

Brown trout 3 -- -- 9 1 1 1 -- 1 -- 16 

Flannelmouth razorback 

hybrid -- -- -- -- -- -- -- -- -- 2 2 

Channel catfish -- -- -- -- 1 -- -- -- -- -- 1 

Black bullhead -- -- -- -- -- -- 1 -- -- -- 1 

Total 84 8 -- 47 104 204 92 39 105 331 1,014 

Hoop net hours -- 641 -- 1,408 1,300 1,307 1,832 704 2,536 1,384 11,112 

Trammel net hours 456 56 -- 118 111 120 103 7 -- -- 972 

Total net hours 456 697 -- 1,526 1,412 1,427 1,935 711 2,536 1,384 12,084 

F. Middle Granite Gorge (125.0–129.7)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 35 1 15 17 51 101 30 99 134 191 674 

Humpback chub 112 10 29 17 13 73 8 33 6 11 312 

Rainbow trout 33 1 54 18 -- 47 9 27 7 6 202 

Bluehead sucker 12 2 6 3 2 18 41 8 1 -- 93 

Speckled dace -- -- 1 -- 49 20 3 1 2 1 77 

Common carp 35 1 3 -- 2 -- -- 2 2 3 48 

Brown trout 7 2 13 9 -- 3 -- -- -- -- 34 

Fathead minnow -- -- -- -- 6 1 -- -- -- -- 7 

Channel catfish -- -- -- 1 -- 1 -- -- -- -- 2 

Green sunfish -- -- -- 1 -- -- -- -- -- -- 1 

Total 234 17 121 66 123 264 91 170 153 1,451 

Hoop net hours -- 645 3,105 3,137 1,431 1,292 1,663 1,579 2,331 1,019 16,201 

Trammel net hours 1,270 65 2,436 208 116 117 86 80 17 -- 4,393 

Total net hours 1,270 709 5,540 3,345 1,547 1,409 1,749 1,659 2,347 1,019 20,594 
30

G. Havasu Creek inflow (155.8–160)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 132 3 29 17 32 206 118 347 687 174 1,745 

Speckled dace 7 1 5 -- 51 189 85 27 1 1 367 

Bluehead sucker 184 -- 3 1 6 27 16 7 5 -- 249 

Humpback chub 6 5 4 2 1 18 50 35 46 9 176 

Rainbow trout 11 -- 19 13 1 62 14 15 18 7 160 

Common carp 21 2 2 4 -- 1 -- 2 6 3 41 

Channel catfish -- -- 2 1 3 2 -- -- -- -- 8 

Fathead minnow -- -- -- -- 5 1 1 -- -- -- 7 

Striped bass 1 -- -- 1 -- -- -- -- -- -- 2 

Brown trout -- -- 1 -- -- 1 -- -- -- -- 2 

Red shiner -- -- -- -- 1 -- -- -- -- -- 1 

Total 362 11 65 39 100 507 284 433 763 194 2,758 

Hoop net hours 111 608 2,479 2,202 950 1,255 1,401 1,448 2,368 478 13,300 

Trammel net hours 795 57 147 149 73 110 55 36 34 -- 1,457 

Total net hours 906 665 2,626 2,351 1,024 1,365 1,456 1,485 2,402 478 14,758 

H. Pumpkin Spring (212.5–216.0)

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 18 2 -- -- 11 72 60 245 248 384 1,040 

Speckled dace -- 2 -- -- 2 151 39 54 13 284 545 

Humpback chub 6 -- -- -- 2 18 10 9 5 42 92 

Common carp 33 3 -- -- 3 -- -- 9 3 5 56 

Fathead minnow -- -- -- -- -- 19 18 1 -- 7 45 

Channel catfish 23 3 -- -- 8 1 1 -- -- -- 36 

Bluehead sucker 1 -- -- -- -- 7 2 1 -- 1 12 

Striped bass 3 -- -- -- 5 -- -- -- -- -- 8 

Rainbow trout 4 -- -- -- -- -- -- -- -- 3 7 

Black bullhead -- -- -- -- 3 -- -- -- -- 1 4 

Flannelmouth razorback 

hybrid -- -- -- -- -- -- -- 1 -- -- 1 

Total 88 10 -- -- 34 268 130 320 269 727 1,846 

Hoop net hours -- 623 -- -- 557 342 703 2,043 1,804 1,235 7,308 

Trammel net hours 622 57 -- -- 56 57 43 -- -- -- 836 

Total net hours 622 681 -- -- 613 399 746 2,043 1,804 1,235 8,144 
31

I. Total at all aggregations

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 971 8 59 72 329 748 412 1,140 1,238 1,790 6,767 

Rainbow trout 1,527 86 175 128 31 334 211 231 755 199 3,677 

Humpback chub 1,289 49 65 68 140 291 387 219 141 397 3,046 

Speckled dace 8 3 12 1 143 644 131 554 18 287 1,801 

Bluehead sucker 354 25 11 7 78 130 132 43 14 13 807 

Brown trout 199 2 44 26 4 9 3 -- 2 -- 289 

Common carp 135 9 9 6 10 6 3 20 11 15 224 

Fathead minnow -- 2 -- -- 56 26 30 21 1 8 144 

Channel catfish 42 3 3 2 12 4 1 -- -- -- 67 

Black bullhead 4 -- -- 1 5 -- -- -- -- 1 11 

Striped bass 1 -- -- 1 4 1 1 2 -- -- 10 

Plains killifish 4 -- -- -- -- -- -- 1 -- 2 7 

Flannelmouth razorback 

hybrid -- 2 -- -- -- -- -- -- -- -- 2 

Unidentified sucker -- -- -- -- -- -- -- -- -- 1 1 

Red shiner -- -- -- -- -- -- -- -- 1 -- 1 

Green sunfish -- -- -- -- 1 -- -- -- -- -- 1 

Total 4,534 189 378 312 813 2,193 1,311 2,231 2,181 2,713 16,855 

Hoop net hours 606 4,801 10,345 10,339 8,957 7,226 10,205 12,290 15,943 10,742 91,454 

Trammel net hours 9,106 477 2,993 779 759 763 560 195 290 99 16,022 

Total net hours 9,711 5,278 13,338 11,118 9,716 7,990 10,764 12,485 16,234 10,842 107,476 
32

J. Total at locations other than aggregations

Species 1990–93 2002 2003 2004 2006 2010 2011 2012 2013 2014 Total

Flannelmouth sucker 298 6 26 18 18 157 105 24 567 1,635 2,854 

Rainbow trout 508 30 3 10 1 113 28 50 155 161 1,059 

Speckled dace 20 2 15 33 42 142 52 10 11 512 839 

Common carp 253 6 9 4 -- 17 -- -- 4 4 297 

Channel catfish 177 1 4 4 -- 1 -- -- -- -- 187 

Bluehead sucker 140 21 1 1 -- 2 6 -- 11 2 184 

Humpback chub 5 8 2 -- 1 5 3 1 19 58 102 

Red shiner -- -- -- -- -- 72 5 -- -- 1 78 

Striped bass 40 -- -- -- -- 1 -- -- -- -- 41 

Fathead minnow -- 2 -- -- 6 8 9 -- -- 11 36 

Brown trout 8 11 -- -- -- -- -- -- 1 -- 20 

Unidentified sucker -- -- -- -- -- -- -- -- -- 15 15 

Black bullhead 3 -- -- -- -- -- -- -- -- -- 3 

Walleye 2 -- -- -- -- -- -- -- -- -- 2 

Threadfin shad 1 -- -- -- -- -- -- -- -- -- 1 

Largemouth bass 1 -- -- -- -- -- -- -- -- -- 1 

Black crappie 1 -- -- -- -- -- -- -- -- -- 1 

Total 1,457 87 60 70 68 518 208 85 768 2,399 5,720 

Hoop net hours 929 1,981 1,452 1,102 624 845 969 1,378 6,377 7,158 22,816 

Trammel net hours 5,850 225 152 151 41 135 63 6 226 53 6,902 

Total net hours 6,779 2,206 1,604 1,253 665 981 1,032 1,384 6,603 7,210 29,717 
33

Table 5. Alternative models of adult humpback chub catch. 

[ΔAICc, Akaike information criterion corrected for small sample sizes relative to the model with the smallest AICc;

K, number of parameters]

Model ΔAICc K

Gear + Period + Location – Non-Aggregation + LCR Inflow:Period 0 15

Gear + Period + Location + LCR Inflow:Period 0.9 23

Gear + Period + Location – Non-Aggregation 5.06 13

Gear + Period + Location 5.13 21

Gear + Year + Location – Non-Aggregation 11.44 23

Gear + Year + Location 13.73 31

Gear + Period + Aggregation + LCR Inflow 19.21 7

Gear + Year + Aggregation + LCR Inflow 28.09 17

Gear + Period + Aggregation 33.84 6

Gear + Year + Aggregation 41.36 16

Gear + Location 65.56 19

Gear + Location – Non-Aggregation 67.26 11

Gear + Aggregation + LCR Inflow 75.29 5

Gear + Aggregation 81.07 4

Gear + Period + LCR Inflow 116.97 6

Gear + Year + LCR Inflow 133.51 16

Gear + LCR Inflow 174.51 4

Gear + Period 178.16 5

Gear + Year 192.01 15
34

Table 6. Number and percent of humpback chub classified by sexual condition and secondary sexual characteristics by 

location of capture with river miles in parentheses, 1977 to 2014. 

[Please note that various investigators made visual determinations of these morphologic features and may have made slightly different 

determinations; also not all investigators consistently collected this information; and the information varied with season. R = Ripe: 

Extrudes gametes with gentle pressure. S = Spent: fish appears to have released gametes. NR = Not ripe: no gametes extruded. U = 

Undetermined. C = Color: Orange color at base of pelvic and pectoral fins and on belly. T = Tuberculate: Tubercles present. B= Both 

colored and tuberculated]

Aggregation

Female Total 

number 

of 

females

Percent 

of ripe 

females

Sexual condition Sexual characteristics

R S NR U C T B

30-Mile (RM 29.8–36.3) 4 1 40 31 8 2 76 5.3%

Little Colorado River inflow (RM 57.0–

77.2) 7 8 196 1,072 35 22 1 1,283 0.5%

Little Colorado River 402 335 17,178 7,941 2,019 410 110 25,856 1.6%

Bright Angel Creek inflow (RM 83.8–92.2) 1 2 3 0.0%

Shinumo Creek inflow (RM 107.8–110.0) 35 32 6 1 1 67 0.0%

Stephen Aisle (RM 114.9–121.0) 11 13 24 0.0%

Middle Granite Gorge (RM 125.0–129.7) 12 83 2 3 95 0.0%

Havasu Creek inflow (RM 155.8–159.2) 20 22 1 2 42 0.0%

Pumpkin Spring (RM 212.5–216.0) 1 4 3 8 12.5%

Aggregation

Male

Total 

number 

of males

Percent 

of ripe 

males

Sexual condition Sexual characteristics

R S NR U C T B

30-Mile (RM 29.8–36.3) 11 41 27 2 11 2 79 13.9%

Little Colorado River inflow (RM 57.0–

77.2) 25 2 139 965 28 81 1,131 2.2%

Little Colorado River 7,560 29 13,415 6,934 2,122 3,755 2,363 27,938 27.1%

Bright Angel Creek inflow (RM 83.8–92.2) 1 7 5 8 12.5%

Shinumo Creek inflow (RM 107.8–110.0) 8 52 22 23 4 6 82 9.8%

Stephen Aisle (RM 114.9–121.0) 3 11 1 1 14 0.0%

Middle Granite Gorge (RM 125.0–129.7) 11 4 85 8 100 11.0%

Havasu Creek inflow (RM 155.8–159.2) 9 25 1 4 34 0.0%

Pumpkin Spring (RM 212.5–216.0) 3 1 1 4 0.0%
35

Figure 1. Map of study area identifying Glen Canyon Dam, the Little Colorado River, and the original nine 

humpback chub aggregation locations.
36

Figure 2. Original (black bars) and revised (gray bars) aggregation boundaries. Original from Valdez and Ryel 

(1995), revised from this study.

1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221

River Mile

Original Revised
37

Figure 3. Length-frequency distribution of humpback chub captured by hoop nets (panel A) and trammel 

nets (panel B) in the Colorado River between Lees Ferry and Lake Mead from 1980 to 2014.
38

Figure 4. Predicted catch of adult humpback chub per 24 hours for hoop nets during three time periods

(Period 1, 1991–93, in black; Period 2, 2002–6, in gray; Period 3, 2010–13, in light gray). Error bars 

represent 95 percent confidence intervals. “Non-Aggregation” includes all locations not previously defined 

as an aggregation.
39

Figure 5. Predicted catch of adult humpback chub per 2 hours for trammel nets during three time periods

(Period 1, 1991–93, in black; Period 2, 2002–6, in gray; Period 3, 2010–13, in light gray). Error bars 

represent 95 percent confidence intervals. “Non-Aggregation” includes all locations not previously defined 

as an aggregation.
40

Figure 6. Pridicted catch of adult humpback chub per 24 hours comparing the terms “Location – NonAggregation” and “Location” from the two most highly supported models (table 5). The estimates are shown 

for a common gear (hoop nets) and time period (Period 3) to facilitate comparison. The term “Location –

Non-Aggregation” groups all non-aggregation locations together forming one estimate (above horizontal 

dashed line). The term “Location” estimates each river location seperately. Unbounded estimates are due 

to sections of river with low or no catches of adult humback chub.
41

Figure 7. Comparison of predicted catch rates for hoop nets (catch/24 hours) and trammel nets (catch/2 

hours), showing the effect of translocated fish at Shinumo Creek and Havasu Creek inflow aggregations. 

Point estimates and 95 percent confidence intervals for both hoop and trammel are from a generalized 

linear model (negative binomial errors) fit with all fish (black points and error bars) and fit without 

translocated fish (gray points and error bars).
42

Figure 8. Catch per unit effort (fish/hour) of humpback chub ≥300 millimeters (mm) total length (TL)

(panel A) captured by trammel net, and catch per unit effort (fish/hour) of humpback chub ≤100 mm TL 

(panel B) captured by hoop net by river mile main-stem Colorado River, 1981 to 2014. Note the y-axis of 

panel B is log scale. Vertical lines represent upstream boundary of aggregations and approximate river 

miles (RM) at 30-Mile (RM 30), Little Colorado River (RM 61), Shinumo Creek (RM 108), Middle Granite 

Gorge (RM 127), Havasu Creek (RM 157), and Pumpkin Springs (RM 213). Each point represents an

individual sample. 

A

B
43

Figure 9. Frequency of humpback chub captures by size class at each aggregation on the Colorado 

River between Lees Ferry and Lake Mead, 1990 to 2014 aggregation sampling trips. Number of fish 

captured may include individual fish captured more than once. Panel A, 30-Mile (river mile [RM] 29.8–36.3);

panel B, Little Colorado River inflow (RM 57–77.2); panel C, Bright Angel Creek inflow (RM 83.8–92.2);

panel D, Shinumo Creek inflow (RM 107.8–110.0); panel E, Stephen Aisle (RM 114.9–121.0); panel F,

Middle Granite Gorge (RM 125.0–129.7); panel G, Havasu Creek inflow (RM 155.8–159.2); panel H,

Pumpkin Spring (RM 212.5–216.0).

n=152

n=1,967

n=25

n=197

n=87

n=312

n=176

n=92