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APPENDIX D: 

PROPOSED ACTION TRIGGERS FOR THE MANAGEMENT OF HUMPBACK CHUB 

COLORADO RIVER, GRAND CANYON 

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APPENDIX D 

DRAFT FINAL 

Proposed Action Triggers for the Management of Humpback Chub 

Colorado River, Grand Canyon 

November 2015 

Developed by an Ad Hoc group of Grand Canyon Aquatic 

Biologists from USFWS, USGS-GCMRC, AZGFD, NPS, USBR 

(Kirk Young, David Van Haverbeke, Scott VanderKooi, David Ward, 

Charles Yackulic, Mike Yard, Brian Healy, Melissa Trammel, 

David Rogowski, Marianne Crawford) 

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PURPOSE 

Mechanical removal of nonnative species is a controversial issue in the Colorado River through 

Glen and Grand Canyons. A spring 2015 meeting of Grand Canyon biologists (NPS, USFWS, 

AZGFD, GCMRC) to assess current trout removal triggers resulted in a concept of early 

conservation measure intervention to maximize conservation benefit to humpback chub (HBC) 

and minimize the likelihood of mechanical predator removal. 

Many factors affect HBC population dynamics such as water temperature, turbidity, and water 

volume in the Little Colorado River (LCR). This restrains available conservation actions that can 

be implemented in the event of a declining population of HBC. We can translocate juveniles and 

young of the year to other areas within and outside the LCR system, juvenile HBC can be headstarted at a hatchery, and we can attempt to remove predators. Other conservation tools may 

include parasite control (although this is unlikely from a population standpoint), non-native fish 

control in the LCR, and protect from over-utilization for scientific purposes. 

Methods to actively manage temperature releases from Glen Canyon Dam sediment 

augmentation below the Paria River are not included in the Long-Term Experimental 

Management Program (LTEMP), for Glen Canyon Dam. Inclusion of infrastructure options 

including these were eliminated from detailed study in the LTEMP alternatives for a variety of 

reasons. We mention them here because these methods may still represent the most important 

potential conservation tools that could be used for the long term conservation of HBC in Grand 

Canyon and the concepts should not be lost. 

While healthy wildlife populations are rarely static, trigger objectives include prescribing actions 

to reverse/ameliorate impacts in order to maintain the LCR HBC population within an acceptable 

range; and, secondarily to reduce reliance on mechanical removal of predators. For the purposes 

of these triggers, it is assumed that the primary drivers of HBC population dynamics are 

interspecific interactions with non-native species, especially rainbow trout, and low water 

temperature in the mainstem of the Colorado River (Kaeding and Zimmerman 1983; Douglas 

and Marsh 1996; USFWS 2002; Coggins et al. 2011; Yard et al. 2011). It is suspected that cold 

water temperatures suppress growth and thus subject young HBC to predation for extended 

periods of time. The approach described here puts the emphasis on managing humpback chub as 

opposed to managing predators. Predator removal will only occur if other conservation measures 

do not appear to be effective in maintaining targeted HBC population levels. 

Two Tier Approach 

Two tiers of sequential actions were identified; the first would emphasize conservation actions 

that would take place early during an adult or sub-adult HBC population decline. The second tier 

would serve as a backstop prescribing predator removal (Threat Reduction) if conservation 

measures did not mitigate a decline in HBC abundance. 

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ACTION TRIGGERS 

Tier 1 Trigger – Early Intervention Through Conservation Actions: 

1a. If the combined point estimate for adult HBC (adults defined ≥200 mm) in the Colorado 

River mainstem LCR aggregation; RM 57-65.9) and Little Colorado River (LCR) falls 

below 9,000 as estimated by the currently accepted HBC population model (e.g., ASMR, 

multi-state). 

-OR1b. If recruitment of sub-adult HBC (150-199mm) does not equal or exceed estimated adult 

mortality such that: 

1) Sub-adult abundance falls below a three-year running average of 1,250 fish in the 

spring LCR population estimates. 

-OR2) Sub-adult abundance falls below a three-year running average of 810 fish in the 

mainstem Juvenile Chub Monitoring reach (JCM annual fall population estimate; RM 

63.45-65.2). 

Tier 1 Trigger Response: Tier 1 conservation actions listed below will be immediately 

implemented either in the LCR or in the adjacent mainstem. Conservation actions will focus on 

increasing growth, survival and distribution of HBC in the LCR & LCR mainstem aggregation 

area. 

Tier 2 Trigger - Reduce threat using mechanical removal if conservation actions in Tier 1 

are insufficient to arrest a population decline: 

Mechanical removal of nonnative aquatic predator will ensue: 

If the point abundance estimate of adult HBC decline to <7,000, as estimated by the currently 

accepted HBC population model. 

Mechanical removal will terminate if: 

Predator index (described below) is depleted to less than 60 RBT/km for at least two 

years in the JCM reach and immigration rate is low (the long term feasibility of using 

immigration rates as a metric still needs to be assessed), or 

Adult HBC population estimates exceed 7,500 and recruitment of sub-adult chub exceed 

adult mortality for at least two years. 

If immigration rate of predators into JCM reach is high, mechanical removal may need to 

continue. These triggers are intended to be adaptive based on ongoing and future research (e.g., 

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Lees Ferry recruitment and emigration dynamics, effects of trout suppression flows, effects of 

Paria River turbidity inputs on predator survival and immigration rates, interactions between 

humpback chub and rainbow trout, other predation studies). 

ACTION TRIGGERS BACKGROUND AND RATIONALE 

Tier 1 Trigger Target 

Adult Humpback Chub population target: 9,000 

Using an age-structured mark-recapture (ASMR) model, Coggins and Walters (2009) estimated 

the adult population of the LCR aggregation of HBC in 2008 was approximately 7,650 fish 

(6,000-10,000 fish considering a range of assumed mortality rates and ageing error). Using a 

multi-state model, Yackulic et al. (2014) obtained point abundance estimates of adult HBC 

between ~11,000-13,000 from 2009 through 2012. This increase in adult abundances roughly 

coincides with the significant increase of adult HBC that first appeared in the LCR post-2006 

(Van Haverbeke et al. 2013). We suggest a population estimate of 9,000 adult fish as a desired 

future conditions target. Estimates falling below 9,000 would trigger additional conservation 

actions to increase recruitment until the HBC population recovered to 9,000 adult fish. A 9,000 

adult chub target is below the most recent estimate of ~11,000-13,000 individuals and would 

preclude conservation measures from being initiated immediately, but also provides a “buffer 

zone” above 7,000 adult fish, at which point mechanical removal is warranted, as prescribed in 

the 2011 high flow Biological Opinion (USFWS 2011). 

LCR and mainstem (LCR aggregation) population targets: 2,000 and 7,000 adult HBC, 

respectively 

We separate the 9,000 total adult target number into an LCR component (2,000 adults), and a 

mainstem Colorado River component (7,000 adults). It is estimated that ~82% of adult HBC 

reside in the mainstem Colorado River during the non-spawning season (Yackulic et al. 2014, p. 

1015). This proportion was based on estimates obtained during September/October 2011, so this 

proportion would be expected to vary, possibly considerably, on an annual basis. Nevertheless, 

objectives to maintain 2,000 adults in the LCR and 7,000 adults in the mainstem during the nonspawning season (i.e., September/October) are proposed. A desired target of 2,000 adults in the 

LCR is reasonable because the average fall population estimate for adults was 2,380 (SE = 518) 

from 2007-2014, compared to the average level of 789 adults (SE = 281) from 2000-2006 (Van 

Haverbeke et al. 2015). 

LCR Humpback Chub recruitment target 

To maintain a population of 2,000 adult HBC in the LCR during the non-spawning season, there 

must be sufficient recruitment of sub-adult chub (150-199 mm size class). We estimate that a 

sub-adult chub population of 1,250 fish annually, as measured during the annual spring spawning 

season in the LCR is sufficient to maintain the adult HBC target population. This number is 

derived from an assumption that the annual adult mortality rate in the LCR is estimated at 0.35 

(Yackulic et al. 2014, updates Yackulic pers. com). Hence 2,000 x 0.35 = 700 new adults 

needed annually to replace adult mortality. To annually recruit 700 adults, we estimate that 1,250 

sub-adults are annually needed (i.e., not all sub-adults will survive into adulthood). If annual 

mortality for sub-adults in the LCR is 0.44 (Yackulic et al. 2014, updates Yackulic pers. com.), 

then 700/(1-0.44) = 1,250 sub-adults needed to offset adult mortality. Hence, if the three-year 

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running average point population size of sub-adult chub measured during the spring season in the 

LCR drops below 1,250 fish, additional conservation measures would be triggered (Figure 1). 

A three-year running average is used for sub-adults because production of younger life stages of 

HBC can be highly variable (Van Haverbeke et al. 2013). For long-lived species such as HBC, 

reduced recruitment of sub-adults in any one year can be compensated in subsequent years with 

increased recruitment. Three years is considered a reasonable timeframe from which to trigger 

actions to minimize large changes in adult HBC numbers. 

Figure 1. Running three year averages (± 95% CI) of sub-adult humpback chub abundances based on closed spring markrecapture studies in the Little Colorado River (Van Haverbeke et al. 2013, 2015). For example, the bar for 2003 represents the 

average abundance of the 150-199 mm size class of humpback chub for 2001, 2002 and 2003 obtained in the Little Colorado 

River during spring monitoring (note: error bars are large because of typically large annual variability in the abundance of this 

size class). Additional conservation measures would have been triggered during 2003-2006. The red line represents a trigger 

value of 1,250 sub-adults, below which conservation measures would be initiated. 

Mainstem LCR aggregation recruitment target 

To maintain a population of 7,000 adults in the mainstem LCR aggregation reach outside of the 

spawning season, there must be sufficient recruitment of sub-adult fish (150-199 mm size class). 

The boundaries of the LCR aggregation in the mainstem traditionally extend from RM 57 

(Malagosa Crest) to 65.9 (Lava-Chuar Rapid)(Valdez and Ryel 1995). Since 2009, most 

mainstem monitoring efforts in the LCR aggregation reach have focused in the JCM (Juvenile 

Chub Monitoring) reach (RM 63.45-65.2), which is below the LCR and contains ~18% of the 

adult HBC population found in the mainstem LCR aggregation reach (Yackulic et al. 2014). If 

~18% of the population is in the JCM reach, then the desired number of adult chub to maintain in 

the JCM reach is 7,000 x 0.18 = 1,260 adults. Annual adult mortality in the mainstem LCR 

aggregation is estimated at 0.15 (Yackulic et al. 2014, updates Yackulic pers. com.). To replace 

the adults in the JCM reach each year would require 1,250 x 0.15 = 189 adults. Annual mortality 

of sub-adult chub in the mainstem is estimated at 0.3. Replacing 189 adults annually would 

require 189/(1-0.3) = 270 sub-adults. Approximately 1/3 of sub-adult chub grow to adult size 

each year, and accordingly it may take ~3 years1 for a chub in the mainstem to transition from 

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the sub-adult to the adult size class (Yackulic et al. 2014). Therefore an acceptable target 

population of sub-adults in the JCM reach each year would be 810 (270 x 3 = 810). As with the 

LCR component, a running three year average of <810 sub-adults in the JCM reach would 

trigger conservation actions (Figure 2). 

The above scenario assumes that population recruitment dynamics are operating more or less 

equally throughout the LCR aggregation reach in the mainstem, which is likely not true. Most 

juvenile chub exiting the LCR are displaced downriver from the confluence (Valdez and Ryel 

1995). As such, we might expect that recruitment into adulthood might be more prevalent 

downstream of the confluence. As such, the proportional number of sub-adults measured in the 

JCM reach may not reflect the number actually needed to annually replace a total 7,000 adults. In 

other words, the JCM reach proportional calculation of 810 sub-adults could be low. For 

example, consider that if the JCM reach harbors a higher than average percent of the mainstem 

sub-adult chub that recruit into adulthood, then even more than 810 sub-adults in this reach may 

be needed to maintain a population of 7,000 adults. 

Figure 2. Running three year average abundances (± 95% CI) of sub-adult humpback chub (150-200 mm) abundances based on 

multi-state model in the mainstem Colorado River in the Juvenile Chub Monitoring reach (data from Yackulic pers. com.). For 

example, the bar for 2014 represents the average abundance of sub-adult humpback chub in the Juvenile Chub Monitoring reach 

during 2012, 2013, and 2014. The red line represents the approximate value of a 3-year running average of 810, below which 

conservation actions would be enacted. 

1

The mainstem LCR population recruitment scenario assumes temperature in the LCR 

mainstem reach is suitable for growth. If LCR mainstem temperatures are cold (do not 

exceed 11 oC during the year in the JCM reach), HBC will take longer to reach 

adulthood, experience greater mortality, and therefore require a larger number of subadults targeted to maintain the adult population objective. Target number adjustments 

will be made prior to implementation of LCR mainstem trigger actions if thought 

necessary. 

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Tier 1 Trigger response – HBC Conservation 

It is expected that the conservation actions proposed below will assist in ameliorating HBC adult 

losses or recruitment failures from predation. First, ongoing translocations in the LCR above 

Chute Falls (~300 fish/year) as well as outside the LCR population (e.g., to Havasu Creek, etc.) 

will continue, regardless of Tier 1 triggers are met or not. New conservation actions will include 

expansion of existing activities coupled with experimental actions: 

 LCR - Expand translocation actions in the LCR by collecting an additional 300-600 

young of the year (YOY) HBC and move to above Chute Falls in October. 

 LCR - Assess efficacy of transporting larval HBC (April/May) into Big Canyon and 

above Blue Springs in the LCR system. Evaluate growth and survival of these 

transplants; 

 Mainstem LCR Aggregation - Larval fish will be removed from LCR (April/May) and 

head-started at Southwest Native Aquatic Resources and Recovery Center (SNARRC). 

Once fish reach 150-200 mm they will be translocated to the mainstem LCR reach the 

following year (currently grow-out space at SNARRC is limited to 750 HBC, use of fish 

for this purpose would reduce numbers available for other actions, e.g. Havasu, 

Shinumo.); 

 Additional conservation actions as identified and evaluated. 

Tier 2 Trigger Targets 

Aquatic Predator index 

A trout or aquatic predator index is proposed as a means to terminate mechanical removal should 

it become initiated. Essentially, this is the level (60 predator index fish/km in the JCM reach) at 

which mechanical removal becomes a futile exercise (i.e., very small return for a high amount of 

effort). The predator index concept was originally intended to serve as an index whereby 

mechanical removal would be initiated (e.g., mechanical removal would be initiated once trout 

levels reached a certain density (~760 index fish/km in the JCM reach). However, because of 

uncertainty of the actual predation rates of trout on HBC (at differing temperatures, densities, 

turbidities, etc.), and on its population level effects on HBC, determining an appropriate density 

of trout at which to initiate mechanical removal is highly uncertain. 

A predator index will be developed in the JCM reach to weigh each probable predator by its 

ability to prey on HBC. The index calculates predator densities by incorporating additional 

species besides rainbow trout and makes assumptions about their relative predation rates 

compared to rainbow trout. For example, brown trout are estimated to be about 17 times more 

predacious on HBC than rainbow trout (Ward and Morton-Starner 2015). Additional predators 

(e.g., smallmouth bass) could be included through an assignment of their piscivory level relative 

to rainbow trout. Thus, relative piscivory can be captured in a rainbow trout equivalent predator 

index (Table 1). For species for which population estimates cannot be estimated with 

mark/recapture methods, capture probabilities or relative abundance (e.g. catch per unit effort) 

will be used to estimate the population and incorporate into the density matrix. Also, for certain 

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species regarded as potentially very piscivorous and dangerous (e.g., small mouth bass, green 

sunfish), targeted removal efforts for these species may be initiated immediately, regardless of 

meeting any type of threshold. If initiated, mechanical removal would be terminated once the 

relative predator index declines to 60 in the JCM reach for two years or HBC recover to a target 

level. A predator index of 60 in the JCM reach likely represents a point at which there is very 

diminished return for effort expended, and is roughly equivalent to densities at which mechanical 

removal was deemed to be not worthwhile as an effective tool to pursue in the past (i.e., 

mechanical was terminated). 

Table 1. Hypothetical predator index. The predator index assigns a relative piscivory rate of 17 to brown trout (Ward and 

Morton-Starner 2015) and to smallmouth bass (assumed at brown trout rate) and sums the hypothetical numbers of fish. If 

initiated, mechanical removal would be terminated once the relative predator index declines to 60 in the JCM reach for two 

years or HBC recover to a target level. 

Species Number Relative predation 

factor 

RBT equivalent 

Brown Trout 21 17 357 

Rainbow trout 400 1 400 

Smallmouth Bass 1 17 17 

Predator index total 774 

HBC population level triggers 

Continue to use the existing adult HBC population estimate of 7,000, as the trigger for predator 

removal actions, as stated in the 2011 Biological Opinion (USFWS 2011). Population estimates 

of sub-adults are not incorporated in Tier 2 triggers, as in Tier 1 triggers. 

Tier 2 Trigger response – Threat Reduction. 

Mechanical removal of predators from the LCR aggregation reach (& immediate vicinity) will be 

conducted. 

TRIGGER CAVEATS 

 If HBC decline and the identified actions are not working, USFWS, in coordination with 

action agencies and traditionally associated Tribes, will identify future appropriate 

actions; 

 Triggers will be reviewed and modified as necessary (evaluated; new information 

considered and included; etc.), but no less than every five years; 

 Actions and triggers will need to adapt if HBC are found to be impacted by other factors; 

 If estimating abundances of small size classes of chub becomes problematic because of 

population decline (i.e., if numbers get so low capture probability cannot be estimated for 

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each trip), catch divided by the best estimate of capture probability will be used to 

estimate abundance. 

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LITERATURE CITED 

Coggins LG Jr, Walters CJ. 2009. Abundance trends and status of the Little Colorado River 

population of humpback chub; an update from 1989–2008. U.S. Geological Survey OpenFile Report 2009-1075. 

Coggins LG Jr, Yard MD, Pine WE III. 2011. Nonnative fish control in the Colorado River in 

Grand Canyon, Arizona: an effective program or serendipitous timing? Transactions of the 

American Fisheries Society 140: 456–470. 

Douglas M.E., and Marsh P.C. 1996. Population estimates/population movements of Gila cypha, 

an endangered Cyprinid fish in the Grand Canyon region of Arizona. Copeia 1996:15–28. 

Kaeding LR, Zimmerman MA. 1983. Life history and ecology of the humpback chub in the 

Little Colorado and Colorado Rivers in Grand Canyon. Transactions of the American 

Fisheries Society 112:577–594. 

U.S. Fish and Wildlife Service. 2002. Humpback chub (Gila cypha) Recovery Goals: amendment 

andNative Fish Monitoring in Little Colorado River, Grand Canyon supplement to the 

humpback chub Recovery Plan. Denver, Colorado. 

USFWS. 2011. Final Biological Opinion for the Operation of Glen Canyon Dam including High 

Flow Experiments Non-native Fish Control. U.S. Fish and Wildlife Service, Phoenix, 

Arizona, December 23, 2011. AESO/SE 22410-2011-F-0100 and 22410-2011-F-0112. 

Available: 

http://www.fws.gov/southwest/es/arizona/Documents/Biol_Opin/110112_HFE_NNR.pdf 

Valdez RA, Ryel RJ. 1995. Life history and ecology of the humpback chub (Gila cypha) in the 

Colorado River, Grand Canyon, Arizona. Final Report to Bureau of Reclamation, Salt Lake 

City, Utah. Contract No. 0-CS-40-09110. Salt Lake City, Utah. Available: 

http://www.gcmrc.gov/library/reports/biological/Fish_studies/Biowest/Valdez1995f.pdf 

Van Haverbeke DR, Stone DM, Coggins LG Jr., Pillow MJ. 2013. Long-term monitoring of an 

endangered desert fish and factors influencing population dynamics. Journal of Fish and 

Wildlife Management 4(1):163–177; e1944-687X. 

Van Haverbeke, D.R., K. Young, D.M. Stone, and M.J. Pillow. 2015. Mark-Recapture and Fish 

Monitoring Activities in the Little Colorado River in Grand Canyon from 2000 to 2014. 

Submitted to USGS Grand Canyon Monitoring and Research Center, Flagstaff, Arizona. 

U.S. Fish and Wildlife Service, Flagstaff, Arizona. 55 pp. 

Ward, D. W. and Morton-Starner, R. 2015. Effects of water temperature and fish size on 

predation vulnerability of juvenile humpback chub to rainbow trout and brown trout. 

Transactions of the American Fisheries Society 144(6): 1184-1191. 

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Yackulic, C.B., Yard, M.D., Korman, J., Van Haverbeke, D.R., 2014. A quantitative life history 

of endangered humpback chub that spawn in the Little Colorado River: variation in 

movement, growth, and survival Ecology and Evolution 4(7):1006–1018. 

Yard MD, Coggins LG, Baxter CV, Bennett GE, Korman J. 2011. Trout piscivory in the 

Colorado River, Grand Canyon: effects of turbidity, temperature, and fish prey availability. 

Transactions of the American Fisheries Society 140:471–486. 

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