Memorandum

To: Vineetha Kartha, GCDAMP Technical Work Group, Chair 

From: Scott VanderKooi, USGS Grand Canyon Monitoring and Research Center, Chief

CC: Camille Touton, Department of Interior, Counselor to the Assistant Secretary for

Water and Science

Beverly Heffernan, Bureau of Reclamation, Upper Colorado Region Environmental 

Resources Division Manager

Date: December 9, 2015

Subject: Technical review of the Lees Ferry Recreational Trout Fishery Management 

Recommendations

The USGS Grand Canyon Monitoring and Research Center (GCMRC) has prepared this memo 

in response to the following motion that was passed by consensus at the Adaptive Management 

Work Group (AMWG) meeting held in Phoenix, Arizona on August 26-27, 2015,

The AMWG requests the Secretary's Designee direct GCMRC to conduct a technical 

review of the Lees Ferry Recreational Trout Fishery Management Recommendations and 

report its findings to the TWG; and directs the TWG to evaluate the GCMRC review at 

their October 2015 meeting, and report its findings to AMWG at its February 2016 

meeting.

An earlier version of this memo was submitted to DOI and Reclamation on October 19, 2015 and 

shared with the Technical Work Group (TWG) at its October 20-21, 2015 meeting. This updated 

and final draft was revised in response to comments and suggestions received from stakeholders 

and managers regarding points of clarification and topics that were not included in our initial 

review. 

We appreciate the opportunity to review the final draft of the Lees Ferry Recreational Trout 

Fishery Management Recommendations and wish to thank the authors for engaging GCMRC and 

United States Department of the Interior

U.S. GEOLOGICAL SURVEY

SOUTHWEST BIOLOGICAL SCIENCE CENTER

GRAND CANYON MONITORING AND RESEARCH CENTER

2255 NORTH GEMINI DRIVE, MS-9394

FLAGSTAFF, ARIZONA 86001-1600

928 556-7380 Telephone

928 556-7100 Fax
cooperator scientists on multiple occasions during the development of this document. GCMRC 

and cooperator scientists were also afforded the opportunity to provide in-depth reviews of a 

draft that was completed and distributed to GCMRC as well as other agencies and organizations 

in April 2015. The reviews from individual scientists were provided to the authors in May 2015 

and also shared in their entirety with the TWG and the Glen Canyon Dam Adaptive Management

Program (GCDAMP) in an e-mail sent to the GCDAMP e-mail list by Linda Whetton on June 2, 

2015.

We understand and appreciate the challenges of revising reports and manuscripts in the face of 

multiple and often contradictory reviews and wish to recognize the efforts put forward by the 

authors to address the comments and concerns of the many reviewers. We appreciate that several

of the comments and concerns included in the reviews from GCMRC and cooperator scientists 

were addressed in this final draft. Our review follows.

In the final draft of the Lees Ferry Recreational Trout Fishery Management Recommendations, 

we believe the authors have done a good job of synthesizing key science issues in developing 

many of their management recommendations. However, there are some recommendations or 

portions of recommendations that have language and logic used to support them that we don’t 

believe are consistent with current scientific understanding. There are also topics where scientific 

consensus is lacking. We identify areas of concern and discuss topics where there is 

disagreement in our comments below. Comments from GCMRC are limited to scientific and 

technical matters, thus for recommendations where no scientific or technical information is 

presented, we indicate that GCMRC was not able comment. As a science agency, USGS remains 

neutral on matters of policy and cannot support or oppose management recommendations. 

Therefore, we indicated which topics we believed fell in the realm of policy and areas where 

decisions would need to be made by management agencies. As stated above, GCMRC cannot 

comment on these topics or areas. Comments are organized by recommendation or section in the 

order they were presented in the final draft.

Aquatic Food Base Enhancement through Experimental Repatriation and “Bug Flows”

There are considerable amounts of data and a consensus among scientists that growth rates of 

trout in Lees Ferry can be poor and this, in turn, can limit the maximum size these fish can attain, 

so recommendations to explore options to improve growth by improving the food base make 

sense. There is not, however, consensus that experimental “bug flows” are likely to succeed. It 

was observed that past periods of steady flows, the summer and fall of 2000 and Memorial Day 

low flows for overflights for example, did not result in observations of insects from the orders 

Ephemeroptera, Plecoptera, or Tricoptera (EPT) following these events. The counter argument is 

that there was not any sort of organized or sustained efforts monitoring aquatic insects after the 
2000 steady flows so any response may have been unobserved and that other periods of low 

flows may have been too brief or only isolated events, thus not sufficient in duration or frequency 

to elicit any sort of measurable response. In addition, establishing EPT is not the only reason for 

conducting experimental “bug flows”. If the hypothesis that high mortality of eggs associated 

with hydropeaking is a critical factor limiting all aquatic insects, then the proposed flow 

experiment will benefit midge and blackfly production even in the absence of an EPT response. 

Given these reasons, GCMRC supports conducting experimental “bug flows” to improve our 

understanding of the aquatic food base in the Lees Ferry reach and the factors controlling 

productivity and diversity.

It should also be noted that there is uncertainty as to whether producing a more diverse 

invertebrate community is the only way to increase trout growth. For example, there are many 

lakes in British Columbia where large trout are produced in lakes with very small but highly 

abundant Daphnia and midge populations. One can grow larger trout with small bugs if the bug 

density is high. The authors may also wish to consider a broader range of alternatives than those 

presented in the document. For example, what about stream fertilization? There are many 

examples of increased production in small streams, and a few examples in very large systems 

(Arrow Lakes Kootenay Lake, Kootenai River below Libby Dam).

Any decisions concerning potential translocations of EPT species historically present in Glen 

Canyon are the responsibility of management agencies. If approved, experimental translocations 

could help answer questions and allow for the testing of hypotheses related to why EPT species 

are currently absent from Glen Canyon. We believe that this type of experimental approach 

would also speed learning.

Dam Operations

No scientific or technical information is presented regarding MLFF. In addition, any decision 

regarding revision of operations is a policy matter. For both these reasons, GCMRC cannot 

comment on this recommendation. We also note that revisions to Glen Canyon Dam operations

are being evaluated as part of the ongoing Long Term Experimental and Management (LTEMP) 

Environmental Impact Statement (EIS) process. As mentioned above, we support conducting 

experimental “bug flows” to improve our understanding of the aquatic food base in the Lees 

Ferry reach and the factors controlling productivity and diversity.

Minimum Flows

There is little scientific evidence to date to support the recommendation to maintain minimum 

flows at 8,000 cubic feet/s (cfs) or at any other specific flow. We note that there have been 

months with 5,000 cfs minimum flows (most recently in fall before the fall steady flow 
experiment) coincide with periods of normal growth and recruitment. We agree that research to 

evaluate effects of lower flows and to develop scientifically based minimum flows should 

continue. As stated above, revisions to Glen Canyon Dam operations are being evaluated as part 

of the ongoing LTEMP EIS process.

Fall and Spring High Flow Experiments (HFEs)

It is correct that the 2008 spring HFE enhanced the aquatic food base in Glen Canyon which, in 

turn, improved recruitment and survival of young rainbow trout. Conducting additional spring 

HFEs would provide additional information on how the aquatic ecosystem downstream from 

Glen Canyon Dam responds to these flows. It would also provide scientists an opportunity to 

quantify the responses of different resources and test a variety of hypotheses including those 

listed by the authors (i.e., controlling New Zealand mud snails, increasing aeolian sand 

availability and transport, re-establishing “natural ecological processes”). Spring HFEs are 

allowed under the HFE protocol currently in place, but require adequate sand inputs from the 

Paria River during winter and spring months in order to be triggered. Any decision to deviate 

from the HFE protocol is a policy matter on which GCMRC cannot comment. Potential revisions 

to the HFE protocol are being evaluated as part of the ongoing LTEMP EIS process. 

Experimental Trout Management Flows

The authors state in this section that they believe the best approach to controlling trout densities 

is through increased invertebrate diversity and avoiding flows that result in excessing spawning 

and recruitment. It is likely that increasing diversity will provide some degree of stability for the 

invertebrate community and by extension redistribute the availability of invertebrates across 

more seasons (currently highest drift availability and growth occurs during the late spring early 

summer). This could benefit fish populations and also result in greater proportions of larger food 

items available to fish which, in turn, could improve growth particularly in larger fish. It should 

also be noted that an increase in food availability for fish could result in more spawning and, if 

environmental conditions are conducive for age-0 survival in summer and fall, an increase in 

recruitment.

The suggestion that trout management flows should only occur when the trout population is 

stable and includes a healthy abundance of all size classes is constraining and may be missing the 

underlying purpose of these flows. Trout management flows will only have utility when 

populations are becoming or already are unstable such as when recruitment rates are very high or 

populations are at unsustainable levels. It should also be noted that trout management flows are 

very likely to have a negligible effect on mature age classes (approximately 3 to 6 years old), and 

are designed to only impact young-of-year trout. Since the trout population is composed of 

approximately six age classes, trout management flows are likely to only affect one of the six 
year classes making up the population at any one time. Given this, it seems unlikely that trout 

management flows pose a risk to the fishery or could result in a catastrophic loss to the fishery. 

Furthermore, trout management flows can be implemented in a forward titration mode to ensure 

that the resulting recruitment does not drop below the level required to achieve fishery objectives 

in the long term. This would be a very conservative approach so we add the caution that it would

likely require many iterations, thus a considerable amount of time. We agree that a well thought 

out experimental design is a critical need prior to the implementation of any trout management 

flows. 

We also note that there is not consensus that trout management flows are the best approach to 

managing the Lees Ferry trout population. The argument is that there is evidence that simple food 

webs, that are inherently unstable, are at least partly to blame for the boom-bust cycles in the 

Lees Ferry trout population. If correct, then efforts to address the root causes (e.g., by increasing 

invertebrate diversity and food web complexity or avoiding flows that result in overabundance of 

young trout) would help resolve issues of instability in this population.

Equalization Flows

The 2011 equalization flows did appear to have a strong effect on young rainbow trout survival 

and subsequent recruitment. The resulting year class led to the highest densities of rainbow trout 

ever observed in Glen Canyon and appeared to trigger a “boom and bust” cycle in the trout 

population that Arizona Game and Fish Department (AGFD) and GCMRC have monitored over 

the last few years. However, any recommendation to deviate from current equalization guidelines

is a policy matter that GCMRC cannot comment on.

Fishing Regulations

While we understand the rationale for encouraging anglers to harvest trout in Lees Ferry, this 

would likely be a numerically ineffective method of reducing trout abundance in this reach of the 

river. This would be particularly true during periods of high trout abundance similar to what has 

been seen in recent years. We recognize, however, that revising fishing regulations is a 

management decision, thus is the responsibility of management agencies.

Marble Canyon Trout Fishery

Our previous comment applies here as well. We understand the rationale for encouraging anglers 

to harvest trout in Marble Canyon, however, this would likely be a numerically ineffective 

method of reducing trout abundance in this reach of the river and downstream. This would be 

particularly true during periods of high trout abundance similar to what has been seen in recent 
years. Again, we recognize that revising fishing regulations is a management decision, thus is the 

responsibility of management agencies.

Riparian Vegetation Restoration

Any decision concerning riparian vegetation restoration in Glen Canyon is the responsibility of 

the National Park Service. As noted previously, the aquatic food base in Glen Canyon has low 

diversity and may be insufficient to support larger trout. Introduction of wood could improve 

productivity by providing habitat and refugia for species that comprise the aquatic food base as 

well as young trout. Experimental additions of dead tamarisk to the Colorado River in Glen 

Canyon could be used to test this hypothesis on a local basis. While riparian vegetation can 

support terrestrial insect abundance, the amount of habitat with shade and cover provided by 

shoreline vegetation in a river the size of the Colorado in Glen Canyon is proportionally quite 

small relative to other habitats available to fish. Given this, population level benefits to fish may 

be small and would be difficult to detect.

Stocking in the Event of a Catastrophic Fishery Failure

There are no criteria used for defining a catastrophic failure in the fishery so it’s unclear when 

stocking would be implemented. Stocking criteria could be based on a number of metrics related 

to biology or population dynamics including trout abundance, survival, or growth. Angler 

satisfaction or catch rates could also be used, but it should be noted that these are likely to be 

more arbitrary and would not be as easy to quantify as measures of population status or trends. 

The authors might consider adding this topic to their list of Protocol Evaluation Panel (PEP)

recommendations.

A notable observation that should be considered with regard to this recommendation is that the 

Lees Ferry fishery has survived over the last two decades without stocking, and has recovered on 

its own from two warm-temperature/high density situations (2004-2006, 2012-2014). While we 

agree with the authors’ recommendation that any proposed stocking should not occur until after 

the causal factors of a failure have been identified and ameliorated, we note that excessive 

biomass of larger fish resulting from very large recruitment events may be the most likely cause 

of failures in this fishery. Prematurely adding biomass in these situations would only exacerbate 

the problem and slow down the natural recovery of the fishery.

We also caution that while stocking may have worked in the past, there is no guarantee that it 

will work now given differences in fish densities, the types and amount of food available to fish, 

and potentially other factors. Even under conditions where densities according to anglers are 

low, there still may be too many fish for the foodbase to support and have desirable growth rates.

In this event, it is likely that stocked fish will simply be out-competed by the naturally produced 
fish even if they are only present in low numbers. Developing a brood stock is a management 

decision. However, we believe it is highly uncertain that the timing of a collapse of the fishery 

could be determined accurately. Furthermore, anticipating a collapse far enough in advance such 

that stocking could occur more quickly than a natural recovery may not be possible.

As noted in our May 2015 review, we are skeptical of the feasibility of translocating trout from 

upper Marble Canyon to supplement the population upstream of Lees Ferry. One key issue is that 

population trends in upper Marble Canyon appear to track very closely with those upstream of 

Lees Ferry so there may not be many fish to move once trout numbers decline to the degree that 

translocations are deemed necessary.

Developing a contingency stocking plan and conducting any associated compliance is a 

management decision, thus is the responsibility of management agencies.

Low Dissolved Oxygen Response Protocol

Dissolved oxygen levels downstream from Glen Canyon Dam are directly influenced by 

conditions in Lake Powell which are, in turn, affected by a complex combination of factors. 

Concentrations of dissolved oxygen low enough to stress and even kill rainbow trout have been 

observed downstream of Glen Canyon Dam. Additional monitoring and reporting of dissolved 

oxygen levels can occur if approved by stakeholders and managers. Dissolved oxygen levels at

the Lees Ferry gage are currently available online at GCMRC’s website 

(http://www.gcmrc.gov/discharge_qw_sediment/station/GCDAMP/09380000). Developing an 

action plan to reduce or avoid negative effects of low dissolved oxygen is a management 

decision, thus is the responsibility of management agencies.

Temperature Control Device

Water temperature is a primary driver of biological processes in aquatic ecosystems. Predicted 

warming of water released from Glen Canyon Dam would increase the likelihood of invasive 

species becoming established in Glen and Grand Canyons. An invasion of warm water fishes 

would almost certainly have a strong adverse effect on native fish populations including the 

endangered humpback chub.

The ability to manipulate the temperature of water releases from Glen Canyon Dam would 

provide a means to experimentally determine flows and temperature regimes that could favor 

desired species and disadvantage undesired species. The recommendation to build a temperature 

control device, however, is a policy matter that GCMRC cannot comment on.
Bypass Tube Electrical Generation

Any decisions regarding structural modification to the Glen Canyon Dam outlet works to allow 

for power generation are policy matters thus, GCMRC cannot comment on this recommendation.

As stated by the authors, releases from the outlet works do immediately oxygenate the Colorado 

River downstream from the dam. They can also cool the temperature of the river when it is warm 

due to cooler reservoir temperatures at the depth of the intakes for the outlet works. It should be 

noted that these effects are temporary, only occurring during releases through the outlet works.

Introduce Turbidity

Rainbow trout predation rates on young humpback chub has been shown to be reduced even 

modest levels of turbidity in controlled laboratory trials. Field data, however, suggest that 

rainbow trout predation rates on young fish can be higher in the Colorado River at moderate 

turbidity levels. Differences between laboratory and field data suggest that in addition to 

turbidity, rainbow trout predation on humpback chub in the wild could be influenced by 

behavioral changes in both predator and prey, environmental conditions, or other factors. Given 

these differences, we believe additional research to improve understanding of how turbidity 

affects rainbow trout predation on humpback chub is warranted. 

In our opinion, increasing Colorado River turbidity by artificially suspending Paria River 

sediment falls into the realms of policy and engineering rather than science thus, GCMRC cannot 

comment.

Monitoring and Measurement of Management Triggers

There is not consensus among cooperating agencies as to the best approaches and methods or 

appropriate level of effort to monitoring the Lees Ferry fishery. GCMRC scientists believe that 

while catch per unit effort (CPUE) based indices can be useful for tracking overall long-term 

trends in fish populations, they have limitations in terms of providing information regarding 

population dynamics (abundance, recruitment) and key process variables (survival, growth, 

movement, etc.) as well as testing hypothesis. Furthermore, GCMRC scientists believe that 

learning from flow alterations, including experimental management flows, will be more rapid if 

monitoring focuses on mark-recapture methods, which provide less ambiguous estimates of 

population responses to management actions.

In recognition of this lack of consensus, the following comment was included in the review of the 

May 2015 draft of this document provided by Scott VanderKooi and Charles Yackulic. Aside 
from the specific reference to line numbers from the earlier draft, we believe the comment still 

applies so have included it here.

Rather than identifying particular projects and agencies to conduct them, we believe it 

would be more useful to focus on what information is needed to 1) understand how 

environmental factors, operations, and management actions affect the aquatic ecosystem 

in Glen Canyon, including the food base and fish populations, and 2) to effectively 

manage the fishery. Some sections are already written like this or close to it, see Lines 

464-467 and 487-489. GCMRC is planning to hold a Protocol Evaluation Panel (PEP) for 

the entire GCDAMP fisheries program in FY2016. We prefer to wait for the 

recommendations of that panel of experts to identify best methods and approaches for 

monitoring the Lees Ferry fishery and Glen Canyon trout population in order to meet the 

science needs of the GCDAMP rather than to have them identified for us and our 

cooperators in this document. 

We agree that additional monitoring for invasive species could improve our ability to detect 

potentially harmful organisms and respond rapidly to mitigate these threats. Were this increased 

effort approved by stakeholders and managers, a carefully designed surveillance plan would help 

ensure this monitoring was conducted in an effective and efficient manner. Water quality 

monitoring downstream from Glen Canyon Dam will continue through FY2017 as described in 

the GCMRC FY2015-17 workplan. Future water quality monitoring will likely be proposed to 

continue in future workplans given the importance of this information.

We appreciate the authors providing recommendations of topics to include in the planned PEP 

review of the GCDAMP fishery program. We will take these recommendations into 

consideration as we work with cooperating agency scientists and others to plan the PEP.

Regarding the recommendation to develop a stock assessment model, Josh Korman provided the 

following comments.

I don’t understand the recommendation to develop a stock assessment model. An annual 

stock assessment model was developed for Lees Ferry as part of the Grand Canyon 

Ecosystem Conceptual Modelling effort (which ran from approximately 1998-2003). 

There was a time where some Arizona Game and Fish Department (AGFD) staff used 

this model (Scott Rogers and Dave Speas), but model use by AGFD eventually stopped 

and I have never seen it used in annual reporting by AGFD to provide a more integrated 

and useful interpretation of the long-term CPUE data. A much more detailed monthly 

stock assessment model was published in 2012:
Korman, J., Martell, S.J.D., Walters, C.J., Makinster, A.S., Coggins, L.G., Yard, 

M.D., and W.R. Persons. 2012. Estimating recruitment dynamics and movement 

of rainbow trout (Oncorhynchus mykiss) in the Colorado River in Grand Canyon 

using an integrated assessment model. Can. J. Fish. Aquat. Sci. 69: 1827-1849. 

This model was used to interpret the historical record of CPUE in Lees Ferry and Marble 

Canyon by the authors. AGFD does not use this model to interpret the CPUE data in their 

reporting. In summary, we already have two stock assessment models. I suggest this 

section be reworded to say that existing stock assessment models should be used to 

provide a more robust interpretation of the CPUE time series if that time series is to be 

continued. No need to reinvent the wheel here.

The 2012 stock assessment modelling effort (Korman et al. 2012) pointed out some key 

uncertainties influencing predictions about the contribution of Lees Ferry recruitment to 

the population of trout near the Little Colorado River confluence area used by endangered 

humpback chub. Those uncertainties led to the Natal Origins project (NO). The NO 

monitoring effort provides direct measurements of key population metrics (recruitment, 

abundance, survival, growth, movement). If that approach continues to be used, a stock 

assessment model isn’t needed, because we measure the demographic parameters of 

interest directly (via mark-recapture methods).