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2010 Biological Modeling Summary

USGS/GCMRC

June 24, 2010

Prepared for Technical Work Group meeting, 29 June 2010

The USGS Grand Canyon Monitoring and Research Center has been pursuing some high priority

questions for the Adaptive Management Program through the use of models. In the biological arena,

GCMRC’s senior visiting ecologist Carl Walters recently led two modeling workshops. The first was

conducted near one of Dr. Walters’ homes, in Cedar Key, Florida in March 2010. The second was

conducted in conjunction with a stakeholder workshop for reviewing sediment modeling at Saguaro

Lake Ranch, Arizona in April 2010. The Cedar Key workshop was an opportunity for Dr. Walters to work

intensively for a week with USGS and cooperating biologists. The resulting models from the Cedar Key

workshop were presented over a day and a half to Technical Work Group members and other managers

at Saguaro Lake Ranch. What follows is a brief summary of the results of the biological modeling,

primarily using the Ecopath/Ecosim platform, provided as a courtesy to the Technical Work Group. This

summary was drafted by Dr. Walters, then reviewed with USGS scientists and GCMRC cooperating

scientist Richard Valdez. Based on this internal review a subjective proportional level of certainty was

assigned to the summary statements to provide some relative quantification of the level of certainty

knowledgeable scientists can be assign to the conclusions at this time. Further data collection and

modeling are expected to help increased the certainty in these conclusions, with the eventual goal that

they are supported or rejected in peer‐reviewed publications.

1. The Little Colorado River is the heart of the native fish community in Grand Canyon. Most

humpback chub spawn there (100%), and bluehead sucker (60%) and flannelmouth sucker (60%)

also spawn in the LCR. Juvenile native fish leaving the LCR can face severe predation pressure

when rainbow trout and brown trout are present in the mainstem Colorado River in the vicinity

of the mouth of the LCR (90%). Predation of approximately 4000 humpback chub exceeds

estimated effective recruitment of 3500 (80%). Natives may also be disadvantaged by relatively

cold water in the mainstem (75%). There is a negative correlation over time between predator

abundance and native fish recruitment, beginning in the 1990s as well as after 2003, when

warming coincided with mechanical removal.

2. Most of the nonnative fish at the LCR mouth are rainbow trout and brown trout and these two

species together have been responsible for the bulk of the predation on humpback chub (90%).

Most of the rainbow trout were spawned in the Lees Ferry reach (100%) and most of the brown

trout were spawned in Bright Angel Creek (80%).

3. The population of rainbow trout at the mouth of the LCR might be effectively reduced with a

removal effort conducted within the first few miles downstream of the Paria River (60%). It may

be that the Lees Ferry rainbow trout are behaving like steelhead trout (same species), where the

fish leave the reach as juveniles, then swim upstream to spawn as they mature.
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4. The following control measures for rainbow trout might be effective (50%) at reducing predation

pressure on humpback chub: removal of downstream migrants (50%), altering ramping rates to

disadvantage rainbow trout eggs (30%), altering ramping rates to reduce juvenile recruitment to

the adult population (50%).

5. Removing brown trout from Bright Angel Creek would effectively reduce the population of this

species and reduce its predation on native fishes (90%).

6. It is not likely (75%) that the LCR can support a much larger population of humpback chub, even

if all predators removed because little excess food production is available. Similarly, there is

little excess food production in the Lees Ferry reach (75%). Probably 80% of the food base

produced in the Lees Ferry reach is consumed, limiting the amount of biomass that will be

produced.

7. When numbers of rainbow trout eggs or larvae are reduced in Lees Ferry then survivorship of

the remaining individuals increases (i.e., compensatory response) (100%).

8. Sand‐mediated backwaters created by high flows do not appear to have any measurable value

for humpback chub (100%). These habitats do appear to be beneficial to native sucker species in

western Grand Canyon.

Dr. Walters is planning to review these findings, and potentially others, with the Adaptive

Management Work Group in August 2010 in Phoenix. Cooperators Ecometric, Inc. (Josh Korman)

and Arizona Game and Fish Department anticipate that they can refine the model for the Lees Ferry

reach over the coming year. Refinement of a model focused on the LCR reach will be more

problematic because fewer data are available there, especially the ages of captured humpback chub,

their movements, and origins. Additional data are being collected (or are proposed for collection)

that could help better inform these models. GCMRC has allocated a limited amount of funding to

continue model development in FY 2011 and FY 2012 (see proposed AMP work plan and budget).