Can Humpback Chub and a Blue Ribbon Trout Fishery Coexist in the Grand Canyon?

Carson Jeffres

Abstract

Maintaining both a population of native Humpback Chub (Gila cypha) and non-native Rainbow Trout (Oncorhynchus mykiss) are stated goals for management downstream of Glen Canyon Dam (GCD) on the Colorado River, yet these species have very different habitat requirements and often times the Rainbow Trout prey upon the Humpback Chub. Habitat conditions in the river downstream of GCD have been extremely altered with moderate flows, cold water temperature, and a much reduced sediment load compared to historic conditions. These physical habitat characteristics create ideal conditions for the non-native trout and have reduced the habitat quality for the chub. In addition to the change in habitat, the rainbow trout and other introduced species predate on the chub. Several management actions from mimicking high flow events to mechanical invasive species remove have taken place to date with differing success. It may also be natural fluctuation such as drought can override the management actions in terms of drivers of the humpback chub population. Recent studies have found that under drought conditions, humpback chub populations have been increasing with warmer water. Under such an altered condition, can both of the species coexist and how does societal desire for recreational trout fishing compete with maintaining a non-recreational fish such as the Humpback Chub? This question will ultimately have to be addressed if a long- term solution is to be found.

Goals of the paper

The goal of this paper is to describe the habitat conditions for both the Rainbow Trout and Humpback Chub and how the alteration of the river below GCD has made the river suitable for Rainbow Trout and reduced habitat quality for Humpback Chub and other native fish. In addition, various management actions have taken place to help both species; the efficacy of these actions will be discussed as well as if there is potential for long-term coexistence of the two species.

Literature synthesis

Habitat conditions have been extremely altered for fishes native to the Colorado River downstream of GCD (Pine III et al. 2013). Currently, flow is relatively homogenous on an annual scale, yet has large daily fluctuations for hydropower generation. Water temperatures are much colder than historic conditions and fluctuate very little on a daily or annual timescale due to the hypolimnetic release from GCD. In addition to flow and temperature changes, the sediment supply has also been disrupted and currently only clear water is released from GCD. The only sediment sources in the Grand Canyon are from tributaries downstream of the dam, mainly the Paria and Little Colorado Rivers. This alteration of the habitat has resulted in the extirpation of three (Colorado Pikeminnow (Ptychocheilus Lucius), Bonytail Chub (Gila elegans), and Roundtail Chub(Gila robusta)) of the eight native fish species in the Grand Canyon (Webb et al. 1999). Of the remaining 5 species, two (Humpback Chub and Razorback Sucker (Xyrauchen texanus)) are listed as endangered and three are relatively stable. The five species that are either extirpated or endangered are all species that have evolved novel morphologies for the natural flow regime of the Colorado River, including large size, narrow caudal peduncles and deeply forked tails. The three species that have relatively stable populations are Bluehead Sucker (Catostomus discobolus), Flannelmouth Sucker (Catostomus latipinnis), and Speckled Dace (Rhinichthys osculus), all of which have more generalist morphologies that are similar to fish found throughout the western North America.

Rainbow Trout were introduced into the Grand Canyon to create a “Blue Ribbon Trout Fishery” in 1964 following the closure of GCD (Korman et al. 2012). In addition to rainbow trout, several amphipods species were introduced as a food source for the trout. The closure of the dam created ideal conditions consisting of cool and clear water for both the amphipods and the trout. In addition to the water conditions, the lack of sediment exposed suitable spawning gravels that were not historically available prior to the GCD. The result was a trout fishery that attracted fisherman from around the world to catch large trout (>7kg) in the arid desert environment. As the legend of the Glen Canyon trout fishery grew, the likelihood of managing solely for native fish declined. Essentially, a new stakeholder group of trout fisherman was created and that group is included into the operation decisions of GCD and other downstream habitat mitigation.

The humpback chub is a fish that evolved in the hell and high water of the historic Grand Canyon and was not officially described until 1945 (Miller 1946), and was the last of the native fishes of the Grand Canyon to be described. The humpback chub is a long-lived fish that can live for more than 40 years (Coggins and Walters 2009). They have a large dorsal hump, a compressed skull, small eyes with a narrow caudal peduncle and forked tail (Figure 1). All of these physical characteristics were evolved in the warm turbid waters of the Colorado River and the fish community that has evolved in that area over the last ~ 3.5 million years. The life history and habitat use of the humpback chub prior to CGD is unknown. Currently, humpback chub populations are relatively low (Figure 2) and they are listed as endangered under the endangered species act.

Figure 1. Humpback chub. Credit: U.S. Geological Survey.

Since the installation of GCD, Humpback Chub have been observed to spawn during the spring (March – May) in the warm tributaries of the Grand Canyon, primarily the Little Colorado River (LCR) (Coggins and Walters 2009). There are no descriptions of main-stem habitat use prior to GCD, mostly due to difficulty of observation and lack of interest in a non-game species. Currently, the main-stem is too cold (minimum of 16°C) for egg survival due to the hypolimnic releases from GCD. After spawning in the LCR, juvenile chub get washed down into the main- stem and can be found near the stream margin where vegetation is present, in talus or shallow backwaters. During the non-spawning season, adults are generally found in large back-water eddies on the main-stem of the Colorado River, but near the LCR with slow velocities and abundant food resources.

Figure 2. Estimated adult humpback chub abundance in Grand Canyon using age-structured mark recapture model and incorporating uncertainty in assignment of age (Coggins and Walters 2009).

In an attempt to mitigate for the altered hydrograph and associated changes to habitat, there has been a series of high flow experimental (HFE) releases from CGD to mimic historic conditions and create lost habitat both for native species and creating beaches for whitewater rafters. There have been a total of six HFEs since 1996. The HFEs have several goals, one of which is to create backwater habitat for native fish. Although one of the stated goals of these flows is to create backwater habitat and simulate historic high flows, the HFEs are relatively small and short in duration compared to the historic hydrograph (Figure 3). In addition, the flows are often times not during the time of year when they would have happened historically and are thus out of sync with the life history of the native fish. Historically, high flows were the result of snow-melt during the spring in the upper watershed and lasted for months. During that time, newly emerged humpback chub would get washed into the margin habitats that were flooded by the spring snow melt from the upper watershed. These margin habitats were relatively warm and the turbid waters provided protection from predators. As flows continued to drop throughout the summer, temperature would warm, approaching 30°C. Currently the HFEs only partially mimic the formation of backwater habitat and do not mimic any of the environmental cues that the native fish use for various life history stages.

Timing of HFEs has been variable over time, ranging from November to May and is currently determined primarily by physical conditions such as sediment accumulation in the river channel and flows in the Paria and Little Colorado Rivers. Only after monitoring following HFEs that occurred in the spring has the importance of timing been shown on the biota within the river. It was theorized that mimicking the general timing of flood events would provide native fish with environmental cues that would benefit the population. Surprisingly, following HFEs in the spring, there was a four-fold increase in recruitment of rainbow trout in the section below GCD (Korman et al. 2012). During surveys downstream near the LCR, there was also found to be an increase in rainbow trout abundance, despite the lack of spawning habitat downstream of Lee’s Ferry. There were so many juvenile trout recruited from the Glen Canyon reach that density dependent effect essentially pushed those excess fish downstream into the habitat that is considered critical for the humpback chub. It was determined that the high flows during the spring coincided with the spawning time of the rainbow trout, a fish species that also evolved to spring during the spring snow-melt recession. The increase in flow increased the amount of spawning habitat as well as quality of rearing habitat for newly emerged fish. Similar to juvenile humpback chub, the newly emerged trout found abundant food resources and flooded margin habitat with slightly warmer water and lower velocities. These habitat conditions allowed for much less mortality during this life stage compared to years without high flows (Kroman et al. 2012). This was an unintended consequence of trying to mimic a natural process for native fish and ultimately increasing the population of their non-native predators. This highlights the complex interactions between the physical processes and biotic response and the competing needs of two fish that evolved in very different environments, but with similar physical cues.

Figure 3. a)Hydrograph from pre and post (1922-2015) GCD at Lee’s Ferry in the Colorado River and b)Hydrograph from November 2014 High Flow Experiment from Glen Canyon Dam in the Grand Canyon. Source: http://www.usbr.gov/uc/water/crsp/cs/images/2014%20HFE%20hydrograph.PNG

In addition to using HFEs to create habitat, mechanical removal of trout (2003-2006 and 2009) has been another method to control non-native fish in the vicinity of the LCR to reduce predation on the humpback chub by trout and other non-native fish (Coggins and Walters 2009, Coggins Jr et al. 2011). From 2003-2005 approximately 22,000 non-native fish were removed from near the LCR by electrofishing and taking the fish out of the system. The apparent result was that the removal was successful in increasing recruitment of native fish into the locations used by humpback chub. In years following mechanical removal, numbers of juvenile native fish in the vicinity of the LCR were higher than pre-removal numbers (Coggins Jr et al. 2011). Confounding this information is that the years following the removal project were drought years and water temperatures were warmer than those preceding the project (Coggins Jr et al 2011). During this period of drought, water temperatures reached as high as 17°C, almost 6°C higher than 1990-2002 data from the same location (Figure 4). In addition, during this relatively dry period, turbidity has been high from local flows from the Paria (Coggins Jr et al. 2011). The increase in turbidity favors the native humpback chub over the non-native trout. This information further complicates what mechanisms actually control the interaction between native fish recruitment and non-native distribution and predation and highlights the complex interaction between physical process and ecological response.

Figure 4. Daily mean water temperature (◦C) observed in the Colorado River (∼river kilometer 99.2) during 1990–2006. Lines indicate locally weighted polynomial regression (lowess) fits to the indicated data set (Coggins Jr et al. 2011).

Managing for multiple conflicting objectives is something that is not unique to the Grand Canyon. As the human footprint continues to grow and our awareness of ecosystem needs and changing valuation of the environment changes this is a scenario that will be increasingly common. Often times, it seems like comprise somewhere in the middle would be a place where both competing interests could continue, but with the humpback chub and rainbow trout in the Grand Canyon it seems that the middle ground is perpetual management to maintain the humpback chub. The current system is so altered from historic conditions that it seems highly unlikely that without continued management the humpback would ever be self- sustaining. An interesting positive of a seemingly bad situation of continued drought seems to be possibly beneficial to the humpback chub, but the societal costs could be challenging. Understanding how physical process ranging from the timing of HFE releases to long-term drought and the interactions of those processes with biota will provide guidance to management of the humpback chub. The Grand Canyon is a heavily altered system and the humpback chub and razorback sucker are the last holdouts of fish that evolved unique morphologies in a very dynamic system and now those morphologies are not advantageous and the dynamism of the system has been homogenized through management. Only time will tell how these unique fish evolve to life in a heavily managed system with heavy competition for resources.

References

Coggins Jr, L. G., M. D. Yard, and W. E. Pine 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. Coggins, L. G., and C. J. Walters. 2009. Abundance trends and status of the Little Colorado River population of humpback chub: an update considering data from 1989-2008. Korman, J., S. J. Martell, C. J. Walters, A. S. Makinster, L. G. Coggins, M. D. Yard, W. R. Persons, and T. Quinn. 2012. Estimating recruitment dynamics and movement of rainbow trout (Oncorhynchus mykiss) in the Colorado River in Grand Canyon using an integrated assessment model. Canadian Journal of Fisheries and Aquatic Sciences 69:1827-1849. Pine III, W. E., B. Healy, E. O. Smith, M. Trammell, D. Speas, R. Valdez, M. Yard, C. Walters, R. Ahrens, and R. Vanhaverbeke. 2013. An individual-based model for population viability analysis of humpback chub in Grand Canyon. North American Journal of Fisheries Management 33:626-641. Webb, R., D. Wegner, E. Andrews, R. Valdez, and D. Patten. 1999. Downstream effects of Glen Canyon dam on the Colorado River in Grand Canyon: A review. The controlled flood in Grand Canyon:1- 21.