Population Dynamics of Endangered Humpback Chub in Grand Canyon

Science Center Objects

Construction of Glen Canyon Dam has led to large changes in environmental conditions of the downriver Colorado River. Whereas the pre-dam Colorado River experienced large seasonal variation in temperature and discharge and was highly turbid, the post-dam Colorado River is far less variable in terms of temperature and discharge and is frequently clear. Many nonnative fish species had already been introduced to the Colorado River or its tributary prior to dam completion, and some thrive in this altered environment. The federally endangered humpback chub is a native fish of the Colorado River that evolved in the pre-dam environment over millions of years and has been able to persist for a half-century in the post-dam environment, alongside introduced non-native species including rainbow trout. The goal of this project is to monitor all life stages of humpback chub (juvenile, subadult, and adult), estimate survival, growth, movement and abundances for various life stages and develop population models to predict responses to potential management strategies focused on either controlling non-native invasive species or restoring aspects of the physical environment.

Background & Importance

Prior to introduction of nonnative fishes and completion of numerous dams, humpback chub were widely distributed in the Colorado River. Humpback chub (Gila cypha) abundance has since declined throughout their range and the species is restricted to six extant populations. The largest of these populations, and the focus of our research, lives in Grand Canyon and is isolated from upstream populations by Glen Canyon Dam. Trends in this population are often attributed to conditions in the Colorado River portion of this river system, which is regulated and highly altered in terms of temperature, turbidity, and flow and which contains many introduced species, including salmonids. This population is believed to have declined during the late 1990s, coincident with cooler water temperatures and higher salmonid abundances.  Humpback chub abundance in Grand Canyon increased during the latter part of 2000s, when water temperatures were warmer and salmonid abundances were lower. Over the last six years this population has been relatively stable. An inability to determine the relative importance of different potential drivers (i.e., environmental conditions versus nonnative fish densities) has often been cited as hampering management decisions.  

A view of the lower Little Colorado River showing the light blue color of the river, riparian grasses, and canyon walls.

A view of the lower Little Colorado River (July 2013).(Credit: Maria Dzul, USGS. Public domain.)

Understanding the relative importance of different factors (e.g., nonnative fish, temperature, turbidity) was difficult until recently because sampling and modeling were focused on adult humpback chub and mostly on the Little Colorado River, where most humpback chub spawning occurs. As a result, researchers were forced to infer early-life history from recruitment to adults, which is difficult in general, and extremely difficult for such a long-lived species, which is now known to have a very complicated life history. In particular, it has recently been shown that humpback chub are double partial migrants (some individuals reside only in the Little Colorado River for many years in a row, while others reside primarily in the Colorado River and only enter the Little Colorado River to spawn every few years). Furthermore, there is extreme variation in the life history of juveniles depending on whether they rear in the Little Colorado River or Colorado River. In recent years, direct studies of juvenile humpback chub early life history in these two systems using mark-recapture methods has led to a much improved understanding of the relative importance of different factors (e.g., environmental conditions, nonnative fish densities) in determining humpback chub population dynamics. These insights have been used to develop predictive simulation models, which in turn are being used to predict outcomes of management alternatives and inform decisions.

General Methods

Mark-recapture studies are ongoing both in the Little Colorado River and a reference site in the Colorado River. We individually mark adult and subadult humpback chub with passive integrated transponder (PIT) tags, and we give size- and trip-specific visual implant elastomer marks to juvenile humpback chub. We use mark-recapture models to estimate parameters of interest, including survival rates, movement rates, growth rates, and abundances. The type of modeling framework is specific to the ecological question of interest. For more details about field methods, mark-recapture models, and statistical analyses, please refer to the listed publications.

Important Results   

Adult humpback chub do not spawn every year, and large adults spawn more frequently than small adults. In addition, there are a subset of humpback chub that reside in the Little Colorado River year round (i.e., residents), whereas other adults move between the Little Colorado River and Colorado River to spawn (i.e., migrants). Growth of all size classes is much faster in the Little Colorado River, however, in some years annual survival in the Colorado River is much higher, suggesting a potential tradeoff between growth and survival.  Environmental drivers of subadult humpback chub growth rates differ between the Little Colorado River and the Colorado River. In the Colorado River, growth is positively affected by mean monthly water temperatures and turbidity duration, but not trout abundance, food availability, or hydropeaking.  In the Little Colorado River, growth is positively related to food availability and temperature, but not turbidity duration. The seasonal timing of growth also differs between the two systems.  In the Little Colorado River, maximum growth occurs from April-June whereas in the Colorado River maximum growth occurs from July to September. We developed a simulation model using mark-recap data from 2009-2012 to support the Long-Term Experimental and Management Plan (LTEMP) Environmental Impact Statement, which did an adequate job of recreating the population trends over the period of 1993-2008.

Future Directions

We will continue to evaluate juvenile humpback chub population dynamics in both the Little Colorado River and Colorado River to refine our understanding and improve our predictive models. Key sources of uncertainty are the drivers of, and variation in both juvenile recruitment and juvenile outmigration from the LCR. In addition, we are working to incorporate detections from autonomous PIT-tag antennas into sampling efforts, as these technologies will allow us to evaluate behavior and size effects on capture probability and minimize biases in our current study design.