Population Dynamics of Threatened Humpback Chub in Grand Canyon Active

Background & Importance

Prior to introduction of nonnative fishes and completion of many dams, humpback chub (Gila cypha) were widely distributed throughout the Colorado River. Humpback chub abundance has since declined throughout their range and the species is restricted to five recognized populations. One of the largest of these, and the focus of our research, lives in Grand Canyon near the confluence between the Little Colorado and Colorado Rivers. This portion of the Colorado River is regulated and highly altered in terms of temperature, turbidity, and flow. Additionally, this river reach contains multiple nonnative fish species, which are numerically dominated by rainbow trout (Oncorhynchus mykiss). Humpback chub population decline observed in the late 1990s and early 2000s coincided 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. Recently, 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.  

Understanding the relative importance of different factors (e.g., nonnative fish, temperature, turbidity) was difficult until recently. Sampling and modeling were focused on adult humpback chub mostly in 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. This inference is difficult for such a long-lived species, which is now known to have a complicated life history. Recently, it has 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 considerable 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. These data inform ecological and economic modeling specific to the question of interest. For more details about field methods, mark-recapture models, and statistical analyses, please refer to the listed publications.

Sources/Usage: Public Domain.

Important Results   

A subset of humpback chub reside in the Little Colorado River year round (i.e., residents), whereas other adults move between the Little Colorado and Colorado Rivers 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. Rainbow trout abundance is negatively related to juvenile chub survival in the mainstem Colorado River, whereas temperature is positively related to growth rates in both the Colorado and Little Colorado Rivers. Food availability and turbidity are also positively related to growth in the Little Colorado and Colorado Rivers respectively. The seasonal timing of growth also differs between the two systems.  In the Little Colorado River, maximum growth occurs from April to June; whereas in the Colorado River, maximum growth occurs from July to September. We have developed simulation model using mark-recapture data to support the Long-Term Experimental and Management Plan (LTEMP) Environmental Impact Statement and the expanded nonnative environmental assessment.

Future Directions

We are continuing 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. Recent work has also focused on quantifying the impacts of ongoing translocations within the Little Colorado River. In recent years, we have begun applying the same approach in the western Grand Canyon where humpback chub abundances are increasing.