Native Fish of the Grand Canyon Allison Dedrick Abstract: Native fish in the Grand Canyon evolved in a habitat that was extremely physically variable but biologically isolated from other river systems. The assemblage of fish species native to the Grand Canyon is highly endemic to the Colorado River basin: most of the species are found in no other river systems. With the construction of dams and other water infrastructure projects and the introduction of numerous nonnative fish species, the environment these fish now inhabit has switched to being more physically constant and biologically crowded. These changes have caused three of the eight fish species native to the Grand Canyon to be extirpated from the canyon stretch and left only one of the native species at population levels high enough to be considered common in Grand Canyon National Park. Four of the eight are listed as endangered and are now monitored and managed in attempts to recover their populations. Freshwater habitat in the Grand Canyon: The Colorado River basin (Figure 1) reaches into seven states and drains an area of at least 600,000 km2 (Graf 1985), with headwaters starting up in Colorado and finally ending in the southeast corner of California at its border with Arizona. Prior to human involvement and water management, the stretch of the Colorado River that passes through the Grand Canyon was highly variable, with flows changing over a range of five orders of magnitude. Before the dams were put in place, a 40-year flow record from Yuma, Arizona showed flows ranging from 0.5 to almost 7,000 m3/s (Minckley 1991). Most of the water comes from snowmelt, causing higher flows in the spring and lower flows during the summer through the onset of winter, though summer storms can raise flows in tributaries temporarily. These widely varying but sometimes extremely high flows created a range of habitats, including deep pools, calm backwaters left by receding high waters, turbulent flow, and eddies. The geology of the rock underlying the Grand Canyon and larger Colorado River basin caused the water to be quite muddy and it was also often warm, particularly in the summer (Miller 1961). Introduction to the fish: The larger Colorado River basin, which includes both the mainstem Colorado River and its tributaries, has fourteen species of native fish. Eight of the fourteen species native to the larger basin were native to the section of the river passing through the Grand Canyon: the speckled dace (Rhinichthys osculus), razorback sucker (Xyrauchen texanus), flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Catostomus discobolus), Colorado pikeminnow (Ptychocheilus lucius), roundtail chub (Gila robusta), humpback chub (Gila cyhpa), and bonytail chub (Gila elegans) (Figure 2). These eight fish were once common to both the upper and lower basins of the Colorado River (Carlson and Mace 1989). Five of these species are still present in the canyon today – the humpback chub, razorback sucker, bluehead sucker, flannelmouth sucker, and speckled dace – but the other three have been extirpated from the Grand Canyon. Figure 1: Map of the Colorado River Basin. The Grand Canyon begins just below the Glen Canyon Dam at Lee’s Ferry. The San Juan River and the Green River in the upper basin hold many of the remaining populations of native fish. Map source: Colorado River Commission of Nevada (http://crc.nv.gov/index.asp?m=maps). The fish in the Colorado River basin show high endemism: six of the eight species native to the Grand Canyon are endemic to the Colorado River basin (Minckley 1991). Only two families of fish are represented in the fish species in the Grand Canyon: minnows (Cyprinidae) and suckers (Catostomidae). The pattern of high endemism is not unique to fish but is found for other freshwater organisms in the Colorado River as well. The Colorado River has the lowest diversity of fish and highest endemism of river systems in the North America, likely due to the system’s isolation and the high variability of temperature and flow conditions in the mainstem and tributaries. The fish assemblage in the Grand Canyon is thought to have existed for about 6 million years, with the genera present since the Pliocene and speciation happening during the Pleistocene and early Holocene (Carlson and Muth 1989). The river system has likely been separated from other river systems for at least as long. **" *" **" **" *" **" Figure 2: Native fish of the Grand Canyon. Clockwise from upper right corner: speckled dace (Rhinichthys osculus), razorback sucker (Xyrauchen texanus), flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Catostomus discobolus), Colorado pikeminnow (Ptychocheilus lucius), roundtail chub (Gila robusta), humpback chub (Gila cyhpa), and bonytail chub (Gila elegans). Species with a red box have been extirpated from the Grand Canyon, while species with a blue box are still present in the Grand Canyon. Species listed as federally endangered are designated with two red asterisks while the two species that are not endemic to the Grand Canyon are indicated with one black asterisk. Illustrations are by Joe Tomelleri (Grand Canyon National Park: http://www.nps.gov/grca/learn/nature/fish-native.htm). Perhaps unsurprising due to their isolation and somewhat extreme environment, fish native to the Grand Canyon share some common characteristics. First, they tend to have a long life span: the Colorado pikeminnow is able to live up to 80 years and the chubs and suckers can live for 20-30 years (Minckley 1991). Since the conditions in the Grand Canyon were once so variable, both within and among years, having a long life span might have been a strategy to ensure that reproduction could be spread out over many years in the hopes that at least some of the years would have conditions favorable for reproduction and recruitment. Additionally, the native fish of the Grand Canyon are also relatively large-bodied. The Colorado pikeminnow can reach lengths of six feet and the chub and sucker species can reach almost two feet in length (Grand Canyon’s Extirpated Fish Species: National Park Service). The speckled dace, however, is neither particularly large nor long-lived, reaching only about four inches in length and living for two or three years. In addition to size and life span, fish of the Grand Canyon also share some physical characteristics. The species tend to have a streamlined shape, with small heads and eyes, large fins, and thick skin. These features might be advantageous for life in fast, turbulent water often laden with sediment (Minckley 1991). Several of the species also have humps behind their heads, most notably the humpback chub and the razorback sucker. The hump was once thought to be another characteristic to aid swimming in fast turbulent water but instead was found to increase drag and the energy required to maintain position in flow (Portz and Tyus 2004). Portz and Tyus argue that the humps are actually an example of convergent evolution in response to the predatory Colorado pikeminnow. By growing a hump, young fish could more quickly grow past the size vulnerable to predation by the Colorado pikeminnow. Current status of Grand Canyon native fish: Four of the eight species native to the Grand Canyon have been listed as federally endangered: the humpback chub, bonytail chub, razorback sucker, and Colorado pikeminnow. Only the humpback chub and the razorback sucker are still present in the Grand Canyon, though both are rare. Of the five native fish species that remain in the Grand Canyon, only the speckled dace is common in the park and it lives primarily in the tributaries. Bonytail chub: In addition to being absent from the Grand Canyon, the bonytail chub no longer has any reproductive populations in the wild. It was listed as endangered in 1980 and was essentially extirpated from the wild before biologists had much of a chance to study it so relatively little is known about its natural behaviors and habitat. By the late 1970s and early 1980s, biologists realized that they were seeing only adults and no young in the Grand Canyon so removed adults to be placed into hatcheries (Upper Colorado River Endangered Fish Recovery Program). Humpback chub: The humpback chub was listed as endangered in 1967. There are six populations that remain in the overall Colorado River basin. Within the Grand Canyon, they are currently found almost exclusively at the confluence with the Little Colorado tributary, which is the largest population. The fish spawn in the warmer waters of the tributary, as the mainstem waters are too cold. They face predation pressure from the trout populations upstream in the tailwater of the Glen Canyon Dam (Historical Native Fishes of Glen and Grand Canyons) and also have trouble with nonnative parasites (Humpback Chub Tributary Tranlocations: National Park Service). Razorback sucker: The razorback sucker was thought to be absent from the Grand Canyon for many decades but recently has been sighted spawning in the Grand Canyon again. Listed as federally endangered in 1991, a population in Lake Mead was thought to be the only remaining wild population. In 2012, however, larval razorback suckers were found at several sites in the Grand Canyon, creating hope that a population might be able to reestablish there (Oskin 2014). Colorado pikeminnow: Listed in 1967 and given full protection with the passage of the Endangered Species Act in 1973, the Colorado pikeminnow has been extirpated from the Grand Canyon for several decades. The last known sighting in the Grand Canyon was in 1972 (Grand Canyon’s Extirpated Fish Species: National Park Service), though populations are still present in the upper part of the Colorado River basin above Lake Powell, particularly in the San Juan and Green Rivers. The Colorado pikeminnow was once the top predator in the Grand Canyon and historically undertook large spawning migrations with high spawning site fidelity.