Colorado, home to the headwaters of the Colorado River, the Arkansas River, the Rio Grande, and the South Platte River, offers a diverse palette of fisheries to the angler and nature enthusiast. The most iconic of these fishing opportunities are those related to trout in the mountain streams and rivers, but visiting and resident anglers are often surprised to learn that Colorado offers quality fishing for other species. Even less well known, and perhaps little appreciated by most, are the numerous native fishes found in the rivers on Colorado’s eastern plains and the Colorado River drainage. Unfortunately, with the wealth of fishing opportunities comes an equal wealth of management problems related to water development, habitat degradation, introduced and invasive species, and disease.
Prior to European settlement, Colorado was home to around fifty-five native species of fish, yet only the cutthroat trout (Oncorhynchus clarkii subsp.), green sunfish (Lepomis cyanellus), orangespotted sunfish (Lepomis humilis), Channel Catfish (Ictalurus punctatus) and black bullhead (Ameiurus melas) would be considered game fish. Another native fish, the Colorado pikeminnow (Ptychocheilus lucius), has the traits of common game fish including large size, piscivory (the eating of other fish), and an aggressive nature. During the settlement and postsettlement periods, populations of the most iconic native fish—the cutthroat trout—declined precipitously due to habitat degradation from mining and logging and from overharvest.
The cutthroat trout belonged to four recognized subspecies: the Colorado Rivercutthroat trout (O. c. pleuriticus), the greenback cutthroat trout (O. c. stomias; the Colorado state fish since 1994), the Rio Grande cutthroat trout (O. c. virginalis), and the yellowfin cutthroat trout (O. c. macdonaldi). The yellowfin went extinct around 1900 following the introduction of the nonnative rainbow trout (O. mykiss). In addition to rainbow trout, brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis) were introduced to “supplement” the native trout populations—in most cases the introductions supplanted the native trout, except in areas where remnant native populations were protected by impassable barriers to migration such as waterfalls. Although nonnative to Colorado, the introduced trout species flourished in rivers and streams that were once home to cutthroat trout, forming the basis of Colorado’s reputation as a state with world-class trout fishing. Colorado’s fish and game management agency, now known as Colorado Parks and Wildlife (CPW), as well as the US Fish and Wildlife Service, built a series of trout hatcheries to help maintain the state’s trout fisheries.
In the past few decades, state and federal agencies have worked to restore some of the cutthroat trout populations through a time- and labor-intensive process that involves locating remnant populations, ensuring they are protected from invasion by nonnative trout species, and reintroducing the native cutthroat trout. These ongoing efforts have sometimes been hampered by our changing understanding of what constitutes a cutthroat trout subspecies, yet, on the whole, they have been successful at providing anglers with at least the opportunity to fish for native cutthroat trout.
Perhaps the biggest recent challenge to Colorado’s river and stream trout fisheries, or, more specifically to the fisheries that were dominated by rainbow trout, was the inadvertent introduction of Myxobolus cerebralis, the parasite that causes whirling disease, in the 1980s. The whirling disease parasite damages the cartilage in the fish’s skeletal system, resulting in deformities (e.g., dislocated spines) that cause the characteristic “whirling” swimming behavior. It is not a threat to humans, but fish afflicted with serious whirling disease infections are likely to perish. Following this introduction, naturalized rainbow trout populations crashed throughout the state, and a major effort was undertaken to both understand the scope of the rainbow trout population declines and develop methods of reversing those declines.
This concerted effort resulted in sweeping changes in hatchery design, management strategies, and revisions of the types of rainbow trout being stocked in Colorado’s rivers. Hatcheries were modernized, decontaminated, and switched over to protected water supplies, and strict biosecurity procedures were adopted so that recontamination with whirling disease would not occur. Management changes included restrictions that kept infected fish out of waters connected to trout-containing rivers. Finally, through the joint efforts of researchers with Colorado Parks and Wildlife, Colorado State University, and other institutions, a variety of whirling disease-resistant rainbow trout, the “Hofer” variety, was extensively tested and has recently been adopted as the primary trout variety raised in CPW hatcheries. As a result of these efforts, rainbow trout are again being found in abundance, though in some areas brown trout remain the dominant species.
Colorado’s Reservoir Fisheries
Unlike states in the Great Plains or eastern United States, Colorado did not have a lot of natural lakes before European settlement. This situation changed, however, as hundreds of reservoirs and ponds were created, often as components of the extensive transbasin water delivery systems. These new bodies of water had the potential to support fisheries, so a large number of fishes were introduced to provide sportfishing opportunities. These fish were primarily species that were native to other regions of the United States, and include popular warm-water sportfish such as largemouth bass (Micropterus salmoides), walleye (Sander vitreus), bluegill (Lepomis macrochirus), channel catfish (Ictalurus punctatus), and white crappie (Pomoxis annularis). Other fishes, including the nonnative trout mentioned above, along with lake trout (Salvelinus namaycush) and Kokanee salmon (O. nerka) were also established in the cooler mountain reservoirs and in mid-elevation reservoirs with significant pools of cold water. To support these fish, forage fish such as gizzard shad (Dorosoma cepedianum) and golden shiner (Notemigonus crysoleucas) were also introduced.
These artificial reservoir and pond systems diversified Colorado’s sportfishing scene, and because many of the lakes and ponds are located near major urban centers, they received a substantial portion of the fishing pressure from resident anglers. As with the trout fisheries, the warm-water and cool-water fisheries are actively managed and supported by CPW. Some of the fisheries, such as those containing walleye along Colorado’s Front Range (the I-25 corridor extending from Fort Collins to Pueblo), provide anglers with opportunities to catch fish of trophy size.
Threatened Fishes of Colorado
The extensive agricultural and urban development of Colorado, and, indeed, of the western United States, has affected the fishes of Colorado, in part because Colorado has the headwaters of numerous major river systems. Downstream water demands often dictate flow in Colorado rivers. Changes in river flow patterns, flow quantities, temperatures, and flow timing typical of Colorado’s regulated rivers (e.g., water released for agriculture, urban use, or flood control) do not necessarily synchronize with the patterns, quantities, and timing to which native fishes are adapted, resulting in declining populations and, in a few cases, extinctions. The most well-known examples of fish affected by the drastically altered river conditions are the federally endangered large species of the Colorado River system: the Colorado pikeminnow, the razorback sucker (Xyrauchen texanus), the humpback chub (Gila cypha), and the bonytail (Gila elegans). Out on Colorado’s eastern plains, small-bodied fishes (those with adult sizes less than one foot in length) are in decline, and some species, such as the stonecat (Noturus flavus) and brassy minnow (Hybognathus hankinsoni), are given special status by CPW. Similar threatened species can be found in the Rio Grande drainage and include the Rio Grande sucker, Catostomus plebeius and the Rio Grande chub, Gila pandora.
Challenges Facing Colorado Fisheries
The challenges facing fish populations and managers of fisheries in Colorado are not unfamiliar ones. As Colorado’s human population grows, the demand for water grows as well, resulting in more frequent drawdowns of reservoirs and increased pressure on rivers. At the same time, Colorado has a finite number of fishing opportunities, and as more people take advantage of those, the amount of fishing pressure mounts — anglers seeking solitude must travel farther afield or fish at unpopular times (e.g., at night or during inclement weather). The specter of invasive fish species and other aquatic organisms (e.g., zebra mussels) has been and will continue to be a serious threat to river and lake fisheries throughout the state. The ill-advised introduction of an undesired species to a novel environment can have drastic consequences for the resident fishes and can set back or permanently alter carefully developed management plans. Finally, Colorado’s fisheries, like those throughout the United States, are facing unknown consequences from the changing climate, and while the final consequences of the changes are still unknown, fisheries managers must be prepared to adapt to novel climatic conditions.