They sound like a collection of colorful misfits—bonytail, humpback chub, squawfish, flannelmouth sucker, stoneroller. They're our native Colorado fishes, a group swimming uphill in a fight for survival.
Most of the fish we're familiar with—rainbow, brook and brown trout, tiger muskie, kokanee salmon, even carp—are not native to Colorado but were introduced as sport fish. In fact, biologists estimate that only 12 fish species were native to the Colorado River drainage within Colorado's borders, and 28 species were native to the South Platte (mostly small fish like minnows and darters).
Since the mid-1800s, the numbers of native fish and fish species in Colorado have been declining. For many species, this decline in quantity and diversity appears linked with the influx of white settlers during the 1859 Colorado gold rush. Some, like the yellowfin cutthroat, have become extinct. Six species are now listed as threatened or endangered in Colorado: The Colorado squawfish, humpback chub, razorback sucker, and bonytail are Colorado River fishes, while the greenback cutthroat trout and Arkansas darter are eastern slope fish. Twenty other natives are species of special concern, meaning their future may be in danger.
Colorado's native fish adapted over millennia to rivers with a cycle of fast, turbid spring flooding followed by clear, low water and drought in late summer, fall and winter. Human settlement wrought great changes. Water is a precious resource in an arid state like Colorado. Our high quality of life here is a result of managing water for human needs. But damming rivers and creating reservoirs greatly altered the natural dynamics of our free-flowing rivers, leaving native fish suited for cyclic conditions that no longer existed.
The Colorado squawfish once migrated upstream great distances to historical spawning grounds, releasing its eggs in summer when river flows were lower and the water was warm. Dams disrupted migration patterns and prevented the fish from reaching spawning sites. Re leases of cold water from deep reservoirs prevented spawning, stopped the eggs from developing, and generally created conditions unfavorable to warmwater fish downstream.
Native fish depend on the natural cycle of spring flooding in other ways. High spring flows scour the riverbed, removing silt and exposing gravel that fish and many insects (the fish's food base) need for depositing their eggs. While water management provides more stable water levels in many rivers (preferred by sport fish), it annually dries up some streams completely or eliminates the natural flooding cycle favored by native fish species.
Channelizing streams for flood control caused other problems. It eliminated streamside vegetation, overhanging banks, and access to seasonally flooded habitat. Lacking the protective backwater areas of a naturally meandering streambed, food, eggs, and young fish are washed downstream.
Water inhabited by native fish has become polluted due to a wide variety of human activities. Acid and heavy metals have leached into streams from old mines and tailings. Spring runoff now carries agricultural and urban pesticides and fertilizer, petroleum residue, and effluents from industry and urban water treatment. Chlorine in treated water released into a river can be toxic to fish even in very low concentrations. High concentrations of naturally occurring selenium released in connection with agricultural land use practices and erosion is suspected to interfere with reproduction of razorback suckers, an endangered species.
Even livestock grazing along waterways can hurt native fish by trampling and destroying streamside vegetation. Erosion of the destabilized streambank results in siltation, causing the stream channel to widen and become very shallow. This, and loss of shading from riparian vegetation, cause water temperatures to rise. These changes are highly detrimental to fish adapted to clear, fast-flowing streams.
Finally, the introduction of non-native fish added to the burdens of Colorado's native species. The new fish species competed more successfully for food and habitat in the altered river environment. Brown and brook trout, for example, are more aggressive than native cutthroat. These introduced trout displace native cutthroat from preferred habitat and sometimes prey directly on the cutthroat eggs and young.
So what is the future for our native fish? The success of the greenback cutthroat trout recovery program shows that we can save our native fish from extinction. With 19 stable populations established in the South Platte and Arkansas rivers, the greenback cutthroat will likely be downlisted from "threatened" to "unlisted" in the next few years. By contrast, the bonytail is extirpated in the wild (except for a nonbreeding population of very old fish in Lake Mohave, Nevada), and there are only two captive populations. Colorado is in the process of establishing a bonytail population at Horsethief State Wildlife Area and hopes to begin breeding them at a native fishes hatchery. The Native Fishes Management Plan (discussed elsewhere in this issue) will concentrate on sustaining native fish species and their habitat. With care and lots of work, we can conserve the legacy of our native fish for the future.