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Science HAPPENINGS August 2019

Precious Native Fish Face Reclassification

The razorback sucker (Xyrauchen texanus) and humpback chub (Gila cypha) are two of four native endangered fish species in the Colorado River basin. Various state and government agencies working in the basin have spent years conducting hatchery rearing and stocking efforts, comprehensive studies on fish populations, and management of non-native predators. In the past year, the U.S. Fish & Wildlife Service (USFWS) has proposed that the humpback chub and razorback sucker should be downlisted from endangered to threatened.

An adult razorback sucker (Xyrauchen texanus). PC John Caldwell.

These fish are endemic to the Colorado River basin, which means that this is the only place in the world where these species have been found. The razorback sucker and humpback chub are aptly named for their respective sharp keel and pronounced fleshy hump just behind their heads. Razorback suckers can live for more than 40 years and grow up to 3 feet in length and is one of the largest suckers in North America. Humpback chubs typically live for about 30 years and grow up to about 20 inches in length. It is estimated that both species evolved about 3 to 5 million years ago, predating humans. It is clear why such an intense effort has been put forth to recover these rare, ancient fish species.

After a species status assessment and a 5-year status review of both fish, the USFWS believe that the fish may no longer be at imminent risk of extinction throughout their range. This suggests that the fish no longer need the endangered classification.

Razorback suckers in the Green River came from near the brink of extinction in the mid-1990s to now when the population is over 30,000 adults. This is due to successful hatchery stocking. It is clear, however, that although stocked razorbacks are reproducing, the wild young-of-year fish struggle to survive to adulthood. This is largely due to predation by non-native species, lack of nursery habitat, and an altered flow regime.

An adult humpback chub (Gila cypha). PC John Caldwell

The largest stable population of humpback chub is 12,000 individuals in the Lower Colorado River and Little Colorado River of the Grand Canyon. Other smaller, but seemingly stable populations exist on the Upper Colorado River in Black Rocks, Westwater Canyon, Cataract Canyon and on the Green River in Desolation and Gray Canyon. Monitoring of these populations has shown that humpbacks continue to thrive and do not currently need hatchery stocking. Humpbacks, however, still need support through flow management and non-native predator control.

It is expected that a final decision will be made about whether the humpback chub and razorback sucker will be officially downlisted or not by October 2020 and October 2021, respectively. If determined appropriate, the reclassification of the fish will include a revised recovery plan for each species. These recovery plans will use the most current scientific information on species needs to increase and support fish survival.

It is no surprise that the rivers which make up such a magnificent landscape are home to such extraordinary creatures. We hope that they can continue to swim about these muddy waters.


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