The Colorado River ecosystem in Grand Canyon is strongly impacted by operation of Glen Canyon Dam... more The Colorado River ecosystem in Grand Canyon is strongly impacted by operation of Glen Canyon Dam. Fluctuating releases of cold, hypolimnetic water from Lake Powell (~8ºC) for peak hydroelectric power generation has had a profound impact on the formerly warm, silty Colorado River. Clear, cold stenothermal conditions preclude successful reproduction and recruitment of most native fishes in the mainstem and introduced trout have become dominant species in the fish community. To assess the effects of dam operations on the native fish assemblage, the distribution and abundance of native and non-native fishes of the Colorado River ecosystem in Grand Canyon were assessed in the first year of a two-year research and monitoring program funded by the Grand Canyon Monitoring and Research Center. Study sites were located along 159 miles of the Colorado River from the Paria River to below Havasu Creek. An intensive area of study was established from just above the confluence of the Little Colorado River (LCR) at river mile (rm) 60 to Tanner Rapid at rm 68.6 (LCR inflow reach). Additional sampling was conducted in major tributaries (LCR, Bright Angel, Shinumo, Kanab, Havasu). Mini-hoopnets, minnow traps, electrofishing, trammel nets, and seines were used to sample fish during 120 days of fieldwork spread over four seasons between March 1998 and February 1999. Some 4,618 fish were captured in the Colorado River mainstem and tributaries. Non-native species dominated the catch in the mainstem Colorado River (1,515/2,565, 59%) and rainbow trout Oncorhychus mykiss was the most common non-native species (n=955, 37%). Of 1,050 native fish captured in the mainstem, speckled dace Rhinichthys osculus (n=463, 18%) and humpback chub Gila cypha (n=395, 15%) were the most abundant. The use of mini-hoopnets to sample fish in mainstem habitats revealed a relative abundance of humpback chub in the 100-200 mm TL size class, a population feature not observed in previous studies. In the smaller tributaries (Bright Angel, Shinumo, Kanab, Havasu), native species dominated the catch (739/847, 87%). Speckled dace (n=370, 44%) and humpback chub (n=133, 16%) were the most abundant native fishes. The LCR assemblage was dominated by native species (963/1206, 80%) with humpback chub (n=515, 43%) and speckled dace (n=308, 19%) dominating. The pattern of appearance, distribution, and abundance of young-of-year (YOY) humpback chub in the mainstem Colorado River observed in our study was consistent with a "source-sink" model of downstream dispersal from the LCR coupled with high mortality from predation in the mainstem Colorado River. The abundance of adult rainbow trout in the LCR inflow reach of the Colorado River coupled with their complementary distribution with juvenile humpback chub in January 1999 strongly suggests that predation by rainbow trout may have been responsible for the near disappearance of YOY humpback chub between late summer and early winter. Mortality rates for small fish are likely exacerbated by fluctuating flows, which disrupt habitat associations and increase the probability of downstream movement and dispersal. Fluctuating flows also reduce the suitability of river edge habitats and food resources needed for rearing small fish. The rarity of small humpback chub downstream of the LCR inflow coupled with high abundance of trout and cold fluctuating flows suggest that conditions for recruitment of YOY humpback chub in the Colorado River in Grand Canyon are extremely poor. In order to provide opportunities for increased survivorship of small humpback chub in Grand Canyon, resource managers should consider ways to stabilize flows and increase temperature in the mainstem Colorado River. Implementation of more natural flow and temperature conditions will increase the stability of shoreline habitat and cover, and likely result in increased food resources and growth rates in small fishes, and decrease mortality associated with picivory.
The Colorado River ecosystem in Grand Canyon is strongly impacted by operation of Glen Canyon Dam... more The Colorado River ecosystem in Grand Canyon is strongly impacted by operation of Glen Canyon Dam. Fluctuating releases of cold, hypolimnetic water from Lake Powell (~8ºC) for peak hydroelectric power generation has had a profound impact on the formerly warm, silty Colorado River. Clear, cold stenothermal conditions preclude successful reproduction and recruitment of most native fishes in the mainstem and introduced trout have become dominant species in the fish community. To assess the effects of dam operations on the native fish assemblage, the distribution and abundance of native and non-native fishes of the Colorado River ecosystem in Grand Canyon were assessed in the first year of a two-year research and monitoring program funded by the Grand Canyon Monitoring and Research Center. Study sites were located along 159 miles of the Colorado River from the Paria River to below Havasu Creek. An intensive area of study was established from just above the confluence of the Little Colorado River (LCR) at river mile (rm) 60 to Tanner Rapid at rm 68.6 (LCR inflow reach). Additional sampling was conducted in major tributaries (LCR, Bright Angel, Shinumo, Kanab, Havasu). Mini-hoopnets, minnow traps, electrofishing, trammel nets, and seines were used to sample fish during 120 days of fieldwork spread over four seasons between March 1998 and February 1999. Some 4,618 fish were captured in the Colorado River mainstem and tributaries. Non-native species dominated the catch in the mainstem Colorado River (1,515/2,565, 59%) and rainbow trout Oncorhychus mykiss was the most common non-native species (n=955, 37%). Of 1,050 native fish captured in the mainstem, speckled dace Rhinichthys osculus (n=463, 18%) and humpback chub Gila cypha (n=395, 15%) were the most abundant. The use of mini-hoopnets to sample fish in mainstem habitats revealed a relative abundance of humpback chub in the 100-200 mm TL size class, a population feature not observed in previous studies. In the smaller tributaries (Bright Angel, Shinumo, Kanab, Havasu), native species dominated the catch (739/847, 87%). Speckled dace (n=370, 44%) and humpback chub (n=133, 16%) were the most abundant native fishes. The LCR assemblage was dominated by native species (963/1206, 80%) with humpback chub (n=515, 43%) and speckled dace (n=308, 19%) dominating. The pattern of appearance, distribution, and abundance of young-of-year (YOY) humpback chub in the mainstem Colorado River observed in our study was consistent with a "source-sink" model of downstream dispersal from the LCR coupled with high mortality from predation in the mainstem Colorado River. The abundance of adult rainbow trout in the LCR inflow reach of the Colorado River coupled with their complementary distribution with juvenile humpback chub in January 1999 strongly suggests that predation by rainbow trout may have been responsible for the near disappearance of YOY humpback chub between late summer and early winter. Mortality rates for small fish are likely exacerbated by fluctuating flows, which disrupt habitat associations and increase the probability of downstream movement and dispersal. Fluctuating flows also reduce the suitability of river edge habitats and food resources needed for rearing small fish. The rarity of small humpback chub downstream of the LCR inflow coupled with high abundance of trout and cold fluctuating flows suggest that conditions for recruitment of YOY humpback chub in the Colorado River in Grand Canyon are extremely poor. In order to provide opportunities for increased survivorship of small humpback chub in Grand Canyon, resource managers should consider ways to stabilize flows and increase temperature in the mainstem Colorado River. Implementation of more natural flow and temperature conditions will increase the stability of shoreline habitat and cover, and likely result in increased food resources and growth rates in small fishes, and decrease mortality associated with picivory.
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