Experimental Flows
In December 2002, U.S. Secretary of Interior Norton approved an adaptive management experiment to be conducted in Grand Canyon National Park. This experiment, recommended by the GCMRC, began in January 2003 and consists of elements designed to provide a better understanding of both sediment and fisheries resources. As part of the current GCMRC Adaptive Management Program, a key objective is to determine whether certain policy actions are improving humpback chub juvenile survival and recruitment. A central part of the fisheries experiment includes reducing the abundance of non-native fishes in a 9.5-mile reach of the Colorado River near the confluence of the Little Colorado River (LCR; RM 56.2-65.7). This experimental manipulation has been implemented in an attempt to better understand interactions between native and non-native fishes, particularly non-native coldwater salmonids and the federally endangered humpback chub.
The Grand Canyon Monitoring and Research Center, at the Direction of the Glen Canyon Dam Adaptive Management Program, began implementation of non-native fish control in the LCR inflow area of the Colorado River in January of 2003 as part of a joint federal action entitled Proposed Experimental Flows and Removal of Non-Native Fishes
. The fisheries objective of this action was to reduce the number of potential predatory and competitor fishes in habitat occupied by the federally endangered humpback chub, Gila cypha. The fish control effort uses electrofishing and had three primary purposes: a) determine the efficacy of this technique to reduce and control the number of non-native fishes in critical habitat for the humpback chub, b) assess native/non-native fish interaction by conducting diet and incidence of predation studies on non-native fishes (primarily rainbow and brown trout), and c) reduce the abundance of non-native fishes in the control reach as much as practicable.
Examination of the preliminary results from the January, February, and March of 2003 removal activities suggests a reduction ~88 % in RBT from the initial January abundance (6,570 fish) following the March trip (779 fish). Theses analyses also indicate very little change in the abundance of RBT between the end of the January trip and the beginning of the February trip (~ 7 fish). However, there was an apparent larger change in the abundance of fish between the end of the February trip and the beginning of the March. Monitoring by the Arizona Game and Fish Department during April indicated the abundance of fish in the removal reach was approximately 80% of estimates obtained the previously year. However, removal efforts in July suggested significant immigration into the removal reach had occurred such that starting abundance estimates in July were approximately 60% of the pre-January level.
Diet analyses are ongoing, but results thus far indicate low rate of piscivory by rainbow trout and high rate of piscivory by brown trout. Increasing hoop net catches of humpback chub through out 2003 may indicate a habitat/survival response by HBC following non-native removal. However, this second year of effort will provide more definitive information regarding these questions.
The success in the first months of the experiment, prompted GCMRC to examine and propose a modification to the original plan for mechanical removal. The modification extended the original area of removal downstream to RM 72.7, adding 7 miles to the area below the LCR. Following the July 2003 trip, most electrofishing and removal effort was focused between river mile 65.7 and 72.7. The notion was that only limited removal efforts would be required in the original removal reach (RM 56.2-65.7) to ensure low abundance of non-native fishes during summer 2003, and that the remaining effort could be expended in the downstream reaches. However, results from all of 2003 now suggest a higher immigration rate than originally estimated and have prompted a further modification. During 2004, we will conduct 4 depletion passes in the original reach (RM 56.2-65.7) and 4 depletion passes in only the upper part of the expanded reach (65.7 - 68.5; Lava Canyon to Tanner Canyon). We predict that this design will allow adequate removal efforts to maintain low non-native abundance and an expansion beyond the originally proposed reach. This compromise should strengthen the experimental treatment and increase both the likelihood that a change in HBC survival and recruitment will occur as well as our ability to detect such an increase.
Experiments in FY05 and beyond will be implemented as per recommendations from the Adaptive Management Work Group as approved by the Secretary of the Interior. GCMRC recommends continuing implementation of experimental treatments under the current experimental flows plan for years 3 and 4.