Fisheries Resources
The Lees Ferry trout fishery has developed a stock assessment model using historic angling data and catch effort data from past monitoring efforts. The model provides a three-to five-year view of the state of this fishery resource and provides an opportunity to evaluate management strategies associated with this fishery (Speas et al., 2001). The monitoring program that is in place through a cooperative effort between GCMRC and Arizona Game and Fish includes the historic fixed sampling sites and new random, stratified sites based on shoreline type. The program’s design is intended to increase sampling areas to better characterize the trout fishery as a whole. Recent data indicate that the fishery is strongly influenced by diel changes in flows and that growth is likely density dependent: The stable flows associated with ROD operations has increased recruitment and the increased numbers of fish has resulted in smaller fish (Speas et al., 2001).
The downstream fishery program has approached the development of a long-term monitoring program in a step-wise fashion to allow for analysis of historic data and to ensure that new monitoring protocols address adaptive management program needs. Steps that have been taken in the downstream fishery program include development of population estimates for rainbow trout and brown trout in the mainstem and for humpback chub in the LCR and its confluence with the mainstem. Stock assessment models of current and historic data suggest that the LCR population of humpback chub has been in decline for over a decade. This downward trend in population abundance is based on an estimated chronic recruitment decline. Multiple hypotheses exist for the apparent recruitment decline including dam operations, tributary flooding, parasitism, predation/competition and mainstem temperature effects. Due to some controversy over appropriate methods to estimate the abundance humpback chub in Grand Canyon, an independent panel of experts has been convened to review current stock assessment methods. A report from this panel will be available at the January 2004 AMWG meeting.
Associated with the 2003-2004 adaptive management experiment approved by the Secretary of Interior, a program of mechanical removal of non-native fishes near the confluence of the Little Colorado was implemented in 2003. This work will continue in 2004 as recommended. Early results suggest the efficacy of mechanical removal of non-native salmonids is quite high (>50%), but that immigration of fish back into the removal reaches is substantial. Therefore, frequent removal of non-native fishes is necessary to maintain low abundance.