Site Occupancy Assessment of an Experimental Flow Regime on the Endangered Humpback Chub (Gila cypha) in the Colorado River, Grand Canyon, AZ

Conserving native riverine fish communities is in part dependent on how regulated river corridors are managed. In order for dams and hydropower operations to be managed optimally with respect to ecosystem integrity, an understanding of the ecological linkages between stream flow conditions, physical habitat and fish community structure is essential. In the Colorado River within Grand Canyon altered river conditions caused by the installation of Glen Canyon Dam have been implicated in the decline of the humpback chub. Specifically, hydropeaking flow schedules or fluctuating flow regimes have caused diel destabilization of nearshore habitats that are thought to be important for humpback chub. Beginning in 2009 a multi-year adaptive management experiment was implemented to contrast the current fluctuating flow management policy with an experimental steady flow regime.  We examined the habitat use patterns of humpback chub using a patch occupancy approach during 2009 and 2010 to assess the influence of this experimental steady flow regime. An occupancy approach was selected because it explicitly incorporates an estimation of detection probability into our assessment of habitat use. Four river trips occurred yearly during July through October 2009 and 2010. River trips conducted in July and August were during fluctuating flows whereas trips conducted in September and October occurred during steady flows. During each trip hoopnets were fished in 60 sites for 12 consecutive 24 hour passes.  In this analysis, humpback chub catch at a given site was partitioned into size categories (50-100 mm, 100-150 mm, 150-200 mm and <200 mm) and converted to a binary sequence (0 if absent and 1 if present) to examine if different patterns of occupancy and detection emerged for different size classes of fish. Site specific covariates included in occupancy models included: shoreline habitat type, hydraulic type, flow regime, mean site depth, and substrate type. Pass specific covariates included the mean daily turbidity and the presence-absence of coexisting fish species.  AIC model selection procedures were utilized to determine the most biologically reasonable model.  Preliminary results suggest that occupancy is high (>0.7) in all sites and that detection ranges from 0.3-0.5.