Study Plan - Biological Resource Responses to Fall Steady Experimental Flows Feruary 2010

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Stability of Nearshore Habitats Constant and low discharge might benefit fish by stabilizing nearshore habitats, if, for example, daily fluctuations in discharge, stage, and/or velocity makes foraging /feeding less profitable relative to when discharge is stable (for example, more energy is expended foraging during fluctuations). The 2000 low summer steady flow experiment (constant discharge of 8,000 cfs from June through August) warmed and stabilized nearshore environments (B. Ralston, U.S. Geological Survey, written commun., 2009), but there is no evidence that this management action is responsible for the decade -long increasing trend in adult humpback chub (Coggins and Walters 2009). The Grand Canyon humpback chub population began its current uptick as the result of increasing recruitment of the cohorts spawned after about 1998. Inferences with respect to the native fish response to the 2000 experiment are weak because of reduced sampling efforts in the years before 2000 and low numbers of native fish present in the system in 2000, limitations that reduce the statistical power of comparisons that were made (B. Ralston, U.S. Geological Survey, written commun., 2009). Further, no process -level measurements of fish growth, such as otolith daily increments or RNA/DNA ratios, were made during this experiment, so the fish response was only evaluated by comparing catch rate data in 2000 with the years before and after, and by comparing the strength of the 1998 -2000 cohorts after they recruited into the adult population relative to the strength of cohorts from years before or after. There was no clear response of biological parameters to short- duration flow treatments that were implemented in 2005. During September and October 2005 fluctuating (7,000 to 9,000 cfs) and steady releases (8,000 cfs) were alternated on 2 -week intervals. Backwater and mainstem habitats throughout the Colorado River ecosystem were sampled for water - quality parameters, invertebrate biomass, and fish relative abundance in early September (fluctuating) and again in late September (stable). Turbidity, which is negatively related to primary production, was significantly lower during the steady discharge treatment, but this decrease was likely because of tributary spates during the fluctuating treatment rather than the stable discharge treatment itself. Zooplankton concentrations were higher during the fluctuating treatment while relative abundance of bluehead suckers (Catostomus discobolus) was higher during the steady treatment. There were no statistically significant differences across flow treatments for biomass of the 11 benthic macroinvertebrate taxa collected, and there were no treatment effects on the relative abundance of the other 10 fish species collected (Ralston and others, 2007). This sampling effort was not repeated during the second pair of fluctuating - stable discharge treatments that occurred in October 2005, so it is tenuous to ascribe the differences that were observed in biological parameters to the flow treatments. Further, no process -level measurements, such as rates of primary production or fish growth, were made during this experiment. Juvenile humpback chub catch rates measured by the nearshore ecology project's pilot study in August 2008 during fluctuating flows were higher relative to catch rates from September 2008 during steady flows. However, large numbers of humpback chub, especially 2- and 3- year -old fish, were captured on both trips, likely because of flooding in the Little Colorado River from July to September that moved humpback chub into the mainstem (discharge of the Little Colorado River at Cameron, Arizona, was above 1,000 cfs for a duration of least 2 days on five different occasions from July to September 2008). Little Colorado River spates, which periodically move humpback chub into the mainstem, will likely complicate interpretation of catch rate /abundance data collected by the nearshore ecology project throughout the remainder of the FSEF.