Monitoring Early Life Stages to Evaluate the Effects of Flow on Long-Term Trends in Salmonid Populations in Large Regulated Rivers

Most assessments of the effects of flow regulation in large rivers on resident fish populations are based on monitoring adult populations. This can result in substantial delays between the time a flow manipulation is implemented and when it is potentially detected in the monitoring data. As well, uncertainty in ageing makes it challenging to assign size classes of fish to the year when they were juveniles and likely most susceptible to effects of the flow changes. In Adaptive Management settings, poor understanding of critical mechanisms regulating fish population sizes make it very difficult to provide substantiated arguments in support of a particular flow manipulation in the first place. In this paper, we demonstrate by example how monitoring multiple early life history stages can be used to overcome these challenges.

Increases in hourly flow fluctuations and a controlled flood from Glen Canyon Dam were implemented to reduce survival of non-native rainbow trout and enhance survival of endangered humpback chub in the Colorado River in Grand Canyon. We assessed whether these flows increased incubation mortality for rainbow trout, and if so, whether the effects were apparent in age-0 and adult populations. Incubation mortality rates for rainbow trout resulting from greater fluctuations in flow were 23-49% (2003, 2004) compared to 5-11% under normal flow fluctuations (2006-2010). Effects of this mortality were not seen in hatch date distributions or in the abundance of the age-0 population, and a stock recruit analysis indicated that strong compensation in survival rates shortly after emergence mitigated the impact of incubation losses. Multiple lines of evidence indicated that a controlled flood in 2008 resulted in a large increase in early survival rates of age-0 trout. Age-0 abundance was over four-fold higher than expected given the number of viable eggs that produced these fish, and this change was apparent in the adult population by 2010. A hatch date analysis indicated that early survival rates were much higher for cohorts that hatched about one month after the controlled flood (~ April 15^th) relative to those that hatched before this date. These cohorts, which were fertilized after the flood, were not exposed to high flows and emerged into better quality habitat with elevated food availability. Our study highlights the need to rigorously assess flow experiments through the evaluation of population-level responses, and the utility of early life history monitoring in this context.