Little fish big river - dam operations and fish life history

Authors: David L. Rogowski

Year: 2014 (xlvi)

Abstract

Glen Canyon Dam on the Colorado River in Northern Arizona was completed in 1966. The resulting cold water releases created a Blue Ribbon rainbow trout, Oncorhynchus mykiss, tailwater fishery below the dam. From the 1970-1980s it was known for trophy size rainbow trout, however over time fewer large fish have been captured during standardized electrofishing surveys. In addition, median size of ripe fish has also significantly declined. This change in life history of rainbow trout is believed to be in response to modifications to Glen Canyon Dam operations. Initially Glen Canyon Dam was operated in response to power demands resulting in fluctuations in water of 4.5 m within a day. There was little natural reproduction occurring, therefore rainbow trout were stocked to sustain the fishery. In 1991 operations at the dam were significantly changed in response to environmental and cultural concerns. Variance in flows was significantly reduced. By the mid-1990s most rainbow trout within the system were naturally produced and stocking ceased in 1998. Since then, angler catch rates have increased to an all-time high (~2 fish/hour); however the number of large fish has declined. It is believed that the reduction in variance of flow (volume of water) accounts for this life history shift. Flow variance is positively correlated with median length of ripe male rainbow trout, as well as percent of large fish (>456 mm) captured during electrofishing surveys. Consistent flow levels have allowed rainbow trout to naturally reproduce every year, thereby increasing the density and competition for a limited food base, and consequently selecting for a fish that matures and reproduces early resulting in smaller fish.

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