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Fish Growth Rate Study
The temperature monitoring study and the surface flow study conducted as part of the Stillwater Sciences' and the University of California, Berkeley's Napa River Basin Limiting Factors Analysis found that temperatures in the summer months were relatively high while flows were relatively low. It was hypothesized that low salmonid abundance may be due to the combination of, elevated fish metabolic needs, due to elevated temperatures, and low food availability, due to dewatered riffles.
To determine whether differences in flows over riffles coupled with warm temperatures in the summer, would create conditions in which juvenile steelhead would have difficulty meeting their metabolic needs and growing during the summer months, Stillwater Sciences conducted a pilot study in the summer of 2001 in eight pools located in two Napa River tributaries, including sites believed to have relatively favorable flow conditions. Juvenile salmonids were captured measured and marked early in the summer season and then recaptured and re-measured to determined net growth over the elapsed period of time. The study documented very limited, or negative, growth rates for young-of-the-year steelhead at all sites, implying that food resources in the reaches monitored were insufficient in summer 2001 to satisfy the base metabolic needs of the fish. These findings support the hypothesis that reduced salmonid feeding opportunities, due to dry riffles, combined with increased metabolic costs, caused by warm temperatures, could ultimately result in small smolts with poor survival during emigration and be responsible, at least in part, for the reported decline of steelhead in the Napa River watershed.
Photo by Jonathan Koehler