Steelhead abundance in seasonally closed estuaries estimated using mark recapture methods
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Steelhead abundance in seasonally closed estuaries estimated using mark recapture methods
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    Seasonally-closed estuaries in central California are important rearing habitat for populations of steelhead (Oncorhynchus mykiss). During periods of estuary closure, juvenile steelhead recruit to the resulting freshwater lagoons, where they may benefit from enhanced growth conditions afforded by inputs of marine nutrients and subsequent increased marine survival, but also face high predation pressure. Accurate estimation of the number of steelhead rearing in lagoon habitat is, therefore, essential for effective management. We implemented a monthly mark-recapture sampling protocol to estimate abundance of steelhead in the Scott Creek lagoon (Santa Cruz County, California) during three years that experienced different patterns of sandbar closure. Specifically, we conducted paired sampling events in which a marking event and a recapture event were conducted each month during the period of sandbar closure. We used recaptures of steelhead individually marked with passive integrated transponders (PIT tags) to assess performance of three methods of abundance estimation; two methods assuming an open population and one assuming a closed population. Monthly estimates of abundance generated using the open population methods were similar to the closed-population method when recapture rates were e10% and the assumption of closure was met. By incorporating each encounter with an individually marked steelhead to inform the estimates of lagoon abundance, the open population methods increased the number of recaptured steelhead in the sampled population, thereby increasing the precision of our abundance estimates relative to the closed-population method. Thus, the open population methods allowed us to more precisely estimate the lagoon population during months when recaptures were very low (0-4%) or the closure assumption was not met. Further, our paired, two-day mark-recapture sampling program provided a consistent sampling routine that could be applied across years with different lagoon closure and population closure dynamics, while minimizing sampling effort and disturbance to the lagoon. Our methods may be broadly applied to bar-built estuary systems throughout central California and will offer valuable insights into ecology and population biology of Central Coast steelhead, which can be applied directly to management of this threatened Distinct Population Segment. [doi:10.7289/V5/TM-SWFSC-555 (]
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