Kathryn L. Watson Influence of Size and Growth Rate in Oncorhynchus kisutch Spring 2010 1 The Influence of Size and Growth Rate on Outmigration Timing in Coho Salmon (Oncorhynchus kisutch) Smolts Kathryn Lynn Watson ABSTRACT Coho salmon (Oncorhynchus kisutch) are on the brink of extinction in central California. The period of salmon migration from feshwater to the marine environment greatly influences prospects for survival in the ocean and subsequent migration to freshwater to reproduce. Smolt scales and other body size parameters from the spring 2009 outmigration of coho salmon on Lagunitas Creek (Marin County, CA) were analyzed to determine the influence of body size and growth rate on migration timing. Fork length was negatively and significantly correlated to the date of migration (r2 = 0.04, p = 0.013). The residuals from the regression of number of circuli on fork length, which can be used as a proxy for growth rate, were negatively and significantly correlated to the date of migration (r2 = 0.10, p < 0.0001). Although both growth rate and absolute size were significantly related to migration date, growth rate appears to exert a stronger influence because it is explains more of the variation in migration date. Faster growing coho are able to leave freshwater earlier, and in doing so they confer several advantages upon entering the marine environment that increase their chance of survival. KEYWORDS Lagunitas Creek, scales, residual analysis, migration patterns, circuliKathryn L. Watson Influence of Size and Growth Rate in Oncorhynchus kisutch Spring 2010 2 INTRODUCTION Coho salmon (Oncorhynchus kisutch), an important component of California’s native biodiversity, are on the brink of extinction in California (Miller 2010). The causes of this decline have been attributed to a variety of factors, including dam construction, diversion of water, over-harvesting in the ocean, greater variability in ocean conditions, and climatic events (Carlisle et al. 2008). In 1996 all coho populations in the West Coast were designated as threatened under the Endangered Species Act (Federal Register 1996) and divided into several Environmentally Significant Units (ESUs). An ESU is a population or group of populations that are (1) reproductively isolated from other populations of the same species and (2) comprise an important component in the evolutionary legacy of the species (Waples 1991). The California Central Coast ESU (CCC ESU), which is located in California between Punta Gorda and the San Lorenzo River (NMFS 2008), is of particular interest for this study. As the largest and most stable coho population in the CCC ESU, the Lagunitas Creek system is where coho salmon have the greatest chances for survival (Moyle 2008). Coho population numbers within the CCC ESU have significantly dropped from between 50,000 and 125,000 individuals in the 1940s to only 6,000 individuals in 1996 (Federal Register 1996). Because of these severe declines, the status of CCC ESU coho was changed from threatened to endangered under the ESA in 2005 (Carlisle et al. 2008). In order to better manage these endangered populations, it is crucial to understand factors impacting their survival. Studying the emigration period of coho salmon allows for better understanding of migration patterns and timing. Coho salmon life history normally occurs over a three year span, during which time they live in both freshwater and marine environments, and have two critical migration periods that characterize their life history. Coho are born in freshwater, where they remain for one year before migrating to the ocean. After eighteen months in the ocean, coho return as adults to their natal freshwater habitat spawn (CDFG accessed 14 Mar 2009). The period of transition between freshwater and saltwater is considered to be a “critical period” because an individual’s prospects for survival and subsequent migration to freshwater to reproduce are dependent on the first year in the marine environment (Quinn 2005). Coho migrate to the ocean because there is a greater abundance of food resources, which allow for faster growth and ultimately prepare salmon for migration back upstream. Migration from the ocean back to freshwater streams offers adult coho safer spawning and rearing sites than theKathryn L. Watson Influence of Size and Growth Rate in Oncorhynchus kisutch Spring 2010 3 marine environment (Beamish 2005). Scheuerell et al. (2009) found that Chinook (O. tshawytscha) and steelhead (O. mykiss) that migrated to the ocean earlier tended to exhibit higher survival rates. Pearcy (1992) found that migration occurs at a time that optimizes the availability of food in the ocean and minimizes the abundance of predators. In other words, salmon enter the marine environment when conditions for growth and survival are optimal. Analysis of fish scales is often a useful method of determining body size, which is an important factor to consider when studying coho migration. Fish scales grow much like tree rings; however, in fish the circuli (dark concentric rings) are laid down sub-monthly for juvenile salmonids. The distance between circuli reflects the amount of growth during that period, and has been shown to correlate strongly with scale growth rate in coho salmon (Holtby et al. 1990; Fischer and Pearcy 2005). Similarly, Fisher and Pearcy (1990) found that the rate of circuli formation was significantly and positively correlated to the rate of overall growth of coho salmon smolts. Many studies have focused on the influence of size during the migration period on survival (e.g. Matthews and Ishida 1989, Quinn and Peterson 1996, Brakensiek and Hankin 2007), but relatively few have studied how body size and growth rate may be related to the specific timing at the onset of migration, especially in California populations of coho salmon. The purpose of this study is to determine the influence of absolute body size and growth rate on the migration timing of coho salmon smolts from Lagunitas Creek (Marin County, CA) using scale samples collected from Lagunitas Creek coho smolts during the spring 2009 migration. I hypothesize that growth rate will have a significant impact on the timing of downstream migration timing, and absolute body size will not significantly impact migration timing. METHODS Study site Lagunitas Creek, located within the California Central Coast
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