In the United States, 28 Evolutionary Significant Units of Pacific salmon and trout are currently listed as threatened or endangered. Ten of these are located, either wholly or in part, in California. Included on this list is California’s Central Coast (CCC) coho salmon (Oncorhynhus kisutch), which have been reduced to only 40-48% of their native range during years with strong brood classes. Interestingly (and alarmingly), most coho within the CCC are found in a single watershed, Lagunitas Creek (Marin County), despite this watershed being located at the southern edge of the coho range. In an effort to understand this pattern, we examined variation in body size of coho salmon migrating from freshwaters to the ocean (“smolts”) originating from the Lagunitas watershed versus three other nearby watersheds, because smolt size has been shown to have an influence on ocean mortality rates during years when ocean conditions are unfavorable. Our objectives were to: (1) test for variation in smolt size among years, (2) test for variation among sites within a year, and (3) test whether variation in abiotic (e.g., precipitation) or biotic (e.g., conspecific density) factors contributed to variation in smolt size among years or sites.
Results/Conclusions
Our preliminary results suggest that there appears to be regional coherence in terms of smolt size within a given year. The exception to this pattern is in Lagunitas Creek, the only flow-regulated watershed in our study, which doesn’t adhere to the regional consistency demonstrated by the other watersheds. Specifically, analysis of among year differences in smolt size indicated that smolts were smaller in 2006 than other years with the exception of Lagunitas smolts. The 2006 smolts were offspring of adult salmon that spawned in the winter of 2004-05, which was a year associated with a large number of adult breeders in these watersheds suggesting the possible importance of conspecific density as a factor determining body size of the resultant smolts. A comparison of smolt size across sites indicated that smolts outmigrating from Lagunitas Creek were generally larger than fish outmigrating from the other sites, and this result was consistent across years. This result suggests that growth conditions in Lagunitas are superior to those in the other sites, possibly due to a variety of factors such as extensive restoration activities on the mainstem of Lagunitas Creek which have improved summer rearing habitat for stream-residing salmonids.
