Date of Award


Degree Name

MS in Biological Sciences


Biological Sciences


Royden Nakamura


Currently, little is known about the demographics of the Oncorhynchus mykiss irideus, or steelhead trout, populations in San Luis Obispo County. Specifically, demographic information including length, age, and condition when first leaving a watershed for the open ocean is lacking. This thesis takes a closer look at a biological and environmental data collected by the California Department of Fish and Game Central Coast Steelhead and Coho Salmon project under the direction of Associate Biologist Jennifer Nelson. The main goal of this thesis is to analyze demographic and habitat data from the steelhead populations of two northern San Luis Obispo county coastal streams, San Simeon and Santa Rosa Creeks.

Habitat mapping surveys were conducted on each stream in order to identify suitable habitat for various steelhead life stages. The data generated from the habitat mapping surveys was compared between the two creeks over the two different survey years (1993 and 2005). The results of these surveys showed that habitat types have changed on San Simeon Creek between survey years while Santa Rosa Creek appeared to remain the same.

Biological inventory methods were conducted on the populations of O. m. irideus in San Simeon and Santa Rosa Creeks during the years of 1993 and 2005. There were very few observations of steelhead moving downstream on both survey creeks in 1993. During the 2005 out-migrant trapping season, the data revealed that non-smolting fish are moving downstream in the watershed in addition to smolting fish. There is evidence that age 1+ and 2+ fish make up the majority of downstream migrants. Older, larger fish tend to migrate downstream earlier in the trapping season. Fork length – weight relationships are not significant between steelhead sampled from out-migrant traps, while those sampled throughout the watersheds by means of electrofishing were different between creeks. Percent frequency distribution of electrofished steelhead reveal that steelhead sampled in 2005 have a similar distribution of total lengths between creeks, where as those sampled in 1993 have a slightly different distribution from each other.

Further analysis of steelhead scales sampled from the populations on San Simeon and Santa Rosa Creeks in 2005 gave greater insight into the growth of these fish. In most cases, steelhead from Santa Rosa Creek have greater growth increments and higher circuli counts per annuli than those surveyed from San Simeon Creek. The majority of scale samples analyzed showed growth since the last annuli mark suggesting an increase in growth over the winter months. The average number of circuli to the first annuli is significantly different between watersheds, as is the relationship between fork length and scale radius.

Analyzing steelhead demographics is important to understanding the life history pattern of steelhead in the South Central California Coast Distinct Population Segment (DPS). Surveys that monitor environmental conditions and habitat help to identify potential limiting factors and risks to steelhead populations. The results of this thesis present such data in hopes to promote continued research and efficient management practices of steelhead trout populations in San Luis Obispo County.

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