Production and Food of Cutthroat Trout in Three Oregon Coastal Streams

Abstract

Unfished populations of coastal cutthroat trout (Salmo clarki clarki) in three small coastal streams in Oregon were studied from April, 1962, until November, 1963. Population size, growth rate, and production were estimated for study areas in each stream. The mean annual biomass of cutthroat trout for all streams was 4.2 grams per square meter (g/m'). Growth in length was greatest in April and May and least in October and November. Production per year averaged 4.1 g/m2 for the three streams, but production levels were about 20 percent higher in one of the three. Food samples from trout stomachs taken during the period of emergence and early growth of coho salmon (Oncorhynchus kisutch) fry suggested that cutthroat trout were not important consumers of young coho. Aquatic arthropods were consistently an important part of the diet; earthworms decreased in importance and terrestrial arthropods increased in importance during the period February-June. This study was undertaken to obtain data concerning the population dynamics of the coastal cutthroat trout in three small streams. The field work was carried on between April 15, 1962, and November 1, 1963. The specific objectives of the investigation were (1) to estimate the size of trout populations, (2) to estimate growth rates of trout, (3) to estimate the production of trout, and (4) to determine the food habits of trout in the late winter and spring. A study of the effect of logging practices on the physical and biotic characteristics of the three study watersheds is under way (Chapman et al. 1961). An important use of the information obtained in this study will be to establish the pre-logging status of cutthroat trout populations. It is planned to conduct another study of the trout populations after logging is completed. Although much has been written on the cutthroat trout (Cope 1958), relatively little is known about the coastal subspecies. Studies on cutthroat trout in Oregon have been made on movement and aging (Sumner 1962), food habits (Dimick and Mote 1934), life history and distribution (Dimick and Merryfield 1945), and spawning activities (Cramer 1940). The distribution, distinc ive features, and life history of this trout in northern California are detailed by DeWitt (1954). Several very thorough studies of population dynamics of fish were used in some r spects as models for the present study. The production of bluegill in a eutrophic lake in Indiana was studied by Gerking (1962). The dynamics of fish populations are discussed by Ricker and Foerster (1948) and Ricker (1958). I wish to thank D. W. Chapman, James Hall, Ray J. White, Homer J. Campbell, Robert W. Phillips, Norman Anderson, David L. Mays, Donald Jensen, Wes Harms, John Neilsen, and the many others who gave encouragement and assistance with this investigation. Financial support was provided by a graduate research assistantship from the Research Division of the Oregon State Game Commission under the leadership of H. J. Rayner.