Masters Thesis

Life history variation in Oncorhynchus mykiss from the lower Klamath River Basin

The lower Klamath River Basin supports the most productive steelhead (Oncorhynchus mykiss) fishery in the state of California, and is one of only several fisheries in the world in which steelhead exhibit the unique half-pounder life history. Half-pounders exhibit a largely amphidromous (i.e., non-reproductive) migration in which individuals return to the river several months after initial ocean entry, overwinter in freshwater, and return to the ocean the following spring. Current information on the incidence and attributes of the half-pounder and other life history forms in the lower Klamath River is not available. Objectives of this study were i) to describe patterns of age and growth, as well as migratory and reproductive history in wild O. mykiss from the lower Klamath River Basin, ii) to evaluate the influencing factors (i.e., size and natal origin) in expression of life history, and iii) to explore the growth and fecundity consequences of differing life history patterns. From August 2007 through April 2009, otoliths, scales, and biological data (i.e., length, weight, and sex) were collected from fish that were captured with hook and line and at weir trapping facilities. To determine migratory history (anadromous versus nonanadromous) and maternal origin (anadromous versus nonanadromous) of O. mykiss, I analyzed otolith strontium isotope ratios (87Sr/86Sr). To determine age, growth, and life history, I performed qualitative scale analysis. Half-pounders were examined to determine sex and incidence of reproductive maturity, and biological data were collected from adult, female steelhead to quantify the relationship between length and fecundity. Otolith microchemistry indicated that anadromous and nonanadromous forms of O. mykiss occur sympatrically in the lower Klamath River Basin. Further, otolith microchemistry suggested that steelhead and rainbow trout may give rise to progeny of the alternative life history form. Including anadromous and nonanadromous forms, O. mykiss from the basin exhibit at least 33 different life history categories at maturity; which differ in the number of years of freshwater and saltwater residency, expression of a half-pounder migration, and number of spawning runs. Among sub-basins, incidence of resident rainbow trout ranged from 1.2% in the Trinity River to 57.2% in the Scott River, and incidence of the half-pounder life history ranged from 11.0% in the Trinity River to 100.0% in the Shasta River. Approximately 7.6% of half-pounders attained full maturity after spending less than one year at sea. Adult steelhead that exhibited the half-pounder life history were smaller and less fecund at age than fish that remained in the ocean; however results of life history analysis suggest that continued expression of the half-pounder phenotype is favored by precious maturation and enhanced survival relative to the ocean contingent phenotype. My finding regarding the ability of rainbow trout to give rise to steelhead has important management implications in the Klamath River Basin, where the scheduled removal of four dams is expected to restore connectivity between the ocean and the upper basin. Following dam removal, resident O. mykiss in the upper basin may be able to contribute towards recovery of the steelhead population.

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