Abstract
Steelhead trout (Oncorhynchus mykiss) are anadromous and iteroparous, but repeat-spawning rates are variable. In this dissertation, the physiological and energetic constraints of iteroparity were evaluated in inland Snake/Columbia River stream-maturing steelhead at selected phases of the reproductive cycle. In chapter 1, histological analysis was used to assess the effects of prolonged fasting on the cellular architecture of tissues in sexually mature and kelt steelhead. We found little evidence of severe cellular necroses in the tissues and observed that many kelts were feeding during emigration. In chapter 2, we examined changes in the lipid, protein and energy density of white muscle samples. At a broad scale, we determined that white muscle lipid was reduced by 94% to levels less than 1% of wet tissue weight from early migration to kelt emigration. In kelts, protein appears to be the only remaining somatic energy source for emigration and likely limits post-spawning survival. In chapter 3, we evaluated the profiles of specific fatty acids in white muscle and liver tissues at three intervals. By the time of kelt emigration, all fatty acids were highly depleted suggesting that even structurally important fatty acids are mobilized for energy during prolonged periods of fasting. In chapter 4, we used blood plasma chemistry to compare physiological metrics between a coastal (Situk River, AK) and inland population (Clearwater River, ID) of steelhead kelts. We observed that plasma analytes related to energy storage and nutrition was higher in coastal kelts, whereas electrolytes and plasma enzymes were more variable. Based on emigration distance and selected nutritional metrics derived from plasma chemistry we suggest that coastal kelts are more likely to emigrate successfully to the ocean and repeat spawn.