Partial Migration of Steelhead in the Puget Sound: an ICP-MS Analysis of Steelhead Otoliths

Rainbow Trout and Steelhead (Onchorynchus mykiss) are endangered species of the salmonid family that are native to the west coast of the USA. Rainbow Trout spend their lives entirely in rivers, whereas steelhead are anadromous and spend part of the life in the ocean before returning to freshwater to spawn. Due to interbreeding of the two types of O. mykiss, the contribution of parental rainbow trout to the steelhead population in the Skagit Basin is unknown and could be a significant contribution to their threatened status in the Puget Sound. This project reconstructs the life history of anadromous steelhead in the Skagit Basin using Sr:Ca ratios of otoliths (fish ears). Otoliths are calcium carbonate structures that accrete during the life of the fish and incorporate trace-elements from the ambient water - acting kind of like a tape recorder. An otolith starts recording when the offspring is still being supplied by nutrients by the yolk. Information collected from their core region may be used to infer whether or not life strategies were mimicked from maternal life habits. Sr:Ca ratio data collected in this study is also compared to ages and habitual staticity estimates using fish scale dating techniques to evaluate the accuracy of scale dating. Our initial results look promising, migration trends between freshwater and salt are clear and indications of maternal contribution have been found in some cases.