Otolith-based Determination of Rainbow Trout Life Histories in the Skagit River Basin

In 2007, the population of Rainbow Trout/Steelhead native to the Skagit River Basin was listed as threatened under the Endangered Species Act (ESA). Since then, research has been conducted to assess population dynamics and migration history using trace elements in otoliths––the “ear stones” found in the inner ear of fish. Otoliths are mostly made up of calcium carbonate and are used for hearing and balance. They grow in proportion to the growth of the fish and incorporate elements from the ambient water and diet. Microchemical analyses were utilized to identify early life history stages, such as hatch and button-up of Rainbow Trout in the Skagit River basin. Juvenile Rainbow Trout were collected from five tributaries of the Skagit River and from the Marblemount State Fish Hatchery. Otoliths were extracted, mounted to glass coverslips, and polished. Elemental analysis was conducted using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), where 23 isotopes of 22 elements were analyzed. Daily ring counts reflected differences in age at hatch and button-up between wild and hatchery fish. Additionally, nearly all wild fish showed an increase of 88Sr in the otolith core, indicating maternal anadromy. Results identify a means to potentially distinguish hatchery v wild origin, maternal anadromy, and point to fundamental early life history differences that may be reflected in later life stages. Ultimately, these results contribute to a deeper understanding of early development in Skagit River Rainbow Trout and emphasize the utility of otolith chemistry in informing management and conservation efforts.