Identification of Maternal Life-History Type and Natal Rearing Origin of Adult Steelhead in the Yakima Basin

Steelhead, a name used to describe ocean-going (anadromous) rainbow trout, represent one of many life-history types within Oncorhynchus mykiss populations. Another common life-history tactic is residency, whereby individuals remain in freshwater until fully mature, often traveling only a short distance from their place of origin over the course of their life-cycle. O. mykiss juveniles do not always emulate their parent’s life-history choices. Evidence from studies of O. mykiss life-history plasticity has led to theories that cross-ecotype production rates may be specific to each watershed and reflect habitat-dependent trade-offs between freshwater survival and increased reproductive output resulting from ocean maturation. We hypothesize that resident rainbow trout play a critical role within steelhead populations and expect that a significant number of their migratory offspring survive to return as adult steelhead. To test this hypothesis we identified the maternal phenotypes and natal rearing origins of adult steelhead in the Yakima Basin by examining geochemical signatures (ratios of strontium isotopes, 87Sr/86Sr) in inner ear bones (otoliths). Much like the rings in a tree, boney material in an otolith is laid down in a concentric circular pattern throughout the fish’s life. Each ring has a chemical makeup unique to the habitat occupied at the time the ring was formed because chemicals in the environment are incorporated into calcified tissues as the fish grows. In this way, the entire life-history of the fish is documented in the otolith, including maternal origin; identifiable in the otolith primordia developed during early life-stages. Based on otolith geochemical analysis we estimate >10% maternal contributions from resident rainbow trout to steelhead production in the Yakima Basin. Geochemical signatures in water samples and steelhead otoliths indicate that resident trout from throughout the basin contribute to steelhead production, but levels of contribution vary between source populations.