Who's Your Mommy?: Use of Otolith Strontium Isotope Ratios 87SR:86Sr to Determine Contribution of Resident Rainbow Trout to the Threatened Steelhead Population in the Yakima River, WA

One of the more difficult issues in the conservation and management of salmonids is the complicated life history variability they possess.  One example of this is the paucity of genetic information that can distinguish resident rainbow trout from anadromous steelhead even though the interbreeding of the two phenotypes is rarely observed.  This is because studies have shown that resident rainbow trout act as founder populations for the migratory steelhead phenotype.  Rainbow trout have been shown to produce the anadromous phenotype, even in completely isolated circumstances.  Thus if genetic data to distinguish the two phenotypes is not possible, how do fisheries managers assess the role of resident rainbow trout on the status of threatened steelhead populations?  Fortunately, there’s another way.  Steelhead undergo oocyte maturation in the ocean where the primordial otolith is formed.  Thus the offspring of steelhead will have a different chemical composition than rainbow trout, which contribute only freshwater derived chemicals.  Using laser ablation we can now sample the primordial otolith and determine the maternal phenotype of steelhead.  In this study we examined the primordial otoliths of 300 adult returning steelhead to the Yakima River.  Of the 300 steelhead approximately 10% had resident rainbow trout mothers.  This study highlights the importance of resident rainbow trout to threatened steelhead and exemplifies the use of otoliths geochemistry in fisheries science.