Application of Otolith Microchemistry to Evaluate Natal Origins, Maternal Origins, and Stock Composition of Rainbow Trout (Oncorhynchus mykiss) in the Kootenai River, Idaho

Historical studies have indicated that (1) the Rainbow Trout (Oncorhynchus mykiss) population in the Kootenai River, Idaho is recruitment-limited, and (2) tributaries in Montana seed a portion of the adult population in the river.  However, little has been done to evaluate natal origins of adult Rainbow Trout in the river, which has hindered management abilities.  An otolith microchemistry study was conducted in 2012-2014 to address whether or not (1) tributaries to the Kootenai River could be differentiated from one another using strontium isotopes derived from otoliths of age-0 trout and (2) adult trout collected from the river could be assigned to natal tributary using otolith strontium signatures.  Five age-0 trout were collected from 11 tributaries to the Kootenai River in November 2012, and 29 adult trout were collected from the Kootenai River during fall 2013.  Results indicated that 9 of 11 tributaries had differing strontium signatures.   Assignment of adults to natal tributaries was high (97%).  Idaho tributaries accounted for 39% of adults, Montana tributaries for 39%, and the mainstem river for 21%.  Results from this study confirm this method’s utility in freshwater systems, and they provide useful information on study design considerations for managers or researchers considering using the technique.