Reconstructing Juvenile Chinook Salmon Life History in the Salmon River Estuary, Oregon, Using Otolith Microchemistry and Microstructure

We quantified the juvenile rearing and migratory patterns of individuals from a population of fall-spawning Chinook salmon Oncorhynchus tshawytscha in Oregon's Salmon River estuary using otolith microchemistry and microstructure. The study confirmed the daily periodicity of otolith growth increments in a natural fish population under field conditions and validated fundamental assumptions about increased otolith strontium : calcium values during entry into saline waters. The otolith results indicated that more than 75% of the subyearling Chinook salmon captured near the mouth of the Salmon River had entered the estuary during the summer and that two-thirds of these fish had spent more than a month in the estuary before capture. Unlike in other Oregon coastal estuaries, in which the fingerling-migrant portion of their life histories is dominant, approximately two-thirds of Salmon River Chinook salmon in upper-estuary marshes were early fry (<50-mm) migrants. A much smaller proportion at the river mouth suggests that many fry migrants did not survive to the lower estuary or passed undetected during ocean emigration. Nonetheless, the otolith results reveal a substantially greater contribution of estuarine-rearing fry to the out-migrant population at the Salmon River than has been reported in other Oregon coastal basins. A small component of fall-migrating fish with long freshwater residence times also occurred at the river mouth. Most of these individuals had migrated quickly through the estuary. Rather than revealing a series of discrete "types'' defined by the predominant rearing patterns in the population, the individual otolith results depict a continuum of freshwater and estuarine life histories that is consistent with reports of considerable phenotypic plasticity in Chinook salmon. Otolith analysis offers the potential to quantify the relative contributions of different juvenile rearing patterns to adult returns.