Using Chemical Tracers to Understand Where Fish Were Born

Identifying where individuals were born is integral for species management and conservation. A common approach for identifying the origin of an individual relies upon chemical tracers (i.e., isotopes) from the environment that are incorporated into an individuals’ tissues. The abundance of certain isotopes differs among habitats and thus can be used as a ‘finger-print’ to tell where an individual resided.

For a threatened population of fall Chinook salmon (Oncorhynchus tshawytscha) in the Snake River of Idaho, isotopes in their earbones (i.e., otoliths) are often analyzed to determine where adults hatched. This information is used to indicate the extent to which different rivers contribute individuals to the adult population. Using isotopes to reconstruct where fall Chinook salmon hatched assumes that isotopes incorporated into the otolith at, or shortly after, hatching are derived solely from the environment. However, evidence exists that at hatching the isotope values in the otolith are influenced by the isotope value in the yolk-sac of a developing embryo, and not solely by the environment. The purpose of this study is to identify when in the life of a developing fish does its otolith acquire isotopes from the environment in order to accurately estimate where fall Chinook salmon hatch. Specifically, we conducted a lab experiment that exposed developing larvae to different ratios of strontium (Sr) isotopes 87 and 86 for a period of 113 days. We used laser ablation inductively coupled plasma mass spectrometry to quantity Sr87/86 in the otoliths of our reared fish and we statistically analyzed this data to determine when in the life of a fish its otolith reflect the Sr87/86 of the water in which it resides.