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CNS isotope values in eye lenses for juvenile and adult Chinook Salmon


Bell Tilcock, Miranda et al. (2021), CNS isotope values in eye lenses for juvenile and adult Chinook Salmon, Dryad, Dataset,


Tracking habitat use and dietary shifts in migratory species is vital to conservation and management. Yet conventional animal tracking often precludes tracking small juveniles at critical life-stages where recruitment bottlenecks often manifest.

Stable isotope analysis (SIA) in consecutive laminae in eye lenses, a protein-rich depositional tissue, has emerged as a promising tool in fishes to develop long-term interpretive records of dietary histories using a single archival tissue. Currently, studies using fisheye lenses to study SIA in diets have primarily been conducted in marine environments using δ¹³C and δ¹⁵N to identify resource partitioning, ontogenetic shifts, and lifelong trophic histories. To date, no studies have examined freshwater taxa nor used δ³⁴S isotopes.

We placed juvenile Chinook Salmon (Oncorhynchus tshawytscha) in experimental enclosures in three different freshwater habitats (hatchery, river, and seasonal floodplain), each with isotopically distinct and well-characterized food webs. This experimental approach allowed us to directly measure diets and quantify tissue turn-over rates in eye lenses as well as the isotopic fractionation among fish tissues (fin and muscle tissue) in distinct habitat types using stable isotopes δ¹³C, ẟ¹⁵N, δ34S.

Bulk eye lens stable isotope measurements were analyzed for juvenile salmon lenses and were found to be consistent with the isotopic values of rearing habitats. Slight additional isotopic fractionation was only found in δ¹³C. We then successfully applied the method to a larger, reproductively mature adult salmon captured in freshwater and inferred juvenile habitat use.

SIA in eye lenses using three dietary isotopes (δ¹³C, ẟ¹⁵N, and δ34S), has significant potential for answering critical questions about migration, diet, foraging ecology, and life-history of migratory aquatic animals on Earth. Such information would have immediate application towards conservation management of diverse species and habitats at multiple scales.


Juvenile fish were collected from a previous field experiement outlined in Jeffres et al. (2020).  Laminae from both eyes of individual fish were combined for bulk analyses to meet minimum dry weight requirements for combined δ¹³C, δ¹⁵N, and δ³⁴S analysis (2mg) and submitted to the UC Davis Stable Isotope Facility for isotope analysis. Individual laminae from a single juvenile salmon from the floodplain was analyzed for δ¹³C, δ¹⁵N, and δ³⁴S to test the feasibility of reconstructing diets over smaller temporal and spatial scales of habitat use.   

In 2014, the carcass of one stray adult Chinook Salmon with intact adipose fin was recovered near our Yolo Bypass floodplain study site. Individual laminae were separated and analyzed for isotopic variations through time and submitted to the same isotope facility listed above. 

Last data set contatins the lens diameters measured in juvenile Chinook Salmon that were reared on the floodplain. Lenses were mounted in Crystalbond resin, sectioned, and measured using established methods for otolith sagittae (Barnett-Johnson et al. 2008).

Usage Notes

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