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

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.