Stable isotopes are commonly used to understand the role of fishes in aquatic food webs. However, variability in species- and tissue-specific isotopic values can affect the inference that is drawn from a stable isotope study. We evaluated differences in stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) among three tissue types (white muscle, caudal fin rays, and eye lenses) for Santa Ana Sucker Pantosteus santaanae and Arroyo Chub Gila orcuttii to inform the design of a stable isotope study in the Santa Ana River, an urban river that is located in southern California.

We used multivariate analyses to test for differences in the stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) among the three tissue types that were collected from Santa Ana Sucker and Arroyo Chub. We also summarized the variability in isotopic values that was recorded over time in fish eye lenses and interpreted this variability in reference to the spatial patterns in isotopic values that have been previously reported throughout the Santa Ana River.

We found that fin ray tissue and white muscle tissue were not significantly different for either isotope or fish species. Fish eye lenses were significantly higher in δ¹³C than muscle tissue, and eye lenses were significantly higher in δ¹⁵N than fin ray tissue for both fishes. We also found a greater range in δ¹³C and δ¹⁵N across eye lens layers for Santa Ana Sucker (δ¹³C = 2.01 ± 0.96‰, δ¹⁵N = 4.93 ± 4.18‰) than for Arroyo Chub (δ¹³C = 0.96 ± 0.65‰, δ¹⁵N = 4.63 ± 1.45‰).

Our results indicate that fin rays may be a viable nonlethal alternative to white muscle tissue for use in a stable isotope study of native fish of the Santa Ana River. Additionally, eye lenses could provide a chemical history of fishes within the river, but species-specific correction factors may be needed if stable isotope values for eye lenses are to be compared with more conventional tissue types (e.g., white muscle).