Analyzing stable isotopes in archival tissues, such as fish eye lenses, is used to document shifts in feeding ecology, diet, habitat use, and to reconstruct life history. Fish eye lenses grow sequentially throughout their ontogeny, resulting in a structure of multiple layers, or laminae. These laminae represent the chronology of the fish's life, much like tree rings. Lenses are protein-rich, which makes them an ideal structure for analyzing light isotopes such as δ¹³C, δ¹⁵N, and δ³⁴S. These light isotopes are primarily integrated into the lens tissue through the fish's diet. As research begins to emerge using eye lenses to reconstruct the life histories of fishes, the need for a reproducible method of delamination grows. For this study, two different researchers peeled lenses from each eye of the same adult Chinook salmon (Oncorhynchus tshawytscha) (n=10 fish). Lens lamina number, diameter (mm), and mass (mg) of each lamina were recorded. Laminae were then submitted for isotopic analysis of both δ¹³C and δ¹⁵N. Isotope values were used as a validation to compare delamination patterns between researchers. δ¹³C and δ¹⁵N values from the lenses were then plotted using both the assigned lamina number and lens diameter to compare the difference between researchers. Analyzing the laminae based on lamina number resulted in significant variability between researchers. However, when lens diameter was used instead of lamina number, isotope patterns throughout the lenses of the same fish were nearly identical. Using lens diameter removes subjectivity between researchers, thereby increasing the reproducibility of the technique and providing a more robust interpretation of the data.

Metadata of eye lens stable isotope data (CN) of 10 2018 fall run Chinook salmon. Fish-specific metadata (e.g., fork_length, site_id) is given and collected by CDFW. Lens-specific metadata (e.g., layer_no, lens_dm) is collected during sample processing. Isotope data is generated by the Stable Isotope Facility at the University of California, Davis.