Enhancing reproducibility in stable isotope analysis (SIA) of fish eye lenses: A comparison between lamina number and diameter

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.