Salmonid eye lens stable isotope values from Maine lakes, collected 2023-2025

Ontogenetic niche shifts are almost universal in fishes and many other organisms and are thus often important to factor in conservation planning. However, some aspects of ontogeny, such as trophic shifts, remain poorly understood in many fishes. Eye lens stable isotope analysis has proven a powerful tool in understanding the ontogeny of some marine species, but few studies have applied this method to freshwater fishes. We conducted the first use of eye lens stable isotope analysis of Arctic Charr (Salvelinus alpinus), a climate-sensitive salmonid, and co-occurring Brook Trout (Salvelinus fontinalis) in four temperate North American lakes at the southern extent of the range of Arctic Charr in Maine, USA. Akin to studies in marine systems, we document patterns in the sizes and isotopic values of the lens structure that agree with expected ontogenetic patterns. Using Bayesian linear regressions, we found that the trophic ontogeny as measured by δ15N of Arctic Charr varied among lakes. Through examining individual lifetime trophic histories (δ15N and δ13C), we found that some Arctic Charr populations increase in trophic position through life while (population lifetime δ15N posterior mean slope = 1.12) others show no substantial change (population lifetime δ15N posterior mean slope = -0.06). Our study demonstrates a useful method of whole-life trophic investigation in fishes that minimizes destructive sampling. The file "CharrEyeLensData.csv" includes δ15N and δ13C isotope values and morphological measurements from individual eye lens layers of Arctic Charr and Brook Trout. The file "CharrEyeLens_SIABaseline_Data.csv" includes δ15N and δ13C isotope values of littoral and pelagic resources from the study lakes. The file "CharrEyeLens_RegressionScript.csv" provides a reproducible analytical method for comparing lifetime trophic experience between populations.