Analysing tissues acting as biogeochemical archives can be a valuable tool when it comes to tracing the life history of fish at various developmental stages. In the case of diadromous fish, stable isotope analysis (SIA) can reveal key insights into their ontogenetic movements across different water systems and related diet changes. Various inert tissues –including otoliths, scales, or bone – have been utilised to understand these shifts, given their ability to retain specific environmental information from their time of formation. However, each tissue type has its inherent limitations (e.g. organic matter content, time resolution). A recent technique using fish eye lenses’ layers as biochemical archives shows promise in advancing research on life-history traits with enhanced precision and may be used as a complementary tool to more traditional tissues analysis. In this work, we propose a detailed investigation of the fish eye lens delamination method, examining multiple layers with the objectives of: (i) determining the amount of material required for SIA of different elements; (ii) identifying the optimal element based on the research question when looking at diadromous fish; (iii) getting insights in the developmental stages of an endemic and endangered eel species, the Aotearoa New Zealand Longfin eel (Anguilla dieffenbachii).