Natal Origin Differentiation Using Eye Lens Stable Isotope Analysis

Natal origin determination can be critical to understanding the movement, recruitment, and stock discrimination of fish populations. We investigated the utility of eye lens stable isotope analysis as a natural marker to determine natal origins, specifically the differentiation of stocked versus naturally reproduced individuals. We compared isotopic values for stocked and naturally reproduced Rainbow Trout Oncorhynchus mykiss in the North Platte River (Wyoming, USA) and found that the metabolically inert core of the eye lens retained hatchery feed values as the fish grew poststocking. The stable isotope values of eye lens cores from stocked Rainbow Trout were > 15 parts per thousand higher for delta S-34 and > 4 parts per thousand higher for delta C-13 than naturally reproduced fish. The values for delta N-15 proved to be a less useful indicator for natal origin than sulfur and carbon. These high carbon and sulfur isotopic values were retained in the core, despite subsequent lens layers showing a dietary shift to natural prey items poststocking. Rainbow Trout formed over 10 layers at age 0, decreasing to an estimated one to two layers during subsequent years. Eye lens stable isotope analysis represents an additional tool for researchers and managers to use natural isotopic markers to accurately assess natal origins and hatchery contributions to fish populations.