Isotope analysis reveals proportional change and site‐selection variation of river and lake‐produced eggs of a landlocked migratory fish

Changes in the proportions of river amphidromous fish, ayu (Plecoglossus altivelis altivelis) in the Lake Biwa water system, Japan, were monitored by stable isotope analysis, based on different delta(15) N and delta(13) C values of prey organisms between the lake and its tributaries. During the 3 month reproduction season, the delta(15) N values of spawned eggs decreased with time. This result implies that there was a shift from lake-produced eggs to river-produced eggs within a reproductive season, based on the observation that adult fish in the lake had previously been shown to have eggs with distinctly higher delta(15) N values in their ovaries than those in the tributaries. This explanation was also supported by the change in delta(13) C values of the spawned eggs. Furthermore, eggs with lower delta(15) N and higher delta(13) C values tended to be spawned at less variable depths, suggesting that females spawning river-produced eggs selected the spawning sites from a narrower range. We conclude that stable isotope ratios of spawned eggs can be indicators of the relative contributions of different food chains and can enable comparisons of reproductive characteristics between types of egg.