Reconstructing seasonal variation in egg-laying strategies: a forensic use of stable isotopes.

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Stable isotopes have proved to be powerful tools to infer habitat use by fishes, both in terms of geographic variation in habitat use and occupancy, and in terms of inferring habitat use at different life-history stages (e.g. Shephard et al. 2007). Most usually, isotopic signatures are quantified for otoliths, scales or somatic tissues. Hard structures that lay down growth increments are useful for determining habitat and diet changes through time as they ‘lock up’ a distinct chemical signature as the fish grows, enabling researchers to look back in time and to infer changes in diet or habitat use (e.g. Cambiè et al. 2016). In this issue, Sawada et al. (2020) have examined the isotopic signatures of the eggs of ayu Plecoglossus altivelis altivelis to infer changes in the proportions of river and lake-produced eggs (i.e. eggs produced by fish living mainly in rivers or the lake) of this landlocked amphidromous fish in the Lake Biwa water system, Japan. They determined the δ15N and δ13C values of prey organisms between the lake and its tributaries against which the isotopic signature of eggs could be compared. As the reproductive season progressed, the δ15N values of spawned eggs decreased with time implying that there was a shift from lake-produced eggs to river-produced eggs within a reproductive season. River-produced eggs, tended to be laid in a narrower range of habitats, which may infer that these fish had access to ‘optimal’ habitat for egg survival. The results of the study highlight the importance of understanding seasonal differences in egg-laying strategies for fish that occupy different habitats and emphasise the importance of ensuring that management of such habitats encompasses these diverse life-history strategies to ensure the resilience of the population to environmental changes.