Diary of a bluegill (Lepomis macrochirus): daily delta-C-13 and delta O-18 records in otoliths by ion microprobe

Otoliths provide information about an individual fish’s environment at ecologically relevant time scales. We used ion microprobe analysis to produce high-resolution δ13C and δ18O time series from two age-4 bluegill (Lepomis macrochirus) otoliths, which provided insight into fish behavior and otolith fractionation processes. Scanning electron microscope images revealed δ13C and δ18O pit diameters of 10 and 15 µm, respectively, corresponding to 1–5 and 2–9 daily increments during rapid otolith growth and 6–9 and 12–25 increments near annual otolith growth checks. Spot-to-spot reproducibility (1 SD) of the calcite standards was <0.2‰ for δ18O and <0.4‰ for δ13C and was small enough to resolve a change in a fish’s ambient temperature of approximately 1 °C. A whole-lake 13C addition experiment elevated the δ13C of the lake’s dissolved inorganic carbon for 56 days during the summer of 2005. Mixing model results indicated that the proportion of dietary carbon in otoliths (M) was similar for both fish (BLG-3, M = 0.45; BLG-12, M = 0.35), but the relation between M and proxies of metabolic rate differed between fish. Otolith stable isotope analysis by ion microprobe can reveal the environmental history of an individual fish and contribute to our understanding of processes that influence isotope ratio fractionation in otoliths.