Enriched stable isotope marking of juvenile golden perch (Macquaria ambigua) otoliths

Stocking of native fish is a standard practice to aid in the recovery and enhancement of depleted populations. However, evaluating the effectiveness of these stocking programs has been hindered because of difficulty in distinguishing hatchery from wild fish, especially for species that are stocked as small, fragile fingerlings. Stable isotopes offer the potential to chemically mark fish with multiple marks that are distinct from each other and from natural signatures. Golden perch (Macquaria ambigua) fingerlings were reared in water enriched in 137Ba and combinations of 137Ba and 86Sr for various lengths of time. Ba and Sr isotopic ratios in the otoliths were determined using laser ablation inductively coupled plasma-mass spectrometry. Fish exposed to increased levels of 137Ba had lower 138Ba/137Ba ratios in their otoliths relative to the natural ratio of control fish and were significantly different in fish exposed to at least 5 µg·L-1 for 8 days or to 15 µg·L-1 for 4 days. Furthermore, eight unique signatures were produced in fish reared in combinations of 137Ba (0–5 µg·L-1) and 86Sr (0–100 µg·L-1) for 24 days. Our results suggest that immersion in water enriched in specific stable isotopes could be an effective means of marking hatchery-reared fish for stock enhancement.