Mass marking farmed Atlantic salmon with transgenerational isotopic fingerprints to trace farm fish escapees

Abstract

Farmed fish sometimes escape and enter natural environments, where they mix with wild fish populations and can have negative effects. Marking farmed fish is a prerequisite for the identification of the origin of escapees and for guiding technical investigations to determine the cause of an escape event and improve farming practices. We tested transgenerational marking with enriched stable isotopes to assess its effectiveness as an accurate, feasible and cost-effective marking method for Atlantic salmon Salmo salar grown in sea-cage aquaculture. We injected a combination of 7 stable isotopes (134Ba, 135Ba, 136Ba, 137Ba, 86Sr, 87Sr and 26Mg) at 4 different concentrations (2, 0.2, 0.02 and 0.002 µg g-1 broodfish) into the abdominal cavity of female Atlantic salmon broodstock. Marking success was assessed in the otoliths of the resulting yolk sac larvae using laser ablation inductively coupled plasma mass spectrometry. Marking was 100% successful with Ba isotopes at concentrations as low as 0.002 µg and for Sr isotopes at 2 µg, when at least 3 wk had passed between the day of injection and spawning. Our results demonstrate that 63 unique fingerprint marks can be made at a low cost using enriched isotopes of Ba (US$0.0002-0.002 mark-1) and Sr (US$0.05-0.13 mark-1). Compared to other mass marking techniques, transgenerational marking of farmed salmon is an economically feasible method for tracing escapees with similarly low costs to delivery by egg bathing or vaccines, and an order of magnitude or more lower than other conventional marking methods.