Immersion during egg swelling results in rapid uptake of stable isotope markers in salmonid otoliths

Determining the value of restocking wild fisheries with hatchery-reared fish requires the ability to identify and quantify the survival of hatchery fish after release. However, to obtain accurate estimates of survival rates, multiple fish identification techniques are often used, making the monitoring of restocking inefficient and costly. Here we test a new immersion marking method to determine its efficiency and cost effectiveness for marking millions of hatchery-reared Atlantic salmon (Salmo salar). Salmon eggs were marked during the egg swelling stage by immersing eggs in a solution containing seven enriched stable isotopes (¹³⁴Ba, ¹³⁵Ba, ¹³⁶Ba, ¹³⁷Ba, ⁸⁶Sr, ⁸⁷Sr, and ²⁶Mg) for 2 h immediately after fertilisation. One hundred percent successful marks were detected in the otoliths of resulting larvae at a concentration of 1000 μg·L^-1 for ¹³⁶Ba and 100 μg·L^-1 for ¹³⁵Ba and ¹³⁷Ba, with no detrimental effects on survival or health of egg and yolk sac larvae. We estimate that seven unique mark combinations can be made at a cost of $0.0001 to $0.0017 (US) per egg and conclude that marking via egg immersion is suitable for low cost, accurate marking of hatchery-reared salmonids destined for restocking purposes.