Using otolith microchemistry to differentiate between stocked and unstocked Australian bass (Percalates novemaculeata)

Understanding the contribution of stocking to fish populations is an essential part of any program to increase fish numbers and abundance. However, subsequent assessments that aim to quantify the success of fish stocking are rarely considered. Juvenile-sized (<200 mm) Australian bass (Percalates novemaculeata), from the upper Snowy River, southern New South Wales (NSW), Australia (assumed to be stocked), wild mature-sized (>400 mm) fish from estuarine areas of the Snowy River, juveniles (15-30 mm) from a commercial Australian bass hatchery facility known to have supplied fish for past stockings in the Snowy River, and wild fish (>400 mm) considered as controls from the Clarence River in northern NSW were used for otolith microchemical analysis. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure Sr:Ca and Ba:Ca ratios at the otolith core and across radial transects to verify natal origins of fish and more specifically, to confirm the origin of fish caught in the river that were assumed to have originated from a recent stocking program. Otolith core Sr:Ca ratios of juvenile fish collected from the upper Snowy River and hatchery fish were not significantly different suggesting they originated from the same source. Significant differences were found in core Sr:Ca and Ba:Ca between known origin hatchery fish and wild fish. Significantly higher Sr:Ca ratios were found at otolith cores compared to the edges in fish collected from the upper Snowy River, suggesting early life was spent in more saline conditions prior to stocking compared to wild fish. Otolith transect data showed a consistent decrease in Sr:Ca ratios and a simultaneous increase in Ba:Ca ratios at approximately 17-59 mm from the core, corresponding with the size at which these fish were stocked. This study has demonstrated that otolith microchemistry can successfully distinguish stocked fish from wild recruits and reveals the timing of natural migrations or artificial translocations.