A needle in a haystack: Strontium isotopes (87Sr/86Sr) in otoliths identify origin of Largemouth Bass from a large Southwest reservoir

Largemouth Bass Micropterus salmoides, a popular warm water sport fish, is routinely stocked in reservoirs throughout the USA to augment wild populations. Evaluating if these supplementations are successful requires distinguishing hatchery-sourced fish from their wild counterparts. From 2011 to 2019, over 467,000 Largemouth Bass fingerlings were stocked from multiple hatchery sources into a large southwestern reservoir (Elephant Butte Reservoir, New Mexico USA) to supplement the sportfish population. To identify hatchery-sourced Largemouth Bass, we measured strontium isotope ratios (87Sr/86Sr) and determined ages using otoliths and dorsal spines. Otolith analysis of 169 fish classified 92.9% (n = 159) to the reservoir with few fish of hatchery origin (2 from Arkansas, 1.2% and 1 from Montana, 0.6%). While stocking over 467,000 fingerlings across eight years appears to reflect negligible stocking success, it is likely that low and variable stocking densities (average 3.0, range 0.29–7.77 fish/ha) likely contributed to the low stocking success in Elephant Butte Reservoir. Dorsal fin spines did not yield accurate age reconstructions and their 87Sr/86Sr were affected by matrix interferences preventing source assignments.