Otolith Chemistry Discriminates among Hatchery-Reared and Tributary-Spawned Salmonines in a Tailwater System

Abstract

The Little Red River tailwater (LRRT) system in Arkansas supports a naturally reproducing population of brown trout Salmo trutta and an economically valuable fishery. Natural reproduction by stocked brook trout Salvelinus fontinalis and rainbow trout Oncorhynchus mykiss is unknown but assumed to be negligible. An artificial tributary to the LRRT was constructed to provide a spawning and nursery refuge for stocked trouts, and shortly thereafter we observed spawning adults and emergent fry in the tributary. As a first step in determining the relative contributions of hatchery- and tributary-produced brook and rainbow trouts recruiting to the LRRT fishery, we distinguished hatchery-reared fry from tributary-spawned fry via otolith chemistry. We used laser ablation inductively coupled plasma mass spectrometry to measure the abundances of cations in otoliths. Significant differences between hatchery and wild stocks were found for six element : Ca ratios. A five-variable (Mg, Mn, Zn, Sr, and Ba) discriminant model was sufficient to distinguish the two brook trout stocks with 100% accuracy, and a two-variable (Sr and Ba) model discriminated between the two rainbow trout stocks with 94% accuracy. Differences in ambient water chemistry between the two rearing habitats most likely accounted for these distinct otolith signatures. Otolith analysis of brook trout parr of unknown origin captured in the LRRT system suggests that natural reproduction is occurring there and that this stock may be supplemented with tributary-spawned immigrants. After interannual variation in habitat-specific otolith chemical signatures is quantified, such an analytical tool would be useful to managers in the LRRT system and elsewhere in determining the relative contributions of various stocks to reproductive and harvested populations.