Use of young-of-the-year Smallmouth Bass as sentinel: a case study by the Mactaquac Dam, New Brunswick, Canada

Effects-based approaches in monitoring programs focus on ecologically relevant responses to better understand impacts in aquatic ecosystems. The primary objective of this study was to explore challenges for using the condition of young-of-the-year (YOY) fish to detect impacts of a hydroelectric dam in a large river. This study sampled YOY Smallmouth Bass (Micropterus dolomieu) monthly at three sites in an area upstream (reference) of a large reservoir and downstream of a hydroelectric dam (nearfield) on the Wolastoq | Saint John River, NB, Canada in 2021. Length, weight, age, stable isotope ratios (δ15N and δ13C, a measure of an organism's diet), and mercury were measured. Using daily increments on otoliths, we found that fish sampled in July and August hatched earlier than fish sampled in September and October and were likely from different cohorts. Spawning was extended over a longer period than anticipated potentially due to the variability of river environments. We found higher concentrations of mercury, higher values of δ15N, and lower values of δ13C in older YOY fish, which is consistent with ontogenetic diet shifts that are well documented in YOY Smallmouth Bass. We conclude that YOY Smallmouth Bass can be used as an indicator of environment condition in effects-based monitoring, but understanding the species ecology in the targeted environment is critical. Sampling at the end of the growing season at consistent locations is required to avoid confounding factors of extended spawning periods and ontogenetic diet shifts.