Evaluating stress recovery and gill morphology during experimental supplementation of an endangered fish species

The efficacy of population supplementation (i.e., stocking) may be limited by physiological stress incurred upon transported and released fishes. In an experimental supplementation program for the critically endangered delta smelt (Hypomesus transpacificus), acclimation enclosures have been developed to facilitate stress recovery. However, the magnitude and duration of the stress response and the optimal time of release from enclosures remain unknown. To address these knowledge gaps, we quantified whole-body cortisol, glucose, lactate, and gill ionocyte morphology at 10 time points across 72 hours following transport to enclosures at high and low densities. Cortisol and glucose peaked within 0.5 hours following transport into enclosures and recovered significantly by 27 hours. Lactate was moderately affected, and transport density did not affect cortisol, glucose, or lactate. Gill ionocyte surface area significantly increased in response to shifts in water chemistry throughout transport, and we observed three distinct types of microvilli on the surface of gill ionocytes, including two never previously reported. This work highlights the importance of ensuring stress recovery during the supplementation of delta smelt and other species.