UC Davis

Physiological stress incurred upon transported and released animals may limit the efficacy of population supplementation - a commonly used conservation strategy designed to recover depleted populations. In a recently developed experimental supplementation program for the critically endangered delta smelt (Hypomesus transpacificus), combined stressors of handling, transport, and shifts in water chemistry are expected to elicit a considerable stress response. Acclimation enclosures have been developed in an attempt to facilitate stress recovery and allow delta smelt to acclimate to field conditions. Pertinent questions surrounding the use of enclosures include determining the optimal time of release from enclosures and ideal transport densities to minimize stress and maximize fish performance. 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. We observed significant peaks in cortisol and glucose within 30 min of release into enclosures, followed by significant recovery of both parameters after 27 hours. Lactate was moderately affected, and we did not detect an effect of transport density on cortisol, glucose, or lactate. Gill ionocytes significantly increased in surface area following transport, and we observed gill ionocytes having up to three different types of microvilli, including two types that have never been reported. Changes in ionocyte morphology is correlated to elevated cortisol levels and suggests greater demand for ion-transport at the release site. This work provides agency managers with data to optimize delta smelt supplementation methodologies and expands our understanding of how fishes respond to targeted stressors.