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.