Plasma Cortisol Levels of Fingerling Rainbow Trout (Salmo gairdneri) at Rest, and Subjected to Handling, Confinement, Transport, and Stocking

Abstract

Fingerling rainbow trout, acclimated to hatchery raceways or laboratory aquaria, had low plasma cortisol levels (≤ 2 ng/mL), with no apparent daily cycle in levels. Netting of cohorts out of an aquarium, without agitation, did not cause a rise in plasma cortisol concentrations in remaining fish. However, following 90 s of handling and confinement by netting, fingerlings had a sharp rise in plasma cortisol to a peak at 15 min, and then a gradual decline to the basal level over 2 h. Gentle agitation and intermittent restraint with a dip net in the aquarium caused a gradual increase in plasma cortisol concentrations. Intense handling and severe confinement caused a rapid increase in plasma cortisol to a plateau, two to four times greater than the peak levels found in fingerlings subjected to the less vigorous stressors; high levels of plasma cortisol were maintained to the median tolerance limit. During a stocking operation, there was a rapid rise in plasma cortisol concentrations during the initial capture from the rearing ponds, and high levels were maintained through to stocking into the lake 6 h and 10 min later, although a small decrease occurred during transit while the fish were in the truck tanks. After stocking, plasma cortisol concentrations in caged fingerlings did not decrease to the basal level until 8 d poststocking.Key words: rainbow trout, fingerlings, plasma cortisol, cycles, handling stress, stocking stress