Physiological Responses of Rainbow Trout <i>Oncorhynchus mykiss</i> to Crowding and Anesthesia with AQUI‐S™

Abstract.— Following exposure to the anesthetic AQUI‐S™, plasma cortisol concentration in immature rainbow trout was measured as (mean) 293 ± 48 ng/ mL, which was significantly ( P &gt; 0.05) higher than the mean concentration in resting fish. Cortisol concentrations remained significantly ( P &gt; 0.05) elevated for at least 24 h after treatment. This was accompanied by a significant increase and decrease in hematocrit and plasma potassium, respectively. These perturbations continued for at least 48 h following recovery from anesthesia. Plasma concentrations of total protein and sodium remained unchanged following anesthesia with AQUI‐S™. Crowding stress is commonly encountered by fish during manipulation in aquaculture situations. Anesthetising fish prior to, and during, manipulation may reduce the associated stress. Changes in cortisol values resulting from crowding (30 min; 0.1 kg/L) during anesthesia with AQUI‐S™ were not appreciably different from those in fish crowded without anesthesia. Thus, anesthesia with AQUI‐S™ at the recommended dose of 17 mg/L did not appear to be effective for alleviating the stress of crowding under the conditions of our experiments.