Juvenile pallid (Scaphirhynchus albus) and hybrid pallidxshovelnose (S. albusxplatorynchus) sturgeons exhibit low physiological responses to acute handling and severe confinement.

Following a 7.5-h transport haul, juvenile pallid sturgeon (Scaphirhynchus albus) showed a small but significant increase in plasma cortisol to 4.7 ng ml(-1) but similar increases did not occur after fish were handled in a net held in the air for 30 s. Subsequent experiments on yearling pallid sturgeon and hybrid pallidxshovelnose (S. albusxplatorynchus) sturgeon using the same 30-s handling stressor failed to evoke increases in plasma cortisol, lactate or glucose. Plasma cortisol increased significantly from about 2 to 13-14 ng ml(-1) in both pallid and hybrid sturgeon during a 6-h severe confinement stressor with handling. Plasma cortisol in 2-year-old pallid sturgeon subjected to the same stressor demonstrated a linear pattern of increase during the initial 1 h. Plasma lactate increased from 1.11 to about 2.11 mmol l(-1) in hybrid sturgeon during the first hour of severe confinement but did not change throughout the entire confinement period in pallid sturgeon. A significant increase in plasma cortisol to 5.4 ng ml(-1) in 2-year-old pallid sturgeon 1 h after being subjected to 30 s handling at 19:00 h but not at 07:00 or 13:00 h suggests that a small diurnal variation in their stress response may exist. Although both pallid and hybrid sturgeons were responsive to stress, they exhibited very low physiological responses compared with those following equivalent stressors in most teleostean fishes or another chondrostean, the paddlefish (Polyodon spathula). Reasons for the apparent low responses to handling and confinement in scaphirhynchid sturgeons are not known but may relate to their evolutionary history, neuroendocrine mechanisms involved in their corticosteroid responses, or anatomy of their interrenal tissue structure.