The Stress Response of Juvenile American Shad to Handling and Confinement Is Greater during Migration in Freshwater than in Seawater

—The physiological responses of juvenile American shad Alosa sapidissima were evaluated during the period of downstream migration in freshwater (FW) and after seawater (SW) acclimation associated with postmigration. Fish were subjected to a standardized, acute handling and confinement stress (3 h). Changes in plasma cortisol, plasma chloride, and hematocrit were monitored for 24 h. Basal levels of plasma cortisol were 5 times as great in FW as in SW fish (34 and 7 ng/mL, respectively). Within 0.5 h, both groups exhibited significant increases in plasma cortisol. The increase in FW fish was 4.5 times as great as that in SW fish. While the cortisol levels in SW fish returned to their basal values within 24 h, those of FW fish remained more than 25 times as high as basal values. Changes in plasma chloride occurred in fish after the initial stress, decreasing in FW fish and increasing in SW fish. This perturbation was overcome within 24 h in SW fish but not in FW fish. Hematocrit increased in FW fish 3 h after the initial stress and returned to normal within 24 h; fish stressed in SW exhibited no change in hematocrit. Significant mortalities were observed in the FW group but not in the SW group. During the period of downstream migration in FW, fish exhibited a heightened sensitivity to acute stress compared with that of SW-acclimated postmigrants. Handling and confinement associated with fish transport and fish passage structures are therefore likely to impact the performance scope and survival of juvenile American shad. American shad Alosa sapidissima are anadromous teleosts indigenous to the East Coast of North America. These fish have also been introduced into the Columbia and Sacramento rivers on the West Coast. Adults migrate into coastal rivers * Corresponding author: [email protected] 1 Present address: Biology Program, University of Northern British Columbia, 3333 University Way, Prince George, British Columbia V2N 4Z9, Canada 2 Present address: Abernathy Fish Technology Center, U.S. Fish and Wildlife Service, 1440 Abernathy Creek Road, Longview, Washington 98632, USA Received February 2, 2000; accepted April 30, 2001 and spawn during the spring (Leggett and Whitney 1972). Young generally remain in freshwater (FW) until the fall, when they migrate downstream. The timing of seaward migration is correlated with decreasing water temperature. In general, the peak migration occurs when the temperature falls to between 16 and 98C (Leggett and Whitney 1972; O’Leary and Kynard 1986). Associated with downstream migration in FW are increased gill Na,K-ATPase, a loss of ion uptake capacity, and decreased hematocrit. These and other physiological changes related to migration are increased and hastened by declining fall water temperature (Zydlewski and McCormick 1997a). American shad maintained in FW past the period of migration (unsuccessful migrants) exhibit behavioral perturbations, including the cessation of feeding and impaired swimming ability (Zydlewski and McCormick 1997a). They also experience high mortality (Howey 1985), which is related to impaired hyperosmoregulatory ability (Zydlewski and McCormick 1997a). On both coasts of North America, populations of American shad are the focus of restoration and mitigation efforts. These efforts may include transport with multiple handling episodes in FW. In addition, downstream passage facilities may entail confinement, handling, and other physiological stressors for migrating fish. While American shad are notoriously sensitive to transport as juveniles in FW, the physiological response to handing and confinement has not been explored. Interestingly, juveniles are more easily handled and show no substantial behavioral perturbations or mortality when acclimatized to seawater (SW; Zydlewski and McCormick 1997a; J. D. Zydlewski, personal observations). It is this apparent difference in sensitivity that is of interest here. Physiological responses to stress are well documented in many species of fish (reviewed by Bar1204 SHRIMPTON ET AL. ton and Iwama 1991) but have not been characterized for American shad. The primary response to a stressor is the release of corticosteroids and catecholamines, which in turn induce secondary responses such as increases in plasma glucose and osmotic perturbations (Mazeaud et al. 1977). Responsiveness to stress has also been shown to change with development. In anadromous salmonids, an increase in responsiveness to stress occurs during the parr–smolt transformation (Barton et al. 1985; Carey and McCormick 1998) as juvenile salmonids prepare for migration into SW. The objective of this study was to compare the stress responses of juvenile American shad in FW and SW, which correspond to those of (1) migrating juveniles and (2) postmigrants that have fully acclimatized to SW. Fish were exposed to a standardized handling and confinement stress and monitored for 24 h. Changes in plasma cortisol, plasma chloride, and hematocrit are reported.