Migration Timing of Atlantic Salmon Smolts Relative to Environmental and Physiological Factors

—We determined the migration timing of fry-stocked smolts of Atlantic salmon Salmo salar, relative to environmental and physiological factors, by using net weirs and counting fences in three tributaries of the West River, Vermont. Smolt migration began in late April and early May when water temperature was 58C, peak movements occurred in early and mid-May at temperatures exceeding 88C, and migration was complete by early June. Within this seasonal window, significant differences in migration timing and gill Na,K-ATPase activity occurred among tributaries. In both years of the study, smolts tended to migrate earlier and exhibit greater gill Na,K-ATPase activity in the warmest tributary than in the coolest tributary. Smolt migration timing differed most among tributaries in mid-May when (1) water temperatures were more than 88C, (2) smolts peaked in gill Na,K-ATPase activity, and (3) discharge peaked, stimulating smolt migration. Smolts captured after the migratory period had lower gill Na,K-ATPase activity than migrating smolts. Relating smolt physiology to migration was crucial for explaining complex interactions among water temperature, discharge, and smolt behavior during both the onset and cessation of migratory activity. Because the period between onset of migration and loss of smolt physiological characteristics may be brief, delays in downstream passage that may occur at dams must be minimized to maximize the successful recruitment of smolts to the marine environment. Atlantic salmon Salmo salar exhibit a highly developed smolt stage (McCormick 1994) typified by physiological events that prepare juvenile salmon for seawater entry and behavioral changes that facilitate their movement to the ocean (Saunders and Henderson 1978; McCormick and Saunders 1987; Hoar 1988). Physiological and behavioral processes of smolting are regulated by environmental changes and mediated and integrated by the pituitary–endocrine axis (Hoar 1988). Because smolting is seasonally labile, optimal periods for seawater entry and survival may be brief (Bilton et al. 1982). Therefore, determining the relation * Corresponding author: [email protected] 1 Present address: Federal Energy Regulatory Commission, Office of Hydropower Licensing, 888 First Street, Northeast, Washington, D.C. 20426, USA. 2 The Unit is jointly supported by the U.S. Geological Survey, Biological Resources Division, the Vermont Department of Fish and Wildlife, the University of Vermont, and the Wildlife Management Institute. Received December 18, 1997 Accepted January 26, 1999 between smolt migration and physiology contributes to an understanding of what controls migratory success. Photoperiod is the most important factor entraining juveniles to undergo the par–smolt transformation and thus be ‘‘primed’’ to migrate (McCormick et al. 1987; Saunders and Harmon 1990). Within the context of photoperiod, water temperature and discharge have concurrent effects on smolt migration and physiology. Increasing water temperature may initiate smolt migration (Youngson et al. 1983; Jonsson and Ruud-Hansen 1985) and either accelerate development (Solbakken et al. 1994) or regression (Duston et al. 1991; McCormick et al. 1997, in press) of smolt physiological characteristics. Peaks in discharge may affect thyroxine levels (Youngson et al. 1986, 1989) and stimulate smolts to migrate (Hesthagen and Garnås 1986; Dempson and Stansbury 1991). Given their interdependence, smolt migration and physiology must be studied simultaneously to understand how water temperature and discharge influence migration timing.