The present study investigated the effects of stress and exercise on seawater (SW) adaptation and cortisol dynamics in juvenile steelhead (Oncorhynchus mykiss). To examine the effects of stress, fish acclimated to freshwater (FW) were subjected for 3 hours to confinement stress in FW, and subsequently SW (25 ppt) was introduced to all tanks. Fish were sampled immediately after the stress treatment, and 1, 7, and 14 days after introduction of SW. Electrolytes, cortisol, glucose and lactate followed the typical pattern that we expected after stress treatment in FW. Fish regained osmotic balance within 24 hours. Glucose concentrations were increasing throughout the experiment and lactate levels stayed elevated during the time spent in SW. IGF-1 did not show an immediate response to stress but after transfer to SW we detected significantly higher concentrations for control fish at days 1 and 14. The differences in IGF-1 levels between stressed and control fish are not reflected in SW adaptability but positive correlations between IGF-1 and electrolyte levels in control fish may indicate its role for osmoregulation. Confinement stress did not impair feed intake subsequently in SW, but our results suggest that feed intake was suppressed by the change of the media from FW to SW. The second study investigated the effects of exercise treatment in FW on SW adaptation and cortisol dynamics in juvenile steelhead. Plasma cortisol and in vitro cortisol secretion by interrenal cells after a 24 hr SW challenge test were neither affected by moderate exercise nor by water temperature (13��C, 21��C), however, plasma osmolality was lower in exercised fish compared to unexercised fish. Half-life (T[subscript 1/2]) of ��H-cortisol was shorter in fish exposed to exercise whereas metabolic clearance rate (MCR) did not respond to exercise treatment. Uptake and retention of corticosteroids in liver and gall bladder were enhanced in exercised fish, and retention of corticosteroids in muscle tissue was longer in unexercised fish. Our findings suggest that exercise likely decreases stress levels in fish and improves the adaptation to seawater (SW) in juvenile steelhead. / Graduation date: 2004

Steelhead and rainbow trout are common names for marine-migratory (anadromous) and freshwater-resident forms of Oncorhynchus mykiss, a partially migratory salmonid fish. Anadromous and resident forms are sympatric and can produce offspring with a life history different from their own (i.e., steelhead parents can produce rainbow trout offspring and vice versa). The expression of these alternative life histories is a plastic response to individual patterns of energy acquisition, assimilation, and allocation during juvenile life stages. Individual performance during early stream life is of particular interest because of potential carry-over effects on subsequent growth and developmental trajectories. In a series of experiments in laboratory streams, I determined the influence of individual variation in energy metabolism on behavior, growth, and life-history expression in O. mykiss. Individual variation in energy metabolism was a strong predictor of feeding territory acquisition by juvenile fish during the transition from dependence on maternal provisioning of egg yolk reserves to independent feeding. Feeding territory acquisition was positively associated with standard metabolic rate (SMR) under conditions of an abundant and predictable food supply. When the density of intraspecific competitors was manipulated, the association between SMR and territory acquisition was strongest at intermediate stocking densities, moderate at the highest stocking densities, and weakest at the lowest stocking densities. However, reducing the spatial predictability of food resources within streams reversed the influence of SMR on competitive outcomes. These experiments determined that variation in ecological conditions during early life stages imposes different selection regimes on juvenile O. mykiss and results in physiological divergence among cohorts. Subsequent rearing experiments determined that behavioral dominance influences rates of anadromy and freshwater maturation, most likely through the association between SMR and territory acquisition. In addition to the effects of behavioral dominance, I observed a significant influence of sex, rearing temperature, and individual growth trajectories on the expression of anadromy and freshwater maturation. Partially migratory populations of O. mykiss maintain an exceptionally diverse portfolio of life-history strategies. Results from this work lend insight into a suite of behavioral and physiological processes influencing individual life histories. / Graduation date: 2013