Stress in fish is ubiquitous in natural and anthropogenic environments, and has the potential to cause major environmental and economic losses. Due to this, there is a need to develop a broad range of tools in risk assessment that facilitate the objective detection and assessment of stressed states in fish. It is well known that the exposure of fish to stressful stimuli activates a highly integrated stress response system that encompasses multiple levels of biological organization. At the

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... smal level, stress hormones (e.g. Cortisol) are released into the circulatory system to mobilize energy stores, and at the cellular level, heat shock proteins (e.g. hsp70) have functions related to preserving the integrity of proteins. Given the similarities between these two levels of stress response (i.e. maintenance of homeostasis), the purpose of my thesis was to determine if the organismal (Cortisol) and cellular (hsp70) stress responses were functionally related. In order to characterize this relationship, I: (1) studied a temperate salmonid (rainbow trout, Oncorhynchus mykiss) and a tropical finfish (mossambique tilapia, Oreochromis mossambicus); (2) measured the stress responses in hepatic and gill tissues; (3) exogenously introduced Cortisol into fish via intra-peritoneal injections or dietary routes; (4) artificially raised levels of Cortisol for acute (24 h) and sub-chronic (28 d) periods; (5) applied different stressors, including heat stress (2 h, +12°C or 2 h, +14°C immediate change), stress hormones (Cortisol), and toxicants (β-napthoflavone; bnf) to fish; (6) characterized the stress responses at the organismal (i.e. Cortisol, glucose, and lactate) and cellular (i.e. hsp70 and glucocorticoid receptor) levels; and (7) applied novel laboratory techniques to describe the association between hsp70 and the glucocorticoid receptor. By studying the stress responses under a variety of experimental conditions, I gained a comprehensive understanding regarding the relationship between the organis [...]