In a broad context, stress can be considered as a biological response that drives physiological systems outside their normal range. Fishes typically respond to short-term, or acute, stress by mechanisms designed to maintain physiological function by compensating for the stress for a while, and then when the stress passes the fish can return to its previous physiological state. If the stress is chronic (persists for a long period of time), however, it may result in are adjustment of physiological set-points and the establishment of a new baseline condition. This is sometimes referred to as all stasis, because rather than returning to its previous physiological state (homeostasis), the organism instead establishes a new baseline condition. This would include changes in gene expression that result in long-term alterations of proteins needed to maintain function under thenew conditions (Iwama et al. 2006).
Physiological responses to stress typically occur in three phases (Barton et al. 2002; Iwama et al. 2006). The primary response is mainly the immediate release of epinephrine, followed by the release of cortisol in teleost’s or 1a-hydroxycorticosterone in elasmobranchs. Epinephrine release and the physiological responses that it initiates can occur in seconds, but do not persist for long. The release of cortisol and the reaction to it, however, begin more slowly and are sustained for a longer period of time. Together, these hormones activate biochemical pathways that lead to the secondary phase of the stress response, which is markedly elevated levels of blood glucose to support an increased metabolism. In addition to elevated blood glucose, the secondary response also is characterized by increased respiration rate, increased blood flow to the gills, and increased gill permeability (Barton et al. 2002). These increases help the fish to take in more oxygen to support elevated metabolism, but also increase the diffusion of water and ions across the gill epithelium, creating more osmo regulatory stress and demanding more active transport, and therefore energy, for the fish to maintain its osmotic balance.
Another part of the secondary response occurs at the cellular level – the induction of stress proteins. These areoften called heat shock proteins (HSPs) because they were initially described as a response to elevated temperatures. However, they are now recognized as a general cellular level response to many types of stress, including temperature, various types of pollution, handling, hypoxia, and pathogens. There are three general categories of stress proteins, based on their molecular weight, and they seem to help maintain the function of other proteins that are critical to cellular biochemical processes by protecting the shape of, helping repair, or helping control degradation of these other proteins. For example, the stress protein identified asHSP-90 apparently is important in protecting the function of the cellular receptor for cortisol, which would help sustain the ability of the cell to respond to this important stress hormone (Iwama et al. 2006). Because stress proteins are a general response to many types of stress, they can be used as an indicator of a fish’s exposure to a stressor, such as unfavorable environmental conditions.
If stress persists, the primary and secondary responses may lead to tertiary responses at the whole-animal or populationlevel (Barton et al. 2002; Iwama et al. 2006). Persistent elevated levels of the stress hormones, especially cortisol,can negatively affect fish growth, condition factor (length3/mass), reproduction, and behavior such as swimming stamina because energy that would have been available for these functions has been diverted to dealing with stress (see, Bioenergetics models).
Several factors can influence a fish’s response to stress. These include sex, because the sex hormones them selves can affect the stress response, and the developmental stage of the individual, because juveniles and adults often will respond differently. A fish’s nutritional state or whether itis affected by an existing stressor also can impact its response to subsequent stress (Barton et al. 2002). Responses to stress can also be seen at all levels of biological organization(Adams 2002; Hodson 2002). Short-term exposure to stressors can lead to changes at the subcellular level as a fish tries to compensate physiologically, but these effects may not have implications at higher levels of organization, such as the overall health of the organism or the status of the population.
Chronic stress can affect fish immune systems, in part because sustained elevated levels of cortisol can suppressimmune function and thereby diminish disease resistance and ultimately survival. Experimentally induced stress designed to resemble the stress of capture significantly impacted the immune responses of Sablefish (Anoplopomafimbria), so that those released as unwanted bycatch might have diminished capabilities to resist natural pathogens(Lupes et al. 2006). And Chinook Salmon smolts exposed to elevated levels of ammonia for 96 h had lowered counts of lymphocytes, which could lead to increased susceptibility to disease (Ackerman et al. 2006). Environmental contaminants may also negatively affect fish immune systems by compromising the protective barriers of skin and mucus, affecting organs that filter pathogens from the blood, and interfering with intercellular signaling. For example, juvenile salmon from Puget Sound, known forits elevated levels of various pollutants, were more susceptible to pathogens because their immune responses were suppressed, and English Sole may also be affected(see Rice & Arkoosh 2002).
Chronic stress also may affect reproduction, and therefore population and community structure. A range of chemical contaminants have been identified as endocrine disrupting compounds (EDCs) because they interfere with some aspects of the hormonal signaling system that regulate the gonads and secondary sex characteristics (Greeley2002). As more potential EDCs are identified in our surface waters, concern increases over the potential impacts on aquatic life, including fish populations.