Factors Affecting the Stress Response

The response to stressors is ultimately determined by the fish’s genetic heritage. However, the stress response phenotype evident during any particular situation is strongly influenced by the fish’s prior history, its extant environment, and its developmental stage.

Genetic Effects. There can be major interspecific differences in the nature of the stress response. While the basic elements are generally similar across groups, the magnitude and dynamics of the physiological responses to stressors can be quite different. Some species appear to respond quite slowly, at least in terms of the HPI axis, compared to others; there is a large variation in tolerance of stressors among different groups of fish.

There are also genetic differences with reasonably high heritability between individuals within a species in the magnitude of the physiological response to stress. Similarly, there are genetically determined be- tween-individual differences in tolerance of stressors, but it is unknown if these are related to the nature of the fish’s physiological stress response.

The physiological stress response factors operate by activating and deactivating specific genes. The genes affected differ between organs. Functional genomics (which genes respond and what they do) of the fish stress response is a relatively new field of study. The liver, immune system, and brain have received the most attention to date, and are all very strongly affected by the stress response.

Environmental Effects. A fish’s prior history and its present environment can affect how it responds to stressors. While the initial stages of the hormonal responses to stress are reasonably invariant in different environments, the persistence of the response and its secondary and tertiary (i.e., whole animal) consequences are very dependent on the nature of the extant environment. Temperature is important in affecting the rates of reactions.

Other water quality variables can also be important as controlling or directing factors. Prior experiences with stressors can also harden the fish and hence lessen the apparent severity of subsequent, similar stressors. Nutritional history of the fish can also modify the magnitude and/or dynamics of certain physiological factors of the stress response, particularly with reference to factors concerned with energy.

Importantly, some environmental challenges, albeit lethal, might not necessarily invoke a general HPI stress response. It appears that stressors that are perceived by the central nervous system are those that elicit such a response.

Developmental Effects. Fish at different ontogenetic stages of their life cycle may respond to stressors differently. The basic elements of the physiological stress response, such as the hormones, are present at very early stages of development, and the system appears functional around hatching or soon thereafter.

Transitional stages such as the onset of feeding by mouth, metamorphosis, or smoltification and reproduction appear very sensitive to stressors in regard to the increased magnitude of their physiological response and decreased tolerance limits (Figure 3).

Figure 3. Top panel: Conceptualization of the performance capacity of a fish’s necessary life functions that are determined genetically and limited by the environment and negatively affected by stress. The length of each arrow (vector) goes from zero in the center to some level of performance at the point (Hb = hemoglobin). Bottom panel: Cartoon representing the performance capacity of a fish as it changes through time, showing a smaller capacity to resist stress during transitional stages

Ecological Consequences. Exposure to acute stressors reduces the fish’s capacity to resist other immediate threats (Figure 3). Longer-term or repeated exposure to relatively brief stressors depresses the capacity of fish during the time they are adapting to carry on other necessary life functions. The fish will have less energy to channel into anabolic processes such as growth, development, and reproduction.

It is likely that longer-term stress can change a fish’s developmental trajectory and thereby alter its life history strategy. For example, fish that are stressed during certain stages of reproductive maturation may change their spawning strategy. Depending on species of fish and the nature of the stressor, spawning could be accelerated, delayed, or totally inhibited. Stress experienced by females can lead to lower egg quality and subsequent progeny fitness.

While general disturbances and deteriorated water quality can cause a stress response, negative social interactions and low hierarchical status also result in a similar response. Fish of low social rank grow less well and tend to have reduced chances of survival. Submissive fish are stressed by aggressive displays, have activated HPI axes, and suffer its maladaptive consequences.

Temperature is one of the most important environmental controlling factors for fish because the fish are cold-blooded. The rates of biochemical reactions that drive physiological processes are directly dependent on the temperature of the water; in general, for each 10^oC increase in temperature, the rates of biological reactions double or triple. Temperature is thus a key variable regarding how fish respond to a stressor, both physiologically and ecologically.

How fish interact with pathogens and parasites is also affected by stress. Stress can increase both the infection rate and pathogenicity of a microorganism.