Individual variability and population level effects

Although a chronic stress response or allostatic load will result in immunosuppression, not all individuals respond in a similar manner. Several studies have indicated genetic variation in disease resistance in Atlantic salmon (Salmo salar) (MacKinnon, 1998 R0ed et al, 2002). However, differences in susceptibility to disease and cortisol responses between strains of fish are poorly understood (Bilodeau et al, 2003). Rainbow trout (O. mykiss) (Fevolden R0ed, 1993) and Atlantic salmon (Fevolden et al, 1993) have been selected for high or low cortisol response. While it is not clear whether the fish have reduced sensitivity, reduced production or increased removal of cortisol, the high-stress response fish had lower immuno-competence. 

However, the association between the cortisol response and disease suscep-tibihty is far from clear. 

Not aU of that variability in response to a stressor is explained by genotype, life stage or circumstances. The magnitude and nature of the response are also partly determined by behavioural strategy. These have also been referred to as behavioural syndromes, personahties or temperaments. 

These strategies are not fixed and can be influenced by the experiences of the fish, for example transportation (Ruiz-Gomezerai, 2008).In mammals and birds early life experiences or even the experiences of the parents can also influence the nature of the offspring through epigenetic mechanisms (e.g. Weinstock, 1997 Gluckman et /., 2007 Rutherford et a/., 2009). In fish maternal condition does have various effects on offspring (e.g. Schreck et al., 2001 Tierney et al., 2009) however, the few studies that have examined such effects in fish have found no effect on susceptibility to infectious disease (Contreras-Sanchez et al, 1998). 

Behavioural strategies are associated with profound differences from the level of gene expression through the metabolome to the neuroendocrine, immune and other systems (MacKenzie et al., 2009). These differences are not only at the level of the neuroedocrine and immune systems but may be mediated through perception since hawks are inherently less responsive to environmental changes. In both hawks and doves an event can be more stressful if it is unpredictable. 

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