Chronic exposure to stressors has a persistent effect on feeding behaviour but not cortisol levels in sticklebacks

Animals are often exposed to changes in their environment that may be perceived as stressful, leading to an acute stress response, but to which they can eventually habituate. However, if habituation is prevented, for example due to the stressors being diverse and unpredictable in the timing and sequence of their appearance, a state of chronic stress may ensue. Organisms exposed to chronic stressors may show altered patterns of glucocorticoid hormone synthesis as well as changes in behaviour (e.g. activity and feeding), but the exact phenotypic effects of chronically stressful environments remain poorly understood. In this study, we exposed adult three-spined sticklebacks, Gasterosteus aculeatus, to an unpredictable chronic stress protocol over a period of 67 days and measured their response in terms of water-borne cortisol levels and behavioural patterns. We quantified activity and feeding behaviour in two contexts: (1) when the stressors were applied and (2) during the resting period between stressors. We did not observe a significant cortisol elevation following a period of chronic exposure nor any change in cortisol levels in response to an acute stressor. The exposure of fish to chronic stressors led to a decline in latency to feed during the resting periods, indicative of an anticipation of future stressors. We observed an increase in activity levels of the stress-exposed fish, but only during the presentation of the stressors. Organismal response to protracted exposure to stressors is energetically expensive; thus, our results may indicate a trade-off between energy-demanding activities in fish subjected to a chronically unpredictable environment.