The stress response of three-spined sticklebacks is modified in proportion to effluent exposure downstream of wastewater treatment works

This study was conducted to investigate whether exposure to wastewater treatment works (WWTW) effluent affects the adaptive stress axis of fish resident within the receiving water. Three-spined sticklebacks (Gasterosteus aculeatus) were sampled from sites downstream of ten WWTWs in north-west England, selected to provide a range of population equivalents between 1,000 and 120,000. Following capture, indices of stress (whole-body cortisol and glucose concentrations) were measured both prior to, and following, a standardised stressor to establish both baseline and stress-induced cortisol and glucose concentrations. There was considerable between-site variation in size, and to a lesser extent condition, of the fish. Pre- and post-stress cortisol and glucose concentrations also exhibited significant between-site variation. A large proportion of the variation in both somatic characters and stress response was explained by variation in the proportion of effluent contributing to total river flow at the study sites. Mass (r2 = 0.35, P < 0.001) and length (r2 = 0.37, P < 0.001) of the fish, and cortisol (r2 = 0.26, P < 0.001) and glucose (r2 = 0.12, P < 0.01) concentrations in unstressed sticklebacks, were positively related to the concentration of effluent across the sample sites. However, in stressed fish cortisol (r2 = 0.32, P < 0.001) and glucose (r2 = 0.14, P < 0.001) concentrations exhibited a negative trend in relation to the effluent concentrations across sites. Individual variation in fish size did not account for the variation in either cortisol or glucose levels. These data are the first in which the activity and responsiveness of the stress axis has been compared across multiple populations of fish and provide the first indication that modulation of the stress axis in fish by anthropogenic factors might be a widespread phenomenon and therefore of greater significance than hitherto assumed.