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

Abstract

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 represent a range of human population equivalents between 1000 and 125,000. Following capture, indices of stress (whole-body cortisol and glucose concentrations) were measured both prior to, and following, the imposition of a standardised stressor to establish both baseline and stress-induced concentrations of cortisol and glucose. 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 varied significantly between-sites. A large proportion of the variation in both the somatic data and the 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 provide the first indication that modulation of the stress axis in fish by anthropogenic factors might be widespread and of greater significance than hitherto assumed.

Introduction

Fish, in common with all vertebrates, possess a suite of neuroendocrine, metabolic and behavioural responses, collectively termed the stress response that are rapidly activated to help cope with challenging circumstances (Pankhurst, 2011). A core element of the stress response, the hypothalamic–pituitary–interrenal (HPI) stress axis, is susceptible to interference by chemicals (reviewed by Pottinger, 2003) including metals, pharmaceuticals, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and herbicides (Bisson and Hontela, 2002, Gesto et al., 2008, Hontela, 2006, Levesque et al., 2003). Chemical interference with the activity of the stress axis has been reported to result in an attenuated release of cortisol, the primary corticosteroid secreted by the interrenal tissue of stressed fish. Although clear evidence of higher-level effects arising from disruption of the stress response is limited, it is reasonable to assume that there may be adverse effects on the fitness of affected fish.

During a recent investigation of the effects of extreme weather events on the stress axis of three-spined sticklebacks (Gasterosteus aculeatus) in rivers in south-west England (Pottinger et al., 2011a) we detected trends in whole-body levels of cortisol which suggested that the magnitude of the stress response following capture was influenced by the proximity of the capture site to a wastewater treatment works (WWTW) effluent discharge. The present study was conducted to evaluate whether these observations constituted evidence that the complex chemical milieu present in rivers downstream of WWTWs, one of the most abundant point-sources of aquatic pollutants in UK waters, can affect the functioning of the stress axis in fish.

To address this aim, resident populations of three-spined sticklebacks were sampled at sites downstream of ten WWTWs in the north west of England serving a range of human populations (1000–125,000). The freshwater three-spined stickleback offers several advantageous characteristics for a study of this nature, including small body size, a wide distribution, local abundance, a short life span often resulting in a single predominant year class, a relatively sedentary lifestyle, and no significance to recreational anglers (Katsiadaki et al., 2007, Pottinger et al., 2002). For fish captured at each site indicators of stress (whole-body concentrations of cortisol and glucose) were quantified in order to assess the status of the stress axis in individuals immediately following capture (unstressed – baseline cortisol and glucose) and after a short period of confinement (stressed – elevated cortisol and glucose). These data were examined in the context of several measures by which the environmental impact of the upstream WWTWs could be characterised: population equivalents, dry weather flow (daily discharge) and modelled effluent concentration at the sample sites.