Responses of wild freshwater fish to anthropogenic stressors in the Waikato River of New Zealand

To assess anthropogenic impacts of point-source and diffuse discharges on fish populations of the Waikato River, compare responses to different discharges and identify potential sentinel fish species, we sampled wild populations of brown bullhead catfish (Ameiurus nebulosus, (LeSueur, 1819)), shortfin eel (Anguilla australis Richardson, 1848), and common bully (Gobiomorphus cotidianus McDowall, 1975) in the Waikato River. Sites upstream and downstream of: geothermal; bleached kraft mill effluent (BKME); sewage and thermal point-source discharges were sampled. At each site, the population parameters, relative abundance, age structure and individual indices such as: condition factor; and organ (gonad, liver, and spleen) somatic weight ratios; and number and size of follicles per female were assessed. Indicators of fish residence and in some cases exposure to contaminants in discharges were analyzed. Bile chemistry of brown bullhead and shortfin eel was assayed, liver and muscle metal levels were analyzed for brown bullhead and shortfin eel respectively, and stable isotopes of C and N in common bully were measured. Bile, metal and isotopic signatures gave strong evidence that fish had been resident at sites for some time before sampling. Signatures of bile and metal contaminants showed contamination was localised to discharge areas. Gradients in stable isotopes in common bully showed evidence of changes in water sources and anthropogenic effects along the river. Biochemical variables, hepatic ethoxyresorufin-O-deethylase (EROD) and plasma steroids indicated exposure and response of brown bullhead and shortfin eel to pulp and paper contaminants at the BKME site. Physiological (blood) variables showed fish largely responded in a predictable way to elevated water temperatures at discharge sites at time of sampling, however total haemoglobin of brown bullhead and common bully blood failed to increase at the BKME site despite elevated temperatures and low dissolved oxygen.