Endocrine disrupting compounds (EDC) are environmental contaminants that disrupt reproduction, development and behaviour in aquatic organisms. A thorough evaluation of the impacts of EDCs on aquatic organisms is currently limited by a lack of robust biomarkers in small model fish, particularly for assessing EDCs with (anti-)androgenic activity. Male sticklebacks build nests using spiggin, an androgen-responsive glycoprotein, which can be used to assess (anti-)androgenic exposure. EDC assessment in the field using threespine stickleback and the spiggin biomarker is limited to coastal and estuarine environments. However, their freshwater relative, brook stickleback (Culaea inconstans), also possess spiggin and their widespread distribution suggests that they may have applications as a bioindicator of EDCs in freshwater systems. Therefore, the overall objective of this thesis was to determine if brook stickleback are a suitable bioindicator species for EDCs by evaluating their response and sensitivity to estrogenic and (anti-)androgenic chemicals. Basal transcript levels of spiggin in kidney and vitellogenin in liver were first measured in wild-caught brook stickleback using qPCR and found to be differentially expressed in males and females. Brook stickleback were then exposed to two model compounds, 17α-ethinylestradiol (EE2) and 17α-methyltestosterone (MT), at 1, 10 and 100 ng/L for 21 days (sampled at 7 and 21 days) via static-renewal to determine the responsiveness of these transcripts to exogenous hormones. The effect of hormone exposure on condition factor, organosomatic indices and histopathology of kidneys was also measured. Exposure to MT and EE2 significantly induced spiggin and vitellogenin transcripts in female kidneys and male livers, respectively. Exposure to EE2 also significantly increased the hepatosomatic index in females after 7 days and in both sexes after 21 days whereas the gonadosomatic index was reduced in females after 21 days. An increase in kidney epithelium cell height was also observed in MT-exposed females and males after 7 days. These results mirror those of threespine stickleback and suggest that brook stickleback are responsive to androgenic and estrogenic chemical exposure and more specifically, possess quantifiable and sensitive biomarkers for exposure to compounds with androgenic activity. In a third experiment, female fish were co-exposed to MT at 500 ng/L and an anti-androgen (flutamide; FL) at 25, 150 and 250 µg/L for 14 days (sampled at 4 and 14 days) to validate this bioassay for the evaluation of anti-androgens using the same endpoints as in the previous two experiments. In females, exposure to MT increased spiggin transcript levels and nephrosomatic index (NSI) but co-exposure to FL did not result in a significant suppression of these endpoints because of high inter-individual variability. In males, exposure to MT increased NSI and co-exposure to FL resulted in a reduction in this endpoint, illustrating anti-androgenic effects. Although the response of brook stickleback to hormone exposure was endpoint-specific and was at times lower than other small model fish species, the ability to simultaneously assess estrogenic and (anti-)androgenic chemical exposure in a single fish using quantitative endpoints is an advantage exclusively held by members of the stickleback family. The results of this thesis suggest that brook stickleback hold promise as an additional small fish model for the evaluation of EDCs, with potential application in EDC biomonitoring in the freshwaters of North America.

Many chemicals present in the aquatic environment can interfere with physiological functions in fish. Exposure to chemicals can be revealed by the use of biomarkers. Induction of 7-ethoxyresorufin O-deethylase (EROD) activity is a commonly used biomarker for exposure to CYP1A inducers such as dioxins and polyaromatic hyrdrocarbons. Vitellogenin is a frequently used biomarker for estrogenic compounds in various fish species whereas a biomarker for androgens, spiggin, is only found in sticklebacks. The main objectives of this thesis were to evaluate gill EROD activity as a biomarker and the three-spined stickleback as a model species in ecotoxicological studies. EROD activities were measured in gill, liver and kidney in rainbow trout (Oncorhynchus mykiss) caged in urban areas in Sweden. EROD induction was most pronounced in the gill. Also in fish caged at reference sites, with an expected low level of known CYP1A inducers, a marked gill EROD induction was found. One suggested inducer in rural waters is humic substances (HS). To evaluate the EROD-inducing capacity of HS, three-spined sticklebacks (Gasterosteus aculeatus) were exposed to HS of natural or synthetic origin. Both kinds of HS caused significant EROD induction. Gill EROD activities were also induced in sticklebacks exposed to ethynylestradiol (EE2) and β-naphthoflavone (βNF), alone and in combinations. Production of vitellogenin was induced in sticklebacks exposed to ≥50 ng EE2/l and a significant decrease in spiggin production was observed in individuals exposed to 170 ng EE2/l. Results from this thesis further strengthen the contention that gill EROD activity is a very sensitive biomarker for CYP1A inducers and that the stickleback is a suitable biomonitoring species, especially for exposure to CYP1A inducers. The finding that not only classical CYP1A inducers but also HS and high EE2 concentrations stimulate gill EROD activity is of significance for the interpretation of biomonitoring data.