Induction of hepatic ethoxyresorufin-0-deethylase (EROD) activity and accumulation of chlorophenolic compounds typical of bleached-kraft mill effluent (BKME) in fish sampled downstream of a pulp mill on the St. Maurice River, Quebec, Canada, provided evidence of chemical exposure to BKME. In comparison, fish sampled over the same distances and in similar habitats in a noncontaminated reference river, the Gatineau River, demonstrated low EROD activity and contamination levels. Accelerated growth of white suckers occurred between 2 and 10 years of age in both rivers at downstream stations relative to upstream stations, suggesting the existence of gradients of nutrient enrichment independent of BKME contamination. The impact of BKME exposure was expressed as reduced investment in reproduction, as revealed by greater length at maturity, reduced gonad size, and more variable fecundity. These effects were not obvious in simple upstream-downstream comparisons, but became evident when fish from the uncontaminated Gatineau River showed increased gonadal development and reduced age and size at maturity in response to enhanced growth rates.Keywords-Bleached-kraft mill effluent EROD MFO 317

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A previous study on the effect of pulp and paper effluents on white sucker (Catostomus commersoni), conducted in 1991 and 1992, in both effluent-exposed and reference rivers showed that fish grew faster at downstream sites than at upstream sites. However, in contrast with fish from a reference river, fish exposed to effluent showed no decrease in age or size at first maturity or increase in gonad size or fecundity in response to greater growth. The objective of the present study, conducted in 1993, was to test if differences in measures of growth and sexual maturation between fish populations in exposed and reference rivers would vary from year to year and whether such variation would affect conclusions concerning the effects of pulp mill effluents. Although size at age, fecundity, and age at first maturity varied between the two studies, patterns of demographic responses to pulp mill effluents, relative to reference populations, remained the same, i.e., conclusions about effects of pulp mill effluents did not change despite variation in demographic variables between years and sites. This study reaffirmed the view that only by including the fish populations in a reference river could effects due to bleached kraft mill effluent be separated from effects due to nutrient gradients.

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The gut microbiome of fish contains core taxa whose relative abundances are modulated in response to diet, environmental factors, and exposure to toxicogenic chemicals, influencing the health of the host fish. Recent advances in genomics and metabolomics have suggested the potential of microbiome analysis as a biomarker for exposure to toxicogenic compounds. In this 35-day laboratory study, 16S RNA sequencing and multivariate analysis were used to explore changes in the gut microbiome of juvenile Lates calcarifer exposed to dietary sub-lethal doses of three metals: vanadium (20 mg/kg), nickel (480 mg/kg), and iron (470 mg/kg), and to two oils: bunker C heavy fuel oil (HFO) (1% w/w) and Montara, a typical Australian medium crude oil (ACO) (1% w/w). Diversity of the gut microbiome was significantly reduced compared to negative controls in fish exposed to metals, but not petroleum hydrocarbons. The core taxa in the microbiome of negative control fish comprised phyla Proteobacteria (62%), Firmicutes (7%), Planctomycetes (3%), Actinobacteria (2%), Bacteroidetes (1%), and others (25%). Differences in the relative abundances of bacterial phyla of metal-exposed fish were pronounced, with the microbiome of Ni-, V-, and Fe-exposed fish dominated by Proteobacteria (81%), Firmicutes (68%), and Bacteroidetes (48%), respectively. The genus Photobacterium was enriched proportionally to the concentration of polycyclic aromatic hydrocarbons (PAHs) in oil-exposed fish. The probiotic lactic acid bacterium Lactobacillus was significantly reduced in the microbiota of fish exposed to metals. Transcription of cytokines IL-1, IL-10, and TNF-a was significantly upregulated in fish exposed to metals but unchanged in oil-exposed fish compared to negative controls. However, IL-7 was significantly downregulated in fish exposed to V, Ni, Fe, and HFOs. Fish gut microbiome exhibits distinctive changes in response to specific toxicants and shows potential for use as biomarkers of exposure to V, Ni, Fe, and to PAHs present in crude oil.