pulp mills are required to perform routine regulatory testing to ensure the effluent they release from their water treatment plants is not harmful to humans and the environment. the key components within the effluent that cause toxicity include factors such as total suspended solids, chemical oxygen demand, concentration of resin and fatty acids and the colour of the effluent. in december 2013 a pulp mill in terrace bay, on experienced three failures when their effluent caused 90 to 100% mortality to tested rainbow trout. in this study, the objective was to determine the potential cause of this failure. prior to this failure, mechanical issues resulted in an increase in pulp fibre with in the mills secondary treatment system. since mortality was observed in trout tests and not daphnia tests, the increase in fibre was suggested as a likely cause. acute toxicity screenings were conducted using effluent that had been spiked with sludge obtained from the primary clarifier, mimicking concentrations observed during the failure. there was 10% mortality in one of the untreated samples and no mortality occurred in any of the spiked samples. the lack of fibre induced mortality combined with increased levels of sulphur suggested volatile sulphur compounds as a potential cause for the failure. given their volatile nature, current test methods are not suitable for testing these compounds especially at low concentrations. another objective of this study was to determine if the toxicity of effluent could be measured using a crystal violet staining (cvs) assay applied to piscine cell lines as an alternative to in vivo methods. [...]