| Summary: | Despite a reduction in exploitation of Atlantic salmon stocks, there continues to be a decline in many populations. The factors regulating these populations remain poorly understood, although contamination of the freshwater environment has been implicated as a causative factor. The major aim of these studies was to investigate the impact of environmental contaminants on salmon throughout their life cycle. Laboratory studies were carried out to assess the impact of environmental levels of pesticides (atrazine, cypermethrin 1'. and diazinon); brominated flame retardants (hexabromocyclododecane, HBCD and Tetrabromobisphenol A, TBBPA); and the pharmaceutical caffeine, on salmon during smoltification, reproduction and embryo development. Exposure to 5.0jlgrl atrazine for 81 days in freshwater reduced migratory activity over a 28 day period that coincided with the peak smolt run, while exposure to caffeine for 5 days in freshwater had an impact on the osmotic balance ofthe smolts, but this was not sufficient to induce mortalities upon transfer to seawater for 24 hours, either individually or in combination with HBCD. Exposure to 5ngrl HBCD over the peak smoltification period (30 days) did disrupt plasma thyroid hormone levels, although HBCD and TBBPA had no impact on olfactory responses in mature male salmon parr. Atrazine had no effect on plasma sex steroid levels in adult female brown trout, although exposure to environmental levels for 5 days prior to ovulation reduced the survival of the fertilised eggs, particularly when fertilisation occurred in atrazine-spiked (0.5jlgrl ) water. Fertilisation of gametes in water containing cypermethrin and diazinon (2 minute exposure) also affected both the number of fry that emerged and the timing ofthat emergence. The results of these studies show that the presence of common contaminants within the freshwater environment can have impacts on the physiology and behaviour of salmon. These impacts may affect the ability of salmon to survive at sea 'and to return to the natal river to successfully spawn. This may have significant implications for salmon populations in the wild.
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