Effects of sublethal sodium dichromate exposure in freshwater on the salinity tolerance and serum osmolality of juvenile coho salmon,Oncorhynchus kisutch, in seawater

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Abstract
To simulate a potential field situation in which seawardmigrating juvenile coho salmon,Oncorhynchus kisutch, pass through Crpolluted freshwater (FW) before being exposed to seawater (SW), the effects of sublethal FW exposure to Cr (as sodium dichromate) on salinity tolerance and serum osmolality after transfer to SW were investigated. SW survival was significantly decreased in salmon exposed in FW to 0.23 mg Cr/L for four weeks or to 0.5 mg Cr/L for two weeks and transferred to 20 or 30 parts per thousand (%) salinity, respectively. Serum osmolality, measured after two weeks of FW exposure to 0.0 or 0.5 mg Cr/L and after transfer to 20 or 30% SW, was significantly higher in Cr-exposed fish from one-half to two days after SW transfer but not at the end of the FW exposure period or at seven days after SW transfer. These effects in SW occurred at Cr exposure concentrations which had no effect on survival or serum osmolality in FW. It is suggested that Cr acted non-specifically on osmoregulatory epithelia to cause excessive dehydration in SW, and that measurement of salinity tolerance and serum osmolality may be sensitive and ecologically realistic indicators of sublethal toxicity in aquatic animals.

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