Publication Summary

A seawater challenge was used to determine if low concentrations of the aquatic herbicide Hydrothol 191® affect the osmoregulatory performance of chinook salmon smolts (Oncorhynchus tshawytscha). Chinook smolts were exposed to 0, 50, 100, or 200 mg/L Hydrothol 191 (endothall acid equivalent) for 96 hours, followed by a 24-hour seawater challenge.

A seawater challenge was used to determine if low concentrations of the aquatic herbicide Hydrothol 191® affect the osmoregulatory performance of chinook salmon smolts (Oncorhynchus tshawytscha). Chinook smolts were exposed to 0, 50, 100, or 200 mg/L Hydrothol 191 (endothall acid equivalent) for 96 hours, followed by a 24-hour seawater challenge.

Fish exposed to 200 mg/L Hydrothol 191 suffered 45% mortality following the seawater challenge. There was 5% mortality among fish exposed to either 50 or 100 mg/L Hydrothol 191. Condition factors, plasma sodium levels, and gill ATPase activities were not significantly different among groups (one-way ANOVA, p<0.05) following exposure to seawater. Plasma sodium concentrations in all groups of fish, except those exposed to 200 mg/L of the herbicide, were consistent with normal seawater adaptation for chinook smolts ((170 meq). Smolts exposed to 200 mg/L Hydrothol 191 had slightly higher plasma sodium levels (173.0 meq/L), but the difference from normal levels was not significant (student's t, p<0.025).

Results of this study indicate that low concentrations of Hydrothol 191 do not cause a decrease in the osmoregulatory capacity of chinook smolts. The cause of mortality is unclear, but may have been respiratory distress due to gill inflammation.