Effects of the Aquatic Herbicide Endothall on Survival of Salmon and Steelhead Smolts During Seawater Transition

In the Pacific Northwest, salmon and steelhead protection and population recovery efforts have led to careful consideration of potential effects of chemical exposure on sensitive life-stages. In particular, delayed effects of herbicide exposure on anadromous species as they transition from freshwater to seawater has become a concern previously overlooked by standardized toxicity testing for chemical labeling. For example, the widely used aquatic herbicide endothall has relatively low toxicity to salmonids following initial exposure; however, it was unclear whether endothall exposure affects the survival of smolts (seagoing juveniles) during transition from freshwater to seawater. Previous studies relied on small sample sizes and inappropriate life-stages, and generated contradicting results. To resolve uncertainty about endothall effects on anadromous salmonids, coho (Oncorhynchus kisutch), fall Chinook (Oncorhynchus tshawytscha), and steelhead (Oncorhynchus mykiss) were exposed to a ten-day seawater challenge (30 ppt) following acute exposure to the herbicide Cascade^®, which contains the acid equivalent of the dipotassium salt of endothall.  Exposure concentrations ranged from 0 to 12 mg/L endothall for 96 hours.  The seawater challenge yielded mean survival rates of 84%, 83%, and 57% for 0, 3-8, and 10-12 mg/L exposure groups, respectively. Toxicity results did not differ significantly between species, but steelhead were most sensitive to the seawater challenge in all treatments. Mean plasma sodium levels of surviving fish were the same for control and treatment groups, indicating that osmoregulatory processes were not impaired by endothall exposure.  Results suggest that the threshold for effects of endothall (dipotassium salt) exposure on smolts during seawater transition is likely >8 mg/L, but <12 mg/L –far less than the LC50 of 230-450 mg/L for salmonids in freshwater. This finding emphasizes the importance of conducting seawater challenge experiments with species that migrate between freshwater and marine environments before defining chemical toxicity levels.