Endothall (7-oxabicyclo-2,2,1-heptane-2,3-dicarboxylic acid) is a contact herbicide which has been approved by the U.S. Environmental Protection Agency (EPA) for use in programs aimed at controlling nuisance organisms such as Eurasian water milfoil (Myriophyllum spicatum). Milfoil has become such a problem in local freshwater systems that Seattle authorities have approved the use of endothall in circumstances in which other eradication methods are not effective. This study was performed to determine the toxicity of endothall to juvenile chinook salmon (Oncorhynchus tshawytscha) and the effects of the herbicide on the ability of this anadromous fish to enter seawater.

Two 14-day flow-through toxicity tests were conducted using nominal concentrations of endothall ranging from 0 to 105.7 ppm of acid equivalent. Four-day toxicity tests were also conducted using the two lowest concentrations (1.5 and 3.0 ppm) to approximate the recommended treatment levels. Upon termination of the toxicity tests, the fish that had been exposed to a noeffect (no mortalities) concentration of the herbicide (≤55 ppm) were utilized in the seawater entry tests.

The 14-day LC₅₀ was calculated to be 62.5 ppm. The seawater entry test proved to be the more sensitive indicator of toxicity, as the fish that had been exposed to endothall levels as low as 3 ppm for four days died when placed in seawater. Survival of the control fish and the fish exposed to 1.5 ppm was acceptable, as was survival of all the fish transferred to clean fresh water. Some behavioral and gross anatomical changes were observed, and histopathological examination revealed hypertrophy of branchial epithelial cells in the fish exposed to 10 ppm or more of endothall.