**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**
Michael A. Boogaard 1, Steven Gutreuter 1, and Cynthia S. Kolar 2
1Upper Midwest Environmental Science Center, 2630 Fanta Reed Road, LaCrosse, WI 54603
2U.S. Geological Survey, JOhn W. Powell Building, 12201 Sunrise Valley Drive, Reston, VA, 20192
The lampricide 2',5-dichloro-4'-nitrosalicylanilide (niclosamide) has been used in sea lamprey control operations as an additive to the lampricde -trifluoromethyl-4-nitrophenol (TFM) since 1963. The addition of 1% niclosamide by weight to TFM effectively reduces the amount of TFM required to kill sea lamprey larvae by about 40%. There is no current information, however, on the influence of niclosamide on TFM toxicity for ratios other than 1% niclosamide to TFM. We conducted a series of toxicological exposures with TFM and niclosamide at ratios ranging from 0.1% to 2.25% niclosamide to TFM against larval sea lampreys to determine the increase in TFM toxicity from varying levels of niclosamide addition. Exposures were conducted in water at pH 7.0, 8.0, and 9.0 to evaluate the influence of pH on the contribution of niclosamide to TFM toxicity to larval sea lampreys. Results of the exposures were used to develop a prediction model to define the lowest ratio of niclosamide to TFM that maintains a contributing effect to TFM toxicity and the highest ratio at which the contribution is maximized. The model predicts additions of niclosamide as low as 0.1% by weight of TFM will increase TFM toxicity to larval sea lampreys by about 8%. A maximum increase of about 54% was estimated at a ratio of 1.5% niclosamide to TFM. According to the model, ratios greater than 1.5% niclosamide to TFM will not result in any additional contribution to the toxicity of TFM to sea lampreys. In addition, the contribution of niclosamide to TFM toxicity was not influenced by pH. For example, the model predicted an increase in TFM toxicity of 31.0% with the addition of 0.5% niclosamide at pH 7.0 and a 30.8% increase for the same ratio at pH 9.0. The prediction model developed from the results of this study will allow treatment managers to more accurately predict minimum lampricide concentration requirements when using TFM and niclosamide in combination for efficacious treatment of streams to control sea lamprey populations.