Publication Summary

In Loomis Lake (Pacific County), the invasive non-native aquatic plants Eurasian watermilfoil and egeria were treated with the herbicide fluridone in 2002. We monitored aquatic plants and fish before herbicide application and for up to three years after treatment. The herbicide treatment resulted in a significant reduction of invasive plants for three years. The native submersed plant community was also significantly reduced, a result from fluridone use at nonselective rates and poor light penetration caused by sediment entrainment. After treatment, the growth of largemouth bass and pumpkinseed sunfish increased, small yellow perch abundance decreased, and larger pumpkinseed sunfish increased.

This is an article in the Journal of Aquatic Plant Management, 47:2009, pp. 31-40.

Loomis Lake, a long narrow shallow lake on the coast of Washington State, had a submersed plant community dominated by the invasive non-native species Eurasian watermilfoil (Myriophyllum spicatum L.) and egeria (Egeria densa Planch.).

In 2002, the whole lake was treated with the liquid formulation of the aquatic herbicide fluridone (1-methyl-3-phenyl-5-[3-(trifluoromethyl) phenyl]-4(1H)-pyridinone). We monitored aquatic plant frequency of occurrence and biomass before herbicide application (2002) and for three years after the treatment (2003 to 2005). The fish population was assessed one year prior to herbicide treatment (2001) and three years post treatment (2005).

Prior to domination by invasive macrophytes, the lake had a diverse native plant community with low growing species in the deep water providing open water. During that time the lake supported a stocked rainbow trout (Oncorhynchus mykiss Walbaum) and warmwater fishery. As invasive macrophytes took over, the native plant richness decreased, the trout stocking program ceased, and small yellow perch (Perca flavescens Mitchill) dominated the fish community.

The herbicide treatment resulted in a significant reduction in frequency (86% for egeria, 84% for Eurasian watermilfoil) and biomass (98% for egeria, 99% for Eurasian watermilfoil) of the invasive species for three years. The native submersed plant community was also significantly reduced for the study duration. We attributed this to fluridone use at a nonselective rate and poor light penetration caused by wind-induced sediment entrainment.

After treatment, the growth of largemouth bass (Micropterus salmoides Lacepede) and pumpkinseed sunfish (Lepomis gibbosus Linnaeus) increased. In addition, the abundance of small yellow perch decreased while abundance of larger pumpkinseed sunfish increased.