1998 Big Lake Survey: The Warmwater Fish Community Before Treatment with Selective Herbicide

Big Lake is a shallow (mean and maximum depth = 4.3 and 7.0 m, respectively), narrow (width = 0.4 - 0.8 km) body of water located in Skagit County southeast of the City of Mt. Vernon, Washington. The lake is fed by Lake Creek from the south, and six unnamed tributaries which are located along the western shore. Surface water exits Big Lake (surface area » 220 ha) at the northern end, through Nookachamps Creek, eventually discharging into the Skagit River. The watershed supports a variety of cold- and warmwater fish species, including cutthroat trout (Oncorhynchus clarki), steelhead trout (Oncorhynchus mykiss), coho salmon (Oncorhynchus kisutch), yellow perch (Perca flavescens), pumpkinseed (Lepomis gibbosus), and largemouth bass (Micropterus salmoides). The aquatic plant community consists of a variety of pondweeds (Potamogeton sp.), but mostly invasive Brazilian elodea (Egeria densa) and, to a lesser degree, Eurasian watermilfoil (Myriophyllum spicatum) [URS undated; Washington Department of Ecology (WDE) 1976; RMI 1999; Washington Department of Fish and Wildlife (WDFW), unpublished data; Jim Johnston, WDFW, personal communication; Jenifer Parsons, WDE, personal communication].

Nearshore residential development at Big Lake is extensive. Concern about the invasion of the lake by Brazilian elodea and the spread of Eurasian watermilfoil led residents to form the Big Lake Management District and, with support from Skagit County, sought grants from WDE's Aquatic Weed Program to implement an integrated aquatic vegetation management plan. In 1998, WDE issued a permit and provided funding to Skagit County in order to treat the lake with the herbicide Sonar^®. (RMI 1999). On July 1, 1998, and every two weeks thereafter for a period of six weeks, a sinking, slow-release pellet form of Sonar^®. was applied to the aquatic plant beds of Big Lake. The benefits of using the herbicide pellets were two-fold: applicators could target specific areas where noxious weeds had colonized and efficacy of treatment was improved since the pellets remained in direct contact with the targeted plants. By October 1, 1998, most Brazilian elodea was damaged or dying-back throughout Big Lake. Eurasian watermilfoil was similarly affected (RMI 1999).

A healthy aquatic plant community is essential for the well-being of many warmwater fish species, which are more likely to be found in areas with aquatic plants than in areas without them (Killgore et al. 1989). Submersed aquatic vegetation provides important foraging, refuge, and spawning habitat (see review by Willis et al. 1997), improving survival and recruitment to harvestable sizes (Durocher et al. 1984). Changes in the standing crop of aquatic plants can alter fish production (Wiley et al. 1984) as well as the structure of the fish community itself (Bettoli et al. 1993). For these reasons, it is important to gather baseline information and carefully review all proposals to limit or control aquatic vegetation for a given lake, especially when the lake supports a popular fishery.

Big Lake is well known for its recreational opportunities, especially fishing for warmwater species such as largemouth bass (Anonymous 1999). In an effort to assess its warmwater fishery, personnel from WDFW's Warmwater Enhancement Program conducted a fisheries survey at the lake in summer 1997. Since it was gathered before implementation of the integrated aquatic vegetation management plan, the baseline information presented here will be useful when monitoring the long-term effects of the Sonar^® treatments at Big Lake.