1997 Silver Lake Survey: The Forage Fish Community after Removal of Aquatic Vegetation by Grass Carp

Silver Lake is a large, eutrophic body of water [surface area = 660 hectares (ha); mean depth = 1.5 meters (m); max depth = 2.5 m] located about 40 kilometers (km) west of Mount St. Helens, between the Cowlitz and Toutle Rivers, in Cowlitz County. The lake is fed by Sucker Creek to the south, Hemlock Creek to the east, rainfall, and groundwater. In addition, two small creeks flow into the lake just west of Hemlock Creek. Surface water exits the lake through a man-made outlet dam located at the east end of the lake. The dominant emergent aquatic plants are yellow waterlily (Nuphar polysepala), bogbean (Menyanthes trifoliata) and, to a lesser extent, watershield (Brasenia schreberi). There are few, if any, submersed aquatic plants in Silver Lake. Details of these characteristics can be found in Parsons (1998) and a recent study (KCM 1998) prepared for the Cowlitz County Department of Community Development.

Until recently, Silver Lake was threatened by excessive submersed aquatic plant growth, including the spread of invasive South American waterweed (Egeria densa) and Eurasian watermilfoil (Myriophyllum spicatum) (KCM 1998; Parsons 1998). For 20 years, dense aquatic macrophyte cover compromised the aesthetic quality of the lake and hindered most water-based recreational activities. These included boating, water skiing, and sport fisheries for warmwater species such as largemouth bass (Micropterus salmoides)(KCM 1998).

In 1990, researchers from Washington State University proposed the introduction of sterile grass carp (Ctenopharyngodon idella) into Silver Lake to reduce the biomass of submersed aquatic plants (KCM 1998 and references therein). Accordingly, 83,000 triploid grass carp (size range: 200 - 250 mm long) were stocked in the lake at a density of about 124 fish ha during May - June -1 1992 (Scherer et al. 1995). A monitoring program was established and, for the next five years, a private contractor studied the impact of the herbivorous fish on Silver Lake’s ecosystem, including changes in the resident warmwater fish community (KCM 1998). By 1994, little, if any, submersed aquatic vegetation was detected in the lake (Scherer et al. 1995).

Currently, much information exists regarding the largemouth bass population before and after stocking grass carp into Silver Lake [Lucas 1986; KCM 1998; John Weinheimer, Washington Department of Fish and Wildlife (WDFW), personal communication]. And although Lucas (1986) characterized the yellow perch (Perca flavescens) and bluegill (Lepomis macrochirus) populations before stocking grass carp, there is little or no information regarding the forage fish community after stocking grass carp. Given its physical characteristics, Silver Lake is well suited for these species. Therefore, in an effort to monitor the success of the restoration program, as well as improve the warmwater fishery at the lake, personnel from the WDFW’s Warmwater Enhancement Program conducted a fisheries survey of Silver Lake during the fall of 1997.