1997 South Lewis County Regional Park Pond Survey: The Warmwater Fish Community Before Implementing Biological Control of Nuisance Aquatic Vegetation and Overproduction of Forage and Non-game Fish

South Lewis County Regional Park Pond (Figure 1) is a small body of water [surface area = 7.1 hectare (ha) or 17 acres] that lies within a severely sensitive aquifer recharge area of the Cowlitz River drainage basin. The pond is located just off the Cowlitz River, approximately 0.8 km south of the City of Toledo, Washington. Springs and subterranean flow from the river feed the pond, which is subject to periodic flooding. There are no outlets. Average annual rainfall in the area is 102 to 127 cm (40 â€" 50”). The shoreline environment is primarily urban. Historically, the pond was used as a gravel borrow pit by a local contractor. In 1987, Lewis County leased and purchased land around the pond [17.9 ha (43 acres) including the pond] in order to develop South Lewis County Regional Park. The park is open from March 1 through October 31; activities include fishing [the pond is stocked regularly with rainbow trout (Oncorhynchus mykiss) and brout trout (Salmo trutta)], swimming, and picnicking. The county maintains a public boat launch, fishing piers, a walking path, and various play areas.

During the 1990’s, the aquatic plant community of South Lewis County Regional Park Pond became dominated by invasive Eurasian watermilfoil (Myriophyllum spicatum). Other aquatic plants include native pondweeds (Potamogeton sp.) [Bill Wamsley, Lewis County Noxious Weed Control Board (NWCB), personal communication]. Concerns about the invasion of Eurasian watermilfoil led NWCB to explore management options available to control the errant vegetation. On August 7, 1995, the county settled on the use of sterile, triploid grass carp (Ctenopharyngodon idella) as a way of reducing the standing crop of Eurasian watermilfoil. Since the mid-1980’s, grass carp have been shown to be a cost-effective aquatic plant management tool in the Pacific Northwest (Pauley et al. 1994). In Washington, public satisfaction concerning the use of grass carp has been moderate to high (Bonar et al. 1996). Subsequently, on February 1, 1996, the Washington Department of Fish and Wildlife (WDFW) issued a permit to NWCB to stock up to 170 sterile, triploid grass carp into South Lewis County Regional Park Pond. On June 7, 1996, 100 grass carp ranging from 254 to 305 mm (10 â€" 12”) total length (TL) were released into the pond. Four months later, an additional 70 grass carp were released for a total stocking density of 24 fish/ha (10 fish/acre).

Beside aquatic vegetation problems, South Lewis County Regional Park Pond harbors populations of undesirable, non-game fish such as northern pikeminnow (Ptychocheilus oregonensis) and largescale sucker (Catostomus macrocheilus). Furthermore, warmwater species such as largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) tend to overpopulate the pond resulting in crowded, stunted fish populations. It is unknown whether this is because of natural or anthropogenic factors. However, it is clear that adequate predation is lacking in order to control overproduction of the non-game and forage fish populations. Consequently, in 1996, biologists from WDFW recommended stocking a hybrid, apex predator - sterile, yearling tiger muskellunge (Esox masquinongy �" E. lucius) - as a way of decreasing the number of non-game species and superabundant, small warmwater fishes (Jack Tipping, WDFW, personal communication). In May 1997 and each spring thereafter, 20 tiger muskellunge were released into the pond by WDFW hatchery personnel for a stocking density of 2.8 fish/ha (1.2 fish/acre). Each fish measured approximately 305 mm (12”) TL and weighed 114 g (1/4 lb).

If the introductions of grass carp and tiger muskellunge into South Lewis County Regional Park Pond are successful, the subsequent changes in the aquatic plant community and species composition of fishes will undoubtedly affect the sport fish community. Whether this impacts the fisheries of South Lewis County Regional Park Pond positively or negatively remains to be seen. 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). Stocking predators can alter the size structure of stunted, prey fish populations (Boxrucker 1992), while a decrease in non-game fish numbers can restore a desirable sport fish community (Wingate 1986; Tipping 1996, 1999). For these reasons, it is important to gather baseline information and carefully review all proposals to limit or control aquatic vegetation and nuisance fish species for a given lake, especially when the lake supports popular fisheries. In an effort to assess its warmwater fish community, WDFW personnel conducted fisheries surveys at South Lewis County Regional Park Pond during spring and fall 1996, and again during fall 1997. Since it was gathered shortly after the introductions of grass carp and tiger muskellunge, the baseline information presented here will be useful when monitoring the long-term effects of these species at the pond.