1997 Black Lake Survey: A Coastal Warmwater Fish Community Before the Introduction of Grass Carp

Black Lake is a shallow (mean and maximum depth = 4.0 and 9.8 m, respectively), L-shaped body of water (surface area = 13.3 ha) located north of the City of Ilwaco on the Long Beach Peninsula in Pacific County, Washington. Precipitation and groundwater feed the lake, whereas surface water flows out a drainage channel at the north end, through Tarlett Slough, eventually discharging into Willapa Bay. Nearshore residential development at Black Lake is minimal; however, Highway 101 runs along the west shore, while a cranberry farm located along the north shore occasionally draws water from the lake for irrigation and harvest purposes. Much of the eastern shoreline consists of a park with primitive boat launch and trail system that allows public access to the lake. Not long ago, a public pier was constructed over the lake off Highway 101. Recreational activities include fishing for warmwater species and trout (the lake is stocked annually with rainbow trout, Oncorhynchus mykiss) and small watercraft use. No motorboats are allowed on Black Lake.

In recent years, the aquatic plant community of Black Lake has become dominated by invasive Brazilian elodea (Egeria densa). In 1999, exotic Eurasian watermilfoil (Myriophyllum spicatum) was discovered at the primitive boat launch along the east shore. Other aquatic plants include ribbon-leaf pondweed (Potamogeton epihydrus), yellow water-lily (Nuphar polysepala), and a variety of emergent types that rim the shoreline (Jenifer Parsons, Washington Department of Ecology, personal communication).

Concern about the invasion of Brazilian elodea led the City of Ilwaco to explore management options available to control the errant vegetation in Black Lake. The city settled on the use of sterile, triploid grass carp (Ctenopharyngodon idella) as a way of reducing the standing crop of Brazilian elodea. Since the mid-1980’s, it has been shown that grass carp can 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). On July 22, 1997, the Washington Department of Fish and Wildlife (WDFW) issued a permit to the City of Ilwaco to stock sterile, triploid grass carp into Black Lake. During September-October 1997, 150 grass carp measuring 229 to 254 mm (9 to 10”) total length each were released into the lake at a density of 11 fish per hectare.

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). A diverse, thriving 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). 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. In an effort to assess its warmwater fishery, personnel from WDFW’s Warmwater Enhancement Program conducted a fisheries survey at Black Lake in late summer 1997. Since it was gathered before the introduction of grass carp, the baseline information presented here will be useful when monitoring the long-term effects of grass carp herbivory at the lake.