1999 Green Lake Surveys: Aspects of the Biology of Common Carp with Notes on the Warmwater Fish Community

To many Seattle residents and visitors, Green Lake is the crown jewel of one of the most popular urban recreation areas in the Northwest. However, throughout much of the latter 20th century, poor water quality hindered recreational opportunities at the lake. For example, during late early summer 1999, the Seattle Department of Parks and Recreation (SDPR) temporarily banned all water-based activities because of a toxic, blue-green algae bloom (Fries 1999). As its name implies, Green Lake is no stranger to algal blooms. However, the late early summer 1999 event was disturbing because it was the first to pose a threat of illness despite SDPR's efforts in the early 1990s to reduce the nutrient (primarily phosphorus) inputs that fuel such blooms. KCM (1995) suggested that the two main impediments to improved water quality at Green Lake were the dense stands of invasive Eurasian watermilfoil (Myriophyllum spicatum) and the prolific common carp (Cyprinus carpio). For example, seasonal senescence and decay of aquatic vegetation contribute to the release of phosphorus, as do high pH values that arise from photosynthesis in macrophyte stands (Scheffer 1998). Common carp elevate nutrient levels through normal digestive processes (e.g., excretion) and bioturbation of sediments (Lamarra 1975, cited in Bradbury 1986; Panek 1987; Scheffer 1998).

A variety of government-approved algi-, herbi-, and piscicides are commercially available for lake and sport fishery restoration. For example, from 1950 through 1972, the Washington Department of Fish and Wildlife (WDFW), formerly the Washington Department of Game (WDG), routinely treated Green Lake with the natural, plant-derived piscicide, rotenone, to eradicate nuisance species that tended to overpopulate and compete with desirable sport fish. In 1991, SDPR treated the lake with buffered aluminum sulfate (a.k.a., alum) to precipitate phosphorus from the water column and to prevent release of sediment phosphorus that fuels nuisance algal blooms. However, in recent years, SDPR discontinued the use of chemical control measures in managing Green Lake and other parks. Subsequently, since 1993, Green Lake restoration efforts have been limited mostly to seasonal removal of submersed aquatic vegetation using a mechanical harvester. The introduction of herbivorous grass carp (Ctenopharyngodon idella) was also proposed as a cost-effective alternative to herbicidal control of Eurasian watermilfoil (KCM 1995; Mueller 1998). Still, no effort has been made to manage the common carp population.

Common carp belong to the minnow family (Cyprinidae). Like many introduced aquatic species, the origin of the Green Lake stock remains enigmatic. A California entrepreneur and the U.S. Fish Commission first imported common carp from Europe into the continental United States during the 1870s. At the time, resource managers were convinced that the "fish of the masses" would augment commercial and recreational freshwater fisheries throughout the country. An ambitious, widespread stocking program ensued. Within 25 years of being transplanted, common carp had become naturalized in most of the major watersheds of the Midwest, eastern seaboard, and Pacific Coast. Their rapid expansion is a testament to their reproductive potential and suitability to a wide variety of habitat types (Fritz 1987).

Common carp were well ensconced in Washington State by the 1890s (WDFW 1999); but it was not until the 1990s that the fish became a problem at Green Lake (KCM 1995; Mueller 1998). WDFW records indicate that common carp were not observed during the period from 1950 to 1972 when the lake was customarily treated with rotenone. However, by early summer 1983, a WDG biologist noted their presence for the first time when conducting a routine fish survey. Within ten years, common carp underwent a population explosion to become the dominant fish in Green Lake (Table 1). Spot checks during 1994 and 1995 suggested a population in decline (KCM 1995). Nevertheless, a comprehensive survey conducted by WDFW in fall 1997 revealed that common carp were indeed dominant, at least in terms of biomass (Mueller 1998).

Little information exists concerning the common carp population of Green Lake. That which is available can be found in two recent technical reports (KCM 1995; Mueller 1998) and WDFW management files. The successful management of this population (and ultimately the water quality of Green Lake) hinges on a better understanding of their biology and ecology. Moreover, common carp may affect angler catch rates of, and compete or interfere with, desirable warmwater sport fishes (Forester and Lawrence 1978; Drenner et al. 1997). To this end, personnel from WDFW's Warmwater Enhancement Program conducted surveys of Green Lake's warmwater fish community, with an emphasis on common carp, during early summer and fall 1999. Furthermore, since it was gathered before implementing additional management activities, the information presented here will be useful when monitoring the long-term effects of future restoration efforts.