1998 Lake Cassidy Survey: A Warmwater Fish Community competing under Conditions of Hypolimnetic Anoxia and Dense Aquatic Macrophytes

Lake Cassidy is a small (surface area = 48 hectares), shallow (maximum depth = 6 meters), seasonally eutrophic body of water located directly east of Marysville in Snohomish County (Figure 1). The lake lies in the Catherine Creek watershed and forms the headwater of Catherine Creek. Water quality is characterized by thermal stratification and anoxia in the hypolimnion during the late summer (Figures 2 and 3) (Drainage Improvement District #8 1998). Based upon Carlson’s Trophic Status Index (TSI) which uses total phosphorus and chlorophyll a data to rate a lake’s trophic status, Lake Cassidy is an upper mesotrophic body of water (Carlson 1977).

Although algal production appears to contribute significantly to microbial respiration and subsequent anoxia in the hypolimnion, much of the productivity of Lake Cassidy and hypolimnetic anoxia probably result from littoral macrophyte growth and decomposition. Over 65 percent of the shoreline to the two meter bathymetric contour is densely vegetated with yellow water lily (Nuphar polysepalum), coontail (Ceratophyllum demersum), and bullrush (Scirpus sp.). The remaining shoreline is moderately vegetated with these and other aquatic plant species (Jenifer Parsons, Department of Ecology, unpublished data).

Lake Cassidy’s shallow basin, low altitude and relief, and largely undeveloped shoreline should provide good habitat for warmwater fish species which have formed the basis for a popular sport fishery there. Habitat suitability and popularity of this warmwater fishery led to a 1982 survey of the warmwater community by the Washington Department of Game (WDG) which found slow growth and poor condition in populations of largemouth bass (Micropterus salmoides), black crappie (Pomoxis nigromaculatus), and pumpkinseed sunfish (Lepomis gibbosus). In 1984, a slot limit was imposed on largemouth bass to increase recruitment to larger size classes, and cursory sampling indicated that growth rates for largemouth bass began improving throughout the 1980s. Meanwhile, the popularity of the warmwater sport fishery has continued to develop. A 1993 angler survey of 354 anglers found that largemouth bass and black crappie were the dominant target species with 32.7 and 20.4 trips/acre, respectively. Anglers seeking to catch any species made up 36.7 trips/acre and trout anglers made up 4.4 trips/acre (Curt Kraemer, WDFW, unpublished data).

Due to Lake Cassidy’s history and popularity as a warmwater fishery, the Washington Department of Fish and Wildlife (WDFW) Warmwater Program conducted a detailed survey of the warmwater fish community in late summer 1998. Our goal was to detect changes in community structure, growth and condition of warmwater fish, assess previous management strategies, and form a basis for other management options that might increase warmwater fishing opportunities in the lake.