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A MODEL FOR STOCKING GRASS CARP IN COLORADO LAKES AND PONDS ERIC D. SWANSON AND ERIC P. BERGERSEN COLORADO COOPERATIVE FISH & WIlDLIFE RESEARCH UNIT COLORADO STATE UNIVERSITY FORT COLLINS, COLORADO 80523 ABSTRACT This grass carp stocking model was designed for a wide variety of users ranging from fishery biologists to private pond owners in Colorado. It is a decision model employing various physical, biological and management attributes to generate a recommended grass carp stocking rate for lakes and ponds. The model is based on a three-year graduate research project coupled with the best available information from the grass carp literature. The model was designed to be simple, yet comprehensive in considering the key factors which influence the effectiveness of grass carp in vegetation control. Four key attributes were selected as factors bearing the greatest influence on grass carp vegetation consumption: water temperature, aquatic vegetation density and distribution, vegetation species, and disturbance factors. Through a step-wise procedure, these attributes are assigned values. In addition to the four attributes, the model considers the management objective for the lake, the size of grass carp to be stocked and whether grass carp are diploid or triploid. The important role of aquatic vegetation in lentic ecosystems is emphasized. recommended levels of aquatic vegetation distribution are 10-25% of a lake's basin area. Lake management objectives dictate the target level of vegetation distribution. For sport fishery management, a 15-25% level is recommended; for multiple use management, 10-15%; and for water storage or aesthetics, 0-10%. Grass carp stocking rates (number per acre) recommended by the model consider consumption, growth, mortality, and behavioral characteristics of fish over time. To maintain target vegetation levels requires a balance of vegetation consumption and production over time. This model suggests a 3-4 year period is required to achieve and maintain regulated vegetation levels using grass carp. INTRODUCTION Aquatic vegetation plays an important role in lake ecosystems. It provides substrate and food for invertebrate communities, cover and spawning substrate for fishes, and food and shelter for waterfowl and other wildlife. Therefore, if a lake is to be managed for sport fishing, it is important to retain some aquatic vegetation. A .recommended level of aquatic vegetation distribution is 10-25% of a lake's basin area. This aquatic vegetation should be present in sparse to moderate densities and generally not emergent. At this regulated level, the benefits of aquatic vegetation in lakes are maintained while detrimental impacts, both biological and recreational, are minimized. 34