Food web changes over fourteen years following introduction of rainbow smelt into a Colorado Reservoir

摘要

Rainbow smelt, Osmerus mordax, were introduced into Horsetooth Reservoir, Colorado, in 1983 to increase prey availability for walleyes, Stizostedion vitreum. The introduction was highly successful. Rainbow smelt abundance reached at least 0.4 fish/m3 within 6 years, and walleye growth improved by 50%. Zooplankton sampling provided the first clues that the Horsetooth Reservoir food web was undergoing dramatic changes in response to the rainbow smelt introduction. During 1989-1994 the abundant rainbow smelt population apparently reduced April and May crustacean zooplankton concentrations from historical levels of 40-80 organisms/L to less than 1.0 organism/L, and a switch occurred in the cladoceran species composition. Standardized sampling with beach seines and gill nets indicated that after 1988 walleye recruitment ceased. Efforts to bolster walleye recruitment by fry stocking in 1992 and 1993 also failed. A recent decrease in rainbow smelt density allowed a resurgence of zooplankton and a shiftin predominance back to a large-bodied cladoceran (Daphnia pulex) in 1995 and 1996. Fish sampling in subsequent years corroborated declines in rainbow smelt abundance and walleye condition. Although hybrids of striped bass Morone saxatilis X white bass M. chrysops have been stocked in recent years, in part to control an overabundant rainbow smelt population, hydroacoustic surveys and bioenergetics modeling suggested that, during 1994-1996, walleye predation alone could have been a significant mortalityfactor limiting the rainbow smelt population biomass. Thus, we expect further declines in rainbow smelt biomass with possible negative consequences for piscivore growth rates and predation rates on prey-sized sport fish. To manage for balance of prey supply and predator demand, regular estimates of the zooplankton, rainbow smelt, and piscivore populations must be obtained.