Research Document - 2011/113
Modelling Spread, Establishment and Impact of Bighead and Silver Carps in the Great Lakes
By W.J.S. Currie, K.M.D. Cuddington, T.J. Stewart, H. Zhang, and M.A. Koops
Bighead Carp (Hypophthalmichthys nobilis) and Silver Carp (Hypophthalmichthys molitrix), two species of Asian carps, have rapidly spread up the Mississippi River system after they escaped from aquaculture facilities in Louisiana and Arkansas, and could now potentially spread into the Great Lakes. Since both species are generalist planktivores, they present a unique invasion risk in that they are very large (>1m), fast-growing fishes that eat at low trophic levels. Predicting invasion probability into novel systems is often difficult because predictions must be based on a foundation of research from systems other than the one being invaded. Most existing research has been undertaken in aquaculture ponds and river systems, which are quite different from large lakes in their thermal regimes, physico-chemical dynamics and food webs. To overcome these difficulties, modelling approaches must be general enough to encompass the wide spectrum of parameters that drive them. Several modelling methodologies are presented to predict the risk of spread, establishment and ecological impact. First, an area-restricted individual-based model is used to predict spread rates and regions likely to attract carps when released from the most likely invasion sites. At even modest movement rates, carps can easily move throughout an entire lake basin within a year making it quite likely they will locate high productivity environments which have abundant food resources for survival (typically embayments such as Green Bay, Saginaw Bay, Bay of Quinte, Georgian Bay). Second, combinatoric mathematics is used to predict the number of adults needed within a basin to have a likely mating pair. The probability of mating is dependant on the number of adults and rivers. A high probability (>50%) of a successful mating requires very few adults (< 10), even if there are a large number of suitable rivers (e.g., 25). Third, an age-structured population model is used to predict the risk of establishing a population and to determine the most vulnerable stage. This approach indicates even a single release of a small number of individuals can result in an established population; the risk increases if mature adults are released or if a “leaky barrier” continually supplies a small number of new individuals. The juvenile stage is the most responsive to management/control activities and age at first maturity has the largest impact on establishment probability. Finally, the ecological impact of an established population is investigated using two mass-balanced food web models for the Lake Ontario offshore food web using high and low dreissenid mussel biomass. The predicted impacts are particularly sensitive to the degree of zooplanktivory. If the carps only eat phytoplankton and the microbial food web, then high Asian carp biomass could be sustained, even in offshore Lake Ontario, but would lead to increased risk of Alewife decline with a subsequent collapse of the Chinook Salmon fishery. Low carp biomass could be maintained with minimal food web consequences, but if zooplanktivory increases, then even small carp populations can lead to a significant decline in Alewife biomass. Thus, disruption of the pelagic food-web by Bighead and Silver carps is highly unpredictable given their extremely flexible diet. These modelling approaches predict that these Asian carps have the potential to spread rapidly through the Great Lakes, establish populations even with small initial abundances, and ultimately disrupt Great Lakes food webs.
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