Since the 1970, sterile grass carp have been transported throughout the United States as a means of macrophyte control, while keeping grass carp from becoming established and invasive. However, recent wild captures of reproductively viable, diploid grass carp in Great Lakes’ rivers indicate there is some risk of population growth and establishment. Yet the majority of wild captures are presumably of escaped sterile carp, and the ultimate fate of population establishment is largely unknown due to mating interference. Using a hierarchical, stochastic population model with a mating function built to reflect the known mate finding behavior of grass carp, we estimate the probability of population establishment as a function of the population abundance, ratio of sterile to reproductively viable individuals, survival, and juvenile recruitment. We then use life history parameter values for grass carp from the literature to evaluate the levels of interference from sterile carp and the introduction effort necessary to overcome demographic stochasticity and the Allee effect that lead to grass carp establishment.
Results/Conclusions
In the absence of sterile individuals, the mating behavior of grass carp results in a weak Allee effect, and the probability of establishment is driven primarily by demographic stochasticity. However, if sterile individuals dominate a population, the Allee effect becomes more influential and the probability of establishment at low population size approaches zero. As such, the presence of wild, but sterile grass carp, which accidentally escaped into the wild, is acting as a barrier to grass carp establishment. However, in light of recent captures of reproductively viable individuals in the Great Lakes, it is possible that grass carp are beginning to establish. The model supports the use of reliably sterile individuals as a mechanism to prevent invasive species establishment by increasing the strength of Allee effects, particularly for grass carp.