Assessing ballast treatment standards for effect on rate of establishment using a stochastic model of the green crab

This paper describes a stochastic model used to characterize the probability/risk of NIS establishment from ships’ ballast water discharges. Establishment is defined as the existence of a sufficient number of individuals of a species to provide for a sustained population of the organism. The inherent variability in population dynamics of organisms in their native or established environments is generally difficult to quantify. Much qualitative information is known about organism life cycles and biotic and abiotic environmental pressures on the population, but generally little quantitative data exist to develop a mechanistic model of populations in such complex environments. Moreover, there is little quantitative data to characterize the stochastic fluctuations of population size over time even without accounting for systematic responses to biotic and abiotic pressures. This research applies an approach using life-stage density and fecundity measures reported in research to determine a stochastic model of an organism's population dynamics. The model is illustrated with data from research studies on the green crab that span a range of habitats of the established organism and were collected over some years to represent a range of time-varying biotic and abiotic conditions that are expected to exist in many receiving environments. This model is applied to introductions of NIS at the IMO D-2 and the U.S. ballast water discharge standard levels designated as Phase Two in the United States Coast Guard’s Notice of Proposed Rulemaking. Under a representative range of ballast volumes discharged at U.S. ports, the average rate of establishment of green crabs for ballast waters treated to the IMO-D2 concentration standard (<10 organisms/m) is predicted to be reduced to about a third the average rate from untreated ballast water discharge. The longevity of populations from the untreated ballast water discharges is expected to be reduced by about 90% by treatment to the IMO-D2 concentration standard.