Management of an invasive marine species: defining and testing the effectiveness of ballast-water management options using management strategy evaluation

Invasive marine and fresh-water species are being spread around the world in ships' ballast water, damaging industries and natural resources. Management policies are being developed nationally and internationally in response to the threat, but these options are not being rigorously evaluated for their potential to meet management objectives. We used management strategy evaluation (MSE) simulation to compare the performance of different management rules for controlling the spread of an invasive sea star, Asterias amurensis, around the southern coast of Australia. A model incorporating population dynamics, oceanographic patterns, and vessel movement was developed to compare the performance of different ballast-water exchange rules at reducing the likelihood of new populations establishing at locations along the coast over time. Static management rules, where ballast exchange was mandated on all voyages, reduced the median likelihood of new invasions from 0.67 with no ballast control to between 0.36 and 0.42 as distance from the coast was varied. Reducing the volume of high-risk ballast water by 95% did not reduce the likelihood of invasion by 95%, but by an average of 21%. Exchanging ballast farther from the coast did not reduce the likelihood of invasion for any of the static management rules. Feedback management rules using a port monitoring programme to assess the risk of transporting larvae between ports were at least as effective as the static rules, but at a significantly reduced cost for this single-species example. MSE provides a method to compare management options against objectives in this uncertain environment, and can be used to evaluate new and expensive treatment options for their effectiveness and value.