Cost-efficient allocation of ship measures and harvest of aquatic invasive species – An application to invasive crabs on the west coast of Sweden

The purpose of this study was to identify cost-efficient combinations of control measures (harvest of established invaders) and prevention measures (ballast water treatment and antifouling to prevent invaders) to achieve targets for the maximum population sizes of two invasive crabs, the Asian shore crab (Hemigrapsus sanguineus) and brush-clawed shore crab (Hemigrapsus takanoi), in interconnected water basins on the west coast of Sweden. To this end, a spatial bio-economic model was developed using transect methods to quantify population sizes and an ocean circulation high-resolution coastal model constructed to estimate connectivity between the water basins. The results showed that both harvest and vessel treatment measures offer cost-efficient solutions, but their optimal levels and timings depend on the choice of spatial target for acceptable population sizes. The costs can be high if increases in populations are to be avoided, but these costs are doubled when the target is to eradicate the populations. The results were also sensitive to parameter values in the population dynamics and cost functions, and to assumptions involved in policymakers' decisions about the targets to be achieved.