Adaptive Multi-scale Sampling to Determine an Invasive Crab’s Habitat Usage and Range in New Zealand

Patterns of local abundance and geographical distribution are often prime correlates of invasive species’ impacts on native ecosystems. Here we adaptively increased the spatial scale of delimitation surveys to determine the local abundance, range and habitat associations of the introduced portunid Charybdis japonica (Milne-Edwards, 1861) in New Zealand. The crab was first discovered in Auckland’s Waitemata Harbour in September 2000, and by April 2002 trapping surveys revealed the invader had spread widely throughout the Harbour. Experiments using three deployment times (1, 3 and 24 h) optimized detection rates prior to larger scale geographic surveys that defined the range of the introduced population. We surveyed >300 sites in coastal waters within the predicted range of larval dispersal and then 14 major shipping ports throughout New Zealand. C. japonica was abundant in the Waitemata Harbour and present in two nearby estuaries, but there was no evidence of spread to other shipping ports nationwide. Subtidal habitat associations were explored in the main area of infestation which indicated that the invader occupied a range of substrata from fine, silty muds to coarse, shelly sands. Although its distribution overlaps with the native portunid crab Ovalipes catharus, the invader was more abundant throughout Waitemata Harbour and occurred in muddy sediments where native portunids are rare. It is not yet clear whether the C. japonica population in New Zealand is self-sustaining, however if it persists and continues to spread, it is likely to have significant impacts on native estuarine benthic assemblages.