Recently introduced Asian paddle crab invades Waitemata Harbour
Science Centres: Aquatic Biodiversity and Biosecurity
A NIWA survey of Auckland’s Waitemata Harbour in April this year uncovered large numbers of non-indigenous paddle crabs. The survey is the first major study of this newly introduced crab species to New Zealand waters.
This invasive crab first attracted scientific attention in November 2000 when two fishers caught them in their flounder nets in the Rangitoto channel near Auckland. They noticed that these paddle crabs had different markings, and were far more aggressive than the native paddle crabs. They sent specimens to New Zealand’s national museum, Te Papa, in Wellington, where they were identified as a Charybdis species (probably japonica) whose distribution was previously restricted to the coastal regions of China, Korea, Taiwan, and Malaysia. It seems that a population of Charybdis has become established in the harbour in the 2 years since they were first discovered.
There are a number of possible ways this invader could have reached New Zealand, but the most likely is that it arrived in the ballast water or ballast intake pipes of ships. It could also have arrived here as larvae or juveniles in fouling communities on vessels’ hulls or, and less likely, as planktonic larvae from established populations up-current of New Zealand. We may never know for sure how it got here. Now that it is here, however, there are a number of questions we must answer. For example, will the crab continue to survive in New Zealand, and, if so, what effects will it have on existing marine communities? How abundant and widespread will the crab become? And how will it impact on our native biodiversity? We can answer these questions only if we understand the biology of the species and how it interacts with its new environment.
The crabs in our April survey were caught by using two types of baited traps, and were found at over 70 locations from the upper Harbour Bridge to North Head. They were trapped in a variety of habitats throughout the harbour at depths of between 0.5 and 12 m (although most were found at depths of less than 6 m). There are now about twice as many of these invasive paddle crabs as native paddle crabs in Waitemata Harbour, and they are more widely spread. It is possible that this species is in direct competition with our native paddle crab and is replacing it in some habitats. We are currently investigating this and a range of other impacts.
Given their rapid spread throughout Waitemata Harbour, and possibly further into the Hauraki Gulf, it seems that these crabs are well established. Research by our marine biosecurity group has begun to assess the impacts of this crab, which has potentially serious consequences for the native biodiversity and marine ecology of the Auckland region.