Pest inquiry

The Asian paddle crab


Introduction

Te Awanui or Tauranga Harbour is a regional treasure. It is one of New Zealand's largest natural harbours and is home to many different native species. The harbour is under threat though, the recent proliferation of the Asian paddle crab has disturbed the ecosystem. As a result, our native mud crab and many other species are under threat. We need to find an effective method to mitigate the harm caused by the Asian paddle crab species in the Bay of Plenty. Asian paddle crabs are aggressive and have the potential to out-compete native crabs and other benthic species for habitat and food. This inquiry focuses on ways to potentially mitigate the effects of the Asian paddle crabs.

The Asian paddle crab is a swimming crab native to Southeast Asia. The Asian paddle crab is relatively large with paddle-like hind legs. Adults have a shell width of around 12 with six distinct spines or spikes on each side of their eyes. They also have a large claw which can be quite painful if it nips you. It ranges in colour from pale green to olive green, to a deep chestnut brown with purplish markings on the shell. Most of the crabs found in the Waitemata Harbour tend to have yellow-orange and brown-orange markings on the shell and legs with white tips on their claws. In New Zealand they prefer to hide in estuaries with firm sand or muddy fine sand of depths of up to 15m. Native paddle crabs nest and hunt wherever there are sweeping sandy beaches and they like areas with some current, like on the Mount Maunganui beaches in Tauranga. Native paddle crabs are not pests and are hunted by recreational fishers.

The Asian paddle crab most likely spread because of human actions, either as a larvae in a water ballast or as hull fouling. The Export/Import nature of the Port of Tauranga probably meant ships from Japan, Korea or Malaysia were the source. Asian paddle crabs can lay up to a hundred thousand eggs and this would be the reason why it was very easy for some eggs to attach or get stuck to hull fouling. It most likely spread to New Zealand accidently. This species has a number of traits that make it particularly invasive, a long larval life that facilitates spread, rapid growth to maturity, high reproductive rates, high environmental tolerance and a broad diet.

In New Zealand the Asian paddle crabs main predators include Snapper, Dogfish and Stingray. The Asian paddle crabs eat shellfish and other small fish. Larger crabs are also known to eat smaller paddle crabs including our native species. Asian paddle crab are a more dominant species than or native paddle crabs, out competing for the same food sources, therefore creating an imbalance in the food chain. It is hard for the prey of the Asian paddle crab because the Asian paddle crabs are very aggressive and very good hunters with a key aspect of stealth.

The Asian paddle crabs outcompete our Native species. It has many predators but it is a intelligent crab. Which means it avoids predators quite easily. This means the Asian paddle crabs predators will hunt other species including our Native Species. However the Asian paddle crab has the ability to compete with some of its own predator's prey which can really affect the food chain in the environment. Adult Asian paddle crabs can produce up to a hundred thousand offspring annually and it is thought that reproduction is limited to seawater temperatures of over 20 °C. Larvae are relatively long-lived and can survive for three to four weeks potentially facilitating spread to new areas. Adults are also capable of swimming large distances.

It was first discovered in New Zealand in 2000 and was declared a pest. In March 2018 efforts were ramping up to detect Asian paddle crabs with more than 600 traps being laid across the Bay of Plenty. However the crabs had rather strong claws so many escaped from these traps with new ones having to be made strong enough for the Asian paddle crab. This invasive species has also been found in the Hauraki Gulf.

Way to mitigate the spread of the Asian paddle crab. BOP Marine bio service is making sure we are cleaning our boat’s hull – ideally keep fouling growth to no more than a light slime layer. The application of antifouling paint to keep boat hulls in good condition can stop the spread of Asian paddle crab larvae and eggs. Ensure your hull is clean and free of fouling before you travelling to new areas. Clean and dry any marine equipment (e.g. ropes, lines and pots) before using in a new area. Inspect areas on your boat that retain water for signs of marine life. Check for aquatic weeds tangled around anchors, trailers and other equipment.

The Asian paddle crab is a more dominant species, than our native paddle crabs. It disrupts the natural food web as predators such as Snapper, Dogfish and Stingrays find it harder to kill. Its aggressive nature and elusiveness mean predator find it a harder target. Overseas cases where the Asian paddle crab has been introduce, populations have not been as rapid to establish due to a more diverse range of predator populations.

Many members of the Tauranga Marine Biosecurity team are now enforcing the use of antifouling paints and insisting that people make sure that their hull is in a good condition before entering Tauranga harbor. These things can stop young Asian paddle crabs from moving to new locations and causing more harm to our native species. The Tauranga Marine biosecurity team believe it will be hard to eradicate the Asian paddle crab.

However in order to manage the population level, we could encourage the community to harvest the Asian paddle crab. Our research has shown that the Asian paddle crabs are good eating, and considered a delicacy in South East Asia. If we are able to encourage recreational/commercial harvesting of the Asian paddle crab, we may be able to manage the population. We even believe there maybe a potential local/export market back into South East Asian community. Due to the limited data and research we were unable to establish the feasibility of this idea.

Overall there is no easy solution to eradicate this invasive species, he best we can hope for is to delay the spread to other parts of New Zealand. Also more research is require to find a viable solution to manage the population of Asian paddle crabs in New Zealand, that allows natives species to survive. Hopefully this essay has informed you on the Asian paddle crab.