Introduction
The Asian Paddle Crab is native to Southeast Asia and is normally found in the waters of Japan, Korea, and Malaysia. It was first detected in New Zealand in 2000 and is now found in the Tauranga, Waitemata, and Whangarei harbours, as well as the Waikare inlet in Northland. The Asian Paddle Crab lives in intertidal to sub-tidal, estuaries, the land between tides, and mudflats.
The Paddle Crab is known for its two back legs that look like paddles, which they use to swim much faster than other crabs, to catch prey much easier, and also to burrow in the sand to hide.k
In January 2018, this aggressive crab was found for the first time off the bridge in Matapihi, causing concerns for the Tauranga Harbour and the native crab population. If we do not stop the spread of this crab or keep numbers under control, our native species would be under threat.
What separates the Asian Paddle Crab from other types of Crabs?
The Asian Paddle Crab is different from other crabs as it has 6 spikes on each side of its body with 5 extra spines on each claw. Their last pair of walking legs are like paddle blades that make it easier to launch themselves through the water, letting them swim faster. These legs also serve as spades when the crab digs backward into the sand to hide and supports them when they are digging up and opening shellfish.
Having a set of paddles for back legs is great for the Asian Paddle crab as it allows it to swim further and faster than other crabs. Most crabs make the intertidal zone (the land between tides) their home because they cannot swim out to deeper water. This is no challenge for the Paddle Crab - their paddles give them the ability to safely swim in waters up to 10 meters deep.
Being fast in the water means they can escape most predators and catch more prey, feeding their large appetite with wider areas to hunt for food.
Where are Asian Paddle Crabs found and what do they feed on?
You can find Asian Paddle Crabs off sandy beaches, harbors, mudflats, and estuaries, where there is firm sand or muddy, fine sand. You’ll find them from low tide to where water is 10m deep.
Asian Paddle Crabs have a huge appetite, feeding mostly on shellfish like pipi, tuatua, and cockles. During the winter when there is not much food, large Paddle Crabs will eat smaller paddle crabs and will also attack and eat larger crabs that have shed their shells.
How do they Breed?
The breeding of the Asian Paddle Crab is surprising because the reproduction rate of the female Asian Paddle Crab is around 85,000 larvae several times a year. Male Asian Paddle Crabs can produce hundreds of thousands of offspring annually. To produce offspring, the Asian Paddle Crab has to be in a water temperature of over 20 degrees. Female Asian Paddle Crabs can store sperm and carry multiple broods annually. Asian Paddle Crabs can mate while the female is still in the hard shell phase and doesn’t need to wait for molting to occur.
How do they Spread?
Humans have contributed to the spread of larvae with the fact that the hulls of some boats become dirty and/or there is some ballast water allowing the Asian Paddle Crabs to breed and produce larvae. The larvae can survive in bad temperatures, also surviving in cold water, warm water, salty water, and can survive in those types of water for three-four weeks. They can also survive big distances from being moved by the tides and currents. The Asian Paddle Crab made itself a home back in July 2016 at the Ngunguru Estuary, which is near Whangarei and it could be why they are increasing in numbers.
Why are Asian Paddle Crabs considered a Pest?
This “nasty nipper”, the Asian Paddle Crab, is a very aggressive crab that competes with other native crabs, fighting them for space and food. Breeding and fast reproduction are a big problem to other native crabs, as with more Asian Paddle Crabs around comes less food and space.
Predators like the dogfish and snapper that eat the Paddle Crab do not have a chance to keep the numbers down because they are being overfished. The Paddle Crab preys on native species like shellfish, fish, and other crustaceans, limiting their population.
Don’t disturb these crabs’ habitat because if you do they can deliver a vicious bite, causing a lot of pain to humans and much worse if you are a fish, shrimp, prawn, or crab. A nip from its large pincers can send a lethal virus called WSSD - White Spot Syndrome Disease - into the blood cells of its prey, causing them to stop breathing.
What impact does the Asain Paddle Crab have on its Food Web?
With the Asian Paddle Crab having a super fast growth rate, they can establish themselves in an area and overtake it. Doing this the Paddle Crab puts a large stain on food supplies like shellfish, fish, and pipis.
Normally in the Food Web predators like dogfish, snappers and stingrays eat these crabs, helping to keep their numbers in control. However, many of these predators are being overfished themselves in New Zealand, allowing the number of Paddle Crab to increase.
If the paddle crabs were allowed to overpopulate an area, they would eat everything, which would drop the number of native crabs and other species in that area, either by eating them or driving them out of the area.
Have Asian Paddle Crabs been found in the Tauranga Harbour?
It is said that in January 2018 they were first spotted in our harbour and Biosecurity Officers believe that they came on the dirty hull of a boat and attached themselves to that while breeding. Also in February 2018, a male and female pair of Asian Paddle Crabs were found near Matapihi Bridge. It is said that they are not 100% sure where they came from and how they arrived in New Zealand. One opinion was that they might have hitched a ride on another boat or floated here. Numbers show so far from 2017/18 to now that 87 Crabs have been found and during this time 1,892 traps have been set.
What are we doing to stop the Asian Paddle Crab today?
You can help prevent the spread of the Asian Paddle Crab by regularly cleaning the hull of your boat, trying to keep the fouling growth to no more than a light gooey slime layer. Applying a thorough coating of antifouling paint and keeping it in good condition helps. Ensure the hull is clean and free of fouling before you travel to a new area. Clean and dry any marine equipment (e.g. ropes, lines, and pots) before using them 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 sorts of equipment. If you see an Asian Paddle lurking around, make sure you kill it on site. If you don’t have a weapon to kill, it make sure to immediately report to the BOPRC.
Cleaning your tools and scuba gear will also help stop the Asian Paddle Crab from spreading. Regional councils around New Zealand have been developing management plans to reduce the risk of transporting marine pests on domestic boats. The best way to catch Paddle Crabs is to use ring pots. These pots have floats attached, and three to six pots are usually set. Pots are baited with fish remains and oily fish types make the best bait. Kahawai and Mackerel frames are their favorites. There are a lot of options that we are doing and if we keep on doing it we might be able to get rid of the Asian Paddle Crab.
Currently, the BOPRC is laying a large number of traps in the Tauranga harbor to stop the Asian Paddle Crab from spreading.
What are all possible solutions to stop the Asain Paddle Crab?
You can help prevent the spread of the Asian paddle crab by regularly cleaning your boat’s hull and trying to keep the fouling growth to no more than a light gooey slime layer. Apply a thorough coating of antifouling paint to keep it in good condition. Keep any antifoul receipts on your boat as evidence for antifoul that you have put on your boat. Ensure the hull is clean and free of fouling before you travel to a new area. Clean and dry any marine equipment (e.g. ropes, lines, and pots) before using them in a new area. Inspect areas on your boat that retain water for signs of marine life and keep all equipment, fishnets, and traps clean to prevent larvae from growing.
Check for aquatic weeds tangled around anchors, trailers, and other sorts of equipment. If you see an Asian Paddle Crab lurking around, make sure you kill it on site. If you see an Asian Paddle Crab and don’t have a weapon to kill, it make sure to immediately report it to the BOPRC. The council is laying traps to keep the crabs out of the water in the harbor and is looking to in the future make a poison to stop the Asian Paddle Crab from spreading.
In the future, technology will have advanced, making it easier to catch them so they don’t potentially become the number one pest in New Zealand. The main source of capturing/killing the Asian Paddle Crab is by placing a bunch of traps all across the harbor then killing them once caught.
Eight schools across Tauranga have a program set up to teach children about control methods for different pests like the Asian Paddle Crab. They aim to teach children how to find and identify the Asian Paddle Crab and learn trapping methods. In Japan, they came up with a way to catch the Asian Paddle Crab by having two pots (a cube-shaped and a cylinder-type pot) and this is still being used today.
Expert in the field of the Asian Paddle Crabs
Our expert in the field of the Asian Paddle Crab was Andy Wills from New Zealand Biosecurity. Through emails with him and reading through information and watching the YouTube videos that he sent to us, he suggests the best way to keep the number of Asian Paddle Crabs down is to continue trapping them and keeping your boat hull clean, and annually applying antifouling paint.
He talks about the council using traps around the Tauranga harbor and even sent a map showing where the council has set traps and which traps have caught an Asian Paddle Crab.
Andy also gave us a table showing how many traps the council has set and the number of Asian Paddle Crabs caught during each year starting from 2017/18 to now.
What he has shared with us has been a great help for us to learn about the Asian Paddle Crab and how having this pest around is a problem. Also, what our council is doing to combat this nasty pest.
Our main questions to Andy were -
Does the Council have an action plan set up to get rid of the Asian
Paddle Crab in the Tauranga Harbour?
Andy's answer to this was no they don’t have one right now but the best way to stop the asain paddle crab is to keep taping them
What control methods are being used?
• Regularly cleaning your boat’s hull – ideally keep fouling growth to no more than a light slime layer.
• Applying a thorough coating of antifouling paint and keeping it in good condition.
• Ensure your hull is clean and free of fouling before you travel to a new area.
• 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.
How many Asian Paddle Crabs have been found in Tauranga?
Andy gave me a map of all the traps they have laid and what one have a carb in it that we can put in our report.
Conclusion
After all our research and communicating with our expert Andy, we believe the best way to mitigate the deleterious effects of the Asian Paddle Crab is to try and eliminate this pest by using traps to keep their numbers under control.
The first plan of attack is to use preventative methods set up by The Regional Council and New Zealand Biosecurity to keep all boat hulls clean and annually reapplying antifouling paint to reduce the overgrowth of weeds, barnacles, and other creatures from attaching themselves to the bottom of the boat.
Checking all intakes and outlets of water and thoroughly cleaning all equipment used in the water to eliminate the spread of unwanted pests is also part of the plan.
Most importantly, if you catch an Asian Paddle Crab you must kill it on sight if you can, or contact the Regional Council who will set more traps in that area, as per their management plan of catching and trapping these aggressive pests in our Harbour.
Finally, keeping control of their numbers in our harbour will give our native crabs and shellfish a better fighting chance.
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