Diving NZ’s Mangroves for something different

by Roger Grace

It is critical to choose the right location, time and tide, however. No point diving after several days of rain, or in the back of the large harbours like Kaipara or Waitemata. But wait for a week or so with no rain, and seek out a site in some of the smaller harbours of New Zealand’s northland east coast, and you could be in for a treat.

If the water is already clear on the open coast then that is a bonus. But the cockles on the intertidal sand flats are pretty efficient at removing a lot of the plankton which can make coastal waters rather greenish. After all, the plankton is their food, and when they have sucked a lot of it out of the water visibility can get up around 15 metres on a good day.

One of the easiest spots near Auckland where you can have a good mangrove snorkelling experience is the Whangateau Harbour south of Leigh. If you have been to Goat Island for the day and are coming back past Whangateau close to the time of high tide, you might give it a try. I think you will be pleasantly surprised. Then once you have a taste for it, you will aim specifically at the right conditions for that ‘perfect’ mangrove experience.

At Whangateau it is best after no rain for a week, and a good spring high tide which usually happens about mid-morning, when it is usually calmest anyway and the lighting is nice. You can enter at the Whangateau Domain about an hour before high tide and drift upharbour with the current to the mangroves on the shoreward side of Horseshoe Island. Another entry point is to walk around the west side of the Whangateau Caravan Park, then it is just a 100-metre swim out to a couple of large isolated mangrove trees on the sand flats.

On the way you can see the twin siphons of cockles busily sucking the plankton out of the water making it nice and clear. There are also lots of mantis shrimps, which retreat down their vertical shafts as they see your shadow. The dark entrance to their burrow appears as you approach. If you want to see the animal, just hover quietly a metre or so from the hole and soon the dark hole will disappear as the sand-coloured animal fills the entrance with its stalked boggly eyes. If you smash a cockle and drop it a quarter metre upstream from the burrow you may see the shrimp dash out and grab the cockle with its sharp front legs shaped like those of a praying mantis. Don’t try to grab the shrimp with bare hands – those front legs can inflict quite a cut!

Approaching the mangroves you will begin to see their aerial roots – the pneumatophores – sticking up from the sand and often covered with little acorn barnacles, busily kicking plankton into their mouths with their feathery feet! Small mangrove plants are also covered with these little barnacles.

Lying around amongst the mangrove roots are clumps of necklace weed. The sheltered harbour version of this common seaweed has very large bubbles or bladders. You may have seen the same weed on the open coast on the lower shore where the bladders are much smaller in the heavier wave action.

Near the mangroves trees you may see schools of yellow-eyed mullet zipping around just beneath the surface. They often shelter under the mangrove tree amongst the branches. You can burrow your way right into the mangrove trees, pushing the branches aside and taking in the amazing vista beneath and through the trees.

But beware of scratches from the same sharp acorn barnacles on the smaller branches, and especially the large pacific oysters which attach in clumps to the larger branches and the trunk of the tree.

On the mud beneath the tree you may also see green olive anemones, their tentacle crowns up to 30mm across spread out to catch small crabs or other crustacea. The crabs make burrows in the mud, and if you sit quietly you may see them come up and explore for something to eat.

Once you have explored the mangroves you may wish to swim toward the harbour channel which here is marked by large white poles. Near the edge of the channel is a sandstone rock shelf covered in necklace weed. There is a 1.5-metre drop-off into the channel itself which is usually full of dozens of parore hooning around at a great rate.

From there you can drift down-harbour and swim around the eastern side of Horseshoe Island, and rest in the sun on the narrow sandy beach. There are usually some oystercatchers and pied stilts roosting there, and if you are lucky you may see a couple of the rare New Zealand dotterel which nests on the end of Omaha Spit at the entrance to the Whangateau Harbour.

I strongly recommend that you carry or tow a diver’s flag on a float, as during the summer small boats are common in this area, and faster boats tow kids on ‘sea biscuits’ and other innovative craft up and down the channel. For your own safety be aware of boat traffic.

If you are into photography there are lots of great opportunities in this area. You may see the odd flounder on the sand, and views beneath the mangroves can produce quite unusual photos.

A night dive in the mangroves opens up a lot more opportunities. So don’t write-off the mangroves as a dive site. They can be a very different and rewarding experience, and a great place to take the kids snorkelling.

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