There’s more than murk in the Manukau

There’s more than murk in the Manukau

By Iain Anderson

Auckland’s western harbour, the Manukau, is full of action and life. Mangroves, beaches and rocky cliffs border the coastline. In most areas the ‘spineless majority’ is abundant. If you take a casual stroll along the shore at low tide you encounter extensive oyster beds, marine snails, and the occasional beached jellyfish. Turn over a rock and you will see dozens of tiny crabs.

Some large creatures use the harbour too: sharks breed in the harbour and it is also frequented by seals. The harbour can be very murky as visibility rarely exceeds three metres, although I have been assured by one of my diving colleagues that sharks don’t attack in murky water. I’m not completely convinced that this is always true. A friend of mine told me of a close encounter with something really big that nudged him aside while he was on a dive near the northern head of the harbour.

Until recently I had never considered diving in this harbour. Curiosity and my friendly colleagues in the Western Underwater Research Team have lured me into the Manukau several times over the past year. The result has been rewarding from an intellectual perspective, and it has been a lot of fun too.

The Team regularly visit a rectangular area (25 metres by 35 metres) of harbour bed, in 10 to 30 metres of water, at Waterfall Bay. Waterfall Bay is located on the northern side of the harbour approximately mid-way between Huia and the heads at Whatipu. The chairman of the team, Mike Percy, told me that the region was first surveyed in July 1993. During each survey dive, teams start at one corner of the area, which is bounded by tethered ropes, and then swim along at least two of the boundary ropes. Important observations can be recorded on a slate, which they carry with them. After each successful dive a survey sheet is completed. Regular visits give the research group the ability to measure changes in animal populations, underwater and topside.

On the rocky shore there is a growing colony of fur seals which sometimes join divers in the water. The first pups were seen about 1994 and the population is on the increase – in July we counted 17 seals.

Dives are carefully timed to avoid ocean swells and strong tidal currents. If you look to the southwest from Waterfall Bay you see the open Tasman Sea. Strong south-westerly winds can produce very large swells. Winds from other compass points (eg. easterlies) generally result in calmer conditions and are preferable for diving.

The Engineering Science Department of the University of Auckland has produced a computer model of tidal flows in the Manukau and Dr. Mark Trew advised me that tidal velocities in the area can reach 1.8 metres per second; this is over three knots. Water velocities are greatest in the mid-tide, between high and low (or vice versa). Western Underwater’s research dives are timed to take place half an hour each side of low tide when the water is at its slowest and turning around, and extreme tides are avoided. The research team will never dive on a tide that exceeds 3.5 metres.

The pattern of water movement near the harbour entrance is complex. Superimposed on the general inward and outward tidal flow there is a net circulation of water – flowing eastward along the northern shore between Whatipu (Paratutae) and Cornwallis (Puponga Point) and flowing westward along the southern shore between Mako Point and the south head. This means that anything dropped overboard at Waterfall Bay might slowly migrate eastward and then out to the middle of the harbour. One of my colleagues, Bruce Ranby, recently lost a dive bag with torch. Repeated dives have failed to locate this. I wonder if this circular flow has placed it out in the middle of the channel?

Visibility was quite bad on the day that Bruce lost his gear. I had a devil of a job keeping alongside him. If I couldn’t touch him, I couldn’t see him. Clouds of silt frustrated photographic attempts. These silt clouds literally follow you around as you disturb the bottom with each kick of the fins. Nevertheless I was able to take a few photographs. Some nudibranchs were around on this day. These slugs were very large, about 5-7cm long and all white. These were probably Aphelodoris luctuosa which have a different body pattern on the east coast – their back is usually covered in large brown patches. On this and other dives I have encountered many colourful encrusting sponges. While swimming back to the boat I ran into a shoal of Pelagia jellyfish. As I tried to photograph them I received some stings to my hands and mouth. Getting close to nature in a wild place like the Manukau can be painful sometimes!

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