Risk of Spreading Marine Pests from Sandspit by Dredge Spoil Dumping at Sea

NZ Hydrographic Chart number NZ531

NZ Hydrographic Chart number NZ531

It is understood that dumping of most dredgings from the proposed Sandspit marina is likely to be at the eastern Great Barrier Island site designated for disposal of marina maintenance dredgings from various Auckland marinas.

I am not in possession of the actual coordinates for the disposal site, but I understand it is east of Great Barrier Island and north of Cuvier Island, in a clear “indentation” in the boundary line for the 12-nautical-mile outer limit of the Territorial Sea in this area (see chart NZ531). This is the closest point of “open sea” to Auckland, where many rules and regulations of the Territorial Sea do not apply.

Jurisdiction for marine disposal at the site is in the process of changing from Maritime NZ to the Environmental Protection Agency. The site would technically be in the NZ EEZ.

As indicated in my recent report on marine pests at Sandspit (Grace, 2014), I believe there is a risk of unwanted marine invasive species established at Sandspit reaching other shores where these pests are not presently found, if marina dredgings are disposed of at sea, even at this relatively remote dump site.

The three species of major concern at Sandspit are the clubbed tunicate Styela clava, the Australian droplet tunicate Eudistoma elongatum, and the Asian paddle crab Charybdis japonicus. The two tunicates apparently have a planktonic larval life of around 24 hours (Biosecurity NZ website). The paddle crab is a good swimmer and can cover considerable distances in open water.

Reference to NZ Hydrographic Chart number NZ531 indicates a tidal reference station “A” inshore from the designated disposal site, which provides the closest tidal information. The table on the chart gives tidal flow direction and speed at this site for both spring and neap tides.

Spring tidal streams range from 0.4 to 0.7 knots, with a mean flow of about 0.55 knots. Neap streams are in the range of 0.3 to 0.4 knots. The residual flow is in a southerly direction (mean 174 degrees). Although tidal streams oscillate, a mean spring flow of 0.55 knots suggests that a surface drift of 13.2 nautical miles would be possible in 24 hours, which could be greatly increased by moderate wind conditions. This would put the northern shores of Cuvier Island, about 12 nautical miles from the dump site, within 24-hour range of drifting tunicate larvae from the offshore dredge spoil disposal site.

There are much stronger currents (2 to 3 knots) around the southern end of Great Barrier Island (Chart NZ531) so once these tunicates were established on Cuvier or Great Barrier shores they could move easily around the shores and become more widespread. Their spread further south into the Coromandel east coast and Bay of Plenty shores would also be likely to be facilitated by spoil disposal from Sandspit to the eastern Great Barrier site.

There are literally millions of colonies of the Australian droplet tunicate in the footprint of the proposed marina at Sandspit, and each of those colonies can produce several hundred tunicate larvae. With many barge loads of dredge spoil destined for the disposal site, the area will be repeatedly inoculated by many millions of tunicate larvae. Sheer weight of numbers would ensure that at least some of them would make it to suitable habitat where they would become established.

In the dredging, transport and dumping process, tunicates included in the dredgings will be highly stressed, which is likely to cause them to spontaneously release larvae. These larvae will not necessarily be carried to the deep seabed along with the bulk of the material and the adult tunicates, but many are likely to remain in surface waters where they can start their “24-hour dash” to the nearest downstream shoreline. It is not clear whether they can settle in deeper reef areas or whether they need shallow water areas to settle.

When it is stated that the tunicates have a larval life of only 24 hours, this is probably an average, and it is likely that some would survive much longer than this, further increasing the chances of some of the larvae reaching the shore from the disposal site.


It is my opinion that larvae of the Australian droplet tunicate would be highly likely to reach a suitable settlement site on Cuvier Island or Great Barrier Island if disposed of in dredgings from Sandspit at the eastern Great Barrier Island dredge spoil dump site.

Larvae of the clubbed tunicate could also reach the same areas, though the numbers of specimens at Sandspit suggest this may be less likely.

Asian paddle crabs, if still alive in the dredgings at the time of disposal, could easily swim the distance to Great Barrier or Cuvier Islands, or to the Coromandel east coast.


Grace, R.V. 2014 Marine pest species at Sandspit, northern New Zealand. www.whangateauharbour.org, January 2014.
NZ Hydrographic Chart number NZ531


  1. Martin Howson says:

    Hi, With respect to dumping of waste material from the Sandspit Harbour Marina reclamation.

    I have just spent a wonderful week out at Great Barrier Island enjoying the pristine water in the area and saw no sign of the white tubular growth that has over the last couple of years been getting more prevalent in the Sandspit River adjacent to the Sandspit Yacht Club and the main wharf at Sandspit. It concerns me greatly that spoil from the proposed Marina is going to be dumped out in vicinity of Great Barrier and along with it this invasive growth which I believe to be Eudistoma Elongatum. Sadly the Department of Conservation do not wish to know about it and passed me off to the Auckland Council who at least had the courtesy to take the details and get back to me.

    I would have thought that DOC may have had some concerns regarding the Bio-Security aspects of this operation.

    Is it too late to stop this madness?

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