Dredging plan will cut deep

Historically, "out of sight, out of mind" has proven an effective policy when it comes to reducing costs, for big and little business alike.

While it may be ethically wrong to muffle, shuffle or even disown damaging behaviour by acts of stealth, the policy's high success rate ensures its long life.

So it is that Port Otago Ltd with its Project Next Generation will next month seek to sneak under the public's nose the largest and most destructive single act of industrial dredging that the Otago coastline has yet seen, notwithstanding our city's proud reputation as the Wildlife Capital of the World.

The proposal, to scrape out the guts of the existing channel, sandbanks and reefs of Otago harbour with a suction dredge and explosives over as short a time as possible (24 hours a day, seven days a week), will result in about 7.2 million cu m of minced sea-bottom being dumped just over 6km off Taiaroa Head, from where it will gradually disperse across coastlines north and south of the deposition zone.

In its draft assessment of environmental effects, the applicant acknowledges "a plume within the water column affecting suspension feeding planktonic animals and birds and fish that feed on them", "an extended area affected by settlement of suspended sediments from the plume" and "physical covering of the benthic community within the disposal site and reducing away from the disposal site".

The last of these effects Port Otago describes further as a substantial loss, mitigated by the fact that the disposal site contains no sensitive or rare species that have been identified.

Neither Port Otago nor anyone else can be sure just how severe the impact will be for species further along the food chain.

How will the dredging impact on Hector's dolphins, Hooker's sea lions, yellow-eyed and blue penguins, and other seabird populations, including the royal albatross colony? What will happen to the salmon fishing? It is no use building a fishing wharf by way of off-site mitigation if there is nothing in the water worthy of catching.

In the history of Dunedin settlement, we have apparently dredged some 34 million cu m of material to date, so that this act might be described as a one-in-30-year event.

In reality, its closest relatives are in the late 1800s, which involved 5.6 million cu m of sea-bottom, and in the 1970s with the development of Port Otago, where an estimated 4 million cu m of bottom was removed.

Even those events took place over four years to a decade, whereas the current proposal is for a much shorter period of time.

In terms of sediment displacement, the proposal is the environmental equivalent of a new tall dam on the Clutha, Think Big foisted on to the undersea bottom-dwellers in the form of 2m of churned mud, weed, sand, reef and shingle.