North Island coho runs were decimated by DDT experiment

Brenda McCorquodale's history column looks at 1950s DDT use on the North Island.

We don’t have to look very far into our history to see some surprising examples of what once constituted ‘acceptable environmental impacts.’

In 1955, North Island foresters realized they had a problem. The valuable stands of timber in the area from the Nimpkish River north were seeing a significant infestation of the black-headed budworm. Worried about the possible impact on timber supplies, the province decided to experiment with a new treatment that was first used to kill parasites during World War II to prevent the spread of malaria and typhus.

Dichlorodiphenyltrichloroethane, known more commonly as DDT, is colourless, tasteless, and almost odourless.

The government knew that the DDT application could have significant effects on the local fish and wildlife, and involved the Department of Fisheries and the Game Commission in the experiment. Flight patterns were designed to avoid rivers.

The treatment on North Island forests involved mixing one pound of DDT with a wood penetrating emulsifier, added to a gallon of diesel oil.  One gallon was then applied per acre by low-flying aircraft, with a total of 155,000 acres being sprayed in effected areas between June 10 and 20, 1957.

The spraying annihilated almost all insects within the treatment area.  Even stream-dwelling insects were almost totally eliminated, as recorded at number of stations sampled prior to, immediately after, and four months following the application.

The impacts on salmon were also catastrophic.  It was hoped that the timing of the application would minimize impacts on local salmon runs, but there were many tens of thousands of juvenile coho salmon and trout within local watersheds at the time.

Thousands of chum fry died as a result of the DDT application in the estuary of the Nimpkish River.  In the Keogh, Waukwass, Klaskish, and Benson Rivers, coho salmon mortality approached 100%. Steelhead trout may also have suffered significant losses. There were also lesser impacts on other North Island streams and rivers.

Researchers concluded that the decimated runs would take many years to rebuild. There were suggestions that juvenile fish from other areas could be flown in to Port Hardy to restock local creeks. Some reports of the day noted that it was good the area was isolated, because it would mean light angling pressure in the affected creeks.

Rachel Carson used the North Island DDT application as an example of the the harmful environmental impacts of synthetic pesticide application in her famous book Silent Spring (1962).

Some local people from Quatsino Sound have reported that the Marble River watershed had a rain event shortly after the DDT application, and that there were piles of dead sea creatures which washed up on the shores of Quatsino Sound in the following days (I would be interested in any further information for our collective historical record).

Brenda McCorquodale is a Port Hardy resident and North Island history enthusiast. If you have any stories or local lore you’d like to share, email her at A collection of her past articles is available on her blog at


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