There is no denying the amount of fight still left in Farley Mowat. Just let him get going on the "evil forces" who are sacrificing the environment in their lust for oil.

The writer, conservationist and conversationalist, who completed what he declared to be his final book nearly three years ago at the age of 89, is irate. A proposal to put an offshore oil and gas well in the Gulf of St. Lawrence will not go away, and Mr. Mowat is aghast at the depths of human folly.

Back in 1984, he wrote a book called Sea of Slaughter that detailed a litany of environmental wrongs in the gulf and on the Atlantic seaboard. The looming development, known as the Old Harry Prospect, holds the potential to unleash more of the same, Mr. Mowat said this week in a telephone interview from Cape Breton, where he and his wife, Claire, spend their summers.

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"I was so appalled by what I discovered when I wrote this book, I could hardly believe that human beings could be so thoughtless, so destructive, so devilish, just plain devilish, all in pursuit of money," he said of Sea of Slaughter. "It took me five years to write the damn thing, and I have never been able to fully reread it since, I get so upset about it."

The spit and vinegar that surfaces whenever Mr. Mowat broaches environmental matters is what prompted those who oppose drilling in the gulf to enlist him in their effort – that and, of course, his literary celebrity.

The Old Harry is a 30-kilometre stretch of the Laurentian Channel off the southwest coast of Newfoundland that could be the largest untapped oil and gas reserve in Eastern Canada. Corridor Resources, a Canadian oil and natural gas company, has held licences to assess its potential since 1996, and wants to drill an exploratory well by 2014.

The name Old Harry was taken from a settlement on the nearby Magdalen Islands. "The oil companies mistakenly – and I think this is wickedly sardonic – have called it Old Harry because they liked the sound of Old Harry. It has such a familiar, pleasant, uncle-y name," Mr. Mowat said. But Old Harry is sailors' soubriquet for Satan, he said. "They don't realize that, what they are doing, is they are calling their company after the devil's own domain."

Mr. Mowat is one of the subjects of Tuesday's season-ending episode of Cinefocus Canada's Green Heroes, a TVO series that profiles environmental luminaries. Another is Mary Gorman, the fisherman's wife turned activist who conscripted Mr. Mowat in her fight against the Old Harry.

Ms. Gorman and her Save Our Seas and Shores Coalition have been challenging the project with everything they can muster, arguing that a spill in the Gulf of St. Lawrence could cause a disaster on a scale even larger than the explosion at the Deepwater Horizon rig in 2010 that killed 11 people and spewed oil into the Gulf of Mexico for 87 days.

Mr. Mowat and his wife summered near the Old Harry Prospect for several years, so when Ms. Gorman called for help three years ago, "I consulted myself and decided I owed something physically to that region," he said. His environmental foundation donated some money to the cause. And he lent his own voice in opposition.

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After the Deepwater catastrophe, "the almost certainty that it would happen again sooner or later in the Gulf of St. Lawrence was really enough to fire me to passionate action," Mr. Mowat said, although he concedes he may not be responding now with the vigour of his earlier years. "At 90, when this all started, I was reaching my peak. And now I have passed that."

It is true that, when he talks to a reporter, he has to sit to collect his thoughts. He also is unavailable in the afternoons because he likes his naps. But don't think he is ready to lie down in the face of what he considers to be environmental sin.

"Not only did Farley fight for Canada in World War Two, he has been fighting ever since to protect our world's vast oceans that were once brimming with wildlife," Ms. Gorman said. "He will go down in history, not only as a literary icon, but as one of the world's earliest and bravest environmentalists, who understands how dependent we humans are on healthy oceans, for oxygen and for life on earth."

Corridor opted not to comment on the statements Mr. Mowat made for this article. Who wants to challenge Farley Mowat at 92?

But Mr. Mowat does not need an opponent in his ring to come out swinging.

"We don't need any more oil than we've got," he said. "We're up to our ass in oil of one type or another – fracking and bracking and all the rest of it – and freight cars full of it coming down on little Quebec towns."