The greatest gift that I have ever received is also my great and enduring curse.

It was June 1975 and I was a crew member on the first Greenpeace campaign to protect the whales. It was off the coast of northern California, 60 miles offshore. Before us, spread across the waters like some invading foreign armada, was the Soviet whaling fleet.

The ships were grey, black, and freckled with rust. From out of the side of the largest vessel, the huge factory ship Dalni Vostok, a steady stream of thick steaming blood poured into the sea.

We felt pretty small on board the 85ft Phyllis Cormack, the halibut seiner we had chartered out of British Columbia, skippered by big no-nonsense Captain John C Cormack.

We were a crew of 13 and I was the first officer.

This was before the 200 territorial limit had been imposed and when the Russians freely fished and whaled up to 13 miles off the shore. The Americans did not like it, but legally there was nothing they could do. Thus it fell to a small band of idealistic young Canadians to challenge the whale killers off the Californian coast.

We had been searching for them since April starting in the north off the Queen Charlotte Islands. It was like trying to find a needle in a haystack and we had no idea of where to even begin looking.

However in early June we received a tip-off from a source I can never reveal in Washington DC that if we wanted to find the Russian whalers, we needed to go south. As we proceeded south, the source supplied us with the movements of the Soviet fleet.

And there we were, 60 miles off the town of Eureka, approaching a massive factory ship and three fast, killer boats, each mounted with harpoons painted the colour of robin's eggs.

We caught them hunting, arriving on the scene just as a harpoon tore into the backside of a young sperm whale. The whalers left it there floating with a radio buoy attached as they took off in pursuit of another victim.

We launched a Zodiac inflatable and I stood in the bow, the bowline wrapped tightly around my wrist for support as we approached the body bobbing up and down in the swells.

I stepped off the Zodiac and on to the carcass of the whale. It was still very warm and blood oozed and bubbled from a gaping raw wound in its side. A long yellow polypropylene rope protruded from the ugly wound. It had been cut and the nylon strands danced about on the surface of the water, whipping rivulets of blood into a salmon pink tinted foam.

The Phyllis Cormack came alongside for our photographer Rex Weyler to take a picture of the whale, using my body as a gauge to measure the animal. It was small, around 22 feet. A young whale.

My bare hand on the whale felt the warmth of its body and the blood on my skin was hot. The whale was lying on its left side and the right eye was open, staring sightlessly skyward. The lower jaw opened and closed with each swell and I could see the lower two rows of pointed teeth.

Suddenly there was a shout from the Phyllis Cormack. The harpoon vessel was returning and they had a fire hose blasting a stream of high-pressure water from the bow.

I jumped into the Zodiac as the Soviets quickly lashed the dead whale to the side of their ship and made way back to the factory ship. We followed and filmed them as they transferred the whale over to the slipway, where it was hauled by cables and winches up on to the flensing deck, where men with long sharp knives waited to flay the body.

More blood gushed from the scuppers into the sea.

And we felt so helpless, so small, and so useless.

After the harpoon vessels transferred their whales, they spread out to resume the killing. We followed one of the harpooners in three Zodiacs as the Phyllis Cormack slowly followed.

It was not long before the Russians found another pod, and once again the chase was on. But this time we were chasing the whalers as they pursued the whales.

Our strategy was simple. We would place our bodies between the whales and the harpoon. We were Gandhi-influenced non-violent advocates, and this was the only tactic we could think of to protect the whales without injuring the whalers.

We had practiced what we would do for over a year and I turned to Bob Hunter, our expedition leader, who was in the boat with me, and said, "Well Bob, this is it. Let's do it."

I gunned the outboard motor, and our inflatable boat roared ahead of the harpoon vessel, with the other two inflatables on either side. Within minutes we were racing ahead of the whaler and behind a pod of eight magnificent sperm whales.

They were racing for their lives, unable to take in enough air for a deep dive. They were spouting rapidly and we could see rainbows sparkling from the mist they expelled into the air. We were so close we could smell their breath, and our objective was to block the path of the harpoon.

Would the Soviets risk killing a human being to slay a whale? The answer to that question was a mystery to us.

But we were tempting them to give us an answer one way or another. As our three inflatable boats raced before the bow, I looked back to see an ape of a man stooping behind the massive harpoon cannon, trying to get one of the whales in his sight. He was not succeeding, and was clearly frustrated.

Suddenly a larger man came storming forward along the catwalk and began yelling into the ear of the Soviet harpooner. The harpooner nodded and crouched down behind his gun as the man who we later identified as the captain stood up and looked down at us with a smile. He brought his finger slowly across his throat, and that was when we realised that Gandhi was not going to work for us that day.

I saw the pod of whales rise up on a swell in front of us just as the harpoon vessel rode up on a swell behind us. As our inflatables descended into the deep trough between the two large swells, a horrendous explosion boomed over the whales.

The explosive-tipped harpoon whizzed through the air above as the attached cable slashed down upon the water close by, cleaving the surface like a heavy sword.

In front of us a female sperm whale screamed in pain as she rolled on her side, with a fountain of hot steaming blood pouring from her. Beside her, the largest whale in the pod rose up from the surface of the sea and dove. As his mighty tail slapped the water with a bang, he disappeared. The other six whales carried on fleeing as the huge male turned to defend them.

For a moment we thought he would attack us. We had all seen the old prints and woodcuts of enraged sperm whales cutting Yankee whaling boats in half with their sabre-like teeth, spilling wounded whalers into the sea.

We had little time to think as the ocean exploded behind us, and this great whale hurled himself from the water trying to reach the man behind the harpoon.

They were ready for him and had quickly reloaded the harpoon gun with a unattached harpoon. As the whale rose up and out of the water, the harpooner lowered the gun, pulling the trigger at point black range. With a thundering explosion the harpoon tore into the whale's head.

He screamed. It was an excruciating cry of pain, shock, and confusion. He fell back into the sea, rolling in agony on the surface in a sea stained scarlet with his blood.

The two dying whales struggled to hold on to life between the harpoon boat and the six of us in three boats, sitting motionless on the swells.

I could not take my eyes off the dying whale closest to us. His tail flayed the sea and pink foam frothed all around him.

Then suddenly the whale was looking directly at me. I saw his huge eye and I could see that he saw me. At that moment he dove once again and I saw pink bloody bubbles coming to the surface, moving closer to our boat. Within seconds the whale's head shot above the surface of the sea and began to tower above, rising higher, but as if in slow motion, and angled so that we could see that his intent was to come crashing down upon us.

And as his head rose ever higher I saw that eye once again, so close that I could see my own reflection in that deep dark orb. Suddenly I was struck with the realisation that this whale understood what we were doing.

His lower jaw hung down almost touching the side of our inflatable boat, so close that I could have reached across and encircled one of the six-inch teeth with my fingers.

His muscles tensed and he stopped rising, and began to slowly slide at an angle back into the sea. I kept eye contact with him until his eye sank beneath the surface of the sea and disappeared.

And so he died.

He could have killed us, but he had not, and the look in that eye has haunted me ever since.

I felt understanding and I knew he knew that we were there to save him, not to kill him. I felt ashamed that we had failed. I felt powerless and angry, frustrated and awed all at once. I felt indebted to him for sparing my life.

But I also saw something else in that eye, and that was pity.

Not for himself nor for his kind, but for us.

An uncomfortable pallor of shame fell over me as I sensed what the whale perceived. It was indeed pity, but pity for us, that we could take life so ruthlessly, so thoughtlessly, and so mercilessly; and for what?

We sat there in our little inflatable boats in the midst of the Soviet whaling fleet with the bodies of a half dozen sperm whales lying lifeless in the swell. I watched the sun begin to set in the west and I remembered that the Russians were killing whales primarily for the valuable spermaceti oil.

Spermaceti oil is valued for its high resistance to heat, and thus it is used in machinery where there is excessive heat. One of the demands for this oil by the Soviets was for use in the production on intercontinental ballistic missiles.

Here they were slaughtering these magnificent, intelligent, socially complex, wondrous sentient beings for the purpose of making a weapon designed for the mass extermination of human beings.

And I thought to myself, are we really this insane?

It is that thought, that unanswered question, that has haunted me every day since.

It is from what I saw in the eye of that whale that has led me to devote my entire adult life to the defence of the whales and the other creatures of the sea, because I know that if we cannot save the whales, the turtles, the sharks, the tuna, and the complex marine biodiversity, that the oceans will not survive. If the life in our oceans is diminished, humanity is diminished and if the oceans die, humanity will die; for we cannot survive on this planet with a dead ocean.

• Captain Paul Watson is a co-founder of the Greenpeace Foundation and the founder in 1977 of the Sea Shepherd Conservation Society. He is currently on board the Sea Shepherd ship Steve Irwin intervening against the killing of whales in the Southern Ocean Whale Sanctuary by the Japanese whaling fleet