My brain froze as I submerged into the cold water of British Columbia’s Shuswap Lake. But the discomfort paled as I watched sockeye salmon – hundreds of cherry-tinted fish with moss-green heads – swirl overhead like a kaleidoscope. At the end of an epic 4,000km journey, they were waiting to swim the final leg upstream to spawn and die.

Over the last four years, these fish had migrated to the Gulf of Alaska and back, navigating by magnetic fields and the scent of their home waters. They had thrashed 485km in from the Pacific, climbing rapids and skirting dams, all the while avoiding hungry predators. Most undertaking the journey – 99.9%, in fact – wouldn’t survive the return trip to the Shuswap, the spectacular lake region 400km northeast of Vancouver. But those that made it would fuel the area’s ecology, delivering rich marine nutrients to land-locked plants and animals. 

This late-September-to-early-November sockeye run, the world’s most concentrated, peaks every four years. Nearly 3.9 million fish came home in 2010 – a record high – and experts predicted a baby boom this season too. So far, it appeared only two million would pull off the feat, but salmon still jammed the Adams River. From the viewing platform at Roderick Haig-Brown Provincial Park, I watched as sockeyes lurched out of the water, fighting each other and wriggling across the shallows.

I longed to get even closer, to scuba dive – yes, scuba dive – with this great scarlet-and-sage flow of fish. BC Parks doesn’t allow sightseers to swim over the spawning grounds here, so I headed 2.2km east, where the river pours into Shuswap Lake. It’s the only place in the world where divers can book a trip to plunge into the middle of a sockeye migration.

As I arrived, seagulls were funnelling over the beach, stark white against the dark-forested slopes of Squilax (Black Bear) Mountain. They treated the salmon run like a sushi conveyor belt, splashing down to dine on live fish, nabbing eyeballs and tearing out chunks of flesh. I focused on an equally busy but more cheerful scene instead. Copper Island Diving control specialist Barbara Gow raced among 23 guests, helping us don the 30kg-plus of gear we’d need to endure the 12C water.

Bright as Chinese lanterns, sockeyes nosed right up to shore and congregated near the river’s mouth, waiting for nesting space to open. The short pause would be their final break before their surge up the 12km-long Adams River, sometimes battling nine-knot currents. “They hold here for two to four days,” explained Copper Island Diving’s owner Paul Downie. “Then it’s like a light switch goes off that tells them to head upstream.”

I dropped into the Shuswap near the river’s smaller, secondary outflow. Cold pierced my body. I fumbled with my equipment, then – whap! – a 7kg salmon carcass, bloated and pale, hit the side of my head. I shrieked despite being largely protected by a mask, mouthpiece and thick, hooded wetsuit.

Already reeling from the flu, I grew more nauseated as I sloshed in the wind-whipped waves. I descended to avoid the chop – not to mention the sour-dishcloth stink of rotting salmon – and entered a dense cloud of migrating fish. A curving wall of golden eyes stared back.

Annoyed by the party crasher at their mating game, at least 100 salmon shot off into the jade water. But one bold sockeye swam within arm’s reach, displaying the hooked nose and needle-sharp teeth that spawning males develop to fight over the ladies. He stretched only 75cm long, but I gave way so I wouldn’t add any stress to his last days. Little Big Man here had survived orcas, bears and eagles along his journey. He deserved to lay his bones down in the place of his birth. 

As I watched the fish labour toward their deaths, a sadness settled in, and I recalled a poem I’d heard a few days earlier while hiking along Adams River Canyon.

“Every day I see or hear something that more or less kills me with delight,” read Phil McIntyre-Paul, the Shuswap Trail Alliance’s executive director. “It was what I was born for – to look, to listen, to lose myself inside this soft world.”

Those lines from Mary Oliver’s Mindful now helped me breathe a little easier as I remembered the lush environment that flows from these salmon. Their deaths feed animals from otters to blue herons and other fish, and fertilize the soil for alders and Douglas firs on the hilly shores. Life goes on, and we shouldn’t flinch away from any of it.

As the sockeyes orbited me – wary of my exhaled bubbles – I realized my dive buddy Carmen had vanished. I popped 5m to the surface and saw her near the beach 30m away. Continuing alone seemed like a bad idea while ill and in unfamiliar conditions, so I set out to join her. But when I rolled onto my back and started kicking, the current boomeranged me out into the lake. Amazing, I thought: millions of forearm-sized salmon swim weeks against torrents like this, yet I couldn’t manage 10 minutes.

A free-diver arrowed over, moving fluidly in his thinner wetsuit and minimal gear. “Want a tow?” he asked. “It’s hard to be in the water when you’re under the weather.”

“True,” I said. But we both knew I just got schooled by a bunch of salmon.

Finally, I flopped into the shallows and decided to pioneer the sport of dry-land snorkelling instead. I had special permission to enter the river – divers are generally confined to the lake -- so I lay on the bank with only my face underwater. While utterly undignified, it quieted my stomach and allowed me to concentrate on the poetry of the migration.

I watched as the salmon squirted forward, only to be shoved back. A tagged sockeye heaved into view – female 34 Z 11 – accompanied by a humpbacked beau. Neither had eaten since the Pacific two weeks ago: their bodies had morphed, narrowing down to just three needs. Swim. Nest. Die.

Then, just as the world didn’t seem so soft any more, I spotted it: a single, translucent raspberry-coloured salmon egg tumbling in the current, aglow with life. 

It just about killed me with delight.