I am trying hard not to breathe. My chest begins to spasm from lack of oxygen. Still, I stay focused.
All around me are sockeye salmon, their crimson bodies and hooked green snouts filling my viewfinder. There are so many that they eclipse the sun; the only light is a small patch of blue sky directly above my head.
I keep shooting. Finally, I can hold it no longer and exhale. The sound of my bubbles startles the fish and they begin to move. I feel shock waves as the massive school changes directions.
I hold my camera up to my face, and cover my head for protection. The sandy bottom becomes stirred, visibility drops to nothing. I steal a few more deep breaths, then make another long breath-hold and begin the process again.
I'm diving solo in Shuswap Lake, at the mouth of the Adams River in British Columbia, Canada. It's mid-October, and the peak of one of the largest sockeye runs in the past 20 years.
The Adams is the colour of blood. Biologists believe that some 5 million fish will enter this 7 mile-long river to spawn this month. Up to a million fish may be at the mouth right now, waiting to migrate upstream.
My dive buddies, photographer Werner Thiele and Dennis Warner of Copper Island Diving, had tried diving here yesterday with me. The effect of the three of us breathing at once seemed to scare the fish - and us - too much. It was decided that shallow solo dives would be the best way to photograph the sockeye.
After a few more breath-hold-on-scuba dives, we head into the river, our goal to witness and record spawning salmon.
Our chosen spot is a short walk from the main viewing area. The Adams is less than 30cm deep in most places. We lie on our sides - not the most comfortable position for hours at a time - and watch the spectacle unfold.
The sockeye are oblivious to our presence. I am repeatedly hit in the face by their fins and often have to push them out of the way to see what I'm doing.
I watch intently, hoping to see a pair mating, an event I have been told is impossible to witness first-hand.
I see a large male approach alongside a female and begin to shudder. His body vibrates rapidly as he brushes against her. It's the first part of the mating game - he is trying to entice her to spawn.
My heart races and my finger stays glued to the shutter release. He repeats his dance. I keep staring, waiting for the sight of eggs, or milt. After several minutes, he loses interest and moves on, and I realise that I've missed it again.
Seeing the salmon from their perspective is incredible. Half a million people from all over the world will travel here for this event but I can't help feeling how little they will see from their dry vantage point. 2002 is the dominant year of the sockeye's four-year life-cycle.
The 4000 eggs that a mating pair leaves behind will hatch next spring. The juveniles that survive will spend the year in Shuswap Lake to mature in size.
The following spring they will travel to the sea, taking two or three years to migrate thousands of miles into the North Pacific and Gulf of Alaska.
Here they reach 3 or 4kg and become the most sought-after of the Pacific salmon, actively harvested by both commercial and sports fisheries.
When they reach about four years old, a pre-programmed signal calls them back to their natal streams to spawn.
In mid-September, the migration begins. The sockeye enter the Fraser River and travel the 250 miles to the Adams in about 18 days, battling strong currents, fishermen and other predators.
On entering fresh water, they stop feeding and their bodies begin to change. Both sexes become a deep crimson red, with olive-green heads.
Males grow humped backs and hooked jaws, and females keep their sleeker shape. They return to spawn in the same stream in which they were born, and die a few days later.
After a few hours of lying on my side in the river, I decide to give my stiff neck a break. I head over to a deeper side-channel. Here I find a dense school of sockeye, sheltered from the current.
I raise my housing to begin photographing, but as I put my other hand down to anchor myself, I inadvertently place it in a pile of decaying salmon.
The feeling is soft and mushy - I'm thankful to be wearing gloves.
The fish don't seem to be moving much here and I wonder if perhaps they have already spawned and are waiting to die.
A large male in front of me stops swimming and begins to sink. As his body lands upside-down on the river bottom, he twitches briefly, then becomes motionless.
I pause, and feel sad for a moment. His remains will add valuable nutrients to this ecosystem and provide for the next generation. Here life and death are so closely intertwined that it's hard to distinguish one from the other.
That evening, my friend Werner and I discuss the day's events. Even though we have both dived all over the world, and have had many incredible experiences in our travels, we are deeply moved by what we have experienced.
In 2006 the offspring of the salmon we swam with today will return to the Adams, and so will we.
Topside view of a large school of sockeye salmon in the Adams River
Dennis Warner illuminates dead sockeye in Shuswap Lake at the river's mouth
a pair of spawned males - dead fish feed many scavengers such as bears, eagles and seagulls
Massive school of sockeye at the mouth of the Adams River
a male (right) and female in the spawning grounds. Females dig nests called 'redds'
the entire river appears to run red with salmon
GETTING THERE: Fly to Vancouver. The main access point for the Adams River is Roderick Haig-Brown Provincial Park, which is a four-hour car drive away.
DIVING & ACCOMODATION: Copper Island Diving (001 250 832 4849) offers both shore and boat dives at the mouth of the Adams River during the spawning season. Snorkelling in the spawning grounds requires special permits. Accommodation is available in the nearby town of Salmon Arm.
WHEN TO GO: The next "dominant run" is in 2006. Numbers vary in non-dominant years. The peak is usually mid-October but salmon are in the river from late September till mid-November. Visibility is variable and water temperature averages 8-10°C.