PICKING YOUR way through rows of rigid tuna corpses lying under a low cloud of chilled air at 5.30am may not be on every tourist's itinerary of must-dos. It's still dark, but it's already crowded and men are bent over the fish sinking the ends of small steel picks into the exposed fish tails, the dull thud of metal hitting frozen flesh resonating around the hall.

In a country known for its piscatorial leanings, the Central Wholesale Market is an unmissable part of any visit to Tokyo. Tsukiji, as it's more commonly known, is the world's largest fish market. It has taken place on the banks of the Sumidagawa River since 1923 and employs over 15,000 people.

The one advantage of jet-lag is that you will be wide-awake for the 4.30am wake-up call needed for a visit to Tsukiji's daily tuna auctions. These start at around 5am in two cavernous halls on the edge of the market and are where some of the most serious business of the day is done. One hall is reserved for frozen tuna and the other for fresh tuna. It's all strictly wholesale, but no one seems to mind the presence of tourists, provided you are willing to observe from a distance and dodge the constant stream of small mechanised trolleys.

The auctioneer rings the bell to signal the start of the auction and the bidding begins. It's a bewildering combination of hand gestures called teyari and bullet-fast Japanese that a friend later tells me is incomprehensible even to many locals. It's fast and furious and over in a matter of minutes - as soon as the price is agreed the fish are unceremoniously loaded onto wooden wheelbarrows and carted away to be carved up for sale. By 6.30am, the floors are being hosed down and the last of the fish are sold.

But to get a true idea of the scale of Tsukiji, you need to jostle your way through the hordes going about their business among the 1,500 stalls. These are in the adjacent hanger-like halls of the main market which between them sell around 2,200 tonnes of fish a day. There are over 450 species of the familiar, weird and wonderful marine life on sale here, stacked in crates, bowls, chilled display cabinets and vats of water.

I wonder why many of the stallholders are wearing wetsuits. I get my answer a few alleys further down, as I am inadvertently drenched by a large fish, which is spending its final few minutes of life thrashing around in a barrel of water with several other fish. I make a mental note: the next time I visit Tsukiji, I'll be wearing an anorak and wellington boots.