The shark was swimming straight towards me. Its mouth was 1.2 metres wide and its body a shadowy torpedo stretching into the hazy green water behind.
Just inches below the surface, the velvety skin caught the light and showed off a pattern of thousands of white spots, as if it had been decorated by a crazed paintball fanatic. Barely seeming to flex its 90cm-high tail, the giant passed me without a flicker of acknowledgement.
As I finned hard to keep up, I measured its size in arm spans – eight metres, more than four times my own body length. But this was a harmless behemoth – a whale shark, the largest fish in the ocean.
Whale sharks migrate vast distances and make seasonal appearances but here in the Sea of Cortez, off Mexico’s Baja Peninsula, they find it hard to leave. “Last year they stayed until late May and they were back again in July,” said James Curtiss, owner of the local Cortez Club diving centre. “They may even be year-round residents.”
The Sea of Cortez has an almost legendary status among divers and marine naturalists. John Steinbeck wrote a book about his voyage here aboard a scientific collecting expedition in 1940, and Jacques Cousteau famously called it “the world’s aquarium”.
The 155,000-square-kilometre gulf is divided between a temperate zone (north of La Paz, the state capital of Baja California Sur) and a warm-water Panamic zone (southward to Cabo San Lucas, a resort city on the peninsula’s tip). The confrontation and subtle mixing of these two ecosystems partly accounts for its richness: some 900 fish species and 32 types of marine mammal gather to feed and breed here.
Massive blooms of plankton mean that even elusive blue whales are seen here, along with the gnarled humpbacks and grey whales that sound and breach in the bay, to the delight of whale-watching parties.
This rich sea is in stark contrast to the forbidding desert at its edge. Driving north from the tourist hub of Cabo San Lucas, you enter a new world: giant cardon cactuses stretch to the horizon where the mountains, the Cordillera del Pacifico, cast a great blue shadow in the midday heat.
This is a wild place where there is more life than the European visitor might at first imagine. On a walk through a small section of a gorge, cutting from east to west across the Baja Peninsula, I was guided by David Alba from Cabo Adventures.
Warning me to stick to the path and not to pick up anything without checking with him, he explained that Baja was home to 17 of Mexico’s 34 species of snake, including the rare tree rattlesnake.
Alba’s love for the desert was clear as he explained how the biggest cardon cactuses weigh 20,000 kilograms and are more than 500 years old, and then showed me the sap from the cinerea bush used by indigenous people as sunblock. Once you start looking, there is an abundance of wildlife in this arid wilderness – in particular, birds, from vulture-like crested caracaras to horned owls and bright vermilion fly-catchers.
Even on dry land, however, there is no escaping the sea. On the broad malecon, La Paz’s main promenade, there is a bronze statue of Cousteau clutching a diving mask and staring out to sea. Further along the esplanade are statues of manta rays, sea lions, whales and dolphins, even a giant clam containing a gleaming pearl.
On land at least, little seems to have changed since my last visit 10 years ago. A few more statues have appeared on the malecon, and at the edge of the bay there are a couple of new hotels. I hope it stays that way, for in my experience mass tourism damages marine habitats.
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That is not to say that the Sea of Cortez has escaped the ravages of human activity completely. Decades of overfishing have unbalanced the ecosystem, taking top predators such as sharks and marlin out of the food chain and leading to a surge in other species.
Before the 1950s, there were no sightings of Humboldt squid, ferocious creatures up to 2.5 metres long which hunt in packs and cannibalise vulnerable members of their own species. Now there are 20 million of them, preying on shrimp and other molluscs.
In the '70s, hundreds of hammerhead sharks were often seen at El Bajo, a sea mount and dive site. These days, sightings are rare because gill-netting and other indiscriminate fishing methods have dramatically reduced their numbers.
According to James Curtiss, however, there are reasons to be optimistic about the Sea of Cortez.
“Sure, there has been overfishing in the past,” he concedes, “but the attitude of local people has changed a lot in the 23 years I’ve been here. Plans for big resorts here have been opposed. I employ ex-fishermen as dive guides and they understand the value of live sharks and manta rays compared to the short-term gains of catching and killing them.”
La Paz seems to be slowly waking up to selling its own charms and conserving its best assets. On a tour of the city I met a group of women running a co-operative that offered guided walks with a cultural dimension, taking in the old cathedral, visiting a local bakery, riding a crowded bus and talking to the barbers in a salon that looked unchanged since the '50s.
Just as local people are adapting to a changing world so, too, is the marine life. At Los Islotes – a jumble of rocks north of Isla Espiritu Santo, a protected island off La Paz – a colony of around 300 California sea lions is thriving, bucking the trend for the Sea of Cortez as a whole, where numbers have declined by 20 per cent over the past two decades.
Why should that be? A few years ago, documentary makers found that the colony was feeding on deep-water fish that sea lions would not normally eat. They had adapted their habits to feed on species beyond the reach of commercial fishermen.
Then and now
All this made me apprehensive about what I would find out there, a decade on. My diving logbook reminded me of encounters with shoals of colourful fish at Espiritu Santo; it told of male sea lions at Los Islotes blowing bubbles underwater to warn me off if I swam too close to their lair. Would they still be there?
On a bright, clear morning with the meekest of swells, I took a boat from La Paz north towards the offshore islands. Javier Olichea, my skipper, has fished and dived these waters all his life.
First he wanted to show me two craggy outcrops festooned with seabirds. El Merito (Grouper Island) is a nesting spot for blue herons and ospreys, while the sheer sides of guano-encrusted La Gaviota are home to a large colony of blue-footed boobies.
After a quick circuit, we sped across open water towards the large bulk of Isla Espiritu Santo, a nature reserve of russet sandstone cliffs, cactuses and deep sheltered bays where the shallows were clear as glass. As we headed around the coastline, Olichea pointed to a distant flash of spray and called out a litany of fish names: yellowfin, manta, marlin.
A repetitive shadow caught my eye and Olichea steered a little further out. It was a large school of long-beaked common dolphins hunting unseen prey. There is nothing common about their appearance, however, distinguished by sleek two-toned flanks and elegant tapering tail flukes.
Sixty, perhaps 80 dolphins flew across the surface, jumping, flipping and standing on their tails as they purposefully herded a shoal of silver fish below. On the glittering surface of the deep blue, they were living expressions of vitality, energy and survival.
At Los Islotes, we heard the reassuring bark of California sea lions. Young pups, golden-haired females and bullish defensive males lay in nooks and crannies sunning themselves after their fishing expeditions.
Slipping as quietly as possible into the water, I swam to within a short distance of the rocks. Almost immediately a young female sea lion joined me and passed within inches of my head.
Finning down towards the rocky slabs 4.5 metres below the surface brought her close, investigating my own clumsy movements while she turned somersaults beside me at dizzying speed. All was well at Los Islotes.
The Sea of Cortez is also famed for its encounters with giant Pacific manta rays, which like whale sharks are harmless plankton feeders. Olichea explained that they are usually seen from August to November. The hammerheads at El Bajo are still there from November to January, he said, albeit in smaller numbers.
I asked him about the mysterious and little-known 'vaquita', the world’s smallest cetacean, which lives north of La Paz and is critically endangered. He told me he had only ever seen two in his life, and most conservationists believe there are fewer than 100 left.
Mexico has, in the past few months, embarked on a renewed effort to capture the illegal fishing boats blamed for killing vaquitas that become entangled in the gill nets laid for the totoaba, a big fish whose swim bladder fetches high prices as a Chinese medicine. President Enrique Pena Nieto has pledged patrol boats and a no-fishing zone to try to save this 90-centimetre-long porpoise, which is so shy it was only definitively identified in the '50s.
Close to the port of La Paz, we steered towards a sand bank known as El Magote. Olichea had promised we would see more whale sharks, and within half an hour I was able to swim with five. One was a juvenile male five metres long, while the largest was approaching eight metres, excluding the tip of its scythe-like tail.
Olichea warned me not to touch the sharks, not because they are dangerous but so as not to disturb their feeding. In this shallower water, green with plankton, I could see only a few feet. The sharks take the tiny plankton with gaping mouths, filtering gallons of water through their enormous gills. Small eyes are mounted in telescopic sockets which retract at any sign of danger, reinforcing their vulnerability despite their size. Their life cycle is still a mystery and no one is yet certain exactly where they go to breed.
But for me, that is part of the allure of so much marine life. Encounters like this bring a sense of privilege: I was swimming with the biggest fish in the sea. To know that they are still coming to the Sea of Cortez, along with all the other marine life we had seen in just one day, is greatly comforting.
This article was written by Tim Ecott from The Daily Telegraph and was legally licensed through the NewsCred publisher network.