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Whale Shark Feast

 FROM DIVE'S ARCHIVE 

We were at a location the local fishermen call ‘the Elbow’, a promontory seaward of the emergent reef, near the continental shelf of Belize at the curiously-named Gladden Spit Marine Reserve. The nearest town is Placencia, some 43km way through a treacherous labyrinth of jagged shallow reefs and hidden sand flats. Waves crashed furiously on the barely-exposed reef top several hundred metres away as the sun sank low on the horizon beyond. Golden light danced on the waves of the wildly churning sea around the small skiff I was about to jump from and into the ominous waters below, accompanied by a research team of five Belizians working for the earnestly named Friends of Nature – a local conservation NGO – and a mad scientist from the US-based charity The Nature Conservancy. 

I entered the water and descended to 25m. The plan was to stay as shallow as possible to maximise bottom time without decompression penalties, while going deep enough to find the objective, which often hovers at 40–65m, before rising. No one said it would be easy, at least not to me. The visibility – maybe 20m – was good for this location; it seemed clearer with the twilight angle of the sun. I swam past a large school of dog snappers (Lutjanus jocu), their vermillion colouring obvious even in reduced light. They were aggregating in preparation to spawn but were not my primary objective. 

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Gladden Spit is part of the Mesoamerican Barrier Reef, the Atlantic Ocean’s largest coral reef, running some 700km north–south, from the tip of Mexico’s Yucatán Peninsula to Honduras’ Bay Islands. It is home to 66 species of stony coral and more than 500 species of fish, and is integral to the interconnected ecosystems and species populations for the Gulf of Honduras and of the entire Caribbean Basin. Here, the dog snappers massed high above the stony corals and gorgonians of the reef, which gently slopes from the shallows to 25m for a considerable distance, then slopes a bit more to 35m, at which point it turns sharply downwards, like a shoulder, 1,000m into the abyss. My buddies and I dispersed in a search pattern, each at the edge of visibility from one another, 50m off the reef edge and looking down into that abyss, searching for schooling fish below.

One of the team signalled that he had found something, so I swam furiously to reach the meeting point while trying to slow my breathing and make my air supply last as long as possible. I met the rest of the group and looked down, making out movement below. As my eyes adjusted to the dark, I saw a mind-boggling number of fish milling in a vast circle. Even from 15m away, I could see that they were cubera snapper (Lutjanus cyanopterus); their silvery, broad bodies and their size – they are, after all, the largest snapper in the western Atlantic, reaching 1.6m in length and 57kg – like no other fish on the reef.

Armed with fierce jaws and sharp teeth, cubera snapper are nocturnal hunters of fish and invertebrates. They are generally solitary animals, but at certain times during the year, they make the journey to Gladden Spit to form mass spawning aggregations of up to 12,000 fish. Some snappers travel in excess of 100km to reach the spawning site. We hovered in the water, waiting for the trigger – the nature of which remains unknown – that would cause the school to rise and begin spawning. All that is really known is that for seven days, a few days after the full moon, every month from March to September, from 40 minutes before sunset to ten minutes after, these fish are going to spawn here. Blame it on the sunset and moon: spawning is cued by time of day, not tide. 

The clock was ticking, painfully. Each second passed like a slow hour as the light grew less and less, my air supply diminished and my dive computer racked up debits; the snapper had all the time in the world.

Dr Will Heyman, the ‘mad scientist’ and my dive buddy, told me that the cuberas go through many different behaviours in their cycle of spawning, including clustering, changing colour, false rises (where the fish make ‘test runs’ but do not spawn), rubbing, twitching, and the eventual release of eggs and sperm. The question is: would I be on hand to witness it, or would the constraints of dive technology and sight keep Nature’s grand spectacle from human eyes? The suspense was electrifying as I stared into the school, which glinted tauntingly in the dark water just beyond my reach. 

Then it happened. The school rose rapidly as a swirling mass, a carousel of fish in motion, covering an area the diameter of a tennis court. I then realised that that was just the uppermost section of the school. As it rose past, the school resembled a large pillar of rapidly swimming, thickly packed fish extending at least 30m in height. The school was so dense that I could no longer see my dive buddies or their bubbles.

The school then formed a cone shape, with the snappers rising to the apex at a ceiling of about 10m, where they released their gametes before turning sharply downward, swimming rapidly back to deeper water. Those fish were followed in rapid succession by the next group, and those pursued by yet more, and so on. The shape of the cone was defined by an internal rising spiral of fish and an external ring of descending fish, not unlike a water fountain. It is believed that the rapid ascent causes the snappers’ swim bladders to greatly expand, helping to propel their gametes into the water at the height of their release, and the desperate swim downward is to avoid a fatal embolism (a few bloated, floating snapper bodies found at the surface afterwards seemed to support this theory).

Like fireworks, the fish at the apex erupted in an orgiastic explosion of eggs and sperm. The eggs are themselves are transparent; it is the sperm – milky-white and dense in consistency – that we saw released. It rapidly expanded in all directions into the water column and dispersed, fertilising as many eggs as possible. Perhaps three million eggs are released each night, absorbing oil droplets containing protein. Six days on, the larvae hatch and a mouth forms, and they begin to feed on smaller plankton. The timing and location of cubera spawning is believed to have evolved to coincide with when there are the fewest predators to prey on the eggs and the maximum amount of food for the larvae.

The sea soon turned into a whiteout, ending all visibility at our present depth. The only way to return to clear water was to descend to 25m and swim away from the expanding sperm cloud. As I attempted to do so, a large object became temporarily visible in the murk. I recognised it as the sweep of the tail of the world’s largest fish. 

It seems that whale sharks (Rhincodon typus) are aware of the mass spawning activities and have joined in the feast to indulge in the abundance of snapper caviar. The eggs are pure protein for the giant planktivores, and as many as a dozen whale sharks showed up to indulge themselves in the floating all-you-can-eat buffet. From a distance, I spotted a whale shark rising from the deep and heading to a spawning snapper group that had moved off from the main action.

As if this was not distracting enough, I then heard a constant beeping over the pounding of my heart. It was the annoying signal from my computer, like some tormenting harpy, telling me I was dangerously low on air. Fortunately, I was just out of a decompression debit and had enough air to do a safety stop, where I prayed that the boat could find me when I surfaced, now the sun was on the horizon.  

I had witnessed one of the most dramatic phenomena in the underwater world. It has only been known about and studied for a few years, but its importance cannot be underestimated. An estimated half a billion cubera snapper eggs are released at Gladden Spit annually, ensuring the survival of the species not just locally, but throughout the wider Caribbean. Thirty-five other species of fish also use Gladden Spit as a spawning aggregation site. As long as the spawning aggregation sites remain marine protected areas, there is hope for the future.  

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