SHE WAS ENORMOUS, as long as I am tall, a solid, compact quarter of a tonne, and she was wheezing as she struggled to drag her bulk down the beach and out to sea.
The beach was riddled with giant “caterpillar” tracks where others had been in the night. But now, as the first light was showing a faint peachy-pink glow on the horizon, there was only her and one other left.
Clumps of sand clung to the salty tears that had been gushing all night from her eye glands. It was still nice and cool, but the sun rises quickly in the tropics, and a turtle caught out on the beach quickly roasts in its own shell.
The first glow reflecting off the waves was drawing others down to the sea. A few dozen hatchlings from earlier nests were charging like clockwork toys, skittering over the sand, instinctively heading at full speed for the rippling shine of the sea.
Genetically programmed not to stop, they swam frantically with all the reserves from the yolk until as far from the shore as possible.
The light that showed them the way, however, was bringing death to most.
Out of the dark sky above came a dozen giant, bat-like shapes. With the wingspan of a golden eagle, and the weight of a chicken, frigate birds are the world’s most agile and spectacular fliers.
As soon as they had a bit of light they swooped in, gorging themselves on as many of the hatchlings as their small crops would carry. Fortunately today there were only 12 birds, so a handful of the hatchlings made it to sea.
The survivors then had to run the gauntlet of the snapper, sharks and the incredible number of morays that live on Ascension’s bizarre pink algal reefs.
It’s obviously still worth nesting here. Ascension, in the middle of the tropical Atlantic, may even be the biggest green turtle breeding ground on Earth.
From here this female turtle needed to head 1250 miles to Brazil before she can feed on the rich seagrass beds on which her kind have been grazing for millenia.
It will take two years at least to replenish the reserves she has used getting here and producing all those eggs.
Modern sea turtles evolved around 40 million years ago, and may have been breeding here for much of that time.
It gets a little harder each year – they have been doing this for so long that since they started the drift of the continents has perhaps quadrupled the distance of their breeding migration.
The big two
Greens are one of divers’ two most frequently encountered turtles, partly because they are abundant and widespread, partly because they spend time around coral reefs in between feeding on nearby seagrass beds.
Adults are massive-bodied (loggerheads reach about the same size but only large leatherback turtles get bigger), and their smoothly rounded ovoid shells, handsome, rounded snouts and smoothly swept flippers make them distinctive.
From a diver’s perspective, they are also rather chilled. If you approach slowly and smoothly, and don’t harass them, they can be remarkably relaxed, both out on the reefs where they often rest and at the seagrass beds where they feed.
I’ve seen snorkellers crowd around turtles as they surface to breathe at the busy Marsa Abbah Dabbab south of Marsa Alam. This is obviously rude, and apparently the source of a certain amount of irritation on the part of the turtles – but is met with admirable tolerance.
In many areas now this level of harassment is almost inevitable. We humans are everywhere these days; we like the warm waters where most turtles live and we like to get “close to nature”.
Seagrasses are low-concentration nutrients – if they want to survive, green turtles need to feed and must learn to ignore us.
The other commonly encountered turtle is the hawksbill. Growing to a third of the bulk of a green, the pinched, pointed hawk-like bill and visible flipper claw, and the scalloped edge to the ray-patterned shell, are diagnostic.
Hawksbills, unlike greens, often feed on the reefs, and I’ve frequently found them chomping their way through big, tough sponges – one of their favourite foods. Sponges and jellyfishes can make hawksbills’ flesh toxic so, unlike greens, they are not hunted by humans for food.
Like greens, when treated with the respect of a slow, calm approach hawksbills can be extremely tolerant, and on a few occasions I have come across a sponge-munching specimen, stayed a while as we looked each other up and down, then pushed off and breathed myself up and away, leaving the turtles to their meals.
Large loggerheads, like greens, used to reach half a tonne. Their huge, angular heads and broad shoulders make them seem even bigger, and a large loggerhead on a dive is an impressive beast indeed.
Their bodies are slightly smaller and less bulky than greens, however, and as with greens we don’t find loggerheads nearly as big as we did a few decades ago – the ancient giants are gone.
You can find loggerheads on reefs all around the tropics, but they are less reef-based and often more standoffish than greens or hawksbills.
They tend to move about more, picking off a broad range of invertebrates from reefs and large gelatinous plankton. They range further, and into colder waters, than greens and hawksbills, taking them away from the reefs that divers visit.
The last and most successful of the turtles regularly seen by divers is the olive ridley, widespread throughout the tropics.
Olive ridleys are small – about hawksbill-sized – and appear somewhat more shy of divers. They are the most abundant sea turtle, but numbers have declined from tens of millions to perhaps fewer than a million nests in a few decades, largely due to hunting.
The famed arribdas – nesting beaches with tens of thousands of individuals – occur at the end of the year off Pacific Central America and at the start of the year off Orissa in India.
They still involve tens of thousands of turtles each year, but these are far smaller numbers than in the recent past.
Down but not quite out
Reef divers see turtles relatively often, so they don’t seem rare. The four species mentioned above have almost global ranges, and populations in the hundreds of thousands.
That’s not even close to the populations or ecological importance of yesteryear – we have clearly had an impact on them as fisheries bycatch or by direct hunting. However, these populations are large enough to be viable for species survival.
Breeding season at these nesting areas is generally whenever their local summer warms the seas to their maximum, and tends to go on for around two months. Adults gather offshore, the smaller males with their long “penis-tails” sometimes piling on top of a single female, making it hard for her to surface to breathe.
The females come ashore several times to lay batches of eggs during the season, and in the second month hatchlings scamper down to the sea every dawn at busy sites.
Unsurprisingly, turtle aggregations attract all kinds of predators to take advantage of nutrient-rich eggs and young, from crabs to various birds, foxes, fish and sharks.
Hatchlings of all sea-turtle species swim frantically offshore until they are past the deadly coastal area. The few that make it will spend the next few years hiding among floating rafts of sargassum and detritus at the edges of deep water, feeding on jellyfish and other small drifters until they are big enough to return inshore.
Humans adore turtles, so many nesting areas are now heavily protected, and programmes safeguard the return of far more hatchlings to the sea than would naturally make it, perhaps compensating for our impact on the adults.
So for these species at least, there seems to be hope. Perhaps.
These days, floating masses over deep water tend to be human-made, plastic garbage or deliberately placed fish aggregation devices (FADs) rather than sargassum. We can’t quantify the impact of this on survival of young turtles, but we know it isn’t good.
We know turtles eat plastic, probably because it looks similar to some of their food. This clogs their guts, so that some starve to death. We also know that FADs cause tens of thousands a year to drown, get hooked or mashed in fishing gear.
Kemp’s and flatbacks
For the last two members of the “normal” sea turtle family, the small Kemp’s ridley from the Caribbean and the little-known flatback from northern Australia, populations are much lower – a couple of tens of thousands of adults of each.
These populations, combined with more restricted ranges, make them less likely to be encountered by divers, and less likely as species to survive garbage and fishing.
The last sea turtle stands apart from its cousins in just about every way. It has its own family, which has been unchanged for probably 100 million years.
Unlike other turtles, the leatherback family was around with the dinosaurs. Leatherbacks, adults averaging 250-450kg, are the largest reptiles on earth.
The only challengers are giant male saltwater and Nile crocodiles which, like male leatherback turtles, very occasionally exceed 900kg.
On average, however, the big crocs are much smaller, and lack the massively muscular build and metabolic power of the leatherbacks, the largest of which in the world are found off west Wales.
Leatherbacks are superbly streamlined, and propelled by wing-like flippers spanning 3m or more in adult males. This drives them at a cruising speed more than double that of other turtles, a knot or so, and allows incredible burst speeds as fast as those of a dolphin – up to 20 knots.
They are built to travel, and do so almost constantly, and better than almost any other non-flying marine animal. Even between nesting episodes, females might swim 100 miles each day foraging. Males range even further, seeking out the richest feeding grounds in water colder than any other reptile could stand.
They can do this because their huge, oil-insulated bodies can maintain a body temperature up to 18° higher than the cold water into which they dive.
Leatherbacks normally frequent moderately cool water, diving to 50m or so for around 15 minutes.
Periodically, however, they plunge much deeper and longer, into water only a handful of degrees above freezing, to assess the local jellyfish concentrations.
A 1270m dive and dives of more than one hour have been recorded.
All this requires energy. Leatherbacks have massive appetites, so they need to find the richest seas in which to forage.
This pushes them into waters as cool as they can manage, and they fan out across the productive cool seas in summer, from July to September in the case of the temperate north Atlantic.
They seek out jellyfish blooms, some of the densest concentrations of nutrients on Earth for an animal that can safely catch and digest them.
To support bodies far larger and more active than those of other turtles, they eat more than a quarter of their own body weight in gelatinous animals every day.
They impale the jellies on spines in their throats, then pre-digest, releasing and belching out much of the water and swallowing a dense soup of protein, vitamins and minerals.
Plastic bags, looking very much like jellyfish, choke up their throats, and untold thousands of leatherbacks die from starvation and suffocation this way.
Leatherback populations have been hit hard by longline and other fishing activities, and from plastic-bag choking, particularly in the Pacific.
Previous populations in the hundreds of thousands consumed hundreds of millions of tonnes of jellyfish each year, but now Pacific leatherback populations are almost extinct, and we are only beginning to guess at the ecological consequences.
In the Atlantic, leatherbacks are doing better. It’s probable that overfishing benefits elatinous organisms, and there do seem to be more examples of large jellyfish blooms in recent years – benefiting animals that feed on jellies.
The north Atlantic has several giant jellies that bloom frequently. The Irish Sea and Bay of Biscay have regular late summer blooms of Rhizostoma cauliflower jellies off Carmarthen and Tremadoc Bays, across the Irish Sea and down the French coast, and the leatherbacks follow.
The south Atlantic, however, has even more leatherbacks, most of them nesting off Gabon around the New Year, and feeding across the open ocean to Ascension, and down to Namibia and southern Brazil and Uruguay.