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Marine Turtles
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Marine Turtles
Once a year, a miraculous invasion takes place on the beaches of
Gabon.
Out in the vast expanse of the mid-Atlantic, adult leatherback
turtles spend most of their lives swimming in surface waters,
feeding on jelly-fish. However, once a year some of these animals
begin a migration towards breeding grounds, the only place where
the sexes ever interact. After breeding, male turtles are free to
return to the open ocean. Female leatherbacks however, turn
themselves to face land and swim towards the shore, beginning
one of the most astonishing and mysterious life-cycles in nature.
By early November, the rainy season is well under way in Gabon .
The days are hot and sticky and the nights are punctuated with
thunder storms. The changing weather brings an explosion of
energy and production from wild plants and animals that have lain
dormant or subdued for much of the long, dusty dry-season. Birds
begin to sing and nest again, forest fruits sweeten the diets of
primates and other frugivores, and in the savannas, new growth
springs from the ashes of the August burning, bringing buffalo and
other grazing species into the open.
At about this time, female leatherback turtles, heavy with eggs, approach the coast of Central Africa. Leatherbacks nest on
both the Atlantic and Pacific coasts of Africa, however of all the nesting sites on the African continent, it is Gabon that
attracts the greatest numbers. Like a magnet, Gabon appears to draw leatherbacks along the equator until they reach
land.
The night is overcast, with merely a hint of moon sliding behind
the clouds. On the beach at Mayumba, palm trees rustle in the
pre-dawn breeze, and the tide is rising. Above the tide-line, the
beach is patrolled by mongoose and genet, foraging for their
next meal. The lower beach is alive with ghost crabs that
scuttle to and fro in time with the rhythmic wash of the tide,
picking their tiny meals from the damp sand. Just beyond the
phosphorescence of the waves a dark shape is discerned.
Through a gap in the clouds the moon reveals that an adult
female leatherback is coming ashore, swimming powerfully
through the surf and grounding on the sand. She is a veteran,
having visited the same section of the Mayumba coast every
three years for over a decade. A hundred yards further down
the beach a young female also emerges from the waves. It is
only the second time in her life that she has touched land.
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Fifteen years earlier, an hour or so after nightfall, the dry sand near the head of the beach begins to move. Just beneath
the surface, fifty hatchling turtles, including our female, are moving upwards. They have crawled from their eggs almost a
meter below, and in the space liberated by their now empty and crushed shells, have begun to dig upwards. By flapping
their flippers, the hatchlings gradually displace the sand above them, forcing it to fall and form a new, slightly higher
platform beneath them. In this way they have, over many hours, inched their way upwards in a shifting air-space, towards
the crucial moment of emergence.
A flipper emerges from the sand, followed by a tiny head, as the first hatchling breaks the surface, and for the first time
sees the world it may inhabit for the next seventy years. It crawls out onto the beach, followed by its nest-mates.
Streaming onto the surface of the beach, the hatchlings now begin the most hazardous journey of their lives. Before these
individuals are able to return here to lay eggs of their own, they must survive an almost overwhelming army of lethal
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predators, from crabs and sea birds to sharks and killer whales, and the most formidable of them all ……. man.
Who?
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All turtles are air-breathing reptiles, who, as a group of animals, have inhabited the planet for over 200 million years (since
the time of the dinosaurs). Most are cold-blooded and live in water for most of their lives, emerging only to lay their eggs
on the sandy banks of rivers or on beaches. Of the 222 species of living turtles, 7 are classed as ‘sea turtles', and of these,
Gabon is visited by 4 species. The most numerous, and that found most frequently at Mayumba is the ‘leatherback turtle'.
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The Leatherback Turtle ( Dermochelys coriacea ) is the world's
largest turtle, and one of the world's largest reptiles (surpassed in
size only by some species of crocodile). Its shell can measure
around 1.6 metres in length and an adult turtle weighs in excess of
500 kg (the largest ever recorded leatherback turtle was caught
accidentally in Wales in 1988. It was 2.91 metres (113 inches)
long, and weighed 916 kg/2,106lbs!). The ‘shell' of a leatherback
is actually a rubbery mantle covered with dark leathery skin,
liberally covered with white spots. The upper jaw has tooth-like
projections used for grabbing soft prey items. The front flippers
are extremely long and powerful, but lack claws. Leatherbacks are
unique among turtles in being warm-blooded (using fat layers for
insulation).
Leatherbacks are carnivorous and feed on a variety of foods including small or juvenile fishes and crustaceans. However,
their main prey consists of jellyfish. To achieve their huge size, leatherbacks eat vast quantities of jellyfish, which they
hunt in the open ocean, either in surface waters, or by diving to depths as low as 1,600 meters/5250 feet (leatherbacks
are the deepest diving species of sea turtles). The only animals that pose a threat to adult leatherbacks are certain
species of large sharks, killer whales, and of course, humans.
Where?
Leatherback turtles have the widest distribution of all sea turtles, and have been recorded
in northerly, equatorial, and southerly latitudes. Being warm-bloodied, they are able to
withstand much colder waters than their cold-bloodied cousins. Leatherbacks range from
New Foundland and Labrador in the northern Atlantic to South Africa and Argentina in the
south. They also range throughout the Indian Ocean, and in the Pacific from the Bering
Sea to Chile and New Zealand. In Gabon, leatherback females nest on almost every
stretch of sandy beach, though there are very few records from the Bay of Corisco, the
Libreville estuary, and the beach between Port Gentil (Cap Lopez) and the mouth of the
Ogooué River. Popular nesting areas in Gabon are situated around Pongara/Point Denis,
between Ozori and Olendé, near Iguela and Gamba, and to the south of Mayumba.
Migrations
Male and female leatherbacks spend most of their largely solitary
lives swimming in the open ocean. However, once every 2 to 4
years, adult females undertake a migration to a breeding site in
order to find a male to fertilize her eggs. Where these meetings
take place is still a mystery, although it is likely to be somewhere
in the vicinity of the coast that is subsequently used by the female
for nesting. After mating, females swim towards land, frequently to
the very same stretch of beach from which they emerged as
hatchlings many years before. The mechanism used for such
extraordinary feats of navigation is not well understood, but is
thought to be based on a sensitivity to the earth's magnetic field.
How?
Nesting
Female leatherbacks are thought to nest between 3 and 6 times per
season. However, they are only likely to breed once every 2 to 4 years.
Females select beaches with steep underwater slopes without reefs and
other obstacles. Crawling ashore at night to avoid dangerous day-time
temperatures, they lumber up the beach until above the high-tide line
(nests made below this are flooded and the eggs lost). Once at a suitable
site, the female begins to excavate a broad depression in the sand, using
her powerful front flippers as shovels. This is to uncover the moist, firmer
sand that will permit the construction of the egg chamber. Having reached
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dig the chamber itself. To do this she uses each hind flipper alternately to
scoop out a measure of sand and place it adjacent to the hole. This
behaviour is automatic in sea turtles; they neither need, nor are able, to see
what they are doing, and it has been noted that individuals lacking a hind
limb will still go through the motions of moving the stump in correct
sequence with the other, present limb.
The digging continues until the female is unable to remove any further sand (at a depth of about 1 metre from the beach
surface). The chamber itself is broad at the bottom and narrows towards the top. The depth at which the eggs are
deposited is crucial, as, similar to many reptiles, the temperature at which the eggs incubate dictates the eventual sex
ratio of the hatchlings. Warmer nests will produce more female hatchlings, and cooler nests, more males. The ‘pivotal
temperature', where the sex ratio of hatchlings is approximately 1:1, is between 29 and 30°C.
Egg laying now begins, with somewhere in the region of 100 eggs being laid. Viable eggs measure about 5 cm in diameter
and become leathery to the touch rather than brittle like hens eggs. Up until this time, the female has been quite
susceptible to disturbance, and bright lights, noise, and physical manipulation may have prompted her to abort the nesting
attempt and return to the sea. However, once laying has begun, she appears relatively oblivious, concentrating only on the
task at hand.
After egg laying, the female uses her hind limbs to fill the egg chamber with the moist, recently excavated sand. As the
level of sand increases, she will use the upper surface of each flipper to firm the sand down. Once the chamber has been
completely filled in, the female begins to cover the whole site with sand thrown vigorously with her fore-limbs. At the end of
this process, it will be practically impossible to pinpoint the actual location of the egg chamber. This is less to confuse
predators than to ensure that the correct temperature and moisture levels are reached for the eggs now lying below.
During the nesting process (which may take up to one and a half
hours), the underside of the female's neck and other areas may
have become bright pink in colour. This is a reaction to the
considerable rise in body temperature experienced – the blood
vessels allowing blood to flow nearer the surface to better
facilitate cooling. The effort and heat rise also cause the female
to gasp noisily and gulp air. Another effect that may be apparent
is the ‘tears' that flow from the female's eyes. Far from being a
sign of the pain of egg laying (as was once thought), the
secretion is a mucus produced to lose salt from the body, and
also may protect the eyes from flying sand during nesting.
Finally the nesting is complete and the female orientates herself
towards the sea. She does this by locating the brightest area of
the horizon, which, in the absence of human habitation, is
generally the open sea. Exhausted, she struggles back to the
waters edge, and with a few beats of her powerful front limbs,
slips through the surf and disappears into the dark.
Egg thieves
Despite being buried almost a meter beneath the surface, turtle eggs are far from safe. The process of adaptive evolution
usually provides for either a few very well protected eggs, or, as is the case with turtles, a great many, rather more
vulnerable eggs. The incubation period for leatherback turtle eggs is roughly 60 days, and at any point during this time the
nest may be liable to attack from predators, or destruction from environmental forces. Perhaps the most voracious egg
thieves are ghost crabs (Ocypode sp). These abundant beach scavengers locate a nest beneath them, then dig a passage
and begin to pull the eggs out. The damage caused by this intrusion is likely to destroy the entire nest, even if only a few
eggs are actually taken at first. These losses can take place either at an early embryonic stage, or may equally affect fully
developed baby turtles just days from hatching. Particularly high tides may destroy nests either by flooding them with
seawater (a frequent occurrence) or by creating ‘cliffs' in the sand, thus exposing the eggs to predation. Other nest raiders
include the Nile monitor lizard (Varanus niloticus), which digs down into the nest, eating many eggs and exposing any
remaining intact to further predation.
Emergence
After a little over two months, those nests that have survived
flooding and predation are now full of hatchlings ready to emerge.
Hatchlings simultaneously begin to tear at, then crawl from their egg
shells. Before beginning to dig upwards, however, they must first
flatten out their bodies, which have for weeks been curled up within
the egg. As mentioned earlier, the newly hatched turtles do not dig
through the sand individually, but by wriggling together, sift down
the sand above them and gradually ‘lift' their egg chamber to the
surface. At this stage, hatchlings are sensitive to the temperature
gradient of the sand. If they sense that the sand above them is
warmer that that below, digging will be inhibited, and the emergence
process halted. This is to avoid emergence during daylight hours
when the temperature of the sand may cause crippling burns, and
when the hatchlings are more visible to predators. After sundown,
when the temperature gradient is reversed and the sand above the
hatchlings is cooler than that below, emergence can continue, with
the young turtles emerging into the darkness. Baby sea turtles, like
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adult females, use the luminosity of the horizon to navigate their
way to the sea.
On reaching the surface, hatchlings become immediately vulnerable to a wide range of predators. Although only a distance of
a few meters, the journey from the nest to the sea may be fraught with hazards. Hatchlings are frequently attacked and eaten
by birds, mongoose, genet, and of course, the ubiquitous ghost crabs.
Once in the water, hatchlings begin to swim directly in an offshore direction. They may continue unswervingly on this course
for hundreds of miles beyond sight of land, using the direction of the oncoming waves for guidance. It would be comforting to
imagine that upon reaching the ocean, the young turtles would finally be able to enjoy some measure of safety. Sadly for the
unfortunate hatchlings, this is not the case. Predatory fish including snappers, kingfish and sharks devour great numbers of
hatchlings as they swim out towards the relative safety of the open ocean.
Soft-shelled and defenceless, the number of individual leatherbacks lost, either during embryonic development or as
hatchlings, is enormous. The chance of a single hatchling ever returning to shore as an adult is probably less than 0.5%. And
yet upon this tiny fraction rests the continued survival of the species. Against apparently insurmountable odds, sufficient
numbers of hatchlings escape the many trials pitted against them, and return as adults to continue the remarkable life cycle of
the leatherback turtle.
Research
Despite many years of intensive research, the majority of a leatherback's life remains shrouded in mystery. It is still unclear
how long it takes for a hatchling to mature into an adult, and in Gabon, we still do not know how frequently females return to
nest, or how many nests are made per female each year. Even mating behaviour, frequently a conspicuous event, has only
ever been witnessed once in leatherbacks. The biggest mystery of all, however, surrounds the whereabouts of leatherbacks
between their entering the sea as hatchlings, and their return to the beach as adults. This period is known even by scientists
as the ‘lost years', and continues to represent a major challenge in our understanding of the sea turtle life-cycle.
In Gabon, almost all current research is focused on nesting
beaches. A new project is now attaching small satellite beacons
to nesting females to automatically track their movements after
leaving the beach, but this study is still in its infancy and technical
issues still limit the amount of information provided by the
method. A well established technique for tracking the nesting
habits of sea turtles involves marking females with small
numbered metal tags. Teams of turtle researchers up and down
the coast of Gabon walk nesting beaches at night to locate
females. Females are first measured and their general condition
noted. A tag is then fitted on the loose web of skin on either side
of the tail. The operation appears to cause little discomfort. A
central database is being constructed so that scientists finding
previously tagged individuals can discover when and where they
were last detected.
By counting the number of fresh nests made every night, it is possible to make estimations of the importance of a given beach
or ecological area for turtle conservation, and to monitor trends in population size. Other information that can be taken
includes that relative to a female's choice of nesting location, the genetic profile of a population, and aspects of turtle health
including the effects of pollution. At Mayumba, turtle research is corollary to controlling the human impact on turtle numbers in
the vicinity of the Mayumba National Park; the team at Mayumba carries out regular night patrols to collect data on nesting
turtles, and daily patrols to count the number of fresh nests and hatchling emergences.
Conservation
The Pacific population of leatherback turtles is thought to have suffered up to a 90% reduction in numbers in the last 20 years.
At this rate of decline, the species could face extinction in many parts of the world within the next few years. Although the
population visiting Gabon appears to be relatively healthy at present, there is no reason for complacency, and if this vital
population is to be maintained, urgent conservation measures are required.
As we have seen, the natural mortality rate in leatherback turtles is
already enormous. In most cases however, evolution has provided
for sufficient balance to be maintained such that populations remain
relatively stable. Human impacts on turtles have, for most of the time
we have shared the planet, been moderate, and many human
populations still collect turtle eggs or hunt adults for local
consumption, at a rate that poses little threat to the survival of the
species. However, in a great many places around the globe, turtles
and their eggs are now targeted as part of a commercial industry,
depleting numbers at a wholly unsustainable rate. In Gabon, despite
a low human population, anthropogenic threats to sea turtles are
numerous. Beginning at nesting, adult females encounter a range of
hazards. Firstly, many beaches are littered with tree trunks that have
floated away from ports such as Owendo, Port Gentil and Mayumba.
These block access to the upper beach and either force turtles to
nest lower on the beach where the nest is more likely to be flooded
by the tide, or induce the female to return to sea without nesting.
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More seriously, females may become trapped by logs, and finding
themselves unable to reverse, die from heat exhaustion.
Females can be disturbed and forced to abort nesting attempts by the unthinking actions of human onlookers. Flash cameras,
bright lamps, dogs, camp fires, the touching of turtles, noise and vibration may all cause the abandonment of a nesting attempt.
A particularly potent disturbance is the use of motorized vehicles on or near the beach during the nesting season. Turtles can
become alarmed and disorientated by cars, motorcycles, and quad bikes, abandoning nesting attempts or heading inland
rather than back to sea after nesting. Quad bikes, used on beaches in the north of Gabon, are also responsible for killing
hatchlings, and even their shallow tracks can become lethal traps for the young turtles as they rush towards the sea, greatly
increasing their chances of being killed by predators. Hotels and private residences can also threaten nesting females merely
by the amount of light given off by their premises. Some properties have powerful lights on the beach, and even those with only
standard lighting contribute to a general brightening of the night sky. As we have seen, turtles use the brightness of the sky to
navigate safely back to sea, and the light disturbance caused by human habitation can be a significant hazard. At Pongara
near Libreville, many turtles become stranded in the savannah beyond the beach having been attracted by the lights of the
hotels at Point Denis. These animals die very quickly if not returned immediately to the sea – an enormous loss of reproductive
potential. Those living near turtle beaches should be encouraged to shade bright lights during the nesting season so as to
minimise the number of strandings and aborted nesting attempts.
Turtle eggs are frequently collected for food by local
people. In some parts of the world, 100% of nests are
raided by egg collectors, leading to population collapse and
local extinctions. The rough location of a nest is found, and
a long stick used to probe for eggs. When a clutch is found,
the hunter digs down and extracts all the eggs. In Gabon,
egg collecting becomes unsustainable when it feeds
commercial urban markets, and this activity will soon
become illegal as leatherbacks gain full protected species
status in the very near future. However, research and
conservation units like the team at Mayumba can effectively
control egg collecting merely by their regular presence on
the beach.
Each year a few adult females are killed on Gabonese beaches, but
this is a relatively rare occurrence as leatherback meat is not
generally considered of great value. Greater losses are more likely
to take place at sea. Tuna fishing boats called ‘long-liners' set out
up to 60 km of line, bearing thousands of baited hooks. These
vessels come from France, Spain, and Portugal, under the terms of
a fishing agreement with the EU. Unfortunately, this method of
fishing is responsible for huge losses to species not sought by the
fishermen, including sharks, seabirds, and sea turtles. In Gabon,
there is a great need to place fisheries observers on tuna boats to
assess the level of impact of this fishery, and thus recommend a
course of action to minimise losses, such as a closed season for
long-line fishing or the use of special new hooks that reduce 'bycatch'. Closer to the coast, trawlers pose a danger to turtles as they
drag their nets through the shallows. Turtles need air to breathe,
and those trapped in trawl nets are easily drowned. Each year many
turtles wash up dead on the beach having been drowned or
otherwise injured by fishing gears. In other parts of the world,
special escape hatches (TEDs - turtle excluder devices) are sewn
into trawl nets to allow turtles to pass through without being
captured, and it is hoped that a similar system can be encouraged
in Gabon.
Leatherbacks are also threatened by the use of standing
monofilament driftnets. These nets stretch over long distances and
are almost invisible. Once again, air breathing animals frequently
drown, or are strangled to death as they struggle to escape. One
other menace encountered at sea is that of floating plastic rubbish.
Leatherbacks frequently mistake plastic bags for jellyfish and
swallow them. Such material may kill a turtle directly, or may lower
its general state of health, rendering it more vulnerable to other
pathogens or threats.
As a host to one of the most important leatherback nesting populations in the world, Gabon has a special responsibility to
ensuring the survival of the species. Conservation and research groups working in Gabon, such as Aventures Sans Frontiers
(ASF), the Wildlife Conservation Society (WCS), Gabon Environnement, Ibonga, and the Worldwide Fund for Nature (WWF),
are all working towards a brighter future for sea turtles. However finding sufficient funds to support conservation programs is
often difficult, and the sea turtles of Gabon are still in considerable peril.
Other Species
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There are four species of sea turtle in Gabon. Although leatherbacks
are the most numerous species and that most likely to be seen, at
Mayumba, you may be lucky enough to come across a nesting Olive
Ridley (Lepidochelys olivacea). This is a classic ‘shelled' turtle, which
in marked contrast to the leatherback, weighs only 35 - 40 kg. It can
be told apart from other hard-shelled turtle species by having
between 6 and 9 pairs of shell plates (costal scutes) on either side of
its central shell plates. Olive Ridleys are the most numerous sea
turtle species in the world, but are less easy to see than the
leatherbacks, and often nest slightly earlier in the season than most
leatherbacks.
The other two species found in Gabon are the Green Turtle (Chelonia mydas) and the Hawksbill Turtle (Eretmochelys
imbricate). They are primarily found in the area of Corisco Bay, to the extreme north of the country. This area is thought to be
one of the most important feeding grounds for these species on the west African coast. Both species graze on seaweeds and
sea grasses which are in short supply along the rest of the Gabonese coast. Both species have four pairs of costal scutes, but
can be told apart as the green turtle has one pair of plates between its eyes, while the hawksbill has two pairs. They are quite
unlikely to be seen on the beach, and use Gabon primarily as a feeding area. Sadly the most likely place to find these
wonderful animals is kept captive in old pirogues on the shore at Cap Esterias, or trapped in small corals, waiting to be
butchered to supply markets in Libreville and Bata. What was once a ceremonial and small scale usage by the Benga people
has now become a commercial enterprise that threatens the survival of the local population. Green and hawksbill turtles will
soon become fully protected species in Gabon, but visitors should be aware that turtle meat and soup is still served in several
restaurants in Libreville and in Cap Esterias. If you are offered turtle products for sale, you should always refuse to buy, and
explain to the vendor the dangers his or her activities pose to the species.
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