Scuba diving with loggerhead turtles in Isla Mujeres
Updated: Jun 23, 2022
The season is about to begin. Learn more about these incredible animals and the ways to encounter them.
Loggerhead turtles (Caguamas)
Isla Mujeres is blessed to be privy to some spectacular seasonal activities. From May to September we see hundreds of whale sharks come to feed just off the coast, from November to February the eagle rays return to hang out at our shipwrecks in huge numbers, and during the year we also get to watch the return of turtles, especially the gigantic loggerheads, to the island for the yearly mating and nesting.
From May to July in the crystal clear, turquoise waters of Isla Mujeres, the life cycle of the loggerhead turtles begin. Females return to their nesting grounds to find eager males awaiting them in the currents off the south end of the small Caribbean island called Isla Mujeres. This seasonal migration creates some unique diving experiences and for non-divers, something magical can be witnessed on land as well.
The night dive is a relaxing, peaceful dive where you will have to perfect opportunity to see large numbers of resting sea turtles.
The drift dive is for the more advanced and adventurous and you will literally be flying amongst dozens and dozens of these mating giants of the turtle world.
Both dives are spectacular and are bucket list dives
At night, between May and October, the females come ashore on the Eastern, windward side of Isla Mujeres to lay their eggs in the sand of the beaches that they themselves hatched upon. By respectfully maintaining your distance, being with a registered guide from the Isla Mujeres Turtle Sanctuary, Tortugranja, and not using a flashlight, you can watch these massive turtles drag their huge bodies, designed for a life in the water, upon the sandy beaches to lay their eggs. These eggs are largely collected by volunteers so that they may be hatched in Tortugranja to give them the best opportunity for survival, free from predators and clumsy tourists.
Turtle releases are organized by Tortugranja between July and October and children line up to receive their buckets of baby turtles to be released into the ocean. These releases draw huge crowds of both local families and tourists.
Of the seven species of sea turtles, four of them live and make their nests off the Eastern coast of Mexico. These include Hawksbill turtles, Green turtles (Tortuga Blanca), Loggerhead turtles (Caguamas), and occasionally the grand leatherbacks.
May to July: mating
May to October: egg-laying
July to October: turtle release
Diving with the Loggerhead turtles
Everything we talk about in this blog leads to some of the most amazing dives that a diver with experience can do.
At South Point (Punta Sur) of Isla Mujeres, from the cliffs above on a calm day, you can see the sheer number of turtles that converge in strong water currents there. Under the water, it is a mating frenzy. At the peak of the season, you are virtually guaranteed to see a staggering number of mating couples.
The drift dive starts with, if the current is in your favor, passing by a huge Spanish anchor and chain. Then the very strong currents carry you flying around the end of the island, taking you a long distance around. This current is perfect for our amorous couples and you will find yourself distracted by one pair only to be carried onto the next and the next and the next.
Dolphins are commonly seen during this time and can be heard on nearly every dive. To see dolphins in their natural habitat is an experience way beyond seeing those in captivity. In fact, it is the ONLY way that they should be viewed. Also common on these dives are sighting some of the 7 types of rays found here, batfish and schools of barracuda.
The strength of these currents means that divers that attempt these dives should have experience beyond initial dive training as extra vigilance is required to stay with your group and your buddy. Being distracted can mean being separated and as such, only those who can respect the ocean and its ways should dive these dives. Good buoyancy is required and the ability to follow an experienced guide and respect their briefings is a must.
If you are unsure of your abilities, then it is recommended that you have a dive on the island previous to doing this dive to retune your buoyancy, refine your weighting and trim and evaluate your abilities. It will be worth it, it will be one of the most amazing dives you ever experienced.
Night dives are another way to experience diving with turtles as many come to rest under ledges and around the reefs of the Manchones National Park. The park is very shallow (only 9mt) with very little current and amazing visibility. Dozens, if not hundreds, of turtles, can be seen on a night dive but we must understand that these animals are at rest (mating and nesting can be hard work) so we should not disturb them nor shine our lights directly at them.
Even on the most normal, shallow dive for the less experienced diver, the chances of seeing some turtles are very high, we see them on nearly every dive during the season. It is awe-inspiring to come across a loggerhead turtle swimming in the same waters as you.
About loggerhead turtles
Common name: Loggerhead turtle (Caguama in Mexico)
Scientific name: Caretta caretta
Family: Cheloniidae
Order: Turtle
Group name : Flotilla
Type: Reptile
Diet: Omnivorous
Sexual Maturity: 17-33 years
Life span: 47-67 years
Size in carapace : 78-95cm/28-37inches
Weight: 80-200 kg/180-440lb
Average weight: 135kg/298lb
Largest recorded: Length: 213cm/84in
Weight: 545kg/1202lb
Classification: Vulnerable - listed in 1978 as threatened under the Endangered Species Act
Loggerhead turtles are considered to be keystone species meaning that they help define an entire ecosystem. Without them, the ecosystem would be dramatically different or cease to exist altogether.
The very way that they feed keeps the ocean floor sediment in balance by digging around the bottom. This changes both its physical structure and the living biological ecosystem, helping the community organization where they forage. The movement of the bottom sediment also results in a rise of oxygen in the water and the overall productivity of the ecosystem benefits and improves because of this.
The Loggerheads feed on large numbers of invertebrates, recycling important nutrients from these hard-shelled prey. The shells of these prey pass through their digestive system and what the turtles excrete falls to the ocean floor as a calcium source, fed upon by other animals.
The turtle shells themselves act as an important habitat for many species of plants and animals. As many as 100 species have been recorded on a single turtle, including barnacles, crabs, and algae. It is unknown if this is a symbiotic relationship that allows the turtle to receive any benefit, although they may provide the turtle some camouflage. One beautiful phrase I read describes this so eloquently by James R Spotila. "Mobile islands transporting hitchhikers across the globe, they are like floating reefs"
Why are they here?
They are the most widely dispersed of all the sea turtles; existing in the Pacific, Indian, and Atlantic Oceans, and the Mediterranean and Caribbean Seas. Dr. Brian Bowen, a conservation geneticist, noted that there are, however, distinctive differences among the loggerheads around the world, and the use of modern molecular genetics has allowed historic relationships to be defined among populations and also within a region. Dr. Bowen, his students, and colleagues used mitochondrial DNA to determine the unique distribution of loggerhead turtles. In other words, they can tell you what happened and how it came about over millions of years of the Earth's evolution. Sea Turtles: A Complete Guide to Their Biology, Behaviour, and Conservation wrote by James R Spotila (1) explains this in very fascinating detail. He donated all proceeds from this book to The Leatherback Trust.
The greatest concentrations of Loggerheads live on the coasts of Mexico, Cuba, and Northern Bahama, up the coasts of the United States, and up to the Eastern coast of Canada. The greatest concentrations in the world exist in Florida with about 67000 nests a year. Mexico has about 1200 nests a year. (1)
Mature females return to lay their eggs on the beaches where they themselves hatched. They do this by navigating Earth's invisible magnetic field. Each coastline has an individual magnetic signature that turtles can remember and use as guides.
Just off the Southern point of Isla Mujeres, the female turtles are drawn to return to this area to mate in the strong currents, and the amorous males await them. This combination of seasonal return and the proximity of these currents to the island allows for an incredible diving experience for the more seasoned diver, unparalleled anywhere. It is not an unusual dive to be amongst 40-60 of these mating turtles at the peak of the season.
What do they eat?
The Loggerhead is omnivorous. Although primarily a carnivore, its diet consists of a variety greater than any other sea turtle.
They feed principally on bottom-dwelling invertebrates such as gastropods, bivalves, and decapods. but the list does not stop there. Their diet also includes sponges, corals, sea pens, polychaete worms, sea anemones, cephalopods, barnacles, brachiopods, isopods, Portuguese men o' war (brave turtles), insects, bryozoans, sea urchins, sand dollars, sea cucumbers, starfish, fish (eggs, juveniles, and adults), hatchling turtles (including members of its own species), algae, and vascular plants. During migration through the open sea, loggerheads eat jellyfish, floating mollusks, floating egg clusters, squid, and flying fish. (3)
Because their diet consists of jellyfish and benthic feeding, it is easy for plastic bags and plastic by-products to be mistakenly ingested.
Since the 1940s plastic has been mass-produced and the impact of this has been devastating to sea turtles. Up to 52% of the turtles in the world have eaten plastic and all of them are at risk.
According to a study from the University of Tokyo (6), loggerheads are consuming plastic in an alarming quantity with 17% of loggerheads consuming plastic upon encountering it and 62% of Green turtles.
This consumption of plastic is devastating. It is basically a death sentence to 22% of all turtles that consume it. Sharp plastics rupture their internal organs and plastic bags block the intestines, causing the turtle to starve to death. Even if they manage to survive this, the plastics can cause them to be unnaturally buoyant which in turn causes their growth to be stunted and leads to slow reproduction rates (7)
Most of the plastic consumed was either clear or translucent (similar to jellyfish) followed by the color white. Black plastics ingested consisted of mainly plastic bags with green and blue being mainly plastic ropes and strings. Red, orange, yellow, and brown were found the least (9). Why is this important? Because it may affect, at least, your consumer choices and habits; although we should all be plastic conscious, avoid all single-use plastics, and dispose of other plastics correctly or avoid them altogether.
How do they eat?
The Loggerheads have huge heads and powerful, massive jaws (hence their name) that are well adapted to crush their hard-shelled prey.
On the anterior margin of their front limbs (forelimbs), they have projecting scale points that they use as "pseudo claws" that help to tear large chunks of food in their mouths. Food is filtered out in the fore region of their esophagus by inward-pointing, mucus-covered papillae that filter out fish hooks and foreign bodies. Further down, the esophagus lacks the papillae but has many mucosal folds.
The digestion of loggerheads, as with many other aspects of this species, is temperature-dependent. As the temperature increases, so does their digestion.
Mating
The male loggerheads arrive before nesting season and wait in the mating grounds for the females to arrive. These are usually located offshore from the nesting beaches and fortunately, in Isla Mujeres, is very close to the Southern end of the island at South Point.
When the courtship and mating begin, the male will circle the female, then approach and bite her neck or shoulder. If she accepts him when he tries to mount her, they will mate. If she does not accept him, she will cover her cloaca and swim to the bottom. A persistent male will wait until she needs to surface for air and try again. (4)
The same claws on the forelimbs that were discussed above for use in feeding are for also holding onto the female during mating. The mating may last for hours and often other males will try to dislodge them by raming and biting the male. It is not uncommon during a dive here in Isla Mujeres to see one or even two other males pilling up on a male and female during mating. A dislodged male is quickly replaced by another.
A female will lay several clutches during a nesting season and will re-mate each time. Sometimes she will mate with several males so a clutch may be fertilized by more than one male's sperm. During this mating and nesting season, the males will remain in the water offshore of the nesting beach and the females will alternate between mating, nesting, and feeding.
For each nesting, she must drag herself onto the beach where she is in great danger of predators. She will spend one to two hours nesting and she will bite if disturbed.
Nesting
Average of 3.9 clutches per season
Intervals of 12-17 days during nesting season
Average of 112 eggs
Eggs weight 32.7g/1.2oz
Returns to nest every 2-4 years
Takes about 1-2 hours to complete the nesting process
Generally at night in the open
The sex of loggerheads is determined by a wider range of pivotal temperatures than other sea turtles. A pivotal temperature is a temperature in which a ratio of 50:50 male: female is produced; generally between 28ºc and 30ºc.
Temperatures of 24ºc to 26ºc tend to produce males and temperatures of 32ºc to 34ºc tend to produce females. Outside of the extremes of these ranges, eggs are just not viable
It takes 45-80 days for an egg to hatch (depending upon temperature)
Hatchlings at the center of a clutch are often the largest, more active in the swimming frenzy of the first few days at sea, and grow faster
At hatching, they weigh approx 20g (0.7oz) and are 5cm/2in length.
Hatchlings use visual cues to find the sea, responding to an arc 180º horizontally and 30º vertically. They appear to orient to the brightest light, the lowest elevation, and can recognize some shapes.
Hatchlings will grow for 2-3 decades before returning to the nest
Female loggerheads will bite if disturbed while laying (with those jaws, something you do not wish to happen, so do not disturb!!)
Migration
Loggerheads are highly migratory and are found in all but the most frigid water on the planet
When hatchlings leave the nest they are 5cm/2in and when they return they are 50cm/20in (Atlantic loggerheads. Other regions are different sizes). For a long time scientists did not know what happened to the turtles in this time but thanks to biologists Alan Bolten and his wife Karen Bjorndal, and the use of molecular genetics, radio and satellite remote sensing (biotelemetry), and computer technology, much was discovered. (2)
They found that after hatching, for the first few days they feed on small animals and inanimate objects such as debris and oil droplets but then find their way to floating sargassum algae. These mats are home to so many forms of life; both plants and animals. It is here that the turtles live at or near the surface and they ride the currents as they leave the continental shelves and enter the open ocean.
They then spend 6-12 years at sea, spending the majority of their time, 75% of it in fact, at the top 5mt/16ft. As they get older they may dive to 200mt/650ft.
The Atlantic loggerheads return to the continental shelf (Neritic zone) when their shells are around 50cm/20in (this size varies depending upon the world