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Whale sharks: the enigmatic marine giant

The skipper cried out, “whale shark”, “whale shark”! I quickly put on my goggles and snorkel, jumped in, and gaped as the biggest fish in the world swam right up to me. I was alone in the water but within minutes two other people from the boat jumped in and joined in my awe as the whale shark floated effortlessly past us. Although it looked like it was moving in slow motion, within seconds, the whale shark was already several metres away and starting to descend. With flowing movements of our fins, we tried to catch up but in vain and we watched as this gentle giant leisurely disappeared into the depths until it was no longer visible.

Whale sharks (Rhincodon typus) have fascinated me for a long time, and it was a great privilege to see one in the warm waters of the Maldives. A species which has eluded researchers for a long time, their life history is riddled with secrets. Whale sharks can be found in the tropical and subtropical waters around the world. They are listed as Endangered on the IUCN Red List of Endangered Species and it is thought that there has been a 50% decline of the population over the last 75 years. Satellite data, acoustic telemetry, photographic identification of individuals and data collected from fisheries are some methods researchers use to study this enigmatic and wide-ranging species.

Studies have found that they can dive up to over 1000m in waters as cold at 3 degrees although this is not common. Being ectothermic, they need to spend extended periods of time in warm waters to regulate their body temperature. Feeding in the warm waters rich with plankton is a good way to recover from deep dives.

Whale sharks are filter feeders, swimming slowly with their large mouth open and sweeping up plankton, and small fish called “pelagic sprat” which can include little tunny, skipjack, yellowfin tuna, fertilized spawn of snappers as well as jellyfish and copepods. It is a species that is frequently spotted during feeding aggregations but their movements outside these aggregations are poorly understood. Some of the largest gatherings of whale sharks (up to 400 individuals!) can be seen off the Yucatan, Mexico, and the Arabian Gulf during the summer months (July, August, September), where they are known to feed on tunny eggs. Other whale shark hotspots include Honda Bay and Donsol in the Philippines, Darwin Island in the Galapagos and Baja California. Except for the Red Sea where the male to female ratio is 1:1, most whale shark hotspots are dominated by juvenile males with some reaching over 80% of the ratio. Researchers have found that 70% of the whale sharks that we see in the Maldives are male and measuring about five metres long. Where the females of this size spend their time is a mystery.

Whale sharks are “oviparous” (egg-laying), or more precisely “delayed/retained oviparous”. This means that whale sharks produce eggs which are laid inside the mother’s womb and the pups hatch in a protective environment, hence the term “delayed/retained”. This rather curious reproduction strategy (also found in nurse sharks) was only discovered in 1995, when a female whale shark was caught off Taiwan. This female had 304 eggs and young at varying stages of development inside her uterus; some embryos still in egg cases, while others already hatched and measured up to 70 cm. Two other females found off the coast of China had 16 embryos and 200 eggs, respectively. Considering the different stages of development in the female found off Taiwan, it is possible that different males sired the embryos, a strategy used by other shark species (e.g., Atlantic lemon sharks).

We are still at a loss as to where juvenile whale sharks spend their time and grow to the size where they become more visible? Do they reach an optimal size within the mother’s uterus before they become viable outside the womb? Another mystery surrounds the whale sharks which are newly born to the size of three metres. There are only three records of whale shark pups measuring around 50 centimetres from the Philippines; the most recent was spotted in December 2020 in Donsol. It is thought that due to their swimming abilities, they young pups were found close to their birthing grounds. Only 12 juveniles measuring less than 1m have been recorded. In which part of the ocean do they grow, what is their feeding ecology and what kind of behaviour do they exhibit in terms of diving depths and time spent in warm waters? According to the literature available most whale sharks encountered around the world measure between four and twelve metres. There have been reports of whale sharks in the Seychelles and along the coast of India reaching 14 metres, but they are unverified. What is their lifespan?

Just like tigers and other terrestrial mammals which are individually identifiable by their coat patterns, whale sharks can be studied using their patterns which are unique to each individual. The predictability of whale shark feeding aggregations have over the years attracted tourists wanting to see these marine giants. Marine tourism has contributed extensively to gathering information and data on whale sharks by providing photographs and videos allowing researchers to identify individuals and determine their movements. Currently 16,962 individuals have been identified with the joint effort of 310 researchers and volunteers and 9,177 citizen scientists.

Whale shark numbers are dropping, and it is partly linked to accidents and collisions with vessels from the shipping industry. With increasing marine traffic and tourist boats, the danger to whale sharks is greater. These slow-moving giants travel close to the surface and cannot make fast twists and turns to avoid oncoming marine traffic, inevitably leading to injuries and deaths. The largest fish in the ocean is also threatened by habitat loss and degradation, the meat and fin industry and polluted oceans.

As researchers begin to unlock the ecological mysteries of these giants, it will be interesting to see the picture that appears when all the data is pooled together. I have a feeling that a whale shark photographed in the Maldives will turn up in Mexico or the Philippines a few years later looking much bigger! Perhaps when we dive in Indonesia next year, we might encounter the same whale shark we saw in the Maldives and contribute to expanding our knowledge of this mysterious giant.

Thank you to Ornella Weideli and Lynn Jula Kessler for your notes and images. Find out more about the exciting work these two marine biologists do on their websites and instagram accounts.

Ornella Weideli: , @ornellaweideli

Lynn Jula Kessler: @lynn_jula


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