Whale sharks are arguably one of the most iconic and beloved gentle giants, and scientists report they have gone MIA (missing in action).  One might imagine that it shouldn’t be hard to spot a fish the size of a school bus in a vast, blue ocean, however a recent scientific study has proven otherwise. After the study was published last Wednesday in the journal Royal Society of Open Science, an outpouring of international public concern and shock has erupted over the mystery of the disappearing whale sharks.  The study primarily analyzed data on whale shark populations of the Ningaloo Reef System in Western Australia, one of the most well understood and researched reefs in the worlwhale sharkd.  The study looked at decades of whale shark data, primarily from Ningaloo, but also from sources around the world, and found two main trends:

  1. Whale shark sightings are becoming scarcer.
  2. The size of whale sharks seen today are drastically smaller than historical trends, indicating they majority of sighted whale sharks are juveniles.

Whale sharks, due to their solitary lifestyle and ability to travel long distances, have not been well studied, creating basic knowledge gaps that further complicate matters.  Scientists do not know the details of their lifestyle, migration patterns, or even concrete global population numbers making it difficult to draw broad conclusions when there are still many unknown variables. Even with these knowledge gaps, researchers are certain that the whale shark population has decreased globally, however they do not fully understand how sharply it has declined over time.  This new data raises the important question: Where have all the adults gone and why have they disappeared?

Shrinking Whale Sharks?

Juvenile whale sharks average six meters (20 feet) from nose to tail, while full-grown adults are at least nine meters (29 feet).  In 1995, whale sharks measuring about 13 meters (45 feet) were frequently spotted, but by the early 2000’s the largest shark was only 10 meters (33 feet), while in 2011 the size again shrank to only 8 meters (26 feet).  As the size of the largest shark on Ningaloo reef decreases, the mean length of that population also decreases, meaning that since the early 2000’s the majority of sharks seen are juveniles.  Whale sharks are known to inhabit the open ocean from India to Belize, making them difficult to study.  Similar reports from other researchers confirms this decrease in appearance of adults, with the exception of two known populations of adult females in the Eastern Pacific.

Whale shark coastal aggregation.

Whale shark coastal aggregation.

Whale sharks have been known to display a behavior called coastal aggregation, where they come near the coastline to feed in a large group.  The most recently documented aggregations have only been composed of juveniles, while in the past many adults also partook in these social events.  Scientists wonder whether the adults have gone to other lesser known locations, or if the adult population has really shrunk so dramatically within the last two decades.  Whale sharks are thought to live until they are 80 years old, so a healthy, viable adult population is necessary to sustain the species, since it takes many years to reach sexual maturity due to their slow growth rate.

 

Threats to Whale Sharks:

The International Union for Conservation of Nature (IUCN) has deemed whale sharks’ status as vulnerable, which falls under the umbrella of threatened species.  The IUCN has already recognized that their population as a whole is declining due to human pressures on the ocean.  Though whale sharks are legally protected by the governments of Australia, Mexico, and China, they still face threats from the fishing industry. Whale sharks are unfortunately still hunted for their fins and oil (from their liver), to be sold on primarily Asian markets as delicacies or traditional medicine.  As a victim of shark finning, whale shark’s fins are sawed off while still alive and then the rest of the body is harvested for its oils.  Whale sharks could also be injured by boat collisions as like true whales, they are large and slow making them easily susceptible to boat strikes.  Recent reports have shown that whale sharks are often bycatch of large-scale fishing operations that use huge mile-long trawling nets to harvest fish.  The various pressures due to human activity could be responsible for the decline in adult sharks.  However, many scientists also believe that natural behavior and biology could be the reason less adults are spotted.

They argue that due to the lack of information on where whale sharks spend most of their time, it could be very possible that older sharks do not partake in coastal aggregation as frequently as youth.  Perhaps it is that the adults prefer the open waters rather than the reefs ecosystems.  Though this is possible it still doesn’t explain why in the 1990’s there were large, adult sharks intermingled inhabiting the Ningaloo reef system while there are hardly any there today.  To find a more definitive answer one of the study’s authors, Mark Meekan of the Australia Institute of Marine Science, recommends tagging and tracking juveniles and adults to better understand their lifestyles patterns.  Despite all of the gray area, whale shark populations are definitely on the decline and it is most likely due to overexploitation.  Whale sharks are truly worth more alive than dead, as there is a bustling ecotourism industry centered around whale sharks in Mexico, Australia, and many areas of South America.

Hopefully the case of the disappearing whale shark will be solved in the near future through further studying this unique species; in addition, the world hopes their population will be able to rebound.