If you have ever planned a holiday with the hope of seeing the world’s largest fish, you will more than likely have come across the term “whale shark season”, but what does this mean and why would whale sharks only occur seasonally?
Whale sharks (Rhincodon typus) are known to aggregate in a number of coastal areas around the world. Aggregations typically occur for a select few months each year and because of their predictability, we refer to them as seasons.
In many cases, the arrival of whale sharks to a certain area coincides with a local boost in productivity, i.e. when there is an increase in the amount of food available. Whale sharks feed on plankton which is made up of microscopic plants and animals. Many marine species including fish, crabs, jellyfish and corals, have a planktonic stage: a period of their life when they are microscopic and live suspended in the water column, often in larval form. Some of these species synchronise their breeding activity, meaning that all of the individuals in an area will release their eggs and sperm into the water column at the same time, often on the very same day (or night as is usually the case). The result of this is that there is a rapid increase in the planktonic community in that location. This is what the whale sharks show up to feed upon.
The animal which spawns and subsequently attracts whale sharks can differ depending on location. For example: at Gladden Spit in Belize, whale shark numbers peak between April and May to coincide with a snapper spawning event; at Ningaloo Reef in Western Australia, they aggregate from April until July in order to feed on coral spawn; and at Christmas Island in the Indian Ocean, they are present between December and March when Red Land Crabs take to the sea to spawn.
The reason that whale sharks visit the Galapagos Marine Reserve (GMR) has yet to be truly established but several recent observations may have given some insight into this. In October 2012, researchers from the Galapagos Whale Shark Project observed a whale shark feeding in an area where various species of snapper and grouper were spawning. Incredibly, this was the first recorded sighting of a whale shark feeding within the GMR. During the same research expedition, the team were able to attach satellite tags to a number of whale sharks. After analysing the movements of these tagged sharks it appears that individuals only stayed within the GMR for a few days before continuing on with what appears to be a large scale migration. Consequently, one theory that is being investigated is that whale sharks may use the Galapagos Islands as some kind of migration waypoint, with some individuals feeding opportunistically en route. We hope that further research will shed light on such questions.
If you would like to help conserve the largest fish on Earth there is still time to donate to our 2013 Whale Shark Appeal.
by Pete Haskell