Where the Boys Are
Whale sharks (Rhincodon typus) are typically studied in feeding aggregations that occur annually in locations around the world. Travel to Djibouti in January, the Philippines in March, Western Australia in May, or Mexico’s Yucatan Peninsula in July, and you will find large numbers of whale sharks massed to feed on a transient food source. Usually this is a fish or coral spawning event, and the sharks arrive yearly like clockwork. As a study population these aggregations are imperfect; most are composed largely of juvenile male sharks less than 8m in length. Adult males, and females of all ages, only rarely join these planktonic feeding frenzies. While these aggregations can tell us only about the life history of young male whale sharks, in their exclusivity they actually highlight one of the most tantalizing questions about these mysterious creatures.
We know little about whale shark reproduction, and the data we do have comes almost entirely from a single pregnant female caught in a Taiwan fishery in 1995. This animal carried 300 developing pups in her uteri; if this number is typical for whale sharks it means a huge number of pups are born each year. These tiny sharks then immediately vanish - only 15 newborn whale sharks have been scientifically documented! Where do all the pups go? Many are probably eaten by large fish and other sharks, but there is still a big disconnect between whale shark fecundity (how many offspring a female has) and the number of small whale sharks seen. Many researchers suspect that newborn whale sharks utilize a unique habitat we have not yet discovered, where they spend the first year or two of their lives growing large enough to survive in the open ocean.
When we first find young whale sharks in significant numbers they are about 2.5m in length, and to an amazing degree they are off the coast of Djibouti. Djibouti has the smallest average size of any whale shark aggregation, as reported in a paper by David Rowat and crew. (David is the director of the Marine Conservation Society Seychelles, and an SRI International Director.) As reported by David and his team, the mean size for Djibouti sharks is only 3.7m, with animals as small as 2m. How old is a 2m whale shark? A newborn whale shark raised in captivity in Taiwan, about 60cm at birth, reached 1.4m before he died at the age of 4 months. Beyond that age there is no information until ~4m in length, where a few aquariums have documented whale shark growth rates. The rapid growth of newborn animals is not sustained - the growth rate of the 4m animal was much slower - so perhaps those 2m sharks are just reaching their first birthdays?
Rowat et al used spot pattern identification to study the Djibouti aggregation, and a separate population in the Seychelles, over a number of years. The Seychelles hosts a different population of animals, with a mean length of 5.8m. They found differences in the number of years that animals visited the two aggregations, with Djibouti animals staying for a shorter period of time than Seychelles. The small size and shorter return period of the Djibouti sharks may indicate that this region is a sort of entry point for young whale sharks, and they stay only long enough to grow to a size where they can compete with larger animals at other aggregation sites. I sometimes explain whale shark feeding aggregations as high schools, with their large populations of adolescent males. By that analogy Djibouti is a whale shark primary school, a place where young animals first enter the whale shark social scene. Where do they come from? Where do they go when they leave? So many questions…..
The paper is: Rowat, D, Brooks, K, March, A, McCarten, C, Jouannet, D, Riley, L, Jeffreys, G, Perri, M, Vely, M and Pardigon, B. (2011) Long-term membership of whale sharks (Rhincodon typus) in coastal aggregations in Seychelles and Djibouti. Marine and Freshwater Research, 62: 621-627 .
It can be found here: http://www.publish.csiro.au/?paper=MF10135