New research led by Australian scientists shows that the biggest fish in the sea are female!

Introducing the whale shark (Rhincodon typus). Officially known as the world’s largest fish, they grow up to maximum known size of about 60 feet (18 meters) total length (TL). Their large size implies that they are slow to grow and therefore have longevity, but data on the growth patterns of whale sharks is very limited. This is a problem for scientists since age and growth data are central to the management of any species, especially one of this immense size. Whale sharks also have low resilience to anthropogenic threats such as overfishing, warming oceans and ship strikes. In fact, declines in abundance have led to these majestic animals recently being classified as Endangered by the IUCN Red List.

This is especially true for whale sharks that call Ningaloo Reef home. A World Heritage-listed site, Ningaloo is found halfway up the West Australian coastline. Although 500 species of tropical fish are found within Ningaloo Marine Park, it is the whale shark that garners the most attention. Large numbers visit every year from April to July, along the world’s largest fringing reef that measures 162 miles (260 kilometers) long. This decade-long study, led by Dr. Mark G. Meekan of the Australian Institute of Marine Science at The University of Western Australia, recorded the growth patterns of free-swimming whale sharks at an aggregation site at Ningaloo.

This whale shark population has been monitored for a while, including having their photos taken for identification and stereo-video measurements of size throughout the years. But how can the scientists tell them apart? This constellation-like pattern sure does look alike! While to the untrained eye they look identical, these spot and stripe patterns are unique to individuals. Therefore, photographs of these patterns taken by snorkelers, divers, and trained scientists can be used as an identifying tag in mark-recapture studies.

A total of 54 (6 females, 48 males) whale sharks were resighted and successfully measured across the 10-year sampling period. The results reveal that female whale sharks grow more slowly than boys but end up measuring longer! These results don’t come too much of a surprise since there exist numerous examples of shark species where females grow to larger sizes than males, including great hammerhead (Sphyrna mokarran), bonnethead (Sphyrna tiburo), blacknose (Carcharhinus acronotus), and shortfin mako (Isurus oxyrinchus) sharks.

This gentle giant has captured many hearts around the world, and in Western Australia (WA) it has become their marine animal emblem! In 2016, the International Union for Conservation of Nature (IUCN) officially declared the whale shark Endangered. The IUCN Red List classifies animals worldwide into categories ranging from Least Concern to Extinct, and with this recent change in classification for the whale shark only two categories (Critically Endangered and Extinct in the Wild) stand in the way of us… well, possibly losing these beautiful animals. Thankfully, they are protected in Australian waters under both state and federal law.