From the data gathered over all those years we have learnt that the number of whale sharks visiting Ningaloo is between 200 and 500, and that most of them are males and immature (3–8 metres long). On average we encounter four or five individuals per day, with a record tally of more than 20 in a single day.

Interestingly, sightings of whale sharks less than three metres long or of females are scarce at the reef, as is the case for most of the aggregations in the Indian Ocean. So where are the whale shark females and pups? It is believed that nursery grounds, where the females give birth and the young spend the first few years of their life, may be found in the open ocean. When whale sharks are born, they are no bigger than 50 centimetres, which makes them vulnerable to all kinds of predators. So staying in open waters until they are bigger and stronger seems like a very smart way to avoid the sharks and other possible predators that frequent coastal areas.

Whale sharks are not sexually mature until they reach seven to eight metres, so our contact with the species at Ningaloo is mostly with ‘teenage’ males. From a conservation point of view, it is important to obtain information about their likes and dislikes and where on the reef they spend most of their time, as these animals constitute the younger generation. Specifically, I am looking at their diet, which will drive their movement patterns.

In order to find out what whale sharks are eating and where they are feeding, we look at the animal itself. Surprisingly, a small piece of it, in this case a skin biopsy, can tell us a lot of things. We base our research on the principle ‘you are what you eat’, a concept that has been used repeatedly in the past and stems from the fact that every time you eat something, the food leaves chemical traces (such as lipids) in your tissue. By looking at these traces in the skin tissue of a whale shark, we can track down the food source.

A whale shark biopsy is a piece of connective tissue no more than two centimetres long that is taken from the skin. To get a sample, an experienced free-diver swims with the animal and, using a modified spear gun, takes the biopsy. Many dietary studies require that the animal dies so that researchers can examine the contents of its stomach or sample other organs. Our method is less invasive while still providing very useful information, an important consideration when studying threatened species such as the whale shark, as we don’t want to kill or harm any individuals. Over the years, we have been building up a significant collection of biopsies by this method and they are now producing some interesting results.

We are noticing that groups of whale sharks showing different diets visit Ningaloo Reef each year, which makes us think they come from different locations. It is likely that these findings reflect the different migratory routes – and different prey – taken by the whale sharks that come to the reef. When our results are coupled with tag data, this idea seems increasingly probable, as there is evidence that some Ningaloo whale sharks travel as far as South-East Asia, for example to the Timor Sea and Indonesia, whereas others prefer to stay around the Australian coast. We like to think of Ningaloo being a whale shark ‘meeting point’, although we don’t yet know why they aggregate here.

Whale sharks are widely known to be pelagic filter feeders as they have often been observed in surface waters feeding on red crab larvae at Christmas Island in the Indian Ocean, fish spawn on Belize’s reefs or tuna eggs in the Red Sea. For many years local fishermen at Ningaloo have seen them feeding in a similar fashion on swarms of krill. This leads us to believe that these tiny shrimps are the main reason that whale sharks approach the coast of Australia. However, even our earliest findings indicate that whale sharks are not just pelagic filter feeders and that their diet is far more complex than we previously thought. It seems that they also feed on prey that lives in deeper waters, such as mysids, fish and zooplankton.

We are at a very exciting stage in whale shark research, as more and more effort is going into it. We hope to gain a better understanding of the species’ diet, which in turn will give us insight into why it forms aggregations. From the little that we know so far, it appears that whale sharks tend to approach coastal areas as they grow, and sometimes undertake long migrations between aggregations. Movements such as these could make them vulnerable to exploitation, as they are likely to cross into national territories where they are not protected. If that is the case, the negative effects of whale shark fishing thousands of kilometres from Australia may be reflected in Ningaloo’s population. In this context, it is essential that countries with whale sharks in their coastal waters cooperate to develop adequate conservation programmes.

At Ningaloo Reef and many other places around the world it has been shown that an ecotourism industry based on whale sharks can be very lucrative, bringing in good revenue for local communities as well as being beneficial to whale shark research. We still have a long way to go, but step by step we will unravel the mysteries of these giants.