Australasian Science: Australia's authority on science since 1938

Hide & Seek

whale shark

Whale sharks are drawn to Ningaloo Reef in autumn when they feed on dense swarms of krill. Credit: crisod/iStockphoto

By Mark Meekan

Whale sharks may be the largest fish in the ocean but they are particularly elusive. Researchers are now using photographic and genetic methods to find out their migration patterns and determine the best conservation strategies to protect them from threats posed by shipping accidents and unregulated fishing.

Whale sharks were first described in 1828 after one was harpooned in Table Bay, South Africa. Fortunately for whale sharks, such lethal interactions with humans proved to be very rare events, and in the following 150 years only a few hundred of these sharks were ever sighted.

Just how difficult it was to find these animals in the open ocean is shown by the fact that the pioneering diver, Jacques Cousteau, only ever encountered two of these sharks in a lifetime spent at sea. Then in the mid-1980s, Geoff Taylor, a doctor who spent his free time exploring Ningaloo Reef in Western Australia, made a remarkable discovery – every year from around March to May the waters just off the reef played host to aggregations of whale sharks.

Taylor’s observations began a revolution in both our attitudes and knowledge of these animals. For the first time researchers could predictably access and observe whale sharks, gaining an insight into the behaviour and ecology of a species that otherwise leads a cryptic life somewhere in the open ocean far from the coast.

Since the pioneering work by Geoff Taylor, aggregations of whale sharks have now been discovered at locations spanning tropical and warm-temperate oceans worldwide, from the coast of east Africa to the Seychelles and the Maldives, off the coast of South-East Asia, in the Philippines, along the Pacific coast of Mexico and throughout much of the Caribbean Sea. The size (up to 18 metres long and 34 tonnes in weight), beauty and gentle nature of these harmless, filter-feeding sharks makes them part of the world’s charismatic megafauna. Because whale sharks are now iconic and a source of fascination to many people, aggregation sites are a huge drawcard for tourism, with most now the subject of profitable industries that allow snorkellers and divers to swim with these sharks in the wild.

In a similar way, these ephemeral but predictable aggregations are also a beacon for researchers, and they prompt many questions. What are whale sharks doing near the reef when their usual habitat is far offshore? What is attracting them to these shallow environments? Where do they go after they leave? And, given that these aggregations are an important source of tourism revenue for local economies in many developing nations, what is the future for whale sharks in a changing world?

The first clue as to why whale sharks might be aggregating at Ningaloo Reef was the observation that abundances of whale sharks seemed to peak around the time of coral spawning. Mass spawning by corals is a spectacular event where colonies simultaneously release billions of packets of sperm and eggs into the water. The oil-filled eggs are a rich food source for many reef animals, and they rise through the water column, forming dense slicks at the surface in calm weather. At this time, whale sharks can be seen swimming at the surface, with their mouths agape, top lip exposed above the water while they collect eggs using the specialised filter plates that line the gills.

But coral spawning only lasts a few days and the whale sharks spend several months at Ningaloo Reef, so we know that coral spawning is probably just a bonus food source rather than the central reason for them aggregating here.

Our recent studies show that whale sharks are drawn to Ningaloo Reef to target dense swarms of a small, shrimp-like krill (Pseudeuphausia latifrons) that are particularly abundant in autumn. We know that whale sharks feed on these swarms both through direct observation and genetic analyses of their faeces, which are packed with the partially digested (and very smelly) exoskeletons of their tiny prey.

The appearance of sharks to target seasonally abundant prey sources also seems to explain the existence of many other aggregation sites – the arrival of whale sharks around Christmas Island to the north of Ningaloo Reef coincides with a spawning event of millions of land crabs; in the Seychelles whale sharks appear when coastal productivity peaks; in the Caribbean Sea aggregations form to target tuna and reef fish spawning events; and in the Coral Sea whale sharks appear when dense schools of small pelagic fish are spawning.

Once the seasonal pulse of plankton productivity diminishes at Ningaloo Reef in mid-winter, the whale sharks move on. But where are they heading to next?

The answer to this question is being gradually revealed by satellite tagging. This type of technology is a popular (albeit expensive) means to track the migration patterns of fish, but one that in the case of whale sharks presents some special challenges. Unlike most other fish, whale sharks cannot be caught and restrained in order to fit a tag. Instead, tags must be attached to free-ranging sharks, usually by snorkellers who swim alongside the animal.

In the past decade funding from Apache Energy Ltd and both the state and federal governments has allowed us to use this technique to attach more than 60 satellite tags to sharks at Ningaloo Reef. These have tracked the animals moving to the north: some into the open Indian Ocean, others to the Indonesian Archipelago and around one-third into the waters of South-East Asia.

Our results suggest that the whale sharks returning to Ningaloo Reef each year are part of a broader regional population that probably ranges across the waters of neighbouring countries to the north, including Indonesia, East Timor, Papua New Guinea and into the eastern Indian Ocean. Where they go beyond these places is difficult to determine because whale sharks are very good at ridding themselves of satellite tags after they have been attached for more than 6 months. For this reason, longer-term migration pathways are still unknown.

The idea that Ningaloo Reef’s whale sharks are part of a regional population is supported by our work that uses body markings as natural “tags” to follow their movements. Because the spot and stripe patterns that occur on the sharks are unique to each individual, photographs of a standard body part (the side of the shark behind the last gill slit and forward of the dorsal fin) act as a “fingerprint” of each animal.

With the aid of ecotourism operators, divers and researchers visiting and working at sites across the Indian Ocean, we have been able to accumulate many thousands of these ID-photos. Simple image-matching software has enabled us to search these libraries for any evidence of large-scale migrations of whale sharks across the entire Indian Ocean.

Exhaustive sifting through this massive data base compiled from Ningaloo Reef, Christmas Island, the Maldives, Seychelles and Mozambique revealed only one match, and this was an animal that swam between the waters of the Seychelles and Mozambique, staying in the region of the western Indian Ocean. Much like our tracking data, the results of this photo-identification study also suggest that if whale sharks are making migrations across the entire Indian Ocean, it is happening very infrequently.

But the genetics of these sharks shows that the story of their migration may be a little more complicated than we might be tempted to believe. Whenever we encounter whale sharks at Ningaloo Reef we take a small skin sample. Over the past decade, these have been contributed to a large collaborative study of their genetics that has involved groups of researchers working on whale sharks throughout the world.

Published last year in the journal Molecular Ecology, our study found little evidence for any genetic differentiation of populations across the entire Indo-Pacific. Only whale sharks in the Caribbean Sea formed a distinct genetic group, a result that was not surprising given that the tips of the continents of South America and Africa, which extend into cool temperate waters, probably form a major barrier for movement of these tropical sharks from the Indo-Pacific into the Atlantic.

But do these genetic results mean that whale sharks are routinely moving between locations right across the Indo-Pacific? Based on distribution patterns and likely movement rates, our recent modelling studies of their dispersal patterns in the open ocean suggest that they are almost certainly capable of doing so within timeframes of 2–3 years.

However, this genetic and theoretical data doesn’t necessarily mean that such broad-scale dispersal is happening frequently, or at least often enough to make a major difference to the management and conservation strategies for the species. This uncertainty exists because genetic studies must always be interpreted in the context of the biology of the subject animal, and in the case of whale sharks this involves some very unusual biology for a fish.

Evidence we are gathering from the first validated ageing study of whale sharks now confirms that these sharks are very long-lived – possibly attaining more than 100 years of age – and grow very slowly, not reaching maturity until they are around 30 years old. This means they have long generation times – a critical point because it requires possibly as little as four migrants each generation to create homogeneity in the genetics of the species at very large spatial scales.

Consequently, the genetic data does not reveal much about the frequency of long-distance migration – it only confirms that it can occur – and the results of our tagging and photo-id work suggest that it is not likely to be happening very often.

Our genetic analyses have, however, uncovered some remarkable data that illuminates both the past and the future of whale sharks in the world’s oceans. Within the whale shark genome there is evidence for population expansion as the ice ages ended, the oceans warmed and the extent of the tropics spread outwards towards the poles. As the area of habitat increased, so too did the population of whale sharks, showing that they were able to take advantage of changing conditions in the world’s oceans and suggesting that warming waters in the future may not necessarily propel their demise.

Equally there are other, more disturbing patterns emerging from this work. Over the past 6 years there has been a decline in the genetic diversity of whale sharks at Ningaloo Reef. Although there are a number of alternative explanations, the most likely hypothesis to account for this pattern is that the numbers of breeding adult sharks is now in decline. If this is the case, it is a worrying trend for the future, both for the sharks and the ecotourism industry that depends on their presence. This genetic analysis backs up evidence of a decline from our mark-and-recapture studies and sighting data reported by the ecotourism industry.

The problem with our earlier modelling work has been that it has relied on a large number of assumptions about the basic biology of whale sharks for which we have virtually no data. For example, it is not known how often these sharks breed or how many pups they may have in a season, which are key variables when assessing population dynamics and resilience.

The addition of this new genetic data, when combined with our earlier work, increases the weight of evidence showing that we need to think carefully about the management of whale sharks in our region and take a very active role in securing their future.

So if there is evidence that whale sharks may be declining in abundance, what are the threats and what can we do to mitigate them? One of the first things we can definitively exclude from consideration, at least in Australia, is the idea that sharks are being harmed in some way by ecotourism. The industry at Ningaloo Reef is very carefully managed by the Western Australian Department of Parks and Wildlife, and the guidelines that they have developed for tourism interactions with sharks are used as a model worldwide.

Our recent work has shown that the number of tourists that swam with a whale shark in one season made no difference to the likelihood of its return to Ningaloo Reef in subsequent years. If anything, whale sharks that were subjected to lots of tourists during their time at Ningaloo Reef were actually slightly more likely to return in subsequent years than those that interacted with only a few snorkellers.

Although whale sharks are treated very carefully and considered to be an important tourism resource in Australia, they are seen in a very different light in some parts of Asia. There is a growing fishery in southern China, where both the flesh and oil of whale sharks are in demand. Past experience in other places in South-East Asia shows that any fishery for whale sharks collapses very quickly, as these long-lived and slow-growing animals simply do not have the life history traits that can tolerate high rates of harvest. This has led to the closure of fisheries as they became uneconomic or, in the recent case of Taiwan, the recognition that the fishery posed a risk to the future of the species.

However, policing and compliance is still likely to be an issue, even where fishing for whale sharks has been banned. Because of the “black market” nature of the trade in whale sharks within South-East Asia, it is very difficult to determine the extent and number of landings and thus the size of the threat it represents to the species. Given the ease with which whale shark products can be found in markets around South-East Asia and China, it is likely that this threat may be substantial.

The other major threat to the future of whale sharks is not a deliberate one – rather it is a by-product of the increasing presence of shipping on the world’s tropical oceans. During the daytime, whale sharks spend up to 90% of their time at the ocean surface, where they bask to warm up after diving. Their presence on the surface and increasing numbers of ships crossing tropical oceans means that the chances of their path intersecting with that of a ship are gradually becoming more likely.

Ship strike is not a new phenomenon. Throughout the early 20th century a fish biologist, Dr Eugene Gudger, collected reports of collisions by ships with whale sharks, many of which ended with the ship returning to port with the dead shark impaled across the bows. His magnum opus on the subject, entitled “Whale sharks rammed by ocean vessels – how these sluggish leviathans aid in their own destruction”, was published near the end of his career in the early 1940s and probably revealed far more about the attitudes of the time to wildlife (i.e. if these animals were too stupid to get out of the way of the wonderful invention of the steamship, they deserved to go extinct) than anything about the biology of whale sharks.

Attitudes may have changed somewhat, but the threat posed by shipping still remains. Our studies show that at some localities around the Indian Ocean, up to 25% of the animals attending aggregations bear the scars of propellers.

The proportion of whale sharks killed outright by ship strikes is not known, but it’s likely to be high. Mortality and injury from ship-strike will be an issue wherever shipping lanes cross the migratory pathways of whale sharks. For this reason, identifying these potential collision zones should be a key objective for management and conservation strategies.

Although there are major threats to whale sharks, there are also significant opportunities for conservation. Ecotourism focused on these animals is now a growing and profitable industry in many localities around South-East Asia. In the best examples, fishermen who once hunted these animals or scratched a living from over-fished reefs now enjoy more lucrative careers as tour guides, transporting tourists to swim with whale sharks in coastal waters. This is an industry that not only uses whale sharks as a renewable resource, it is also one that provides a better life for ex-fishermen and engages local people in the conservation of these animals – a truly win-win situation.

Development of such industries is a key goal for our project because, as our research shows, in today’s oceans the world’s largest fish now has fewer places to hide.

Mark Meekan is a Principal Research Scientist with the Australian Institute of Marine Science in Perth. See http://tinyurl.com/l3s73v9 to help crowdfund this research.