The Whale Shark Book

Whale sharks are the largest of all fishes, fascinating for comparative studies of all manner of biological fields, including functional anatomy, growth, metabolism, movement ecology, behavior and physiology. These gentle ocean giants have captured the interest of scientists and the imagination of the public, yet their future is uncertain. The conservation status of whale sharks was upgraded to Endangered on the IUCN Red List and the species faces a range of intense threats from human activities. Can these iconic living animals, who have survived for millions of years, survive us?

Written by the world’s leading experts in whale shark biology, ecology, and conservation, Whale Sharks: Biology, Ecology and Conservation is the first definitive volume about the world's biggest fish. Chapters include discussions of satellite-linked tags, used to track whale shark movements; genetic sequencing, to examine evolutionary adaptations; even the use of underwater ultrasound units to investigate the species’ reproduction. The editors hope that by collating what is known, they can make it easier for future researchers, conservationists, and resource managers to fill some of the remaining knowledge gaps, and provide the information they need to join the team.

As you work your way through this book, we hope that you will develop a sense of awe and marvel at all of our good fortune to share the ocean, and the planet, with this utterly extraordinary species.

The whale shark is the largest extant fish species and likely the largest fish to have ever lived, yet its closest relatives are bottom-dwelling carpet sharks of unremarkable size. So, what led the whale shark to become the world’s largest fish? The answer may lie in the combination of a planktivorous lifestyle and relatively slow metabolism inherited from their ancestors. The apparent absence in whale sharks of adaptations for partial endothermy, which occur in many other large pelagic species and likely incur significant energetic cost, may provide a release from some constraints on their size. But why are whale sharks not larger, achieving the same gargantuan body sizes as the baleen whales? The authors speculate that the flexible cartilaginous skeleton of whale sharks may place biomechanical limits on their body size. Alternatively, the relatively low food density of the tropical oceans, combined with the energetic costs of filter feeding, may constitute a maximum size threshold past which it is impossible to harvest enough energy to sustain such enormous bulk. Hypotheses surrounding the extraordinary gigantism of whale sharks need rigorous testing, but in the meantime, conservationists should exploit body size bias to improve the conservation plight of this endangered species.

Whale sharks are famously enormous. This fact has many implications for their life history. Their reproductive mode (yolk-sac viviparity) and pelagic habitat have led to them having relatively huge (~300) litters – by far the largest of any shark species – of relatively tiny ~60 cm neonates. While these pups doubtless have a high mortality rate through their early years, the survivors likely have a relatively good chance of surviving once they reach a few meters in length. There is emerging evidence that whale sharks have determinate growth, with the growth rate starting to plateau at around their mature size: 7–9 m in males and an estimated 9–10 m in female whale sharks. While only one pregnant female whale shark has been examined, it appears that the female reproductive cycle takes place on a biennial or longer periodicity. The high juvenile mortality rate and the late age of female maturity (tentatively estimated at 30–40 years) result in whale sharks having a low potential intrinsic rate of population increase.

The nervous system (both brain and sensory structures) provides accurate information to an organism about its environment, in order to generate appropriate behavioral responses. Although understanding of the biology of the whale shark has improved markedly in recent years, remarkably little study has gone into their sensory biology and neuroanatomy. This chapter explores what is known about the major senses (including hearing, chemoreception, vision, mechanoreception, and electroreception) and the regions of the brain that receive inputs from these senses in whale sharks, in the context of a wide range of other shark species. Although based on few opportunistic specimens and/or inferences made from close relatives, current data show the nervous system of the whale shark reflects both its primary habitat and the unique morphological and behavioral specializations exhibited by this species. Future work on how the whale shark senses its environment and how these signals are processed in the brain will enable a greater understanding of the feeding behavior, movement patters, and social behavior of this unique and mysterious fish.

There is very little known about the parasites and pathogens of the world’s biggest fish; there are no publications about viruses, bacteria, fungi or protistan parasites that live in association with whale sharks and only a handful about their metazoan parasites. These include three copepods, one isopod, one digenean flatworm, one rather dubious record of a tapeworm, and an opportunistic leech infection. These certainly do not constitute the entire parasite fauna, but, rather, they reflect the drastic under-sampling of whale sharks. That situation is unlikely to be rectified due to the lethal nature of sampling for many of these organisms, which is not generally possible for an endangered species and so requires opportunistic sampling of stranded animals. There are also other animals, primarily bony fishes, that associate with whale sharks but are neither their predator nor their prey. Many of these appear to use the massive bodies of whale sharks for protection, to travel efficiently or to facilitate their own foraging and feeding behaviors. The precise nature of these relationships deserves dedicated study because some of them seem to be quite complex and nuanced. This chapter reviews published studies on parasites, pathogens and facultative associates of whale sharks.

Population structure is an essential component when characterizing a species; the details of population analysis on both small and large scales contribute information of biological and conservation importance. At its simplest, the term refers to the demographic profile (size, age, and sex) of the individuals within local populations, and the degree and mechanism of connectivity between these populations across the whole range of the species. For whale sharks, understanding population structure can inform the migratory and reproductive behavior of the species, aspects of their life history for which little information is available. One necessary component of determining population structure is field work performed locally with each group of animals. Equally important is genetic analysis, both within and between populations. Genetic studies highlight the past behavior of the species, using traces of migration and breeding events written in its DNA. The combination of field studies of current populations, and genetic studies of the history of past generations, can give a detailed picture of whale shark population structure and connectivity. With threats to whale sharks still evident in many parts of their range, population research that can determine the most effective conservation policies is of pressing importance.

Whale sharks occupy tropical and warm temperate marine waters globally. They spend much of their time in surface waters but are able to dive to more than 1900 m depth. They can display a range of diving behaviors, for reasons which include feeding, thermoregulation and efficient forward propulsion. They tend to be mainly solitary and pelagic, but over a dozen locations have been identified globally where seasonal constellations occur, often timed with high local productivity. Although some individuals remain near these sites year round, in most cases the sharks disperse once the season ends. Inter-ocean basin movements have not been reported, and genetic data suggest the presence of distinct populations. The only predictable seasonal movement reported to date occurs in the Eastern Tropical Pacific, where large adult females move through the Galapagos Islands annually in July, heading west along the Equatorial Front and back again, ending the year off the shelf break of Peru. The reasons for this movement are unknown. Improved technology and analytical tools should help elucidate the drivers of whale shark movements in the future. Climate modeling suggests that their range may shift polewards as oceans warm.

In this chapter, we review the global patterns in the population ecology of whale sharks. Population ecology investigates basic information, such as the life cycle of this ocean giant, and provides a significant portion of the data needed for conservation strategies to be developed. The foundation of much of this research is identifying individual whale sharks from their natural markings through photo-identification. We summarize how whale sharks are identified and explain how thousands of citizen scientists have contributed data from their whale shark sightings to a global assessment of their population ecology. Artificial intelligence data mining is now used as a complementary means to gather additional sighting data. We then review the population structure at aggregation sites around the world and discuss the almost ubiquitous segregation by size and sex. Many whale sharks display site fidelity and return to certain hotspots, while others appear to be more transient. We discuss the implications of this fragmented population structure and conclude with a hypothesis of their ecology at different life stages.

This chapter investigates how whale sharks, as large ectothermic filter feeders, approach life in the low-productivity waters of the tropics and sub-tropics. We outline the foraging strategies that whale sharks use to find patchy prey and examine the senses involved with picking up prey cues. We describe how whale sharks feed, and how their suction-feeding ability – unique among plankton-feeding elasmobranchs – influences the different feeding behaviors used by whale sharks. We also discuss observations that indicate whale sharks are less concerned with the species composition or size of their prey, but instead switch from foraging (searching) to concentrated feeding behavior in patches of high prey biomass. Methods of studying whale shark diet are explained, and we discuss our current knowledge of their diet. Finally, we consider the ecological role of whale sharks, as both a consumer and prey species themselves, and how they store and transport energy through the marine environment.

Display of whale sharks in public aquariums demands the largest aquarium habitats yet designed and presents a number of significant challenges. Nonetheless, these facilities offer extraordinary opportunities to advance our understanding of whale sharks in ways that are different from, and complementary to, field research. In this chapter, scientists, veterinarians and husbandry experts from two facilities that house whale sharks present data and lessons from their care in public aquariums, including acquisition, habitat design, feeding, behavior management, veterinary care, growth, and reproduction. Approaches to drawing blood are discussed and normal blood values for hematology, clinical chemistry, and fatty acid composition are presented. Future successful breeding efforts with whale sharks in public aquariums may prove the best and possibly even the only chance to answer some of the most pressing questions about reproduction in the world’s biggest fish. Beyond the scientific opportunities, public aquariums will continue to provide a way to raise the public profile of whale sharks because far more people visit these facilities than can (or should) participate in wildlife tourism with this species.

This chapter discusses the role of whale shark tourism within the context of incentive-based conservation. Whale sharks are the most-watched shark in the world, with tourism worth an estimated US$1.9 billion worldwide, attracting over 25.5 million people annually at 46 sites in 23 countries. The largest collection of sites is in Asia. Tourist opportunities range from captive aquariums and seapen tourism to non-captive provisioned activities and wild encounters. Whale shark tourism can be an important means to protect whale sharks by providing economic incentives to local communities. However, research suggests that the activity can also lead to negative impacts on individual sharks and overall fitness. Effective management is critical in order to minimize impacts, incorporate community perspectives, build conservation awareness, and ensure a satisfactory tourist viewing experience in line with expectations for a genuine ecotourism activity. Although codes of conduct exist for most sites, these need to be reviewed to meet international standards and assistance provided to ensure that they are implemented and enforced.

Human threats have reduced whale shark populations to the extent that they are now a globally endangered species. Whale sharks are vulnerable to a diverse range of direct and indirect human activities, many of which are understudied. The key conservation concern to date has been fisheries. Prior to the 1980s, whale sharks were rarely fished, but in the 1990s, demand for the species’ meat and fins increased, and they became a target of commercial fisheries, or at least a valuable bycatch. These practices have led to a number of region-specific regulations to limit overexploitation. Today, whale sharks are caught accidentally in fisheries targeting tuna, particularly those using purse-seine nets or gillnets, and occasionally in other coastal net fisheries. Whale sharks are also vulnerable to ship strikes, which are likely a significant source of mortality, although hard to quantify. Tourism, which is considered a mixed blessing for the species, has been linked to physical injuries and behavioral changes. As we enter the 2020s, awareness is increasing of the global threat posed by marine pollution and climate change. Research has only begun to scratch the surface of these complex topics.

The whale shark Rhincodon typus is a globally endangered species, primarily due to overfishing in the Indian Ocean and Pacific Ocean. Their precarious conservation status has been recognized in the form of multilateral agreements to regulate trade and to safeguard their habitats and migratory corridors, and through species protection legislation in many countries within their distribution. The whale shark’s status as the world’s largest fish, and their popularity in tourism and media, has been an advantage in enacting conservation measures. They are an effective and charismatic flagship for marine environmental protection. However, whale shark populations remain at a low level and may still be declining. A variety of direct and indirect threats to their recovery still exist. Whale sharks are at a high risk of extinction through a large portion of their range and a strategic, international conservation plan is needed to focus efforts on high-priority actions to help the species rebound.

Whale sharks are now one of the best-studied shark species. Despite a recent, dramatic, and ongoing increase in research effort since the late 1990s, there are still some major areas of whale shark biology and ecology that remain unknown, unclear, or understudied, which doesn’t say much for our understanding of the other 450-odd species of sharks. This chapter considers the conclusions of the other chapters of this book to generate a list of the most important unanswered questions about this species, laying out a roadmap for research directions in the next 10–20 years. Beyond their interest to the scientific community, answers to these questions would provide the foundations for a true global approach to the conservation of this endangered and extraordinary species.