Department of the Environment and Heritage, May 2005
ISBN 0 6425 5082 4
Information on threats faced by Whale Sharks is relatively poor. Indeed, aside from the threat posed by directed take of Whale Sharks to supply trade in Whale Sharks products, other threats that may be faced by Whale Sharks are less easily monitored, quantified or assessed.
Known predators of juvenile Whale Sharks include the blue shark (Prionace glauca), the blue marlin (Makaira nigricons) and the killer whale (Orcinus orca) (Kukuyev, 1996; O'Sullivan & Mitchell, 2000; A. Goorah pers. comm. to Brad Norman). Although unconfirmed, it is believed that an attack on a sub-adult Whale Shark photographed in northwestern Western Australia in 2003 may have been from a great white shark (Carcharodon carcharias) (R. Mau pers comm. to Brad Norman).
Possible threats to the Whale Shark include threats to the habitat on which they rely. While important information has been collected on this species at Ningaloo and more recently from Christmas Island, there still remains a general lack of knowledge on many aspects of Whale Shark biology, including definitive migration patterns and habitat critical to the survival of this species. The task of identifying possible threats to the survival of the Whale Shark is reduced to little more than an educated guess. In the first instance therefore, the following list of identified threats should be treated with caution - other threats may exist, and the threats listed may not significantly impact on the survival of Whale Sharks in Australian waters.
Whale Sharks are known to be highly migratory, with studies demonstrating migrations of at least 13,000 km over 37 months (Eckert and Stewart, 2001). Current information on migration patterns is scarce, though Norman (2004) is currently investigating movements of Whale Sharks from Ningaloo Reef to Christmas Island to coincide with the mass spawning of the red land crab (G. natalis). Currently there is no information available regarding threats to the migration patterns followed by Whale Sharks.
It is possible that increased levels of noise and pollution resulting from an increase in boat traffic may have a negative impact on the migration patterns normally followed by Whale Sharks, through disturbance to habitat or through disturbance of individual sharks.
Damage to ecosystems on which Whale Sharks rely along their migration routes could negatively impact on the migration patterns normally followed by the sharks. Current information on migration routes followed by Whale Sharks is insufficient to make an informed judgement on the main threats to the migratory behaviour of Whale Sharks, nor on the likelihood of those threats occurring, or of the possible consequences of migration disturbance on the survival of Whale Sharks.
Unlike cetaceans, sharks do not use sound to communicate with each other. However, sharks do sense sound as pressure through their lateral line system, and it is possible that high decibel sounds may negatively impact on Whale Sharks. Experiments have demonstrated that sharks can hear sounds with frequencies ranging from about 10 Hertz to about 800 Hertz (Martin, 2004). The effects of very loud sounds on shark behaviour are not well documented, however it is possible that they could potentially disrupt normal behaviours such as feeding, mating, or migrating from one place to another.
In the past two decades, there has been a global increase in the number of tourism operations providing the opportunity for tourists to swim with Whale Sharks. Tourism operations are generally located where Whale Sharks a reliably found, notably in Ningaloo Reef, the Galapagos Islands, islands off the west coast of Thailand, the Sea of Cortez and Baja California in the eastern Pacific (Colman, 1997), the Philippines, the Maldives, Seychelles and Belize (Norman, 2004). Tourism operations generally involve snorkelling or scuba diving with Whale Sharks.
Taylor (1994) suggests that Whale Sharks do not generally feed during the daytime, when most divers are present, whereas Norman (1999) presents conflicting data, with many sharks observed 'passively' feeding with mouth partly or fully distended to take in prey items. Whale Sharks do show reaction to tourists in particular circumstances, including avoidance to SCUBA bubbles and 'duck-diving' of swimmers near the head of the shark (Norman, 1999). Whale Sharks have the ability to avoid a tourist interaction by diving or swimming away. In general however, provided the tourism industry adhere to guidelines in place to ensure minimal impact, broad behavioural patterns do not seem affected by the presence of humans (Norman, 1999). Taylor (1994) suggests it is unlikely that Whale Sharks would be harmed in any biological sense by being in close proximity to swimmers, especially since the riding and touching of Whale Sharks is illegal in Western Australia. However, it will be important to determine the 'natural behaviours' of Whale Sharks in order to ensure that human activities are not having an adverse effect on the sharks.
The WA Department of CALM has developed a Code of Conduct for Whale Shark interactions (see Appendix 5). The Code includes guidelines on the number of vessels allowed around the shark at any time, the maximum length of time a vessel may interact with a shark, the minimum distance from the shark that vessels are permitted to keep, the number of divers in the water with the shark at any moment, and minimum distances that must be respected between divers and the shark. The dive industry at Christmas Island in the Indian Ocean use similar guidelines (Brad Norman, pers. comm.).
While unregulated tourism activity has the potential to disrupt feeding patterns and to drive Whale Sharks away from critical seasonal feeding grounds (Prop 12.35 CITES), considering the above, it is unlikely that tourism operations in Australia currently pose a threat to Whale Sharks.
The Whale Shark's skin is thicker and tougher than any other species in the world, reaching up to 14cm in thickness, and is covered by dermal denticles (Taylor, 1994). This thick covering is not sufficient to negate the threat posed by boat strike with several reports of Whale Sharks being impaled on the bows of larger ships in other regions (Norman, 1999). However, Whale Sharks may be behaviourally vulnerable to boat strike. They spend considerable periods of time at or close to the surface of the water (Norman, 1999; Gunn et al., 1999) and several sharks bear scars likely resulting from boat contact (see Norman 2004). It is likely that high numbers of boat strikes would discourage the movement of Whale Sharks to a particular area. However, at present there is limited data on the level of disturbance faced by Whale Sharks from boat traffic (i.e. the total number of boat strikes and whether this number is increasing or decreasing).
As detailed in the "Population trends" section of this paper, catch data indicates alarming declines in Whale Shark populations, due to (likely) unsustainable directed take of Whale Sharks (and trade in Whale Shark products). Trade in Whale Shark specimens and products is prohibited in Australia under the EPBC Act. International trade in Whale Shark specimens and products thereof is subject to very strict regulations under the Appendix II CITES listing that the species has enjoyed since 2002. The continued directed take of Whale Sharks in other range states is likely to negatively impact on the Australian population (Norman, 2000).
Whale Shark meat is readily available in Taiwan (Chen & Phipps, 2002). It is reputed to be the world's most expensive shark meat. The main products traded include liver oil, meat, fins, cartilage, skin, stomach and intestines (Joung et al. 1996). Taiwan's Fisheries Administration has recently implemented a system to monitor catch and trade of Whale Sharks (Chen & Phipps, 2002). Data available to date indicates that Whale Sharks caught in Taiwan are relatively small in size, a possible indicator of overfishing (Chen & Phipps, 2002).
Official estimates indicate that up to 100,000 kg of fish are removed from the Ningaloo Reef lagoon every year by commercial and recreational fishers (Taylor 1994). Taylor (1994) speculates that a link may exist between high catch rates of targeted species, and subsequent population increases of Drupella cornus, a gastropod known to cause reef damage. Pogonoski indicates that this may also be linked to high catch rates of carnivorous and/or omnivorous reef fishes such as wrasses (pers. comm. 2004).
The health of the reef ecosystem may impact on Whale Shark numbers in the Ningaloo Reef region. Declining Whale Shark numbers in the late 1980's and early 1990's may have been linked to high rates of coral destruction by the gastropod D. cornus (Taylor, 1996).
Surveys conducted during 1991 indicated that live coral cover was as low as 3.8% and 11.1% at two sites, due to a population explosion of the gastropod D. cornus. The corals destroyed by D. cornus were principally the fast growing Acropora sp which make a major contribution to the amount of coral spawn present in the water during spawning, and act as an important habitat for coral-dependant species (Taylor, 1996).
Although not fully researched, it is clear that the presence and aggregation of Whale Sharks in Australian waters is dependent on healthy marine ecosystems, particularly coral reefs. At Christmas Island, Indian Ocean where there is a very limited coral reef, a major contributing factor for the annual Whale Shark appearance is the spawning en masse of the red land crab (G. natalis). Large volumes of the megalopa of this crab can be found in the waters surrounding Christmas Island in certain years (during the short Whale Shark 'season'), with Whale Sharks observed feeding at night on this prey (Brad Norman, pers. comm.). Threats to the population of this crab on Christmas Island (e.g. crazy ant infestation) if too great, may ultimately affect the appearance of Whale Sharks to this isolated Australian Territory.
Damage caused to the marine environment by pollution, overfishing, the introduction of invasive species, and global warming, present very real threats to the survival of the Whale Sharks in Australian waters.