The conservation status of the widely-distributed whale shark Rhincodon typus is presently listed as 'Indeterminate - Data Deficient'. One of the main hindrances to obtaining biological data on whale sharks that is relevant to determining its 'conservation status' is that this species has rarely been recorded as occurring in sufficient numbers to obtain quantitative data. However, R. typus does form aggregations at Ningaloo Marine Park (NMP), Western Australia, annually between March and June. This has enabled studies to be made of aspects of the biology of R. typus and of the possible impacts of the ecotourism industry on this species.
Using a position provided on vessels involved with the whale shark ecotourism industry at NMP, R. typus was observed on 360 separate occasions in 1995, 1996 and 1997, and it was possible to sex 90.3% of these sharks. The majority of the sexed sharks (84.6%) were male and ranged in length from 4 to 12 m, with a mean of 7.4 m, while the females ranged in length from 4.5 to 8.5 m, with a mean of 6.2 m. The size and degree of abrasion of the claspers was used as an indicator of whether or not a male shark had mated. Using such criteria, it was estimated that male whale sharks start to mature at ca 8 m and that ca 50% are mature by the time they reach 8.6m.
Observations suggested that R. typus feeds by using both suction and flow-through mechanisms. The prey that were observed being ingested included coral spawn, tropical krill, mysids and small jellyfish. The contents of a faecal sample contained parts of the exoskeleton of copepods and the scales of small fishes. The degree of mouth distension, which is assumed to be related to feeding activity, was low during most observation periods.
Photographs of the scars and natural patterning on the skin of individual sharks were used to construct a photographic library for subsequent identification of these sharks. The features used for identifying individual sharks were chosen because they were considered likely to remain for a protracted period. The Whale Shark Photo -identification Library that was produced provides details on the characteristic features of 52 R. typus that were present at NMP. Six individuals were recorded at NMP in both 1995 and 1996, four in both 1996 and 1997, and one in both 1995 and 1997. No identified whale sharks were recorded in all three years.
Rhincodon typus was distributed widely throughout NMP, with most boat and aerial sightings lying within 1 - 2 Ism of the reef crest between Tantabiddi and Turquoise Bay. Rhincodon typus was typically sighted in water depths of 10 to 30 m. The sharks were predominantly travelling parallel to Ningaloo Reef, with significantly more moving in a northward than southward direction. Acoustic tracking of R. typus in 1997 suggested that this species remains within NMP for extended periods and is at the surface for ca 17% of daylight hours.
The number and species of fauna observed to be associated with R. typus were recorded, and a new species of copepod, Pandarus sp. nov., which lives on the skin of R. typus has been described. Golden trevally (Gnathanodon speciosus), miscellaneous trevally (Carangid sp.), remora (Remora sp.) and slender suckerfish (Echeneis naucrates) were common. The prevalence of Pandarus sp. nov. was inversely proportional to the number of Remora sp. and E. mucrates in 1996, while the opposite was true in 1997, suggesting that Pandarus sp. nov. were preyed on by these diskfish.
Rhincodon typus is the basis of the ecotourism industry that operates within NMP each year. While there was considerable variation in the number of tour vessels searching for whale sharks at NMP each year, the greatest mean number of vessels operating per week in successive whale shark seasons were 6.7 during Week 8 (April 19 - 25) of 1995, 6.1 during Week 7 (April 12 - 18) of 1996 and 6.9 during Week 8 (April 19 - 25) of 1997. The greatest mean numbers of whale sharks sighted per week in each year were 5.1 during Week 14 (May 31 - June 6) of 1995,4.2 during Week 6 (April 5 - 11) of 1996 and 4.1 during Week 8 (April 19 - 25) of 1997.
Tourists, who were permitted to swim alongside R. tyus, interacted with sharks for a mean period of 19.3 rnin in 1995, 14.2 min in 1996 and 9.5 rnin in 1997. The reduction in the duration of interaction in three successive years suggests that, over time, R. typus may have become slightly less tolerant of the ecotourism industry at NMP. The mean minimum distance between vessel and shark during each interaction was 20.7 m in 1995, 21.3 m in 1996 and 31.0 m in 1997. The mean minimum distance between tourist and shark during each interaction was 1.5 m in 1995, 2.05 m in 1996, and 2.1 m in 1997. The mean minimum distance of vessel and tourist from R. typus during each individual interaction decreased as the duration of the interaction increased. Therefore, both R. typus and this industry must be carefully monitored to ensure that the impacts of humans are kept to a minimum and thereby ensure that whale sharks return to NMP each year.
An ethology of whale shark behaviours, which included banking, porpoising, diving and eye-rolling, was produced in an attempt to determine whether there is evidence that the ecotourism industry has a negative impact on R. opus at NMP. The frequency of behavioural change was greatest in the first 0 - 5 min of an observation. Eye-rolling by R. typus was recorded as a reaction to flash-photography, while banking was often recorded when SCUBA was used and/or tourists swam beneath the head of the shark. The swimming speed of R. typus at NMP was rarely too fast for tourists to maintain proximity to the sharks.
Several sharks possessed both recent and healed scars, which were probably inflicted by vessel contact. The recent wounds indicate that vessels had caused injuries to R. typus within NMP. These individuals tended to display a higher frequency of avoidance behaviours and reduced interaction times.
Recommendations are provided to the Western Australian Department of Conservation and Land Management which are aimed at reducing the potential deleterious effects of the ecotourism industry on the whale sharks at NMP.
| Identifer | oai:union.ndltd.org:ADTP/221904 |
| Date | January 1999 |
| Creators | Bradley Michael Norman |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Bradley Michael Norman |
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