Triton Population Studies

Project title:
Population studies of the Giant Triton (Charonia tritonis)

Project summary:
Population studies on the giant triton will be conducted in the immediate regions of starfish control programs on reefs that have tourist operations. There is reason to suspect that tritons may aggregate in the vicinity of large numbers of dead or injured starfish. In the past, the triton’s generally low abundance has prevented population studies except under such conditions. These conditions occur rarely and represent the only real opportunity to study this rare and highly specialised predator.

Project Description:

(a) Need (why you think the project is necessary?)
The giant triton (Charonia tritonis) is a large and extremely beautiful shell that is being collected and killed by humans for the shell trade. Most professional and scientific divers have never seen a live giant triton and while it is generally accepted that they are rare, they can be seen for sale in any shell shop in the world. While there are numerous negative accounts, there is little positive data at present on which to base any estimate of their real population density. There is a general belief that it is not possible to undertake a meaningful population study on such a rare species.

Environment Australia intends to propose that the giant triton be listed in Appendix II of the CITES Treaty and an NHT application to monitor domestic trade in this species has already been lodged. Previous attempts to list this much-exploited species have met with objections. If Environment Australia intends to rely on trade data alone, without any direct supporting evidence of the low population density of this species, it may be difficult to justify this listing. With no knowledge of the habitat preference of the giant triton it is very difficult to prove that it is endangered. Without appropriate study, the lack of population data may be seen as no evidence in support of the proposition that they are endangered. This would be an extremely undesirable outcome for the proponent as well as the giant triton.

This project will involve a number of tourist operators in basic research on an endangered species and will increase public awareness of the importance of invertebrates in the coral reef ecosystem. This project will allow some estimate to be made of the large and small-scale distribution of this species and will provide the first substantial quantitative data on the abundance of this species.

(b) Objectives (what you hope to achieve?)
There is reason to suspect that tritons may be found at greater than average abundance in the vicinity of starfish control programs as they are attracted to the scent of injured or dead crown-of-thorns starfish. If it can be shown that there are localised regions in which they are sufficiently abundant to estimate their density, then this will provide a baseline and suggest regions for further sampling. If it can be shown that 99% of the tritons on a coral reef can be found in 1% of the reef area then that is a valuable statistic in the management of this species. A 2-week survey by 30 navy divers in 1986 located 12 specimens in an area of 50 hectares at Grub Reef. A more detailed survey by two divers in 1988 located 7 specimens in just one hectare in the vicinity of a starfish control program on John Brewer Reef and half of these specimens were located in the first 2 weeks of study. A 2-week survey by four divers conducted in 1990 did not locate even one triton in a region well away from starfish outbreaks and control programs.

We expect to show that the population distribution of the giant triton is extremely non-random and that it is very difficult to locate any specimens except where the giant triton has aggregated in the immediate vicinity of human-controlled starfish aggregations. We expect to show that the triton is extremely rare over most of its geographic range and that its population numbers are not stable at any one location due to variations in starfish abundance. We also hope to show that there is a positive correlation between starfish numbers and triton numbers when examined on this small spatial scale compared with the large scale predictions of the Predator Control Hypothesis that a negative correlation should exist between predator and prey. In addition to density estimates, this project will provide detailed information on triton prey preference, feeding, movements and reproduction.

(c) Methodology (how you will do the project?)
The study areas of this project will comprise several reefs of the Great Barrier Reef that have tourist operations that have recently completed or are presently undertaking a crown-of-thorns starfish control program. These control programs do not collect the starfish but involve the injection of individual starfish with a solution that kills the starfish but leaves no harmful residue. The tourist operators can provide repeated and relatively inexpensive access to these study areas and are usually extremely cooperative with research such as this once they have reached the point of initiating a control program.

On each reef, the region immediately surrounding the starfish control program will be searched in detail for giant tritons using line transects and SCUBA. It is intended to map the entire study area and search it repeatedly so that no specimens of giant triton are missed. Each specimen of triton that is located will be measured around the spiral and also in total length, its faeces examined, sexed and tagged on the shell with a small ultra-sonic transducer tag. The triton will then be placed back in the position it was found. The use of an ultra-sonic receiver will facilitate the relocation of tagged specimens. Each time a triton is relocated its position will be mapped and its faeces will be examined for evidence of predation on starfish. The species of starfish last fed upon by a triton is clearly indicated by the type of spines and other ossicles in the faeces.

(d) Outputs/Outcomes (when completed, how will this project have made a direct contribution to tackling the problems/issues listed above?)
This project will provide quantitative data on triton abundance at a small number of locations on the Great Barrier Reef. It is expected that this project will demonstrate that the giant triton is extremely rare over most of its geographic range. Given the extensive trade that occurs in this species, such data will justify its protection under Queensland law and support the need for its international protection.

This project will establish the feeding rate and prey preference of the giant triton and should demonstrate that it is a specialist and voracious starfish predator.

This project will provide valuable information on movement, growth and reproduction of the triton and it is expected that this will reveal that the triton is long lived. Knowledge of the breeding habits and life cycle of the triton should give further insight into the mariculture potential of this endangered species.

(e) Long-term maintenance and implementation. (how your project outcomes will be maintained in the long term?)
Convention in Trade in Endangered Species (CITES) listing will ensure that international trade in this species is regulated. The further use of mariculture techniques to restock depleted areas could be carefully considered if it was thought that triton populations exercised some control over numbers of crown-of-thorns starfish. If the main diet of the triton is confirmed to be crown-of-thorns starfish, then this is another reason for CITES members to support the listing of this species on Appendix II of the CITES Treaty.

The direct community (tourist and tourist operator) involvement in this project will ensure the long-term maintenance of existing triton populations in their general area of operation. Federal and State Government initiatives, together with information that will be available at our North Queensland project office and web site will assist future conservation of this species.

The knowledge of triton behavior and population biology provided by this project will help ensure the long-term survival of this beautiful species.