Project Manta (THE UNIVERSITY OF QUEENSLAND)
The manta ray, Manta birostris, is the world's largest ray, reaching a disc width of up to seven metres. It is an iconic species with a worldwide distribution. As a harmless giant of the oceans it attracts considerable interest from the general public and is the subject of commercial dive-tourism industries worth millions of dollars in many parts of the world, including Australia.
Manta rays are targeted for fisheries in various parts of the world. Recent demand in East Asia for manta ray products (e.g. gill arches for traditional medicines) poses an immediate threat to the species and has caused a dramatic increase in fishing pressure throughout southeast Asia and eastern Africa. This has led to significant population declines in many regions. It is listed as Vulnerable on the IUCN Red List for Threatened Species and is not currently protected in Australian state or federal waters.
In Australia, manta rays along the eastern coast occur in relatively shallow waters from Narooma in southern NSW to the tip of Cape York in the north, a distance of nearly 4,000 km. However, although the rays are commonly seen and dived with at certain sites, there is relatively little is known about most aspects of the biology and ecology of manta rays (see side box), and there is no scientific data available on the Australian east coast population.
"Project Manta" wishes to rectify this disturbing lack of data through a multi-faceted study of the biology of manta rays along the eastern seaboard of Australia. The project aims to engage scientists, industry partners and the general public in a comprehensive study of manta rays that will simultaneously enhance our knowledge of the species, generate economic and social benefits and provide a basis for long-term monitoring of the environment.
Despite being listed as Vulnerable on the Red Ust there are currently no population size estimates for manta rays on the east coast of Australia. Project Manta aims to rectify this lack of knowledge through passive (photographic) and active (genetic) sampling.
Photographic identification of individual manta rays
The advent of inexpensive underwater housings and digital cameras has resulted in a rise in the number of SCUBA divers taking underwater photographs. General interest in manta rays is reflected in the large number of recreational divers who have complied photo catalogues of mantas which regularly visit their favourite dive sites. Many of these catalogues span over a decade.
"Project Manta" aims to utilise this large pool of underwater expertise to create a diver network covering the whole eastern seaboard and will focus on capturing images of the ventral surface of manta rays. The pattern on the underside of an individual manta ray is as unique as a fingerprint, providing a non-intrusive method of identifying individuals.
The Manta Database will be web-based, allowing divers to submit images electronically in digital format. In conjunction with this, specific scientific dives will utilise an underwater laser system to measure individual rays. We anticipate that the photographic identification portion of the project will run indefinitely, and much of the research that follows hinges on data compiled from it. The information from the Manta Database will allow us to:
- Calculate the population size;
- Determine the sex ratio and any spatialltemporal variations; . Determine how animals move along the coast through time;
- Determine the size-frequency pattern for this population;
- Explore growth rates;
- Determine recruitment rates (pregnant females and pup numbers);
- Determine mortality rates (loss of individuals from the population).
This component of the research will engage and involve the diving community along the whole east Australian coast. The outcomes will be of immense scientific value in increasing our understanding of the species and will be of immediate use to the dive industry and marine parks management.
Manta rays have a worldwide distribution in temperate-tropical waters, but it is unknown whether this is a single population or if it could be more realistically modelled as a series of local populations. If populations are effectively local in nature, then there are important implications for management. This question of biogeography (biological and geographical limitations) is best answered by using a genetic analysis approach.
We will collect small (5 mm diameter) biopsy samples from individual manta rays using biopsy corers on SCUBA. Samples will be examined to explore the genetic diversity within the eastern Australian 'population' and will be compared, collaboratively, with manta ray samples from other regions of the globe. Samples for genetic analysis have already been obtained from Mexico, southern Africa, Hawaii, Indonesia and Japan.
Photographic identification allows for the monitoring of movements of individual animals at a relatively coarse level (presence / absence at particular sites at particular times). While these data are extremely important, it is also of value to understand how individuals move on a much finer scale of resolution, both in terms of time and space. Project Manta will utilise the best in tracking technology, from acoustic tags to monitor movements of individuals on local and coast-wide scales, to satellite tags to monitor large scale migration patterns.
Direct acoustic tracking:
Small acoustic transmitters will be attached to the dorsal surface of individual manta rays. Each tag emits an ultrasonic 'ping' that is detected with a boat-mounted hydrophone and receiver. A positional fix, depth and temperature information will be collected every 15 minutes during a 24 hour period to determine how individual manta rays move during the day and night-time periods.
Remote acoustic tracking:
A number of individual animals will be tagged using special acoustic tags, each of which has a unique identification code that allows the presence of that tag (=animal) to be logged if it is within 800 metres of an underwater listening station (VR2). Project Manta will deploy listening stations at sites that will provide good data on the daily use of a known manta ray location. This will allow the presence/absence of animals within a particular area and/or movements of animals between VR2-monitored areas to be determined for periods of up to several years. This technology is compatible with existing deployments of VR2s by other research groups around Australia, allowing greater coverage than would be achieved by a stand-alone system.
Satellite Tracking: Migration Patterns
It is already well established by the recreational dive industry that manta rays are presence at some dive sites only seasonally. The question is; "Where do they go?"
To understand the large scale migratory patterns of individual manta rays, Satellite tags will be attached to the dorsal surface of individual animals. Data on the animal's behaviour and environment are collected by the tag, summarised and transmitted back to the Argos satellite system. As mantas are wholly pelagic animals, Pop-up Archival Transmitting Tags (PAT) will be used. During deployment the PAT collects detailed depth, temperature and light-level data from which large scale movement/migration can be determined. It releases itself from the animal and floats to the surface on a user-specified date and transmits the stored data to the Argos system.
Behaviour: Interpretation of Movement
While knowledge of how animals move is extremely important, it is the interpretation of these movements that gives us a greater understanding of the animal and its place within the ecosystem. Direct (diver) and remote (Crittercam) behavioural observations will help interpret the site fidelity displayed by individual manta rays. For example, manta ray aggregations around the world have been attributed to the presence of cleaner fish stations, food availability and mates. Through understanding the behaviour of the animals on a fine temporal scale (hours) interpretation of the larger movement patterns can be unravelled.
Presence or absence from a site could be attributed to human and/or natural disturbances. In some areas, boat traffic, coastal development and diver interactions have been attributed to a decrease in numbers of manta rays from popular dive sites. Additionally, predation by large sharks has been attributed to low survivorship of manta's in some regions.
This project aims to explain the presence and absence of mantas from a known site through recording boat strike marks, attempted predation (shark bites) and direct recreational diver/boat/manta interactions. From this, best practice guidelines for sustaining a local manta population can be created. This research sub project will be of particular value to numerous potential stakeholders, particularly dive/island resort operators, GBRPMA and other marine park/protected area management agencies.
Food Resources/Remote Sensing/Oceanography
Plankton is essential food for manta rays, but there is relative little information on the abundance, composition and nutritional quality of plankton within the areas that mantas frequent. Many areas are used by mantas on a seasonal basis. Can this movement be attributed to the availability of food resources?
To address this question, boat based plankton tows will be conducted during and outside of "manta season" in locations that mantas are known to frequent. Lab based studies of plankton composition and density will follow with a subset of these samples analysed for nutritional and energetic content. Differences in quantity and nutritional quality of plankton during and outside of manta seasons will be compared.
Remote sensing represents a true multi-scale sampling tool in both temporal and spatial domains. If the presence and absence of manta rays is found to change relative to the plankton composition, then broad scale changes may be picked up through remote sensing techniques. This mapped information would help us to predict previously unidentified manta aggregations and help to explain some of the movement patterns observed during the large scale tracking.
A longer-term goal of the study is to use manta rays as an indicator species to assist in monitoring the effects of environmental change. Global warming has caused marked changes to the global oceanic conditions. Changes in water temperature, current patterns, and ocean acidification are all effects that may have dramatic consequences on the distribution, movements and behaviours of manta rays. By correlating large scale oceanographic variables with manta distribution and movement may make them the perfect candidate as an easily spotted bio-indicator of global oceanic health.
Want to get involved?
contact: PROJECT MANTA
Dr. Kathy Townsend
Assoc. Prof. Mike Bennett
MORETON BAY RESEARCH STATION
The university of Queensland
25 Flinders Avenue
PO BOX 138, DUNWICH QLD 4183
Phone: (07) 3409 9058
Fax: (07) 3409 9839