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POPULATION ECOLOGY OF MANTA RAYS (Manta birostris)

OVERALL OBJECTIVES

The manta rays are the largest living ray in the ocean. Measured by their wingspan, individuals have been known to reach over 7m in disc width. Like many other large marine megafauna, manta rays are planktivores, feeding on small marine invertebrates and occasionally small fish.


As human pressures increase worldwide manta ray populations have declined in many areas where they were once common. There is a notable contrast between the few reasonably protected populations, such as in Yap and the Hawaiian Islands, where economically valuable ecotourism and dive operations exist, and populations in areas where fisheries target the species, such as along the coast of Africa, South East Asia, and in Mexico. Sadly, due to many of their life history traits mantas are highly vulnerable to over-fishing, and there looms a genuine threat of localised extinction of certain populations.

Currently there are no comprehensive management programs for manta rays anywhere in the world, yet they are listed by the IUCN as ‘near threatened’ or ‘vulnerable’ to extinction throughout their distribution. Acquiring accurate information on population dynamics, lifespan, reproductive parameters, growth rates and natural mortality rates is crucial to understanding the conservation requirements of a species. Additional information on the life history of manta rays is sorely needed to supplement the paucity of existing data.


Site fidelity, movement patterns and habitat usage are also essential pieces of information needed to properly manage a species. This type of information can highlight an animal’s potential susceptibility to fishing pressures and help determine critical habitats and seasonal migration routes. Once biological and ecological information has been acquired however, real efforts need to be made to protect these identified areas.Beyond this, fishing, diving, etc. must be regulated to some degree in order to ensure the sustainability of fisheries and eco-tourism in specific locations that might otherwise receive intense anthropogenic pressure.

Eco-tourism has proven to be an excellent way of generating a positive, sustainable balance between protecting populations of animals while creating an economically viable form of tourism for the area. In 2002, the Big Island of Hawaii generated over $2.5 million US dollars in combined revenues from manta related activities in their coastal waters. This figure is astonishing when compared to the $250-500 US that a dead manta can yield in the fish markets in the Philippines, Indonesia and China where as many as 1,500 individuals have been killed in a single year.


Our preliminary findings combined with local conservation efforts continue to raise the profile of manta rays in Mozambique. Our long-term goals are to contribute scientifically to the known ecology of this species, use our data to put pressure on illegal fishing activities and promote their conservation worldwide.

AIMS and EXPECTED OUTCOMES

The proposed study on the manta population off the coast of Mozambique is the first of its kind in Africa. The existence of a reasonably large, year round population of manta rays provides a unique opportunity to study their movements and aspects of their biology and life history.


Non-intrusive underwater photography is being used for the majority of the study. Each animal sighted on the local reefs is identified by a ‘fingerprint’ of markings on their ventral surface as well as by distinctive scars. Once an individual has been identified, their details are logged in a computer database and their re-sightings tracked over time.

 

Additionally coded acoustic tags are placed in a random selection of individuals. These tags relay information to listening stations on the reef, giving us an uninterrupted view of how these individuals use cleaning station on the selected reefs.

For the past few years we have begun collecting genetic samples of identified individuals in the population. In time, we expect to have tissue samples for the majority of the mantas in the area.


Specific aims for this project include:

To ascertain rough population estimates along this stretch of coastline and track population composition seasonally.

To determine if this population of manta rays is closed or open with respect to its movements and to establish if philopatry exists for specific areas.

To establish how manta rays use near-shore and estuarine environments, and specifically to identify critical habitats such as feeding areas, cleaning stations, and areas used for mating and birthing.

Using specimens from the directed fisheries in the area and individuals caught in the Natal Sharks Board nets, determine the dietary preferences and seasonal shifts in the diet of this manta population, determine the fecundity and size at maturity for both sexes and to begin modeling age and growth characteristics for this species.

Document the courtship and mating behaviour of the local manta ray population and determine the reproductive season for manta rays off the coast of Mozambique.

To determine through re-sighting events of known individuals, the gestation period, age at maturity, and reproductive periodicity for this species.

To quantify and track predatory shark bite marks on the local population of manta rays.

To identify different species of internal and external parasites in Manta birostris and the cleaner fish that remove them.

To track the annual number of manta rays and devil rays killed by local directed fisheries.

CURRENT FINDINGS

  • Over 600 individuals have been identified at Manta Reef, making the Mozambican Manta Photo Database the largest scientific manta database in the world.

  • Over 40% of the total identified population has been re-sighted using non-invasive photographic tagging techniques and acoustic technology.

  • From the identified population we have determined that 80% of the population is female and 20% is male.

  • 90% of the male population is mature and at least 55% of the female population is mature.

  • Courtship behaviour and mating events have been studied and described from many different events during the last three consecutive summers.

  • Over 60 different pregnant females have been sighted and 8 different newborns (1.2-1.6 metres) have been recorded on local reefs in the summer months.

  • A detailed dissection of a full to term embryo was conducted in late 2005.

  • 76% of all identified mantas bear significant injuries from shark attacks.

  • In conjunction with the bite mark analysis, I have also begun observations and experiments on the cleaning behaviour that occurs on local reefs. At least five different species of cleaner fish have been identified in cleaning events on mantas.

  • Listening stations have been placed on local reefs with cleaning stations. Thus far, not a single manta has been recorded cleaning on these stations at night. Additionally listening station data show that is not uncommon for mantas to spend up to 2-6 hours per day cleaning on a reef.

International Research

Comparative manta ray research is being conducted in other international destinations including, Mexico, Australia, Yap and Indonesia. Some of this research is directed at comparing population sizes and strutures in these different areas while other research focuses more on obtaining valuable biological data from mantas killed in target fisheries. We are also continuing our efforts to genetically sample and compare populations around the globe to better understand their movement patterns and relatedness.

As manta populations are frequently small in size, intense fishing pressure can threaten the stability of local populations in a few years. It is simply not sustainable, especially in south-east asia and where there are large markets for products like cartilage and branchial plates which are sold at exorbadent prices (up to $50.00 USD per kilo) in Asia.

 

 

 

 

 

 

 

 

 

 

 

 

 

The more information that can be collected the better chance we have at understanding the biology of this species and the threats it might face. The first step is to ensure that the conservation status of manta rays is properly assessed and measures are in place for the sustainable management of populations, particularly in places where they are targeted or where dive eco-tourism industries exist.