The behaviour and ecology of Alfred mantas (Manta Alfredi) in the Maldives

Abstract

At the commencement of this study all data on Manta rays were combined under the single species Manta birostris, and there was little information available on their ecology. Manta rays were frequently reported throughout the Maldives, and tourist manta-watching is an economically important activity in this region although no previous study of manta numbers or movements had been made. An individual identification method was developed based on the unique ventral marking pattern of each animal in order to record the identity and frequency of visit by mantas to the numerous cleaning and feeding areas. Information on sex, estimated size, and the markings pattern were collated in an Access database along with date/time and location of each animal sighting. Mantas were also observed and recorded photographically and on video being cleaned by cleaner fish and during feeding activity so that typical behaviours could be investigated. Analysis of the ID records and sightings data of over 1900 individually identified mantas has shown that the smaller reef-associated Manta alfredi is the predominant species in the Maldives. The majority of mantas (~74%) have been sighted only once, but this is probably due to a relatively low survey rate, with the remainder re-sighted between one and 34 times. Individual mantas which were re-sighted appeared confined to an atoll, visiting a number of cleaning stations around the atoll and migrating between east and west sides so as to be predominantly on the leeside to the prevailing monsoon. Only ~1% of mantas were reported in more than one different atoll with 270 km being the greatest distance between sites where an individual manta was reported, a range not previously recorded. A population study estimated populations of around 537 mantas in small atolls and 811 in large atolls. The population was ~65% female, and females were significantly larger than males. Reproductive periodicity appears low with only 26% of likely mature females ever being sighted pregnant over a 5 year period. Females which were reported pregnant produced offspring less frequently than every two years, on average. This low reproductive rate might put this small population at risk if local fishing practices changed. i The behaviour of mantas at cleaning stations was investigated as well as the identity and abundance of cleaner fish species. Feeding strategies are described and appear to show that feeding is a co-operative behaviour in mantas. These preliminary behavioural studies provide early insights into the ecology of this species.