Population estimates of Alfred mantas (Manta alfredi) in central Maldives atolls: North Male, Ari and Baa

Abstract

The Alfred manta, Manta alfredi, is economically important in the Maldives due to the large number of tourists attracted to SCUBA dive and snorkel with them. It is important to understand the size of populations around main diving points and throughout the Maldives so that manta based tourism activities can be developed more successfully and the impact of tourism can be monitored. Using natural markings to identify individual manta rays, Petersen and Jolly-Seber probabilistic estimators were used to assess population size for main dive sites and the central atolls of North Male, Ari and Baa. Between 2000 and 2009, 1835 individual mantas were identified from 3373 sightings. There was a significant gender bias with a male: female ratio of 1:1.8. Only 24.7% of manta rays were re-sighted between 1 and 34 times with a maximum gap of 7 years and 11 months between sightings. Estimates for populations around main dive sites were 181-562. The estimated population of M. alfredi for North Male atoll is 537 (SE±53.2). Estimated atoll populations were extrapolated to obtain an estimate of Manta alfredi population for the entire Maldives of 9,677 individuals.

Population dynamics of the reef manta ray Manta alfredi in eastern Australia

“…Although more pronounced than in eastern Australia, female-biased sex ratios were also observed in the Maldives (1.8:1) (Kitchen-Wheeler et al 2012) and Mozambique (3.5:1) (Marshall et al 2011a). By contrast, the M. alfredi population at Maui Island had no significant bias .…”

“…M. alfredi showed greater site affinity at LEI than in Mozambique and the Maldives. Over 40 % of identified individuals in Mozambique were resighted at least once in the study area over a 4-yr period (Marshall et al 2011a), and 36 % of the identified individuals at North Male Atoll revisited the same site over a 9-yr period (Kitchen-Wheeler et al 2012). M. alfredi exhibit site affinity for several locations within a certain range, with individuals travelling seasonally up to 270 km in the Maldives (Anderson et al 2011), 400 km in Japan (Marshall et al 2011b) and up to 650 km in eastern Australia (this study).…”

“…This decline may be linked to local anthropogenic pressures (Marshall et al 2011a) or natural predation (Marshall and Bennett 2010a). Population estimates for M. alfredi in the Maldives from Kitchen-Wheeler et al (2012) are difficult to interpret as the sampling design and analysis appear to violate assumptions of the models used, and the model selection process was not reported. Together, these studies suggest that subpopulations of M. alfredi generally number in the hundreds within defined areas across years.…”

“…total number of individuals in the population over the study period, assuming no mortality) of 890 individuals in [2003][2004][2005][2006][2007]. A population size of 537 individuals was estimated around North Male Atoll in the Maldives (Kitchen-Wheeler et al 2012). Assessment of population dynamics through a combination of mark-recapture modelling and photo-ID data can provide valuable information for the conservation and management of a species.…”

Long-term body pigmentation changes on a manta ray (Mobulidae)

“…Individual manta rays have been identified from patterns of ventral markings in recent studies to assess the population size of M. alfredi in Hawaii (Deakos, Baker & Bejder, 2011), Mozambique (Marshall, Dudgeon & Bennett, 2011c), the Maldives (Kitchen-Wheeler, Ari & Edwards, 2012), and Australia (Couturier et al, 2014), or the seasonal occurrence of M. birostris in Brazil (Luiz et al, 2009). To identify individual manta rays, Luiz et al (2009) and Deakos et al (2011) used the entire ventral pattern of each individual sighted, whereas Marshall et al (2011c) defined the standardized region as a rectangle-shaped area extending posterior from the anterior margins of the anterior gill slits, including the pelvic fins.…”