Characteristics of a manta ray Manta alfredi population off Maui, Hawaii, and implications for management

Late maturity, few offspring and a residential nature are typical characteristics of Manta alfredi that make this species vulnerable to localized anthropogenic threats. Improving its life history information is crucial for successful management. A total of 229 surveys was conducted from 2005 to 2009 at a manta ray aggregation site off Maui, Hawaii, to qualitatively and quantitatively describe the abundance, movements and temporal habits of this population. Photo-identifications revealed 290 unique individuals, but a discovery curve showed no asymptotic trend, indicating that the number of individuals using the area was much larger. Resightings and manta ray follows revealed that this population and a population off the Big Island may be independent, island-associated stocks. High resighting rates within and across years provided strong evidence of site fidelity. Findings were consistent with a population of manta rays moving into and out of the Maui aggregation area, with a varying portion of the total population temporarily resident at any given time. Males, accounting for 53% of all individuals, resided for shorter periods than females around the study area. Manta rays were usually absent at first light with numbers increasing throughout the day. More frequent mating trains were observed during the winter months. Shark predation was evident in 33% of individuals, and 10% had an amputated or non-functional cephalic fin. This small, demographically independent population appears vulnerable to the impacts from non-target fisheries, primarily from entanglement in fishing line, and could suffer from exploitation by commercial, unregulated 'swim-with manta ray' programs. Management on an island-area basis is recommended.