Ecology and social behavior of a resident manta ray (Manta alfredi) population off Maui, Hawaiʻi

Abstract

Findings from this study broaden our understanding of the ecology and behavior of Manta alfredi. Paired-laser photogrammetry, photo-identification, and active tracking were used to investigate a M. alfredi population off Maui, Hawaiʻi. A total of 286 surveys were conducted between 2005 and 2010, resulting in 309 different individual manta rays, of which 163 were sized. A discovery curve showed no asymptotic trend, indicating the number of individuals using the area was much larger than the total identified. Resights and manta follows revealed a minimum home range spanning the four-islands within Maui County with high site fidelity to the study area within and across years by most individuals. There was no evidence of movements to the Big Island, suggesting there are two independent, island-associated stocks. Males, accounting for 51% of the population, were estimated to reach sexual maturity at 2.8 m disc width ( DW) and females at 3.37 m DW. Maximum sizes were much smaller than those reported in other parts of the world indicating geographic variation in size for this species. The mean pregnancy rate was estimated at 0.56 pregnancies/female/year with larger females pregnant more often and more likely in consecutive years. Significantly more mating trains and pregnant females occurred during the winter season. No direct physical competition was observed between males but evidence of mating trains lasting more than one day suggests endurance rivalry, during which larger males may benefit due to greater energy reserves. The absence of individuals less than 2.5 m DW suggests age class segregation is occurring in this population. Shark scars were evident in 24% of individuals and 10% had an amputated or non-functional cephalic fin, likely caused by entanglement in monofilament fishing line. The Maui aggregation site appears to be an important staging area for breeding and parasite removal by members of this population. Late maturity, low fecundity, and the residential nature of individuals in this population make them particularly vulnerable to localized, anthropogenic threats. Implementing successful management practices to reduce the threat of entanglement and of unregulated "swim-with manta ray" programs is recommended.