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

Late maturity, few offspring, and a residential nature, typical of Manta alfredi, make this species particularly vulnerable to localized anthropogenic threats, and much less likely to recover from depleted populations. Understanding the population characteristics and reproductive ecology of this species is critical for its successful management. Paired-laser photogrammetry, combined with photo-identification and active tracking were used to describe the population characteristics, demographics, habitat range and use, and reproductive ecology of a resident population of manta rays (Manta alfredi) in Hawai'i. Paired-laser photogrammetry proved to be a simple, non-invasive, accurate (mean error of 0.39%), and precise (CV = 0.54%) method for sizing free-ranging manta rays. A total of 286 surveys were conducted between 2005 and 2010 at a known aggregation site off Maui, Hawai'i. A total of 309 different individual manta rays were photo-identified. 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 home range spanning Maui County waters with visits to the four-islands but did not include the Big Island, suggesting there are independent, island-associated stocks. High resight rates within and across years at the study site 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, accounted for 53% of all individuals, and resided for shorter periods than females around the study site. Manta rays were usually absent at first light with numbers increasing throughout the day. Shark predation was evident in 24% of individuals, and 10% had an amputated or non-functional cephalic fin, likely caused by entanglement in monofilament fishing line. Repeated measurements on 154 different manta rays, produced a mean CV of 1.46%, providing further support for the paired-laser system. Sexual dimorphism was evident with the largest female (3.64 m DW) 19% larger than the largest male (3.05 m DW). Sexual maturity in females, based on evidence of pregnancy and mating scars, was conservatively determined to be 3.37 m DW. The DW at which 50% of the males were likely to begin maturation and accelerated clasper growth was between 2.70 and 2.80 m DW. The absence of individuals smaller than 2.50 m DW suggests age class segregation may be occurring in this population. Although mating trains and late-term pregnant females were observed at all times of the year, they were more likely to occur during the winter months. Females seemed capable of ovulating multiple times during a year if their initial mating attempts are unsuccessful. Estrus may last at least several days based on repeated sightings of the same female in a mating train over several days. Sexual maturity appears delayed in both males and females until their body size exceeds 90% of their maximum size, an indicator that large body size provides a reproductive advantage. Larger females had higher pregnancy rates, and were more likely to reproduce in successive years. 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 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.