Makako Bay, located on the west coast of Hawaii, is the location of a popular manta ray night dive, during which SCUBA divers sit on the sandy/rubble bottom during each 30 to 80 minute dive and observe the manta rays drawn into the bay by zooplankton attracted to artificial lighting. An estimated 20,000 to 25,000 dives are conducted annually at this site, far exceeding the recommended maximum of 5,000 to 6,000 dives prior studies have determined to be sustainable. Other studies have shown that emerging zooplankton biomass is positively correlated with benthic structure complexity, such as coral reefs, suggesting that the unsustainable diver levels may lead to substrate destruction and decreases in both emergent zooplankton habitat and biomass. Subsequently, manta ray abundance may decrease in response to the lower levels of zooplankton, hypothesized to be a significant food resource of the manta rays. The purpose of this study, therefore, was to determine (1) if there is any evidence of habitat destruction in Makako Bay; (2) if zooplankton diversity and abundance is lower in Makako Bay as a result (when compared to a control site); (3) if zooplankton and manta ray abundances are correlated; and (4) the significance of seasonal and lunar cycles on manta ray and zooplankton abundance.

Zooplankton communities within two sites, Makako and Hoona Bay Kailua-Kona Hawaii were quantitatively and qualitatively analyzed using benthic emergence nets, surface and vertical plankton tows. Zooplankton captured in emergence nets were used to compare coral and sand/rubble sub-sites within each bay. Surface and vertical tows were conducted two hours before and after sunset to study diurnal differences. Total zooplankton abundance was found to be equal in both bays, with the dominant emerging taxon being a calanoid copepod, Labidocera sp. Significant differences were noted in total taxa emergence between sub-sites, with the Hoona Bay coral sub-site having the highest number of taxa emerging and the Makako Bay sand having the lowest. Additionally, the Makako Bay sand site had the lowest abundance of emerging zooplankton if the dominant taxon, Labidocera, was removed from the analysis. Comparisons of the zooplankton assemblages captured in the emergence nets, surface tows, and vertical tows indicate that the emergent fauna is absent/rare in the water column in the daytime, but dominates the water column zooplankton assemblage at night. Thus, the zooplankton the manta rays are consuming during the night dives are originating from the substrate within the bay. Additionally, Spearman rank correlations were significantly positive between manta ray abundance and zooplankton abundance, further demonstrating the importance of zooplankton abundance in manta feeding activities. Analysis of long-term data collected by the Manta Pacific Foundation indicates that zooplankton numbers have decreased from 1998 through 2007. This result, when coupled to the observation that Makako Bay had lower species richness than Hoona Bay, lower abundance (with Labidocera removed) and receives a level of diving that has been deemed unsustainable, suggests that the emergent zooplankton of Makako Bay is exhibiting signs of impact. As manta rays abundance is correlated with zooplankton abundance, a decrease in manta rays is expected unless regulations are imposed on the dive operations.