Food Availability
Warming oceans are causing changes to ocean acidity, oxygen content, current circulation, and primary production, which will all ultimately affect the wider food web. Biogeochemical models project a reduction in zooplankton biomass globally by 10%. However, the decrease could be as high as 50% in tropical regions. How this broad-scale reduction will affect localised manta and devil ray feeding grounds is not clear, but the likely outcome will be a reduction in zooplankton biomass for mobulids and other zooplanktivores to feed on. The knock-on ramifications of reduced prey availability are likely to have major impacts on mobulid ray populations and distributions.
A selection of zooplankton species collected from a manta ray feeding site in the British Indian Ocean Territory.
Cleaning Station Habitat Loss
As sea surface temperatures rise, coral reefs worldwide are experiencing longer, more severe, and more frequent bleaching events. It only takes water temperature to rise by a couple of degrees for a few sustained weeks, for these bleaching events to cause mass mortality of corals. This has been seen explicitly in the Maldives in 1997 and more recently in 2016. As a result, coral reefs are severely degraded globally.
Reef building corals provide three-dimensional complexity to their ecosystem, providing a home and shelter for thousands of species of tropical marine animals, including dozens of cleaner invertebrate and fish species.
Cleaning stations on coral reefs are important aggregation sites for reef manta rays. They visit these sites to be picked clean of irritating parasites and to have any wounds cleaned by an army of little cleaner fish, which in return feed upon the parasites, dead skin and mucous of the rays. Cleaning stations also provide an important location for the manta rays to thermoregulate, avoid predation, socialise, court and mate. Therefore, the continued degradation of coral reefs globally is likely to have a negative effect on the health of reef manta rays also. Furthermore, a reduction in visitations by manta rays to cleaning stations will also be damaging to the dive and snorkel tourism industry, which in many regions is the economic driver of protective legislation for manta rays and their reef habitat.
