Manta ray preferences revealed

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Image credit: Fabrice Jaine, PLOS ONE (2012)

If the thought of a calm moonlit night out on a tropical island makes you want to be at the beach, you may have something in common with the reef manta ray. Researchers studying the habits of manta rays in the waters off an island in the Great Barrier Reef have found that the rays aggregate in larger numbers when wind speeds are low and tides are particularly high or low (because of a new or full moon). Published today in PLOS ONE, their findings may help understand how manta ray populations are affected by climate and ocean conditions, they say.

This type of insight is particularly important because reef manta rays are classified a ‘vulnerable’ species by the International Union for Conservation of Nature (IUCN), just one step away from being endangered. Though they have few natural predators, they are hunted for their supposed efficacy in traditional Chinese medicine and are frequently trapped accidentally in fisheries. In several regions around the world, their populations are thought to have declined by about 80%, and globally, the IUCN estimates a drop of approximately 30% in the last 75 years. Little is known, however, about what factors predict their behavior or abundance in a particular geographical area; such knowledge is critical to conservation efforts, particularly in the context of a changing climate.

To begin addressing this question, the authors of today’s paper recorded weather and water conditions around the island and correlated these to the population of rays around the island and the kinds of behaviors they displayed. To study the animals themselves, the researchers enlisted volunteer SCUBA divers and tour operators on the island to watch for rays and their behavior. Another paper recently published in PLOS ONE uses a similar approach to track the decline of baiji, a freshwater dolphin in the Yangtse River. The authors of this paper surveyed local fishing communities along the river to estimate the abundance and decline of large species in the ecosystem, tracking the river dolphin, finless porpoise and two large fish now considered critically endangered or possibly extinct. They found that plotting the decline in frequency of sightings reported by the locals could help establish the dynamics of species decline and extinction.

Protecting larger animals like dolphins and manta rays can help more than just a single species.  In addition to acting as attractive poster children for conservation efforts, these animals frequently act as indicator or umbrella species. As indicators, their presence (or absence) can be a marker to monitor environmental conditions known to impact other species in the area, and targeting conservation efforts towards these animals can help conserve the habitat of several other species. Information on whether manta rays can serve as indicator or umbrella species is sparse, but research such as this study should help to answer this question.

Citations:

Jaine FRA, Couturier LIE, Weeks SJ, Townsend KA, Bennett MB, et al. (2012) When Giants Turn Up: Sighting Trends, Environmental Influences and Habitat Use of the Manta Ray Manta alfredi at a Coral Reef. PLoS ONE 7(10): e46170. doi:10.1371/journal.pone.0046170

Turvey ST, Risley CL, Barrett LA, Yujiang H, Ding W (2012) River Dolphins Can Act as Population Trend Indicators in Degraded Freshwater Systems. PLoS ONE 7(5): e37902. doi:10.1371/journal.pone.0037902

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