Giant manta rays are deep-sea predators, study finds

By AG Staff Writer 30 November 2016
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A long-term study has confirmed much of the diet of giant manta rays is made up of deep-sea fish rather than zooplankton as previously thought.

UNTIL RECENTLY, LITTLE was known about the feeding habits of giant manta rays, which are found in tropical and temperate waters worldwide and can grow up to 7m across and weigh up to 1350kg.

What was known was based on observations of the enormous rays feeding on zooplankton near the ocean’s surface – but now, scientists have found zooplankton makes up less than a third of their diet.

Researchers from the University of Queensland (UQ) and the Marine Megafauna Foundation have spent the past six years studying the diets of giant manta rays on Isla de la Plata off the coast of Ecuador.

“We studied the giant manta rays’ diet using biochemical tests, such as stable isotope analysis, which works on the ‘you are what you eat’ paradigm,” explained Katherine Burgess, a PhD student at the UQ School of Biomedical Sciences.

“These tests can determine what animals have been eating by examining a piece of tissue from a muscle biopsy from a free-swimming animal,” said Katherine, who added that the normal invasive approach of studying stomach contents would have been inappropriate for a species listed as vulnerable from over-fishing.

Using this method, the researchers found up to 73 per cent of the giant manta ray’s diet was from “mesopelagic” sources – that is, marine organisms, including fish, that live between 200m and 1000m below the ocean surface.

The researchers said they hope the findings, which are published in the Royal Society Open Science journal, will help conservation efforts to protect giant manta rays, which have seen a drastic population decrease in the past 20 years.

“The deep ocean is the next frontier for open ocean fisheries, and we are only just realising the potential reliance on this zone by threatened marine megafauna,” said Professor Anthony Richardson, a scientist with UQ’s School of Mathematics and Physics and the CSIRO’s Oceans and Atmosphere Flagship.