by Kandy Curran
Science Network Western Australia
FILTER feeding manta rays provided astounding performances off Broome’s coast over Easter, with the more commonly occurring white manta performing continuous backward, mid-water somersaults close to the coast as the tide peaked.
Dr Rory McAuley, Shark and Ray Sustainability senior research scientist at the Department of Fisheries WA says there are at least four species of devil rays (Mobulidae) that occur in Western Australian waters, although two of these species have only recently been identified (in WA) from single confirmed records.
Two of these species are the larger manta; the others are the smaller more agile mobula ray, often mistaken for juvenile manta ray.
Frazer McGregor, who has been researching manta rays for six years and heads a Murdoch University Research Station in Coral Bay, confirmed from photographs that these three were the coastal or reef manta ray(Manta alfredi), which can grow as large as 4m from wingtip to wingtip.
Mr McGregor says barrel rolling is a common feeding technique, used only when there is a very high density of food in the water column.
“It is the more common white bellied manta rays that do it the most, possibly because prey are attracted to the light being flashed again and again by their white belly as they roll,” he says.
Indeed, the water was soupy with what appeared to be miniscule shrimp clearly visible around the manta’s cavernous mouths as they fed at Riddell Beach in Broome. Two other all black manta rays involved in the feeding frenzy, surged through the water using their wide, wing-like pectoral fins to propel them, whilst their cephalic lobes (fleshy-looking flaps on either side of the head) funnelled the food into their mouths.
With the mantas so close to shore, suckerfish and juvenile golden trevally could easily be seen attached to and around the manta ray’s mouths. These ‘hitchhikers’ use the manta rays for protection, get a free ride throughout the ocean and can feed on scraps of food and skin parasites attached to the manta rays.
Mr McGregor says sightings of manta ray aggregations at this time of year in Roebuck Bay are likely to be associated with coral spawning events along the coast which happen around each of the autumn full moons and in turn lead to large swarms of small crustaceans such as sergestid shrimp which are a favorite prey of manta rays.
So how do we conserve and keep manta rays healthy and abundant in our local waters?
“If you are in the water with a manta ray, keep 3-4 metres away and if boating try and resist the temptation to get closer than 10 or 20 metres as it may affect what they are doing,” Mr McGregor says.
Water quality is a big issue for manta rays as they are very fussy about what they eat.
“The quality of the water dictates whether specific prey, such as sergestid shrimp are available, how high their densities are and therefore how long mantas remain in any specific area,” he says.
The presence or absence of manta rays is likely to be a very good indicator of the overall health our coastline from Shark Bay to the very north of the Kimberley.”
One of the best ways to look after the water quality in Roebuck Bay and surrounding waters is to plant a native garden and design it to trap rainwater. Native plants do not require fertiliser, which can flow off properties during rain and into stormwater drains that discharge into surrounding waters.
Find out more about gardening to keep fish stocks and surrounding waters healthy, at the North West Expo on May 4 and 5 and visit Garden the Roebuck Bay Friendly Way at www.roebuckbay.org.au.
This article is an Inspiring Australia community contribution.