Where the reef manta doth roam

Reaching up to 5.5 meters in length, the reef manta ray ( Manta alfredi ) is the second largest species of ray in the world.  As a group, rays are highly threatened and the reef manta ray is no exception.  Already listed as vulnerable on the IUCN Red List, it is thought that the global population of the reef manta is in decline.  The threats to the ray primarily come from fisheries that target them for their meat, fins, and the aquarium trade, but they are also at risk from being struck by boats, and from becoming entangled in fishing gear, line lines and nets.  These critters need our help if their population is to stop declining.   

Protecting mantas isn’t just important for the manta’s themselves, or even the wider food web of which they are a part.  Manta’s are captivating creatures, so much so that in some places in the world, manta’s drive a tourism industry all of their own.   Indonesia has the fourth highest number of manta ray tourism sites in the world, bringing in an estimated U$15 million a year to the Indonesia economy.  Indonesia also happens to be home to a substantial manta-targeting fishery which brings in around $442,000 a year.

To help reduce the human impact on the rays and ensure the continuation of manta ray tourism, Indonesia has set up three manta ray-specific marine protected areas, where the rays should be (in theory) safe from human harm.  The problem is that manta rays are pelagic species and like all pelagics they move around.  A study back in 2008 using telemetry data suggested that the reef manta rays in Indonesia’s Komodo National Park showed site fidelity, meaning individuals were regularly re-sighted within the Park.  However, the data also suggested that the rays were moving beyond the boundaries of the protected areas, but to where was, at that time, unknown.  Just this year Indonesia declared manta rays a protected species.  As a result all catch of manta rays are now prohibited in Indonesia’s Economic Exclusive Zone.  This is great news, but Elitza Germanov and Andrea Marshall from the Marine Megafauna Foundation note removing all manta fisheries from Indonesia’s waters is likely to take a long time.  They argue that if we really want to take action to protect these enigmatic creatures, then we need to make sure that conservation strategies are based on sound science.  And this means understanding the ray’s movements.

To gather this movement data the researchers utilized “Manta Catcher” – the first global database of manta rays.  Conceived by Andrea, Manta Matcher is at its heart a collection of photographs of manta rays, taken by amateur divers and dive operators.  Yes that’s right, you can identify one reef manta from another!  Each reef manta has a unique patterning on its underside – the ventral side, sort of like black spots.  As a bonus, you can also tell what sex the ray is as well as get an idea of its ‘maturity status’ by looking at the claspers (for males), pregnancy bulges (for females), and mating scars on their pectoral fins – those huge great big wing-like fins on the sides of their body. 

Focusing just on images taken in Indonesia between 2006 and 2014, the researchers were able to identify 820 individual reef mantas from 2,604 images.  And because each photograph had the dive location attached, they were able to work out where each individual manta had been.  As suspected, most of these manta’s had been spotted in more than one location.  Let’s take a look at one reef manta as an example, with the rather fetching ‘name’ of INRA0050A.  Over the space of 8 days, this chap was photographed in 3 different locations.  On a straight line course, this would be a distance of 48 kilometers.  Other rays have straight-line movements of 450 kilometers and one of up to 650 kilometers over a 6 month period!  Of course it is unlikely that these rays moved in a dead straight line between these locations, but this is an incomplete picture of the reef manta’s movements.  The images were taken by people looking for the mantas, and thus at known manta aggregation sites. 

What is missing is where the mantas go in between these dive hotspots.  Never-the-less the study highlights something quite important for manta ray conservation.  Marine protected areas are great.  We should keep them!  But unless Indonesia’s protected areas become much larger than they are then they simply won’t protect the reef manta ray in its habitat.  We aren’t just talking about protecting the rays from fisheries targeting them either.  The study also indicated that some of the rays are crossing heavy shipping lanes, and it wouldn’t be at all surprising if the rays were to be spotted outside of Indonesia’s waters, where they are afforded even less protective measures.  By understanding where the rays are the researchers argue, we can truly start to create a workable conservation policy.

The paper appears in the open access journal PLoS ONE - you can read it here 10.1371/journal.pone.0110071

If you want to have a look at MantaMatcher yourself, their website (including details of how to submit encounter records) is http://www.mantamatcher.org/

Image:  Reef manta ray Manta alfredi at Dharavandhoo Thila, Maldives.  This image was taken by Shiyam ElkCloner who has put it up on Wikimedia for our enjoyment.

#science #marinescience #sciencesunday #citizenscience #mantaray #spatialecology #pelagic