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Study shows krill is important food source for Mobulid Rays

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A new study by scientists at the Marine Megafauna Foundation (MMF), Large Marine Vertebrates Research Institute Philippines (LAMAVE), the University of Queensland, and Scripps Institution of Oceanography was published in Royal Society Open Science last week, providing novel insights into the food sources of four mobulid ray species found in the Bohol Sea, Philippines.

Mobulids have slow growth and reproductive rates, making them vulnerable to overexploitation. Targeted and incidental fishing continue to cause population declines in various locations worldwide, and, as a result, Manta birostris and Mobula tarapacana have been listed as ‘vulnerable’ on the IUCN Red List of Threatened Species, and Mobula japanica and Mobula thurstoni as ‘near threatened’.

Dietary studies can inform fisheries and help reduce by-catch through the identification of critical feeding areas, where the occurrence of mobulid rays and targeted fishes overlap. For direct, short-term assessment of diet through stomach contents analyses, access to dead specimens is required, which can be challenging for rare or internationally protected species such as mobulids.

For this study, scientists were granted access to a targeted mobulid fishery which operates out of Jagna, Bohol in the Philippines from November to May. Stomach contents of all examined mobulid species were dominated by the krill Euphausia diomedeae (91% of stomachs contained the species), suggesting that this zooplankton species is abundant during this period. The larger mobulids also contained small mesopelagic fishes in their stomachs in addition to krill.

“Our results show that krill can be an important food source for large filter feeders living in tropical seas with surface waters that are low in plankton at the surface during the day. Krill migrates vertically, staying deep during the day and shallow, including at the surface, at night when the mobulids were caught”, commented Dr Chris Rohner, principal scientist at the Marine Megafauna Foundation. “Some mobulids had empty stomachs, while others had just finished a huge meal, which shows that tropical mobulids have a boom-and-bust strategy, feeding in dense prey patches when they are available and then undergo a period of starvation until they find the next prey patch.”

Stomach content studies provide a direct, but short-term snapshot of a species’ diet, while biochemical analyses can provide an indirect, but longer-term view. In 2016, manta ray scientists at the Marine Megafauna Foundation in collaboration with Proyecto Mantas Ecuador published a study using non-lethal muscle tissue sampling and stable isotope analysis, which revealed that Manta birostris found in the waters off Isla de la Plata, Ecuador, largely feed on prey originating from the mesopelagic zone (200 to 1,000 meters below the ocean surface) rather than on surface zooplankton. The new study from the Philippines now provides direct evidence to the theory that large planktivores in the tropics and subtropics heavily feed on mesopelagic migrating prey.

Photo: Gonzalo Araujo, LAMAVE


The Marine Megafauna Foundation was created in 2009 to research, protect and conserve the populations of threatened marine megafauna around the world. ‘Megafauna’ are large marine species such as sharks, rays, marine mammals and sea turtles – marinemegafauna.org

Large Marine Vertebrates Research Institute Philippines (LAMAVE) is the largest independent non-stock non-governmental organization dedicated to the conservation of marine megafauna and their habitats in the Philippines. LAMAVE strive for conservation through scientific research, policy and education -
 lamave.org

Gear Reviews

Gear Review: Gemini Switch Box from Lungfish (Watch Video)

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In a video shot exclusively for Scubaverse.com, Jeff Goodman reviews the Gemini Switch Box from Lungfish.

For more information, visit www.lungfishdivesystems.com and www.facebook.com/divelungfish

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Dive Training Blogs

Jump into… A career in diving

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A career in doing something that you love… I have heard so many times that diving is just a hobby and not a career. A career by definition is ‘an occupation undertaken for a significant period of a person’s life and with opportunities for progress.’

I started diving at the age of 17. I became a PADI Divemaster and from this point progressed to an Open Water instructor, to Staff Instructor, to Master Instructor, to Course Director. Surely by definition this is a career path? The only difference (in some cases) that would dispute this matter… the controversial subject of pay!

I am 100% not going to say that no dive centres in the world pay. I myself do, and I know others that do, too. It does however seem to have become very much the norm, that the ‘because I enjoy it’ philosophy has eradicated the UK diving career path for years. Divers volunteering their help for little or no reward (again… not everyone before you stop reading). To eventually realising, that they are doing hard work, for not much to gain… even paying to carry on doing courses, and to become an instructor to work for that centre. What is all that about?!

If you are the type of person to be happy with that, that is completely fine, so long as you are happy. I was at one point… and then realised that I had invested a lot of my time and money, and when this realisation hit, started to feel undervalued. The instructor I was ‘working for’, for a free hot chocolate at the end of the day, would sit in the cafe whilst I taught in the 3 degree waters in the middle of winter. Obviously the paying customer had booked his course through this person and not me… I was happy with a hot chocolate and having fun… but aren’t all of the best careers the ones that we do not see as work. They aren’t all volunteer roles. 

Those of you looking for a career in diving, don’t be put off. There are places that you can work, and a career in diving can literally take you all across the world. Those saying that there is no money in diving… ignore those guys too. There is. Obviously working for free is never going to get you there, but if you want to do it, then do it. There are plenty of places not only looking to employ scuba instructors, there are other jobs at aquariums, conservation roles, the Navy and many others for you to take a look at. 

There are also grants to look at for education, the open water instructor course, or anything else after that is not exactly cheap… but still nonetheless worthwhile.

So, please do not take away the fact of diving being a career. It is. The only thing that I will leave you with (dropping a bombshell), is that if we accept the fact of ‘working for free’ then it will never change and still be hard to make a career in diving… I mean, of course there is limited need when there is still the alternate option for a business to have free labour. 


Clare began Duttons Divers at just 19 years old and a short while later became one of the world’s youngest PADI Course Directors. Find out more at www.duttonsdivers.com

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