In Pursuit of Plankton: The Effect of Climate Change on Manta Ray Food Sources

Michelle Carpenter

For Michelle Carpenter, who lives and breathes diving and researching manta ray behaviour in Zavora, Mozambique, climate change is one of the most frightening threats to these majestic animals. Here, she talks about plankton blooms, climate change, and manta sightings – a reminder that everything is connected in our wonderful watery world.  

Once upon a time, you lived in a vast desert, which was beautiful albeit lacking in sustenance. Oh, your favourite food source is there all right (let us imagine that they are miniscule fireflies that one can survive and thrive off), eating hundreds of them in a meal - but they only appear periodically. In places where optimum conditions occur, these fireflies will light up and aggregate together to produce a beautiful ball of light – a guiding signal to find your meal. Thus, only the right temperature, light, and scarce nutrients in the desert sand produce these rare pockets of life. In a controlled environment, this would be easy, but we live in a wonderfully dynamic, unpredictable, world, where air and water are ephemeral. Although monitoring patterns can help predict where these special pockets occur, nature is idiosyncratic and often variable! Therefore, your life in the desert depends upon and revolves around you somehow finding these oases. 

This analogy explains the trials and tribulations of what it’s like to be the largest ray in the world, filter-feeding on the world’s smallest marine animal: plankton. Manta rays, Mobula birostris (reaching up to 8m) and Mobula alfredi (reaching up to 5.5 m), are a type of cartilaginous fish called a ray – flappy, large-finned creatures that look like graceful kites in the water (see all the photos!). They spend their lives gliding through the ocean in pursuit of mates and cleaning stations (where parasites are removed and social aggregations occur). Much of the ocean is a vast desert where food is scarce – and even harder to find when their food source is plankton! Plankton is a term for any organism that cannot move against water currents, and this includes phytoplankton (algae which photosynthesize) and zooplankton, which feed on phytoplankton and also one another. 

Manta madness! Photos: Michelle Carpenter

Plankton blooms are an incredible phenomenon, which only demonstrates to us how interconnected our blue planet is. Animals that die in the ocean eventually fall to the seafloor, driving decomposition and nutrient production. This cold, nutrient-rich water remains at great depths until a strong physical process, known as upwelling, carries it up to shallower waters. Upwelling can be driven by the wind, or by convergence of certain currents. When this happens, the clear blue water suddenly becomes full of essential nutrients and the phytoplankton goes crazy, bringing in a plethora of zooplankton, fish, and filter feeders; ultimately creating hotspots of green, soupy-filled biodiversity [manta food!] where upwelling occurs. 

Southern Mozambique is one of such places. Here at Marine Action Research in Zavora, we are lucky to have both upwelling and an abundance of cleaning stations in our backyard, bringing in a large aggregation of manta rays. Using photographs of their unique belly spot pattern to identify individuals like a human fingerprint, we have identified 566 individual manta rays. Mozambique has the largest decline in reported sightings of manta rays in the world - up to 98% for Mobula alfredi, in Tofo, a town 90 km north of us. However, in Zavora, we have not experienced the same decline. Manta rays are IUCN Red listed, and are threatened by direct fishing for Chinese medicine, and by-catch from longlines, trawlers, and gill nets. Fishing pressure and dive tourism may have driven the drastic sightings decline, but the principal threat to our blue planet is climate change and ocean acidification. We are beginning to see the effects of this, and the future is terrifying. As the sea surface temperature rises, there will be changes in the ocean currents, which will affect the aggregation propensity of plankton into blooms. Excess carbon dioxide in the atmosphere also dissolves in seawater to create an acidic ocean and begins to harm the plankton themselves. 

Now, imagine yourself back in the desert, and it’s warming up. The fireflies may not survive in the same desert anymore. Or when they do appear, they might be in an entirely different composition or abundance. We have to change our entire lifestyle, or, go hungry and suffer ourselves – the end. Thus, as you can see, something that seems so far removed from a manta and their food source (carbon dioxide emissions and temperature rises), will have catastrophic consequences for all animals great and small, which cannot function without their food sources.

So what can we do to help?

Manta ID in progress and a successful ID! Photos: Marine Action Research, Marine Megafauna Foundation, Marine Action Research

Like all actions, they can be small, individual and simple. Recycle, reduce your carbon footprint, help mantas live in marine protected areas, use less fossil fuels. However, I believe the most liberating fact is that knowing everything is connected. You can find solace in the fact that: when you walk instead of driving your car, this will help manta rays, filter feeders, and the ocean; no matter where you are in the world. We can achieve small steps as an individual, but by coming together, step-by-step in our daily lives; we can all aggregate together and create a significant change – just as a bloom of plankton does for a manta!

And just before I leave you with another picture of these beautiful marine animals – an optimistic inspirational quote from author David Mitchell:

“Your life amounts to no more than one drop in a limitless ocean…yet what is any ocean but a multitude of drops?” David Mitchell.

Mantas 10.JPG

This blog was written by Michelle Carpenter, a master’s student at the University of Cape Town, who studies manta behaviour. She is passionate about climate change and education, and is currently living at the Marine Action Research station, Mozambique. You can follow her Instagram at @sol.fins.photography_, and check out @marineactionresearch for internship opportunities.

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