Basking sharks (Cetorhinus maximus) have been found to have swarming behavior in the autumn, and no one knows exactly why

One of my favorite things about animal science is that there’s still a lot of strange behavior seen in species that we haven't seen before, and have absolutely no idea why they do this thing. And that’s exactly what has happened with the basking sharks (Cetorhinus maximus) lately when research found that they go into a swarm in the autumn.

A basking shark in the process of eating. Image by Greg Skomal / NOAA Fisheries Service, posted as Public Domain.

The basking shark is the second biggest species of shark, after the whale shark (Rhincodon typus). An adult can reach up to 8 meters / 26 feet in length, but fear not, they do not attack humans. In fact, they don’t eat meat at all. The basking sharks are actually slow-moving filter feeders that mostly eat zooplankton, but can also fill up on small fish and invertebrates if they happen to end up in the mouth of the basking shark.

The sharks are found pretty much all over the ocean, with the exception of the warm waters close to the equator that it will only cross if it has to. Although it is found in most parts of the world, seeing a basking shark is rather rare, since they tend to swim either solitary or in very small groups, and spend most of their time in the depths of the ocean.

The range of the basking shark. Image by TheEmirr, posted with the Creative Commons Attribution-Share Alike 3.0 Unported license.

Like most large sharks and whales, it is threatened with extinction due to over-harvesting, and it is classified as Vulnerable by the IUCN Red List. While most nations do not harvest basking sharks these days, many populations are still struggling with recuperating from losses during the 20th century.

Shark swarming of the basking sharks

During research on the North Atlantic right whales (Eubalaena glacialis), scientists have been deploying drones in order to survey the area to count the whales left. However, during the survey they also got footage of some interesting behavior from the basking sharks.

What they saw was that these mostly solitary sharks would form huge aggregations, with up to 1,398 whales in a single aggregation. At other swarming they only found a few hundred, but this is much higher than what even the rare groups tend to have, which is less than 7 in over 99 % of the cases.

The aggregations has only been seen during the late summer or at autumn, and a total of 10 swarms has been seen since the first one was found in 1980. During the biggest recorded aggregation of 2013 with almost 1,400 sharks, there was a high abundancy of zooplankton in the area, which could be a potential clue when it comes to finding out what this swarming is all about.

Just look at how small the human is compared to the basking shark! Image is Public Domain.

And that’s pretty much all we know of the swarms! There are of course some hypotheses surrounding the swarms, and the most popular one is that they aggregate in order to reduce the drag that is caused when swimming with their huge mouths open. Just like how you can see birds fly in formations to prevent drag, some scientists believe that these sharks will swim in the same formations to save energy.

This could mean that they meet up at certain locations where there is a lot of zooplankton and feast on these while also conserving energy by reducing drag. Maybe it even is an important annual event each autumn that is critical for them to have if they want to survive the winter?

It’s a shame that scientists mostly have aerial photos to work on, because there are surely lots of more clues if they were able to get closer and witness their behavior underwater. But since they have found 10 aggregations in the last 40 years, there is a of course always a chance they they could be more lucky the next time and spot it before it is too late to get samples. It would certainly be interesting to learn more about the mysterious behavior.

Join in on the research if you see a basking shark!

I love citizen science, and there happen to be a pretty cool citizen science project with the basking sharks. The project called “Spot a Basking Shark” is led by National Marine Fisheries Service (NMFS), and wants to know where and how many basking sharks you have seen.

So if you see one, just head over to their website and fill out the form, and your sight could actually help scientists understand the behaviors of the basking shark!

Thanks for reading

I think it's pretty cool when scientists find strange behaviors such as swarming in a mostly solitary species, and it certainly goes to show how little we know about some animals. The ocean is still mostly unstudied, with thousands or even millions of small secrets that are waiting to be uncovered.

Hopefully this small little discovery about the aggregation of basking sharks can lead to a better understanding of this amazing shark that almost looks more like a whale then a shark. Anyway, I hope you enjoyed the post, and I really hope to see your down in the comment section ;)

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