Tuna Have Been Seen Rubbing Themselves Against Sharks And We Don't Blame Them

Imagine you're a big yellow fin tuna, miles from shore out in the blue, and you start to feel a little itch near your eye. Maybe it's a scratch that heals, or maybe it's a tiny crustacean.

Do you do anything? You don't have the strength to pick it up. You don't have cleaner wrasses nearby to take care of it.

We discovered how tunas and other fishes solve this problem after poring over thousands of hours of video. You might think of sharks as the answer.

Big fish prefer rubbing shoulders with sharks

We found that fish in the ocean use sharks to scratch against each other.

The scratching removes dead skin and parasites. The fishes have a variety of parasites but their environment makes it difficult to remove them.

Several fish and sharks were recorded in our research. The fish preferred to eat sharks over other fish. Smaller fish are less likely to get eaten by larger sharks, possibly because of the risk of being eaten.

There are small tooth-like structures in shark skin. In pre-industrial times, it was used as a surface against which to scratch.

The fishes scratched their heads and sides more than other parts of their body. The eyes, nostrils, gills, and the side of a fish's body are all affected by parasites.

The fish species differed in how they used their teeth. The tunas were lined up behind the shark and brushing against it.

Using underwater cameras to spy on ocean wildlife

While analyzing thousands of hours of underwater video taken with baited camera systems left to drift at sea, we discovered this behavior. We looked at the footage, identified, counted, and measured everyone we saw.

Population trends are determined by the data we collected. We noticed some strange behaviors while watching these videos.

We saw a huge yellow fin tuna approaching a silky shark from behind and rubbing against it's tail before cruising off. There was a similar interaction between two sharks before it was too late.

We were able to log the details of each interaction between fish and sharks from all over the world.

Why scraping matters: healthy oceans need healthy shark populations

It is difficult to study the open ocean.

There are very few direct observations of the behavior of animals in the ocean. These animals may be new to us but that doesn't mean their interactions aren't interesting.

Fit animals are more likely to reproduce and pass on their genes to the next generation as a result of parasites being removed. The fishes might be getting a benefit from being near sharks.

If sharks become too low for fishes to find their scratching posts, what will happen? There would be a loss of fitness in these fishes.

Given the decline of shark populations in the ocean, this is an important question. A number of species have declined off the coast of Australia.

The decline of shark populations could have an effect on relationships.

In remote regions with relatively healthy populations of sharks and large tunas, we only observed scrapers. It is possible to see the weird and wild things in the ocean at remote locations.

There are marine protected areas where sharks and fishes can be found. More of these areas could help restore and preserve these behaviors.

What's next?

We will continue to check out the waters.

The work may show other species involved in these interactions.

Christopher D H Thompson is a research fellow at the University of Western Australia.

Under a Creative Commons license, this article is re-posted. The original article is worth a read.