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Behavior

Release the Robots! How researchers are studying stressed sharks

Have you ever wondered what happens to a fish after you catch and release it? Does its behavior change? Does it survive after it’s been taken out of the water? Researchers are now using remote controlled robots to answer these very questions!

Raoult, V., J.E. Williamson, T.M. Smith, and T.F. Gaston. 2018. Effects of on-deck holding conditions and air exposure on post-release behaviours of sharks revealed by a remote operated vehicle. Journal of Experimental Marine Biology and Ecology 511:10-18. https://doi.org/10.1016/j.jembe.2018.11.003

Background

Often both commercial and recreational fisheries catch fish that are not the fish they’re looking for, and at Obi-Wan Kenobi’s request they are released back into the ocean. Just, kidding, no Jedi mind tricks here, but sharks, in particular, are often caught unintentionally and released after capture. These sharks are alive when released, but this does not mean that they survive after they have been let go. In fact, the stress of capture can alter a shark’s blood chemistry, impairing its reflexes and reducing its ability escape potential predators.

Researchers mainly study the effects of capture stress in sharks and other fish through blood sampling. SCUBA divers have even observed stressed fish behavior by following them after they were released, but this is more difficult to do with sharks. Post-release behavior of stressed sharks has been observed in tanks and natural enclosures, but not in open water. Since these enclosures are not the shark’s natural habitat, there is a chance that their behavior in the tank may be abnormal. This is why Dr. Vincent Raoult and his team decided to study the behavior of released sharks with a remote-operated-vehicle (ROV).

ROVs are underwater robots that are controlled remotely via a tether. They can go places that divers cannot get to, which allows researchers to observe fish behavior in many different conditions. With that in mind, researchers set out to determine: if an ROV could be used to effectively track sharks, if shark behavior varied between ROV tracked sharks and sharks observed in a holding tank, and if a shark actively swimming away after release is a good indicator of normal behavior and well-being.

The ROV waits in the water to track a newly released shark (Photo Credit V. Raoult).

How did they do it?

The piked spurdog (Squalus megalops) and draughtboard shark (Cephaloscyllium laticeps) were used in the experiments. Both species are often caught and released during fishing in Tasmania, Australia. Piked spurdogs are fairly small sharks, less than 2.5 ft, from the dogfish family, while draughtboard sharks grow larger, up to 5 ft, and are a species of catshark. Both of these species are thought to be hardy, often surviving the stress of capture.

The sharks were caught on longlines and in otter trawls and immediately put into three wading pools and allowed to rest and recover for 24 hours. One pool was designated the control group and would receive minimal additional stress, the other two experimental pools were either released into the ocean and followed with the ROV or placed into a different recovery pool after they were stressed. To mimic conditions experienced on a commercial fishing boat, experimental group sharks were stressed by being exposed to air for 15 minutes before being measured and released into the ocean or recovery tank. Control sharks were simply picked up from their tank and measured before being placed in a recovery tank to be monitored or released to be followed by the ROV. Sharks placed in the recovery pool were monitored with a GoPro camera, while those released into the ocean were followed about 6.5 ft away by the ROV, equipped with a camera. All sharks were monitored for 5 minutes after handling.

A draughtboard shark rests on the bottom of the sea floor (Photo credit Wikimedia Commons).

To understand how the stress influenced the sharks, tailbeat frequency, or the number of times the shark swished its tail to swim, was measured at 15 second intervals over all of the 5 minute videos. Researchers then used statistics to determine if there were significant differences between treatments (control, holding pool, or ocean release), explore if immediately swimming away was a good indicator of later fish behavior, and determine if deck exposure influenced activity levels.

What did they find?

Dr. Raoult and his team found that there were significant differences in tailbeat frequency between treatments and species. Both spurdog and draughtborad sharks had significantly lower tailbeat frequencies after they were exposed to air and placed into a recovery pool when compared to control sharks placed in the recovery pool. Spurdog released back into the ocean had dramatically lower tailbeat frequencies after air exposure when compared to the control. At times there was a difference of almost 20 tail beats between the two! That is the difference between a fish effectively swimming away from a predator and becoming someone’s lunch. Tailbeat frequency after air exposure and ocean release for draughtboard sharks was lower than the control group, but the difference was not as dramatic as it was for spurdogs. In general, sharks that were released into a holding pool had lower tailbeat frequencies than those released into the ocean.

So what’s it all mean?

Generally, sharks that were exposed to air on the deck of the boat were weaker swimmers, with lower tailbeat frequencies, than those that were released immediately. This emphasizes the importance of quick release for sharks. Remember that all of these sharks survived significant stress from traditional fishing gear, like longlines and trawls, before the study. Unlike stressed sharks, control sharks recovered and were swimming normally in less than two minutes after being measured, showing that rapid recovery is still possible even after significant previous stress.

Despite large differences in tailbeat frequency between control sharks and stressed sharks, these differences did not happen until more than 30 seconds had passed after release. This means that for these species immediately swimming away does not mean the shark will continue to behave normally. After the first 30 seconds of observation, many stressed sharks had periods where they were not swimming at all followed by short bursts of movement. Though most of us think of sharks as large top predators, many sharks are small like the piked spurdog and draughtboard shark, and they could easily go from predator to prey if their ability to swim is compromised. Additionally, many sharks need to generate forward momentum to pass water over their gills to breathe and an inability to swim could cause the shark to suffocate.

A piked spurdog tracked by the ROV is unable to swim after stress. This shark later recovered after resting on the bottom (Photo credit V. Raoult).

In general, sharks that were released into the ocean were more active than those that were held in the recovery pool, suggesting that recovery pool observations are not always an accurate way to assess post release recovery behavior. The ROV was able to successfully track and record the behavior of both piked spurdog and draughtboard sharks after they were released. This means that ROVs are a viable way to study the behavior of accidentally captured species after they are released. This information is important for improving handling practices for fish, which inherently will decrease stress and increase the potential for accidentally caught fish to live. When it comes to studying post-release behavior in sharks, it seems that ROVs are the droids we’re looking for.

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