Sharks and Storms

Written by Dr. Michelle Heupel

Stories about animals reacting to storms are not uncommon. We’ve all seen news reports of dogs running away before earthquakes suggesting they knew it was coming. This is really interesting, but dogs are mammals and considered to be fairly smart – can fish, which are not considered to be terribly bright, also sense these kinds of events?

This question is one I have had the luck (or misfortune) to test several times now. I have been tracking the long-term movements of sharks in coastal systems for many years now and one of the consequences of working in the tropics is we get tropical storms that come ashore. The first time we got a glimpse into how sharks respond to tropical storms was in 2001 when Tropical Storm Gabrielle made landfall in central Florida. I was tracking a population of juvenile blacktip sharks in a small bay and was in my third year of tracking their movements. Blacktip sharks in this bay are resident for 6-8 months during the summer before migrating south for the winter. They typically do not leave the area until migration time, and they definitely don’t leave in large groups.

As Tropical Storm Gabrielle approached the coast we didn’t have enough time to remove our array of acoustic receivers (listening stations) from the water so we left them in and hoped for the best. When we downloaded the data a couple of weeks later (luckily all of the equipment was still there) what we found was that all of the sharks we had been tracking left. Not only did they all leave, but it turns out they all left 7 hours before Gabrielle hit the coast! This was an amazing finding and one never recorded before. Digging further into the data we found that the sharks most likely responded to the decrease in barometric (air) pressure that comes along with these big storms. This means these small sharks could sense this storm coming and swam out into the deeper water of Tampa Bay to get out of harm’s way. What was even more amazing is that after the storm had passed and things in the bay returned to normal all of our tracked sharks came home and went back to swimming around the area like normal.

blacktip

                                                            A juvenile blacktip shark tracked in Florida.

So it looks like sharks can sense big changes in their environment, like tropical storms, similar to the response of dogs to earthquakes mentioned above. But was this a fluke? Is it unique to this one species? The answers to those questions were answered thanks to a much bigger storm occurring half way around the world. In 2010 I was tracking a variety of sharks in north Queensland, Australia. While we were tracking sharks in this region we experienced Tropical Cyclone Yasi. This was an enormous Category 5 storm (the highest category packing 130 mph winds) that spanned most of the Queensland coast and came ashore not far from our study site. What we found this time was that 4 of the 5 shark species we were tracking left the bay. At the peak of the storm all of the acoustic receivers we use to track the sharks stopped recording data. This made it look like everyone had left the area, but really the wave action was so strong the units could no longer hear the shark transmitters over the rushing waves. Once the receivers could hear again it was clear that one group of sharks had not left. All of the blacktip reef sharks we were tracking had stayed in the bay during the storm. This species is typically found in reef habitats and was living on an inshore reef area inside our study site.

 storm2

Category 5 Tropical Cyclone Yasi’s path to the Queensland coast. The yellow box shows our study site.

Why would blacktip reef sharks stay during this huge storm while all the others left? This is a difficult question to answer because all of the sharks we were tracking were of a similar size, so there was no obvious reason for them to react differently. The one difference between the sharks we were tracking was that all the sharks that left were coastal. They can live in bays anywhere along the coast, so if they leave and need to find a new bay they can do that. Blacktip reef sharks, however, need reef habitats to live in. So maybe the blacktip reef sharks stayed because they didn’t want to risk not being able to find another reef. In the end most of the coastal sharks came back to the bay, like we saw in Florida and went back to their home areas. The question still remains though, why exactly didn’t the blacktip reef sharks leave?

reefshark

A blacktip reef shark, the shark that didn’t leave when the storm came.

Studying the response of sharks to storms has been something of a happy accident and I feel fortunate to have had the chance to find out how sharks react to these events. It’s great to know they have good instincts and can get out of the way when a big storm is coming, and nice to know that when the sun comes back out that they can find their way home. We have a long way to go before we fully understand all of these behaviours, but this is a very interesting start to that process.

If you’d like to learn more about studies on sharks and storms you can take a look at these two publications:

Heupel MR, Simpfendorfer CA and Hueter RE (2003) Running before the storm: blacktip sharks respond to falling barometric pressure associated with Tropical Storm Gabrielle.  Journal of Fish Biology 63: 1357-1363

Udyawer V, Simpfendorfer CA, Chin A, Knip DM, Heupel MR (2013) Variable response of coastal sharks to severe tropical storms: environmental cues and changes in space use. Marine Ecology Progress Series 480: 171-183

If you can’t find them online you can request free copies of these papers from James Cook University here: http://researchonline.jcu.edu.au/view/jcu/8E8414EC56EF0825BDA1AEE96370CA5E.html

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