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The Social Lives of Sharks

A group of Port Jackson sharks under kelp at their mating aggregation site in Jervis Bay, NSW. Credit: Johann Mourier

A group of Port Jackson sharks under kelp at their mating aggregation site in Jervis Bay, NSW. Credit: Johann Mourier

By Culum Brown

Tracking technology reveals that Port Jackson sharks have buddies of similar age and gender, and can navigate across Bass Strait to the same breeding grounds.

Sharks have an interesting place in our collective psyche. People have always feared sharks, but ever since Hollywood’s depiction of white sharks in the movie Jaws, our fear has been magnified.

Much of this fear is irrational, even in countries where white sharks do occasionally kill people. In Australia, only 47 people have been killed by sharks over the past 50 years, an average of 0.9 per year. Far more people are killed by horses (7.7 deaths per year), cows (3.3 deaths per year) and dogs (2.7 deaths per year). Kangaroos and bees are next on the list.

So, one might question why we are terrified of sharks when statistics clearly show that there are many animals that rank far above them in terms of real risk. Of course, all deaths by animals pale into insignificance when we compare them with motor vehicle statistics or homicide.

Part of the reason for this irrational fear of sharks stems from our fear of the unknown. Sharks are the rulers of the sea, an environment that is both unfamiliar and unknown to humans. Swimming at the surface with the dark inky depths bellow us, our imagination kicks into overdrive.

But again we need a reality check. The vast majority of the 1000 or so species live on the sea floor and don’t even have pointy teeth. If one bit you, you’d be lucky to get a nasty bruise. The irony is that we know very little about the vast majority of these animals, even the ones we fear so much.

As top-order predators, sharks play important ecosystem roles. In many ways the number of sharks in the sea is something of a health barometer for the marine ecosystem as a whole. Trouble in the food chain below sharks exerts bottom-up influences on shark numbers, and likewise changes in shark numbers at the top of the food chain have unpredictable impacts on the food chain below them. Given that many sharks are long-lived and reproduce very slowly, any harvesting of sharks is largely unsustainable and population recovery takes a very long time.

The Fish Lab at Macquarie University has been studying shark and ray behaviour for the past 6 years in a bid to further understand these much-maligned animals. At the heart of our work is applying modern technology to try to figure out where sharks and rays go, and why.

To date we have used acoustic tags to track the behaviour of 140 Port Jackson sharks. We work on these great little sharks because they are highly abundant, hardy and very easy to work with.

We use them to try out all sorts of new methods for studying shark behaviour. Once proven, the technology can be applied to other species that we may be interested in managing for fisheries or conservation. We are also using this approach to study smooth sting rays, the biggest marine stingrays in the world, and manta rays, which are arguably the most majestic creatures in the sea.

Acoustic tracking technology is improving all the time, and we use it to address a very broad range of questions. In the Port Jackson shark case, we have monitored their movement and migration in Sydney Harbour, Port Stephens and Jervis Bay.

Our data show that these little sharks (large females are only about 1.5 metres long) migrate to Bass Strait and back every year. They are very long-lived animals – males mature at about 10 years of age, and females in their teens – so we can follow the behaviour of individuals for very long periods of time. Our largest acoustic tags last 10 years so we can see the same individuals following the same migration route year after year.

These sharks not only come back to the same bay, but in Jervis Bay they come back to the same rocky reef. This remarkable feat is on par with salmon migration in the Northern Hemisphere.

Our tracking data also show that males migrate faster than females and the speed of migration is fastest when travelling south, likely because they can “surf” the East Australian Current. Males come to breeding reefs early and intercept females as they arrive. The females stick around longer to lay their eggs in rocky crevices before heading back south.

The eggs take around 10 months to hatch. We have no idea what the young sharks do, but eventually they start migrating with the adults. How do they know where to go?

These sharks show very high site fidelity over many years, and this made us ponder that perhaps they have preferred social partners at these breeding locations. But to address social interactions we needed to monitor their movements on a much finer scale. The typical acoustic receiver has a detection range greater than 400 metres radius, so even if sharks are detected simultaneously in the area it does not necessarily mean they are interacting in any meaningful way.

So we switched to a different receiver technology that allows us to adjust the detection range. We dialled them down to alternate between 60 metre and 10 metre detection ranges. At this finer scale we could be reasonably certain that sharks detected at the same time were interacting.

We also have receivers that can be mounted on the animals themselves. As the shark swims around and encounters other tagged sharks, the interaction is stored on the tag. These animal-borne receivers only have a detection range of 4 metres.

Analysing data from a network of receivers is very complicated, but in essence we want to know if sharks are having meaningful social interactions or if they are just mutually attracted to the breeding reef. To answer that question we turned to social network analysis.

Social network analysis allows us to investigate social interactions in very sophisticated ways. The basis of the analysis is a comparison of the relationships observed with a completely random set of relationships generated by mixing up all possible interactions using computer simulations.

The results of that analysis clearly show that the social networks of Port Jackson sharks are not random – they have preferred partners. While we often see mixed-sex groups of sharks stacked on top of one another in caves and adjoining sand flats, our results indicate that males normally associate with males, and females with females. Further, they tend to hang out with individuals about the same size (i.e. age). So, for the most part, their social interactions are very similar to our own: Port Jackson sharks tend to have best buddies of the same sex and age.

The results of this work have some interesting implications, not least of which is the fact that sharks and rays are clearly capable of recognising one another and preferentially associate with specific individuals. They are also capable of remarkable navigation feats, returning to the same reef after a round trip exceeding 1000 km! We should not be all that surprised by this, as many elasmobranchs have very large brain to body size ratios, quite similar to mammals. All that brain power must be there for good reason.

Our work shows that sharks are not the mindless killers we often read about in the media: they are neither mindless nor killers. Quite clearly we need a major rethink about our attitudes towards sharks based on fact rather than fiction. Sharks have far more to fear from us than we from them.

Dr Culum Brown is Editor of The Journal of Fish Biology and Animal Behaviour, and Director of Higher Degree Research in Biology at Macquarie University.