Where do sharks go and when? What areas do they use? These are important questions for understanding the role of sharks in ecosystems because the answers will tell us when and where sharks are going to influence their prey populations. Answering these questions is also important to the conservation and management of sharks because it allows for the designation of critical habitats, such as nursery grounds, that can be protected.

But how do we figure out where the sharks are and where they go? It turns out fishers can be very helpful in this respect. Let’s face it, fishers on the whole, whether commercial or recreational, are very good at catching fish. In fact, fishers often know the hotspots for a given species before biologists do, and we biologists often use this information to pick our study sites and species. So by working with fishers, we can learn where, when and how often they catch sharks, and then design appropriate studies to research the movements of the sharks and their encounters with fisheries. Talk to enough fishers over a large enough area and get enough conventional identification tag recaptures and there’s a good chance that you can piece together a general migration picture. For example, in shortfin mako sharks in the western Atlantic Ocean, we see a northward progression of fisheries captures from the mid-Atlantic Bight in the USA to the Grand Banks off Canada as the seasons change from spring to autumn. Presumably, the sharks are following the warm waters north.

Data of this type are called fishery-dependent data because they are collected directly from commercial and recreational fisheries. Although they are extremely useful to biologists, they do have some drawbacks. Not only are we are at the mercy of the fisheries, but data collected in this manner are not standardised. Many fisheries are not targeting sharks, so depending on their target and local regulations, the gear, bait types and time of year spent fishing can differ between fisheries. The fishing locations also reflect the best places to catch the target species, not necessarily where the sharks we are studying are. So when it comes to shark movements and habitat use, fishery-dependent data can tell us that sharks are in an area where they are being caught, but the lack of capture information for an area during a given time of year doesn’t necessarily mean the sharks are not there. The lack of captures for an area could be because no one was fishing there. And even if sharks are being captured in a certain area, that may not be the best place to catch them for studies.

Although fishery-dependent data are certainly useful and can provide broad insights into potential fish movements, the biases that come with them make it difficult to answer fully questions about the details of shark movements and habitat use. What we need are additional sources of data that are not dependent on fisheries – we need, in fact, fisheries-independent data. One way for scientists to collect such data directly is to conduct fishing surveys themselves in a standardised scientific format. But because of all the fishing effort needed, these methods can be very labour intensive and therefore costly. And it is still possible that sharks, being highly mobile, may leave the survey area.

So how can we figure out where wide-ranging shark species are going in a way that is independent of fisheries and yet also provides the detailed information we need? If only the sharks could tell us! But it turns out that they can – by using a technique called satellite telemetry. This method makes use of specialised tags that can ‘talk’ to satellites, essentially saying, ‘Hey, I’m over here!’ The location of the shark can then be relayed back to researchers sitting in front of their computers in their comfy offices. It’s just like a tracking device you’d see in a spy movie. After the shark has been tagged, it will tell us where it is for as long as the tag’s battery lasts; we don’t have to recapture the shark, so we have achieved fisheries-independence!