New research published by scientists at ZSL’s Institute of Zoology is helping to reveal the mysterious movements of sharks
Monitoring the behaviour of marine wildlife can be difficult. Because they’re underwater, it’s not easy to observe their behaviour, especially for long periods of time, unlike with terrestrial species. Acoustic telemetry can be used to detect the movements of marine species, without having to observe them.
In acoustic telemetry, animals are attached with small tags that emit a specific frequency, which are picked up by receivers. These receivers then store the time and individual identification of the animal. These technologies can be used to see how marine wildlife move around and utilise specific areas of interest, such as coral reefs or sea mounts, although there are often gaps in the data in areas where no receivers have been positioned. ZSL’s researchers have presented a unique gap analysis of telemetry data to help understand tagged animal movements and their use of space where acoustic coverage is otherwise unreliable or impractical.
ZSL’s lead researcher, Michael Williamson, tells us more about this latest study and what it could mean for scientists studying the behaviour of the planet’s more mysterious inhabitants.
Can you give us an overview of your research?
As part of research being conducted in the Indian Ocean, across the Chagos Archipelago, several species of reef-associated sharks have been tagged, using acoustic transmitters, to see how they use and move around this important marine sanctuary. I’m using data on two species; grey reef sharks and silvertip sharks, two of the most common shark species in the region, to investigate the movements of these two species. One of the issues we have is that the Marine Protected Area (MPA) is very large (640,000 km²), and we can’t place receivers everywhere. Lots of regions are too deep for us to dive to place and collect the receivers (and the data they gather), and the MPA is very large so we need to work out different ways to analyse the data to find out where the animals might be, even if they are not being detected continuously on our receiver network.
Why is it useful to know what a shark, or any tagged animal, is doing?
If we know where sharks like to spend their time, this can help inform the ecology of the species; what habitats do they like to use, and how do different species share space. This in turn can inform conservation. The British Indian Ocean Territory (BIOT) MPA is a very large area to cover and monitor, and illegal fishing is a problem within the MPA. By understanding where sharks spend most of their time, we can figure out what are the most important areas to monitor and enforce within the MPA to mitigate illegal fishing.
You’ve described acoustic telemetry as being like an oyster card – can you tell us a bit more about that?
In acoustic telemetry animals are attached with small tags that emit a specific frequency, which are picked up by receivers. This works similar to an Oyster card, albeit it a slightly different technology. When an animal with an acoustic tag attached comes close to a receiver (usually within 500m) it is detected, like when you tap on or off at a tube station. We have a network of receivers around the MPA which work a bit like tube stations. So we know when an animal visited one place and where it went to next but not what it did in between and this is what our study is trying to estimate.
Our receivers are not in such a dense network as the underground network, so we developed a new method to find out if sharks spent time on the reef, or if they spent time away from the reef. This method takes time into account to assess the directionality of the shark’s movement. For example, we know shark 1 went from King’s Cross to Victoria and did it in a short amount of time. So, we can most likely say it went straight on the Victoria line and stayed within zone 1. However, shark 2 also went from King’s Cross to Victoria, but took three hours. We can say it probably didn’t go directly there but may have travelled a longer method, so most likely didn’t stay in zone 1 during that time.
By doing this we can see if sharks spend time on coral reefs (zone 1) or away from coral reefs (away from zone 1), and we can look how different species spend time in different areas (on reefs or away from reefs) in areas where you don’t have receivers to detect them.
What we found was that grey reef sharks tend to spend more time on coral reefs, whilst silvertip sharks spend more time away from the reefs, showing that these species utilise the habitat in the MPA in different ways, which backs up previous work done on these two species from satellite tracking, and shows that this method could be useful not only for us, but lots of other researchers around the world which may have similar issues with a lack of receivers, or a very wide area to monitor animal movements.
What’s next for your research?
Our aim is to use this method, and others, to see how sharks change their movements over time, particularly how they change their movements following coral bleaching events. Coral reefs are important ecosystems for many shark species. If coral bleaching occurs, and destroys their habitats, we want to know how this impacts how sharks use the BIOT MPA, which can aid their protection and conservation as outlined above.
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