In April of 2015, researchers headed out to the Chagos Archipelago in the Indian Ocean to service acoustic receivers they had dotted around the archipelago, and to download tag data from the 95 grey reef and silvertip sharks they had tagged a year prior. Little did they know that, over the course of 10 days during the previous December, 15 of their tagged individuals had been illegally fished.

The Chagos Archipelago, otherwise known as the British Indian Ocean Territory, boasts one of the largest marine reserves in the world. The BIOT Marine Protected Area was created in 2010 and comprises 210,040 square miles of marine habitat, including an archipelago of seven atolls, at least 70 islands, and some of the world's most pristine coral reefs.

The British government created the reserve to conserve the archipelago's biodiverse ecosystems, amid concerns that increased fishing pressure from neighboring countries, including India and Sri Lanka, would decimate populations of important marine species. The sheltered atolls of the BIOT attract several species of pelagic and reef shark; here, mothers can give birth without their young being immediately threatened with predation, and juveniles can learn to hunt in lagoons before heading into the open ocean.

Researchers have been recording shark populations in the BIOT since the 1970s, and it has become clear that the numbers they are witnessing now are not what they should be.

"[Researchers] recorded sharp declines in reef shark abundances ... which the creation of the marine reserve in 2010 hoped to reverse," says David Tickler, a researcher from the University of Western Australia studying the effects of illegal shark fishing in the Indian Ocean.

Sudden declines in populations of any species are never good news; however, sharks are particularly vulnerable to these changes due to their ambling life history. "Sharks are very slow-growing," says David Jacoby, a Zoological Society of London researcher studying shark social networks in the BIOT. "They're very late to mature and they have very few offspring, so if you suddenly remove a large proportion of the population that are aggregating ... then the recovery is long and the impacts can be huge."

Acoustic Telemetry of High-Fidelity Reef Sharks

Acoustic telemetry is becoming an increasingly popular method of data collection for marine conservationists. Researchers from ZSL, the University of Western Australia, and the Stanford University Hopkins Marine Station have collaboratively been using acoustic transmitters to track the movements of reef sharks around the marine reserve since 2013.

"Acoustic tags transmit a series of sound pulses (a bit like Morse code), which are picked up by hydrophones [receivers] to locate the position of a tagged animal," Tickler says. "Depending on local conditions around a hydrophone, accuracy of position fixes can be a few hundred meters.... [Therefore] acoustic telemetry is much better suited to relatively resident animals, such as reef sharks, whose movements are typically too short to be reliably detected with satellite tags."

After rigorous development and strict ethical reviews, Tickler and colleagues acoustically tagged 47 grey reef sharks and 48 silvertip sharks in the BIOT. They monitored the sharks throughout 2013–14.

"When we were putting these tags in, we wanted to find out what areas within the marine protected area and the archipelago are more or less important at certain times of the year than others," says David Curnick, a ZSL researcher investigating the relationships between marine reserves and sharks. "Seeing whether there is social clustering both in time and space, you can then direct monitoring [of illegal activity] and enforcement to these areas."

Illegal Fisheries Target Sharks

In 2015, MRAG Ltd. published a paper detailing the extent of illegal fishing in the BIOT. The results suggested that the removal of reef fish has increased since 2013, and that sharks are being deliberately targeted. Between 2010 and 2015, 91 percent of illegal fishing vessels in the BIOT had sharks on board, and when present, sharks made up 79 percent of the catch.

This data concurs with global fisheries statistics; the mass-capture of endangered sharks has been recorded off the coast of Argentina, along the Mid-Atlantic ridge, and in the Galápagos Marine Reserve where 6,000 sharks were found onboard one vessel. These jaw-dropping figures are fueled by several factors; however, the demand for shark fin soup is by far the largest threat facing shark populations.

Shark fin soup is a traditional Chinese dish often served at weddings and other special occasions in China. The soup usually contains meat from the dorsal fin of the shark, which gives the soup its texture, while other flavorings, such as chicken, provide the taste.

Since 2011, following a star-studded campaign, shark fin soup consumption in China has decreased by a staggering 80 percent. However, growing popularity of the dish in other areas of Southeast Asia, such as Thailand and Vietnam, could indicate shark fin's disassociation from the expensive, exclusive meat it once was, and represent an increase in its availability, thus making the policing of biodiverse areas like the BIOT even more critical.

Acoustic Telemetry Data Uncover Illegal Fishing

Collecting data through wildlife tracking does have limitations, including their inevitable malfunction, damage to the tag, or the migration or predation of a tagged animal. Even though acoustic tags have a life expectancy of up to 10 years, a certain degree of tag loss is expected.

Before April of 2014, shark researchers in the BIOT saw an average loss of 4.1 tags per month. However, in December of 2014, 15 tags stopped recording data over the course of 10 days. The scientists investigated these potential causes of tag loss, but all seemed unlikely given the baseline tag loss rate that had been established.

"We'd heard about a big illegal fishing incident from the fisheries officer in [the] BIOT during the April of 2015 expedition," Tickler says. "We didn't think much more about it until we plotted out the tag detections and saw a big drop off in activity during the same month. When we looked at the sighting and arrest reports from the marine reserve's patrol boat, we found the sightings had all occurred just after our tags stopped transmitting."

The BIOT patrol vessel encountered 17 suspected illegal fishing vessels in December of 2014, which coincided with the dates the tags stopped transmitting. Authorities caught two of the vessels and found a combined catch of 359 sharks on board; grey reef and silvertip sharks made up half their catch. Using historical fisheries data, and assuming that this was a typical catch, over 2,000 sharks may have been caught in December of 2014 alone, including more than 200 grey reef sharks and almost 900 silvertip sharks. This would have constituted a loss of one-third of the shark population in that part of BIOT.

After modeling the fishing events and tag losses, the team was able to prove that there was a link between the loss of 15 tags and the illegal fishing vessels found in December of 2014. "Our study showed that a small number of tags (fewer than 50) was sufficient to detect an illegal fishing event," Tickler says, "most likely because we were monitoring one of the key target species of the illegal fishery."

Using Acoustic Tags to Tackle Illegal Fishing in the BIOT

The research team in the BIOT showed that, by combining acoustic telemetry data, historical fisheries data, and recorded illegal activity, conservationists can successfully implicate illegal fishing vessels. However, acoustic technology has a way to go before it can be used as a routine tracking method for illegal activity in marine reserves.

"In this case we were only able to analyze the data several months after the tagged animals had been caught," Tickler says. "New tag technologies being developed will allow tags to report their location the instant they are removed from the water, pinpointing fishing and allowing direct interception or observation of the vessels."

Until data can be uploaded from tags faster than it is currently, the extent of illegal fishing will remain elusive. But the data the team has and is continuing to collect is already helping them to understand where sharks are going to be and when, which is essential if marine reserves are to reach their full potential.

"If we can identify ecological hotspots of shark aggregations or areas where you see high movements between specific areas, and we know at what time of year they occur," says Jacoby, "then we can direct the enforcement vessels to be there at that time on the off-chance that fishermen also have that information and are purposely targeting that area."

With acoustic technology improving each year, the potential for it to help conservationists monitor and detect illegal fishing in marine reserves is increasing. Alongside acoustic technology, ZSL researchers are carrying out research to evaluate the effectiveness of sensors borne by aerial and aquatic drones, paired with the use of satellite imagery, to identify illegal activity.

Tickler is optimistic that technology will enable conservationists to make significant progress in coming years. "Managing our impact on marine ecosystems will be a vital challenge in the coming decades, and technology that increases transparency and fosters more sustainable use of the ocean will be needed to ensure we do not exceed its limits," he says. "Using animals as sentinels, both to detect illegal activity and to monitor environmental conditions, extends our capabilities and reach, making us more effective stewards of our oceans."

This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.