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Where one predator meets another: tracking sharks and fishing effort

  • Fishing boats kill over 100 million sharks each year, many of which are caught unintentionally (bycatch) and may be discarded at sea without being recorded, so data on their mortality are poor.
  • Researchers used satellite telemetry and publicly available global fishing locations in Global Fishing Watch to compare movements of 10 blue sharks to fishing activity in the northwest Atlantic Ocean.
  • Within a 110-day period, two of the 10 tagged sharks surfaced near three different boats that were likely fishing, based on their movement patterns.
  • The research team wants to better understand how fishing fleets can limit fishing in areas when and where sharks and other non-target species gather or migrate each year.

Technology above the clouds is helping scientists study sharks beneath the waves.

A new initiative combines shark movement data with publicly available vessel identification data—both transmitted to researchers via satellite—to identify where sharks and fishing activity intersect in time and space.

A predator of sharks

As the ocean’s top predators, sharks play an essential role in marine ecosystems. They tend to reproduce slowly, and high rates of catch-related mortality may help to explain why populations not recovering.

A blue shark, distinguished by its long, thin head and body, plies the coastal seas off California. They spend most of their time in the open ocean. Photo credit: Mark Conlin, National Marine Fisheries Service, SWFSC Large Pelagics Program

Overfishing, both purposeful and accidental, has caused shark populations to plummet worldwide. Humans are estimated to kill over 100 million sharks every year. Nevertheless, the harvest of oceanic sharks is largely unregulated and poorly enforced, so data on shark mortality remains scarce.

Catch data reported by fishing vessels were the only available source of mortality data but are considered to be of questionable quality for both commercial and non-commercial shark species.

Researchers have begun to use satellite tracking tags to independently document sharks’ movements, interactions with fisheries, and mortality, and their findings suggest catch data are even less accurate than previously thought.

Another study published earlier this year found that fishing killed 12 of 40 of satellite-tagged shortfin mako sharks, a mortality rate 10 times higher than what fishers report. Moreover, this rate is greater than the maximum sustainable yield for the species, suggesting fisheries are overharvesting these sharks, either purposefully or accidentally.

A shortfin mako shark, taken during juvenile shark survey of California’s Channel Islands. They are the world’s fastest shark and can jump up to 9m (30 ft) high, making them a target for game fishing. Photo credit: Mark Conlin, NMFS

Where fish meet fishers: combining telemetry and transponder data

More recently, researchers have compared movements of satellite-tagged sharks to the activity of commercial fishing vessels in the northwest Atlantic Ocean, processed and displayed on Global Fishing Watch.

University of Miami shark researcher Neil Hammerschlag explained in a project video: “Several different species have declined upwards of 90% primarily due to overfishing, so it’s really important for us to determine where and when these individuals are vulnerable to fishing and what might be the best strategies to put in place to have effective conservation management.”

Hammerschlag contributed to a 2016 study that tracked the Global Positioning System (GPS) movement data of the Spanish and Portuguese longline-vessel fishing fleets in the North Atlantic. The study showed an 80% overlap of fished areas with shark habitat hotspots, suggesting that sharks are susceptible to fishing exploitation in many of their preferred areas.

In the current study, Hammerschlag and fellow shark researcher Austin Gallagher from the NGO Beneath the Waves tagged and tracked 10 blue sharks (Prionace glauca), highlighted in this video:

The wide-ranging, pelagic blue shark is the most frequently caught bycatch fish in some regions. Estimates of up to 20 million blue sharks are killed every year, mainly as bycatch (unintentionally) by longline and driftnet fisheries, as well as for sport fishing and, by Spanish and other European fisheries, for the fin trade. Sharks considered bycatch are often discarded at sea without being recorded, which means mortality statistics are very likely underestimates.

The researchers attached a SPOT-6 (Specific Position or Temperature) transmitter to the dorsal fin of each shark and monitored its movements in the northwest Atlantic Ocean between June and September 2016. Sharks were monitored for between 20 and 68 days in this initial effort. SPOT tags transmit location information via satellites whenever the tag is out of the water; that is, when the shark swims at the ocean’s surface.

An even more elongated blue shark off southern California. Photo credit: Mark Conlin, NMFS

The NGO Oceana combined the blue shark satellite telemetry data with fishing vessel locations in Global Fishing Watch to examine the overlap of fishing activity with shark locations in the continental shelf area south of Nantucket Shoals in the Northwest Atlantic Ocean. Global Fishing Watch’s free online platform allows users to monitor commercial fishing vessels across the world in near-real time.

Commercial fishing vessels in many countries must carry and use an Automatic Identification System (AIS) that integrates GPS with radio transceivers and other electronic navigation sensors. AIS devices broadcast the ship’s identity, location, and intended direction. The publicly available information is used by ships to help avoid collisions and by maritime authorities to track and monitor ship movements from coastal base stations or, increasingly, from satellites.

Global Fishing Watch harvests data from the AIS transmissions and applies algorithms to monitor and examine these billions of signals to determine which ships are actively fishing. The platform identifies “apparent fishing effort” of each boat using algorithms that assess AIS data, which are collected by terrestrial and satellite receivers to find the specific patterns of speed and direction that suggest fishing activity. The platform clarifies that it is possible some fishing effort may not be captured, and some movements that suggest fishing may in fact represent another activity.

A close-up image of the study results, including GPS locations of tagged sharks (in pink) and fishing vessels (in light blue), using the interactive Global Fishing Watch map. Image credit: Global Fishing Watch

Global Fishing Watch also displays the AIS vessel movement data on interactive maps that anyone can view. You can now explore the fishing and the shark movement data on its own project workspace.

The project team identified four occasions during the 110-day study period where a tagged blue shark came to the surface less than one kilometer from a vessel that was likely fishing. One shark surfaced near a single fishing boat, while another surfaced near three different vessels.

Data on satellite-tagged blue sharks moving through the northwestern Atlantic Ocean and encounters with fishing boats in first 3 months of the study. Image credit: Global Fishing Watch

A call for marine tracking data

The shark study suggests that combining satellite-based tracking and vessel monitoring can help researchers and fisheries managers study interactions between marine animals and fishing boats.

The researchers, Oceana, and other Global Fishing Watch partners want to better understand and communicate how fishing fleets can limit or avoid fishing in certain spots where sharks and other non-target species gather or migrate each year.

Wessley Merten, Oceana’s former data and fisheries analyst for Global Fishing Watch, posted, “Any type of tracking data can be integrated into Global Fishing Watch, whether it be sharks, or seabirds or sea turtles or other types of fish, and a lot of that information can be pretty telling as to how those animals are interacting with boats.”

Hawksbill turtles are highly migratory and use a variety of habitats, from the open ocean to coral reefs, lagoons and mangrove swamps. Like other marine turtles, hawksbills are endangered by human activity, on land, where females lay their eggs, and at sea. Photo credit: Caroline S. Rogers, NOAA, Creative Commons 2.0

Oceana’s senior campaign director Beth Lowell acknowledged the need for more data to expand upon this initial proof of concept. She suggested to that “…as more data is ported into the tool, more trends will arise and researchers will be able to see in time and space how sharks operate among fishing activity.”

Lowell hopes that scientists tracking animals in the field will continue to build the species database by sharing satellite tracking data on sharks and other species dating back to 2012. “Improvements in satellite tags and the quality of the data will help this grow exponentially,” she said to Smithsonian, and the information “can help inform fisheries managers and others on the best way to restore our oceans and protect vulnerable species.”

One strategy recommended last year by a multi-institutional research team is for the 170+ member countries of the International Maritime Organization to more consistently require and enforce use of AIS devices.

More locally, researchers and managers can use the combined species-fishing location data to predict bycatch risk and recommend management actions to reduce it. These actions could include setting no-take periods in certain areas where bycatch species are most likely to be caught or modifications to fishing gear that helps reduce bycatch of at-risk species.