Satellite Tracking Has Shed Light On The Remarkable Odyssey Of Silky Sharks
The sun hung directly above azure waters of the northern Galapagos Marine Reserve, casting a glare as a small boat bobbed gently amidst the swells, its bow cutting through the calm surface with practiced ease. Against the backdrop of the rugged, volcanic coastline of Wolf Island, a team of dedicated scientists waited – patiently – for its target to show.
A species of requiem shark found in tropical and warm temperate waters worldwide, silky sharks (Carcharhinus falciformis) typically inhabit pelagic environments, ranging from the surface to depths of around 200 meters (656 feet). With a name derived from the smooth texture of their skin, which feels silky to the touch, they are characterized by their streamlined bodies, large eyes, and long, rounded snouts. They are known to often form large aggregations, particularly around seamounts, oceanic islands, and along continental shelves. This island was one of those known aggregation sites... so where were they?
"There!" a voice shouted, as a gray dorsal fin sliced through the water surface with ease. The air thrumming with anticipation, the team works swiftly, their practiced hands expertly guiding ropes to secure the predator alongside the vessel. Amidst the chorus of seabirds and the lapping of waves, the scientists affix a fin-mounted satellite tag to the female shark, marking her as a scientific datapoint – a crucial one.
Like many of their pelagic counterparts, silky sharks face an uncertain future due to overfishing driven by their life-history traits. Slow growth, late maturity, and small litter sizes render them vulnerable to exploitation. And their vulnerability is exacerbated by their behavior. Silky sharks tend to congregate around tuna schools and drifting objects, making them easy targets for industrial fishing fleets, particularly in the Tropical Eastern Pacific. They're also frequently caught as bycatch in fisheries targeting other species, and their fins are highly valued in the shark fin trade. These pressures have led to their listing as Vulnerable on the IUCN Red List of Threatened Species.
"Despite their potential importance as top predators in oceanic food webs and their contribution to both artisanal and industrial fisheries landings across the Tropical Eastern Pacific, little is known about the spatial ecology of silky sharks in this ecoregion," the authors of a recent study , that reports the longest (in duration and distance traveled) migration for the sharks, explain. "Few peer-review movement studies on silky sharks using satellite telemetry for the Tropical Eastern Pacific have been published to date, with most of these using pop-up satellite archival tags to focus on post-release survival following capture by industrial fishing fleets." These studies documented a maximum linear distance traveled of approximately 1,988 miles (3,200 kilometers), with the majority of tags resurfacing within a radius of less than 621 miles (1,000 kilometers) from the point of tagging.
But this study, led by Dr. Pelayo Salinas de León of the Charles Darwin Research Station and Nova Southeastern University, follows the journey of a female silky shark tagged in the Galapagos Marine Reserve, uncovering a migration spanning over 16,777 miles (27,000 kilometers) – one of the longest recorded for any pelagic shark species. "The Tropical Eastern Pacific is a shark hotspot and the oceanography around this part of our blue planet has a lot to do with it. The northern equatorial upwelling front, which brings cold-nutrient rich waters to the surface, combined with the tropical sun is the perfect recipe for ocean productivity that sustains ocean pelagic food webs. While we are not sure why silkies head out to this 'cold tongue,' we hypothesize that these migrations might be related to foraging, mating or birthing (or a combination of these)," he says. Similar seasonal migrations for foraging purposes have been observed in other shark species, highlighting the importance of these nutrient-rich regions in sustaining marine predators. "We presently have another 88 silkies being tracked so hopefully this new data will share more light on the why."
The recorded migratory pathway also indicated the extensive utilization of international waters by this female shark, with over 99% of the tracked time spent within international waters to the west and south of the Ecuadorian Exclusive Economic Zone surrounding the Galapagos Islands. "[Revealed] by the satellite monitoring of industrial vessels through the Global Fishing Watch platform, this area is heavily fished by national and international fishing fleets, and this 'overlap' results in high levels of fishing mortality for many shark species," says the team. This, Salinas-de-León and his co-authors argue, underscores the need for collaborative conservation efforts beyond individual jurisdictions. Steps have been taken to mitigate silky shark bycatch in industrial fisheries, but much remains to be done, especially in regions where they're still targeted by artisanal fleets. "[Marine Reserves serve as crucial guardians of nearshore ecosystems, safeguarding habitats like mangrove forests and coastal reefs along with their resident species like corals and reef fishes. They also offer refuge to highly migratory species such as coastal and pelagic sharks during certain life stages. However,] the highly migratory nature of pelagic sharks means that sooner or later they will leave the reserve to follow their more basic instincts in the search of foraging areas, mates or birthing grounds," Salinas de León explains. "This highlights the need to combine the use of marine reserves together with sustainable fisheries management around protected areas, including the high seas, if we really want to save sharks from extinction."
As apex predators, silky sharks play a crucial role in maintaining ecosystem balance, and their decline could have far-reaching consequences. Advancements in technology are providing crucial glimpses into the intricate workings of marine life, revealing the interconnectedness of ecosystems beneath the waves. However, it's crucial to acknowledge the sobering truth that every aspect of our planet's ecosystems is intricately interconnected. This means that events unfolding in the remote reaches of the Galapagos Islands can reverberate across the entire marine environment. "Pelagic sharks are in big trouble mainly due to overfishing, with over 75%of these species now threatened with extinction. If we want to save them, it is time to act," stresses Salinas de León. "Pelagic sharks do not belong to one single country, they migrate between the waters of many nations and also across the high seas, that belongs to all humanity. So we need to come up with urgent trans-boundary management actions that stop unsustainable fishing practices and give silkies and other pelagic sharks a chance for recovery."
