Lee Today

They Just Keep Swimming and Swimming and Swimming – Silky Sharks are Setting Records

Main Take-a-Ways

• Silky Sharks cover vast distances (thousands of miles per year) covering large geographic areas
• Silky Sharks frequent the ocean’s depths as well as shallower waters to feed
• Silky Sharks are one of the most heavily fished species; make up a large number of fins in the global fin trade
• Research Helping Protect These Sharks; More Needs to be Done

Smooth, slender and swift. We’re talking silky sharks, one of the ocean’s most inquisitive predators that turn out to be true travelers, roaming open seas and diving to depths of more than 300 meters (985 feet). Yet their tendency to feed on schools of tuna and to spend most of their time in relatively shallow waters (less than 100 meters, or 328 feet, deep) renders them extremely vulnerable to industrial fishing.

Silky sharks are among the most heavily fished shark species in the Tropical Eastern Pacific. Not only do their fins make up one of the largest proportions of the global fin trade, but their tendency to spend time on the high seas outside of the Marine Protected Areas of the region also puts them at risk of being incidentally taken as bycatch by industrial fishing fleets.

Shark satellite tagging carried out by scientists from NSU’s Guy Harvey Research Institute (GHRI) and Save our Seas Foundation Shark Research Center(SOSF-SRC), the Charles Darwin Foundation and the Galápagos National Park Directorate is shining a light on the travels of vulnerable silky sharks. The expansion of marine protected areas goes some way to protecting these sharks during their migrations, but new record distance and time tracking data reveal silky sharks travel massive distances and that much more must be done to save them from extinction.

Since 2014, research scientists from these organizations have been working jointly to discover more about the migration patterns of silky and other open-ocean sharks globally. By uncovering where and when the sharks spend their time, knowledge generated can better inform conservation plans and management actions needed to protect the animals during their travels.

In 2021, researchers expanded silky shark satellite tagging to include the iconic Galápagos Marine Reserve. The silky shark tracks can be followed on NSU’s GHRI website – select the silky shark option from the pop-up menu.

So, what have researchers learned after the tagging of 47 adult silky sharks during 2021? The distances they swim are massive!

One female shark (Silky 1) travelled nearly 7,000 kilometers (4,350 miles) in seven months (February – September 2021), visiting the UNESCO World Heritage Sites and marine protected areas of Galápagos, Isla del Coco National Park in Costa Rica and Malpelo Flora and Fauna Sanctuary in Colombia along the way. But, more importantly, she spent most of that time in unprotected areas in a region where there are high levels of illegal, unreported, and unregulated fishing.

Another female shark (Silky 39), tagged on July 17, 2021, took the “gold medal” for the furthest distance travelled between locations: more than 16,300 kilometers (10,100 miles) in 10 months. That distance is more than three times the previous record for this species – and this shark is still being tracked!

“These very long, energetically costly walkabouts by some females but not others raises the question, ‘why?’,” said Mahmood Shivji, Ph.D., a research scientist at NSU’s Halmos College of Arts and Sciences, who is co-leading this study. He is also the director of NSU’s GHRI and the SOSF-SRC.

It’s possible that environmental factors, such as shifting ocean currents, which drive hotspots of tuna and other prey, are at play, but more work is needed to say with greater certainty.

While Shivji’s question of ‘why’ remains unanswered for now, the ‘where’ is beginning to come into focus. Over the past 12 months, Silky 39 and her fellow travelers moved far and wide.

Some sharks made pit-stops at multiple marine protected areas and UNESCO World Heritage Sites across the region, including Isla del Coco National Park and Malpelo Flora and Fauna Sanctuary. However, most of the silkies also spent substantial time in unprotected areas with high fishing activity. But it’s not just their swimming that sets records.

Silky shark fins make up the second-highest proportion of fins found in the global market. Fins from up to two million silky sharks contribute to the global fin trade each year. The extremely high fishing pressure these sharks are facing causes dramatic population declines and has resulted in their recent uplisting to “Vulnerable” on the IUCN Red List of Threatened Species.

Dr. Pelayo Salinas de León, senior marine ecologist at the CDF and a 2021 SOSF Fellowship Grantee, oversees this tagging work in his ‘office’ – the Galápagos Islands. Darwin’s living laboratory is home to large shivers of scalloped hammerhead sharks, huge whale sharks and the tiger sharks. This shark sanctuary supports the largest shark biomass reported on any reef in the world and the silky shark is one of the dominant pelagic species here.

Sharks play an essential role in keeping marine ecosystems healthy and balanced. The biggest threat to their survival is overfishing, so taking these spatial overlaps into account, he explained, is a critical step towards reducing ongoing population declines.

There is hope for silkies and other sharks, though.

At COP26 in Glasgow 2021, the presidents of Colombia, Costa Rica, Ecuador and Panama signed a declaration to establish a new ‘marine corridor’ in the Eastern Tropical Pacific that expands protection and links existing marine protected areas (MPA) to stretch over 500,000 square kilometers (193,000 square miles), an area the size of Spain. This expanded marine protected area could increase protection for sharks on a critical marine migratory route. But does it go far enough to protect sharks that we now know travel much further than previously thought?

Of course, expanding marine protection across the Tropical Eastern Pacific is a great step. It will help protect the sharks during some of their travels, but silky sharks don’t know where these MPA lines on a map are, and frequently adventure to adjacent areas open to fishing. And even a MPA isn’t completely safe, since illegal, unreported and unregulated fishing of sharks across the ETP continues to be an issue. MPAs need to be complemented by an urgently needed, ambitious, and comprehensive management plan for silky shark populations across the ETP.

The Save Our Seas Foundation Shark Research Center is on a mission to understand the world’s sharks and rays. For the founder of the Save Our Seas Foundation, His Excellency Abdulmohsen Abdulmalik Al-Sheikh, it all began with silky sharks.

“My love for sharks started with silky sharks,” he explained. “Their tremendous migrations continue to amaze and inspire me. I hope that learning more about where they go, and when and why, can really help us target how best to protect them.”

_{Source:  https://news.nova.edu/news-releases/nsu-research-scientist-and-shark-expert-working-on-international-study-of-silky-sharks/}