Seamounts are gathering hubs for ocean predators

Deep below the ocean’s surface, underwater mountains rise from the seafloor, creating landscapes few have explored. These seamounts, often teeming with life, stand in stark contrast to the vast, open ocean surrounding them.

New research has revealed that these hidden peaks hold far more predators than the open sea, drawing large numbers of sharks and other marine hunters.

A study led by the University of Exeter and the Ascension Island Government found that seamounts in the South Atlantic Ocean support astonishing levels of marine life.

The research focused on three seamounts near Ascension Island, uncovering why these locations serve as prime gathering spots for predators.

Scientists discovered that shallow seamounts, with peaks less than 100 meters below the surface, were particularly rich in marine life, including sharks and tuna.

Why do seamounts attract predators?

Scientists have long suspected that seamounts act as hubs for marine life. Some of these underwater mountains create nutrient-rich upwellings that boost phytoplankton growth, which forms the foundation of ocean food chains.

However, this new study found no evidence of such upwellings at the seamounts near Ascension Island. Instead, the researchers observed a different pattern. Marine life increased at every level of the food web, even without a surge in phytoplankton.

Zooplankton, which consume phytoplankton, were twice as abundant near the seamounts as in the open ocean. Sharks, at the top of the food chain, were 41 times more numerous than in surrounding waters.

“Seamounts have been likened to oases of life in the comparative deserts of the open ocean,” said Dr. Sam Weber from the Centre for Ecology and Conservation at Exeter’s Penryn Campus.

Why seamounts attract predators

Several factors explain why seamounts attract so many predators. The underwater peaks interrupt ocean currents, pushing smaller organisms into concentrated areas. This creates an easy feeding ground for fish, which, in turn, draws larger predators.

Dr. Weber and his team suggest that filter-feeding species benefit from food being swept over the seamount peaks, making prey easier to catch.

The peaks may also prevent prey from escaping into deeper waters, effectively trapping them in a confined space. This allows predators to hunt with greater success, making these locations an efficient source of food.

Seamounts are social hubs

Beyond hunting opportunities, seamounts appear to serve another function. Researchers found that some predators use these underwater mountains as social meeting points.

Sharks and other species may gather to interact, mate, or rest before returning to the open ocean.

“Also, some predators appear to use seamounts as ‘hubs’ to gather, socialise, mate or rest, and as a base to return to after hunting in the open ocean. This may lead to more top predators on seamounts than you would expect based on the amount of food available,” Dr. Weber explained.

The findings suggest that seamounts are more than just feeding zones. They may be essential gathering sites for marine predators, helping them interact and maintain social structures within their populations.

Species found at seamounts

The experts found that certain species consistently gather at seamounts. Among them are Galapagos and silky sharks, along with yellowfin and bigeye tuna.

Some individuals remained at a single seamount for long periods, while others traveled between seamounts up to 80 kilometers apart.

The researchers also noted a “halo” effect around the seamounts, where marine life remained abundant up to five kilometers into the surrounding ocean. This suggests that the influence of seamounts extends beyond their immediate area, affecting a much larger ecosystem.

Conservation for seamount ecosystems

The seamounts included in the study fall within the Ascension Island Marine Protected Area. This vast 445,000 square kilometer zone is free from large-scale commercial fishing and seabed mining, ensuring that marine life can thrive without human interference.

“Our results reinforce the conservation significance of shallow seamounts for many top predators,” Dr. Weber said.

“This research also offers fundamental insights into seamounts’ role as activity hubs and oases for marine species and shows how these remarkable habitats influence the oceans that surround them.”

A deeper understanding of seamounts

Despite their importance, seamounts remain one of the least studied ecosystems in the ocean. This research sheds light on the critical role they play in supporting marine predators and influencing broader oceanic food webs.

Understanding these underwater structures is essential for ocean conservation, as protecting seamounts could help maintain predator populations and overall marine biodiversity.

The study gathered data from a National Geographic Pristine Seas expedition aboard the British Antarctic Survey research vessel RRS James Clark Ross. Funding came from a European Union BEST grant and the UK government’s Darwin Initiative.

As scientists continue to explore these hidden underwater worlds, seamounts may reveal even more secrets about the complex relationships that shape life in the ocean.

The study is published in the journal PLOS Biology.

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