Under-sea mountains are key ‘hubs’ for sharks

Under-sea mountains are key locations for predators – with 41 times more sharks than the open ocean, new research shows.

The study – led by the University of Exeter and the Ascension Island Government – examined three seamounts off Ascension Island in the South Atlantic Ocean.

Two were shallow seamounts, with peaks less than 100 metres below the surface – and these were teeming with vast numbers of predators, including sharks and tuna.

“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 on Exeter’s Penryn Campus in Cornwall.

“However, this hasn’t been studied in detail – meaning we’ve been unsure about why seamounts attract so many marine top predators.”

Some seamounts create upwellings of minerals that support bountiful phytoplankton (tiny drifting plants that are the first link in ocean food chains).

Such quantities of phytoplankton can support increased numbers of other species, from zooplankton (which eat phytoplankton) all the way to top predators like sharks.

But this study found no evidence of increased “primary productivity” of phytoplankton at the Ascension seamounts.

Instead, enrichment of marine life (measured by “biomass” – the total weight of organic material) goes up with each level of the food web.

Zooplankton were twice as common at shallow seamounts than in the open ocean, while shark biomass was 41 times higher.

“Our findings suggest that several factors combine to make seamounts so rich in sea life, especially predators,” Dr Weber said.

“While primary productivity is not higher at the seamounts we studied, filter feeders may benefit from prey being ‘blown over’ the peak, and the peak may also stop prey species from retreating into deeper water to avoid predators. This effectively concentrates food in one predictable spot in the 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.”

The findings suggest certain species tend to gather at seamounts – including Galapagos and silky sharks, and yellowfin and bigeye tuna.

Some individual animals were found to be “resident” – living at a particular seamount most of the time – and others visited both shallow seamounts in the study (80km apart).

The study also found a “halo” of increased marine life around seamounts, extending at least 5km into the open ocean.

The seamounts in the study are all within the Ascension Island Marine Protected Area – a 445,000 square km zone where no large-scale commercial fishing or seabed mining are allowed.

“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.”

Data for the study was collected by a National Geographic Pristine Seas expedition aboard the British Antarctic Survey research vessel RRS James Clark Ross

The research was funded by a European Union BEST grant and the UK government’s Darwin Initiative.

The paper, published in the journal PLOS Biology, is entitled: “Shallow seamounts are ‘oases’ and activity hubs for pelagic predators in a large-scale marine reserve.”