Predator Paradise

Contrary to popular belief, marine predators do not spend their days in the open ocean.

New research from the United Kingdom’s University of Exeter examined three seamounts, or raised subsea platforms, in the South Atlantic Ocean and found that under-sea mountains are hot spots for predators, with a density of 41 times more sharks than found in the open ocean.

The seamounts in the study are located within the Ascension Island Marine Protected Area, a 172,000-square-mile zone that bans large-scale commercial fishing and seabed mining.

Two seamounts were shallow, with peaks less than 328 feet below the surface, and filled with predators like sharks and tuna.

“Seamounts have been likened to oases of life in the comparative deserts of the open ocean,” said study author Sam Weber.

However, until now, scientists were not sure about why so many marine predators dwell there.

Some seamounts cause upwellings of nutrient-rich minerals that support the growth of phytoplankton – tiny, drifting plants that form the base of the ocean’s food chain. The large presence of phytoplankton supports increased numbers of other species, from zooplankton that feed on them to apex predators like sharks.

This study, however, found no signs of increased “primary productivity” – phytoplankton growth and energy production – in the seamounts analyzed.

Researchers found instead that the rise in marine life — measured by “biomass,” the total weight of organic material — becomes more noticeable at higher levels of the food web.

Zooplankton were twice as likely to be found at shallow seamounts as in the open ocean, while shark biomass was 41 times greater.

Several factors are responsible for making seamounts so rich in predators.

Weber suggested that while primary productivity isn’t higher at those seamounts, filter feeders may benefit from prey being pushed over the peaks. This movement may also keep prey from retreating into deeper water, concentrating food in one predictable spot.

Moreover, researchers noted that some predators gather at seamounts simply to socialize, mate, or rest.