Great white sharks recorded diving deep to twilight, midnight zones

Researchers have recorded great white sharks engaging in a perplexing deep-sea behaviour that has mystified scientists.

A new study has recorded large marine predators including great white sharks diving deep to the ocean’s twilight and midnight zones, but what they do down there is a mystery.

The data, published earlier this month in PNAS, analysed the movements of 12 species of large predatory fish, including sharks, swordfish and tunas. It showed the fish made “regular excursions” to the mesopelagic zone, also known as the twilight zone, between 200 and 1000 metres below the ocean surface, as well as the midnight zone, 1000 to 3000 metres under.

Great white sharks were recorded diving as deep as 1128 metres, whale sharks up to 1912 metres and swordfish as far as 2000 metres. The visits went far deeper than the depths at which these species feed, mystifying scientists.

Great white sharks were recorded diving to the midnight zone.

“How, when and where they access the deep ocean certainly varies, but the clear anecdotal answer is that the deep ocean seems like an important habitat regardless of the predator species,” Camrin Braun, study lead and an assistant scientist at the Woods Hole Oceanographic Institution, told LiveScience.

“It’s clear there are good reasons for these animals to dive deep, otherwise why would they all do it?”

The researchers’ findings also showed a correlation between the dives and the location of the deep scattering layer (DSL), an ocean layer that is so densely packed with small fish and other marine organisms it is sometimes mistaken for the sea floor.

This suggests large marine predators feed on bounty of prey that make up the DSL. However, the team also discovered many predators dive far deeper than the DSL extends, for reasons unknown.

Tuna were also recorded in the study. Picture: iStock.

“There’s good evidence for some species or situations in which diving deep is clearly for foraging, so that supported our expectation,” Dr Braun said.

“However, we also find several cases where we can pretty definitively say the use of the deep ocean is not for feeding — or if it is, it represents a totally different kind of predator-prey interaction or mysterious prey resource.”

Dr Braun said the evidence suggested the twilight zone had been overlooked as a critical habitat for ocean life.

“If it turns out that there is indeed more biomass in the twilight zone than in all current marine capture fisheries combined then it’s possible to imagine a kind of mesopelagic ‘gold rush’ to catch and use this biomass,” Dr Braun said.

“There are many ‘ifs’ in this chain and many issues in making mesopelagic fishing feasible, but it seems that biomass may be important for predators. Therefore, we really need to better quantify those links between predators and mesopelagic biomass before we can sustainably harvest or use those resources.”

Originally published as Great white sharks recorded diving deep to twilight, midnight zones