Watch the fastest fish in the world hunt its prey in the open ocean

Sailfishes are the fastest fish in the ocean, clocking in at speeds up to 110 kilometres per hour, so studying them is challenging to say the least.

Up on the surface of the ocean, wildlife photographers have observed groups of swordfish (Istiophorus)hunting together – using their long bill to slash, stun, and eat schooling fishes. But they’re thought to live a mostly solitary lifestyle, and little has been known about their hunting behaviour once they dive beneath the surface of the ocean.

Thanks to the design of a new electronic tag scientists finally have some insight: for the first time they have video footage of a solitary hunting sailfish.

“Most of what you see in the videos is just a lot of blue water. But when I saw the sailfish start to swim really fast toward the surface, I knew something was up,” says Ryan Logan, a doctoral candidate and research associate at Nova Southeastern University in the US, and lead author of the study.

The new research is in the journal Scientific Reports.

Sailfish are known for their distinctive large dorsal fins, which can be taller than the length of their bodies, and their elongated, sword-like upper bill. Their unique features don’t end there though, they can even keep their eyes and brain warmer than the surrounding water, which gives them a distinct advantage over prey when hunting in colder or dimly lit water.

Because of this, it’s assumed that they must burn a lot of energy during the day and must need to eat between group hunting events, but this has never been documented.

The new video shows a 40 kilogram sailfish ascending rapidly to the surface from a depth of about 60 metres in pursuit of a small tuna. Because the tag was mounted on the side of its body the sailfish’s mouth wasn’t in view, but it looks like the sailfish got what it was after in the end, as it returned to normal, calm swimming behaviour.  

“Because you can’t keep a sailfish in captivity, we know surprisingly little about their basic biology. For instance, how much food do they need on a daily basis to survive?” says Logan.

To address this, the team used the video footage, along with other information collected by the high-resolution tag, to estimate how much energy the sailfish burned throughout the day and how much energy it gained back by consuming the tuna.

Based on their estimations, this particular sailfish needs roughly half of a small tuna each day to meet its energetic demands.

“This research improves our understanding of the hidden lives of these majestic, ecologically and economically very important fishes,” adds Dr Mahmood Shivji, a co-author of the study and Director of NSU’s Guy Harvey Research Institute.

In 2021, sailfish were assessed for The IUCN Red List of Threatened Species (which recognises a single species Istiophorus platypterus). The fish are caught as food and sportfish and where it is overfished and overfishing is occurring, the species is listed as Vulnerable.

“Such knowledge is essential to help us better protect the health of these fish and their prey to have a sustainable sportfish industry for many years to come.”