Surface-based group feeding of Indo-Pacific sailfish (Istiophorus platypterus), a highly popular species in recreational fishing worldwide, is commonly observed, yet sailfish are believed to be largely solitary roaming predators. In new research, marine biologists from the Guy Harvey Research Institute at Nova Southeastern University and Florida International University used biologging data and video to examine daily activity levels and foraging behavior, estimate metabolic costs, and document a solitary predation event for a 40-kg sailfish.
Sailfish are known for their distinctive large dorsal fins and elongated front-end bill, which gives them a built-in sword-like weapon for hunting.
These open ocean wanderers can travel both by themselves, or in groups.
Sailfish have been observed to hunt in groups on schooling fishes, using their bill in a slash, stun, and eat the baitfish sequence of actions.
This type of hunting by sailfish has been observed several times by wildlife photographers because it often occurs at the surface of the ocean, where it is easily accessible to human eyes.
However, when sailfish are not engaged in these cooperative group hunting events, they are thought to live a mostly solitary lifestyle.
Sailfish have some other pretty unique features that separates them from many other groups of fish. For example, they can keep their eyes and brain warmer than the surrounding water, which gives them an advantage over their prey when hunting in colder or dimly lit water.
But because of this, it’s assumed that they burn a lot of calories throughout the day, so one could assume that they must also need to eat in between those group hunting events, although this had never been documented.
“Most of the day sailfish dive back and forth between the surface and the thermocline layer, where the water gets cold,” said lead author Ryan Logan, a doctoral candidate in the Guy Harvey Research Institute at Nova Southeastern University.
“The thermocline can concentrate prey that don’t want to enter the cold water, so it looks like the sailfish might be using this to its advantage.”
Logan and his colleagues originally set out to determine how long it took the sailfish to recover after being released by a recreational fisher.
But after the fish had recovered, the video cameras continued to roll, picking up some never-before-seen footage of hunting behavior.
While the video footage provides a unique perspective on the daily lives of sailfish, the researchers caution that most of isn’t that exciting.
“Most of what you see in the videos is just a lot of blue water,” Logan said.
“But when I saw the sailfish start to swim really fast toward the surface, I knew something was up.”
The video shows a sailfish ascending fast to the surface from nearly 200 feet deep, and vigorously chasing a small tuna once it reaches the surface.
The sailfish makes several attempts to catch the little tuna, twisting and turning, speeding up and slowing down, often breaking the surface of the water during each attack.
In the end, it looks like the sailfish got what it was after, returning to a normal, calm swimming behavior.
“These findings change what we know about the daily lives of sailfish,” Logan said.
“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?”
These are questions that this research is helping to answer.
The authors used the video footage, along with other swimming 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, an energy-rich prey item.
“Based on our estimations, this sailfish needs roughly half of a tuna like this per day to meet energetic demands,” Logan said.
The findings were published in the journal Scientific Reports.
R.K. Logan et al. 2023. Hunting behavior of a solitary sailfish Istiophorus platypterus and estimated energy gain after prey capture. Sci Rep 13, 1484; doi: 10.1038/s41598-023-28748-0