A question I'm often asked about Sailfish is "what is the purpose of the huge dorsal fin or sail?"
While free swimming or snorkelling at close quarters with these bold and curious Sailers (bold because they’re not shy to swim close to free-divers & display intimidating postures with their dorsal fins spread high & wide) I have observed that the fin plays an integral part in the awesome feeding behaviour of Sailfish.
Sails, like dolphins & most other schooling pelagics, co-operate together when feeding. One or two Sails act as “herders”, making a series of leaps, crashing loudly or slapping horizontally on their sides to round up a school of baitfish into a tight ball while their mates (approx 10-15 or more) surround the bait & pack them into an even tighter ball. The high dorsal fin acts like a matador’s cape, flashing neon electric blue & purple, encircling and mesmerizing the bait to prevent any from dispersing or escaping. This in turn causes the baitfish to panic & pack tighter together.
Simultaneously, the Sails dive into the bait-ball, 1 or 2 at a time (displaying good manners & etiquette!), swiping & stunning the bait with their sword-like bills. Sails would do well in a buffet line.....
Salt-water fly-fisherman have also observed this behaviour when Sails pursue their teasers on the surface, slashing repeatedly with their bills in an attempt to stun the teasers!
The teamwork is amazing to watch, and is similar to other great predators on land like wolves and lions. The bill can be used in apparent aggression (Sails have even been known to run boats through with their sword-like bills) but it does not appear to be used deliberately to “spear” prey.
The bill also plays another function. It's believed that Sails actually ‘fence’ with their bills and I have attached a picture below of a Sail with a broken-off bill tip embedded in its jaw. Sails exhibit aggressive behaviour during courtship/mating & may also be very territorial, & this may account for the broken off bill.
I believe that each and every Sail has its own unique personality, and this can be seen in the way they take the bait and fight on the line, from long exuberant tail walks on the surface, to frisky leaps & jumps, to head-jerking on the surface & fighting deep below like a Marlin or big Tuna; even charging aggressively towards the stern or the side of the boat at times.
Sails also come in different shades and colours depending on the level of excitement or stress they undergo during a fight, from dark blue, purple and brown to iridescent silver and even the length, thickness & shape of their bills can vary widely.
In short, every trip is different with Sails because of their enigmatic personalities, tenacity, and the situational action of Sail-fishing involving the quality & art of the catch, and the incredible aerial displays & acrobatics of each Sail.