Food habits of Pacific blue marlin
What billfish feed on is of major interest to fishermen for obvious reasons. Where in the water column a marlin feeds has a direct bearing on its catch-ability and depends on what it happens to be feeding on at the time. Marlin are known to feed at great depths, where they are fished by longline, as well as near the surface, where sport fishermen target them.
Most marlin feed on prey usually found in the uppermost layer (Epipelagic, 0-82-fathoms) of the ocean prior to capture. Epipelagic prey account for close to 50 percent of the food items, and by volume they made up more than 90 percent of the food consumed. Epipelagic prey are comprised of a large number of species; mostly consisting of squids, tunas and puffers. Squids are the dominant prey, but tunas — aku, skipjack, kawakawa and yellowfin — are the single most important prey.
This is strikingly apparent with marlin taken by live baiting techniques. Marlin captured using live bait techniques had been feeding on larger prey prior to hookup; the most common larger prey items are small tunas
A prey group that appears to be numerically important (35 percent) is the category of inshore species, such as the larvae of coral reef fish of a quarter- to one-inch in length that are found out in the open ocean plankton during part of their normal life cycle.
Marlin do not have gill rakers like whales do, so they can’t filter feed to separate these small creatures from the water by just opening their mouths and swimming through a school. Instead, they have to actively go after each little morsel one at a time. Ironically, the marlin will usually burn more energy catching the larvae than it gains once it is digested.
This behavior tells us two things. First, marlin have excellent eyesight to be able to go after quarter-inch prey. Second, when a marlin is hungry, it will feed on just about anything.
But while marlin will eat just about anything if they are hungry enough, they, like many other predators, may establish a specific search image for certain prey. For some period of time, they favor pursuit and consumption of prey fitting this visual image. Marlin caught on a specific bait often had stomachs containing more of that same type of bait. In other words, they were caught on what they had been feeding on.
Specific search images probably develop with the increased frequency of encounter of a given prey type. Thus, the more often an individual marlin sees and captures a certain kind of prey, the more positive reinforcement that billfish has for that prey item.
The majority of the marlin in a given area will develop specific search images for the locally most abundant prey. Thus, if a fisherman were to know what prey are abundant at a given time, he could select lures or bait that most closely approximate the appearance of that prey.
Remember, however, that billfish — like most predaceous fishes — are opportunists, and if they are hungry and see a nonspecific search image prey item, they will probably attack it.
What is the bottom line? If a certain type of prey is usually common at one point in time, these predators probably develop short-term specific search images for these common prey items. Thus, if small aku are abundant, I would put my money on either live-baiting or using a lure that looks like a small aku.