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Having read a lot of opinion on Blue Marlin feeding habits, I decided to dig around for research reports on actual stomach contents of this apex ocean predator. The only opinions that count are stomach contents. This subject has been studied more often than I would have thought around the world. Below is a synopsis of what I could find. Included are links to the report and a pdf if you care to dig deeper.
Common themes are:
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 (Japansese word for skipjack), skipjack, kawakawa (another small tuna) 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.
Primarily near-surface pelagic fishes such as mackerels, tunas, and dolphinfish are preyed upon by the blue marlin. Squids, and the occasional deep sea fish have been noted in the stomachs of blue marlin. Considerable disagreement among researchers exists over whether or not the bill is used during feeding. It is believed by some to be used to stun prey with a swift lateral strike or strikes. The blue marlin is capable of consuming prey of relatively large proportions. Blue marlin are not known to feed at night.
The food and feeding habits of blue marlin Makaira nigricans Lacépède, 1802 were investigated using 1052 samples landed on Yonaguni Island, southwestern Japan, from February 2003 to February 2006. A total of 45 prey species consisting of 881 individuals were identified from undigested and partially digested stomach contents, and additionally seven other species were identified from nearly digested stomach contents. The most common prey species was skipjack tuna Katsuwonus pelamis (Linnaeus, 1758), accounting for 35.9% by number and 75.2% by weight, and observed in 62.3% of the samples. Species composition of stomach contents indicated that blue marlin mainly feed on prey near the surface, however, the presence of mesopelagic fishes in the stomach suggested that they also make movements to deeper depths for feeding. Feeding activity was high around fish aggregating devices (FADs) which attracted prey. Large blue marlin feed on larger prey and feed less frequently than do small blue marlin which contained a high variety of prey species.
We analyzed the stomach contents of 52 blue Marlins caught between October 2002 and October 2004 by the sport fishing fleet of Mazatlan, Mexico in the Gulf of California. Blue Marlin feed on 15 food items. According to the index of relative importance (IRI), the most important prey were the frigate or bullet mackerel/tuna Auxis spp. 52% and Jumbo Squid Dosidicus gigas (30%).
Katsuwonus pelamis, (Skipjack) the main item (68%) in the diet of M. nigricans (Atlantic Blue Marlin), is a typical epipelagic species of tropical waters, which reinforce the probability of encounter between both species. Furthermore, the heavy and muscular body of K. pelamis and other scombrids (Mackerel, Tunas, Bonito) is probably a much more important and efficient energy source for M. nigricans, to sustain its high metabolism rate, than small and isolated preys, commonly found in the open ocean. Since most analyzed individuals are adults, it is probable that larger M. nigricans needs larger fish to sustain their metabolism, which is profitable along the tropical waters of the Brazilian coast where schools of K. pelamis are commonly found.
Krumholtz & Sylva (1958) pointed out that the main food items of M. nigricans around Bahamas were also Scombridae (Mackerel, Tunas, Bonito).
Baker (1966) likewise, found the scombrids (Mackerel, Tunas, Bonito) K. pelamis, Scomber japonicas (Chub Mackerel), and Thunnus alalunga (Albacore Tuna) as the main prey items for blue marlin in New Zealand. Block et al.
All three above from this pdf: Blue Marlin-Brazil (w/ references to Bahamas and New Zealand)
As opportunistic feeders, a huge variety of things are found in their stomachs including tiger and nautilus shells from fish on the Great Barrier Reef. A large proportion of fish even contain several billfish spears as remains of the other marlin they have eaten.
Having said that, fish like yellowtail and slimey mackerel are the most common food with squid the most important non-fish prey. Around WA, they will also eat tuna and dolphinfish.
(I could not find hard research on Australian nor African Blue Marlin stomach contents but am sure there is substantial data. I would appreciate a link from someone to add to this information.)