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Marlin are opportunistic feeders

August 24, 2017
BY DONNEL A. TATE , Lahaina News

Marlin are carnivores, and when hungry, they are fearless and undoubtedly the top predator teleost of the oceans. Their stomach contents show that they are opportunistic feeders - feeding on almost anything and everything. Almost every available species has been identified from gut content analyses.

Accordingly, their gastrointestinal tract shows features common to most carnivores, including large-capacity stomachs and short intestines. The abdominal organs of marlin make up about 4-10 percent of the total body mass. Because carnivores feed discontinuously, the stomach needs to be capacious to take advantage of periodic food availability.

The intestines are short because meat or fish is more easily digested than vegetation. Marlin ingest food items whole, without chewing, and therefore must have a digestive system capable of breaking down bones, scales and skin as well as the proteins, fats and carbohydrates in their diet. How long this takes is unknown, but two facts suggest it is rapid.

The first is that marlin are very active fish and need a large food supply. The second is that in order to take advantage of periodic food availability, they need to take in and process as much food as possible when it is available. Large-capacity stomachs and rapid digestion will help them take advantage of periodically plentiful prey.

Marlin are occasionally seen by anglers with their stomachs everted. The stomach is inside out and usually hanging from the corner of the mouth.

Stomach throwing is commonly seen in marlin. The absence of mesenteric attachments and blood vessels, other than those at the esophagus, allow stomach eversion with no obvious damage. The mechanism by which they might swallow the stomach again is unknown, but anatomically it appears feasible and may involve the longitudinal bands of smooth muscle on the outer stomach walls.

Stomach throwing may be a normal activity to help marlin get rid of indigestible items such as hooks, wood, squid beaks and bony debris that they tend to accumulate in order to inhibit injury to their stomachs. However, they do have a tendency to accumulate ulcers at the end of their stomach, possibly due to injuries from spines of prey and other indigestible matter.

The most notable stomach contents I have personally documented were during the 1997 Lahaina Jackpot Fishing Tournament. That was the year that the two 1,100-pound "Grander" blue marlin were caught on the same day. They weighed 1,106.0 and 1,101.5 pounds.

Before weighing the 1,101.5-pounder, I noticed that the tail of a mahi was stuck in its throat. This was IGFA legal and was left in the marlin's mouth for weighing. After weighing the marlin, I noticed something about 3-4 inches sticking out of the belly side of the fish. As I touched it, I knew that it was the bill of a small marlin.

I cut open the stomach of the marlin and pulled out an estimated 20-pound spear-fish. I then pulled out the mahi from its mouth, estimated at 20 pounds. After talking to the captain, he informed me that as they were leadering the marlin next to the boat, it regurgitated a spearfish before they could get it secured.

This big blue was greedy and hungry. It ate a spearfish, a mahi and another spearfish before eating an artificial lure. The spearfish and mahi looked freshly eaten, within a few hours of it being captured. If that last spearfish had stayed in its mouth, they would have won the tournament, losing by only 4.5 pounds and costing them $11,500 difference in first place.

Just last week, one of the charter boats brought in a 293.0-pound blue marlin. Once they were tied up at the dock, the crew mentioned that it had a mahi sticking out of its mouth. They taped the mouth closed so it wouldn't drop its stomach and the mahi out during weighing.

After the fish was weighed, I cut open the belly to check its sex and also pulled out its stomach to see what else it had been eating. At the end of its stomach, I felt the head of a mahi. Thinking there might be two mahi there, I cut open its belly and pulled out the head of a mahi.

I then cut the tape holding the mouth shut, grabbed the tail of the mahi and pulled it out. It was the back two-thirds of the head of the mahi I had cut out of its belly. This mahi was estimated at 25 pounds and three feet long, and by looking at the condition of the mahi, it had probably been eaten less than 3-4-hours before hitting the lure that captured it.

The marlin couldn't, or didn't want to, regurgitate the mahi, and was probably choking on it as it continued to feed, but it still ate an artificial lure - even with the tail of the mahi sticking out its mouth.

Mahalo to Don for the idea.

Aug. 21 is the 32nd anniversary of my first "At the Harbor" weekly column.



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