Did You Know?


Welcome to our Did You Know blog! Billfish and related species, like tuna and swordfish, are amazing creatures and set themselves apart from other fish. The Billfish Foundation would like to share some of the most interesting facts and stories involving these fish. What some of the things that surprised you about these magnificent animals? #didyouknow


April 21, 2019 – Forage Fish Abundance Importance for Billfish

Did you know that billfish have a wide variety of diet throughout their life cycle? This is partially due to the drastic change in size throughout their life cycle. Juvenile billfish (recently born) are about the size of a thumbnail, if not smaller, and some species of billfish can grow from that little size to upwards of 1500 pounds! Due to that drastic change in size, there are many forage options that billfish can take advantage of/prefer at certain stages of their life. At their small juvenile size, billfish will start out consuming micronutrients such as zooplankton.

As they grow bigger, billfish will move on to bigger foods such as squid, hardtails, flying fish, and other small pelagic baits. From this stage forward, squid seem to be a favorite forage of billfish of all sizes. When the bigger billfish, such as marlin, began to reach around 300-pounds, the most popular forage for them are smaller tunas such as yellowfin, skipjack, bonita, blackfin, etc. However, mahi mahi, rainbow runners, mackerels, larger hardtails, and squid all are forage options for these fish as well. Essentially, medium size baits/prey around 5 to 10-pounds. As they continue to grow to their larger weights, their forage is very similar, but they will consume larger versions of the tunas, mahi mahi, mackerels, as well as smaller billfish such as spearfish. For example, many boats in Australia pull giant 40 to 50-pound mackerel on the surface to entice giant black marlin to bite. However, 1000-pound marlin are caught every year in Kona, Hawaii on much smaller artificial lures. This shows not only will larger marlin go after large prey, but they will not turn down the smaller offerings as well. For anglers, the best thing to do is match the hatch of what is being seeing on the water, and to have a variety of baits in different sizes to present to the different targeted fish.

Understanding these unique predator/prey relationships are important when discussing new/modifying fisheries management policies. Currently, the primary fishery management law of the U.S., the Magnuson-Stevens Act, does not include provisions for managing prey or forage fish that provide an important food source for larger fish, marine mammals and birds.  If the Forage Fish Conservation Act (FFCA), a bipartisan bill, introduced on April 10, 2019, becomes law, monitoring and assessments will be required of predator needs, impacts of increased removal of forage fish on other species, established fisheries and fishing communities before any new or increase is harvest could be approved. It is imperative that our policies take these relationships into account since forage fish (like ballyhoo, menhaden, tunas, mahi, and mackerels) are tied so closely to those predatory species like sharks, marine mammals, and billfish.


November 28, 2018 – The Importance of Properly Reporting and Recaptures

Did you know that accurate reporting of tagged billfish leads to better recapture data and better overall data? Tagging billfish is an important practice that many boats take part in today, but what is just as important, is the reporting side of this equation.

Tagging billfish does a lot for the conservation of billfish worldwide, and the sport of billfishing. It provides information into the life cycle, movement, growth, and post-release mortality. Here at The Billfish Foundation, we take a lot of pride in the tagging database that we have built up over the years. However, one of the most important things in this process is proper reporting on the initial tag of the fish, and proper recapture reporting. One of the biggest hindrances in our database is missing or incorrect information on initial tag cards we receive from anglers. The most common information that is missing is the weight or length of a fish. Granted, length can be tough to measure in the water, but an accurate estimated weight measurement is not too difficult to get, and it is very important. Without these measurements, we have no idea how much a recaptured billfish grew in the time between catches. Additionally, accuracy is crucial in these measurements as well. For example, getting a recaptured billfish report that says the fish weighed roughly 250 pounds and comparing it to the initial tag report that says the fish weighed 400 pounds three years before, doesn’t help contribute to the knowledge of the biology of the fish.  Thus, accuracy in these measurements is truly important.  Finally, another important piece of information when sending in tagging and recapturing information is the location of where the fish was caught. Not having an accurate location (with a latitude and longitude) at either the tagging or recapturing point in the process hinders knowledge that can be learned about the movement of the fish. This can lead to bad management in the future.

To promote the tagging process and better reporting, The Billfish Foundation hosts an annual tagging competition worldwide that awards captains and anglers for tagging and releasing the most billfish every year. The competition is split up into multiple categories, to acknowledge the fishermen who tag and release the most billfish by species in each of the world’s oceans. It is vital that anglers/captains/crews filling out tag cards accurately. The whole point of the tagging program is to get accurate data to help our community and better understand/manage billfish. Bad data not only “hurts” the reputation of our community but it will greatly affect our data analysis made from the data submitted. This takes away from the whole point of the tagging competition, which is trying to compile accurate data and award individuals/crews that contribute the most to this process. Thus, please only report accurate data.

In conclusion, the tagging process does a lot to help improve our knowledge about billfish, but more accurate reporting in a couple places could vastly improve this process. Additionally, if sending in letters is hard, The Billfish Foundation now has a new reporting app one can download to make reporting catches easier.


November 14, 2018 – How Deep Can Marlin Dive?


Did you know that blue marlin can dive to incredibly deep depths in the ocean? Blue marlin are one of the top ocean predators and gamefish around the world. Blue marlin are sought after by many fishermen around the world for their fighting skills, acrobatic displays when hooked, and the difficulty associated with catching them. For the most part, fishing techniques for blue marlin consist of trolling artificial lures, dead baits, or live baits on the surface. Granted, some boats pull live baits on a downrigger during hot days, but most techniques are surface fishing applications. Additionally, blue marlin will dive down when tired during a fight, but for the most part they stay on the surface grey hounding and tail-walking. However, blue marlin actually have the capability to dive to deep depths.

In fact, blue marlin can swim down to depths beyond 2,000 feet deep. This was characterized by the work of Dr. Phillip Goodyear, a scientist who works in conjunction with The Billfish Foundation. In Dr. Goodyear’s study, 51 blue marlin were tagged with satellite data archival pop-up tags. These tags stay with the marlin for a period of time and record measurements such temperature, depth, and distance traveled, before popping off the marlin and transmitting the data to a satellite once it reaches the surface. The results of the satellite tags showed blue marlin routinely diving to depths of 400-600 meters (1300~1900 feet), with some fish diving to depths deeper than 800 meters (~2600+ feet) during the day time! There are multiple theories to why blue marlin dive this deep. One is they do it to feed on prey at different depths in the water column, namely species such as squid. Interestingly enough billfish do not perform these deep dives regularly at night.



Also, this work shows how important the use of satellite tags are in billfish research. Despite being effective, we never would have learned information like this from conventional non-satellite archival tags. Thus, this is why satellite tagging expeditions are invaluable in furthering billfish conservation worldwide.



October 30, 2018 – The Evolution of Fishing Gear


Photo Credit – Hatteras Yachts

Did you know that fishing tackle for billfish at the recreational level has changed drastically over the past 40 to 50 years? This list of advancements is quite extensive, so this will just talk about some of the big changes.

Fishing line has advanced quite a lot over the years. In particular, fishing line has become much stronger in terms of breaking strength and knot strength. Before World War II, all fishing line was made of natural materials such as linen, whereas today it is made of manufactured materials. Due to this change in material, nowadays fishing line can often exceed the limit it is rated for. For example, some manufactured lines at 100lbs test have a breaking strength above 130 pounds. Additionally, fishing rods have evolved significantly over the years. When targeting a billfish such as a marlin, anglers were forced to use very large heavy rods in order to land a fish of this caliber. However, fishing rods have become stronger in much smaller models. Therefore, a boat does not have to be set up to use large heavy tackle all the time when targeting billfish. This allows boats of all sizes to target billfish, as opposed to just sportfishers. Another major advancement in fishing gear is the evolution in sonar and fish finders. In previous times, the only real technology used would be a bottom machine to help track the bottom as well as water temperature to detect current breaks. Boats today have sonars that not only mark billfish and bait vertically throughout the water column but also scan to the sides of the boat. Thus, boats can mark bait and billfish in a much larger area than ever before. Some boats have the ability to use this technology to mark billfish up to a mile away in a certain direction, allowing them a huge advantage in catching fish then they once did!

photo courtesy of bigmarinefish.com

These are just some of the few major changes to billfish fishing gear that have taken place over the past 50 years. The main result of the mentioned and unmentioned changes is that the catch rate of billfish has gone up. As long as good conservation efforts are in place, this will not have a negative impact on billfish population numbers. Thus, billfish conservation is important to make sure these and future advancements only have positive effects



October 16, 2018 – Billfish Dimorphism

Photo Courtesy of Kelly Fallon – Kekoa

Did you know that billfish exhibit sexual dimorphism? Sexual dimorphism is when males and females of the same species have different characteristics other than their reproductive organs. These characteristics can be things such as size, colors, and body appearance traits.

For example, lions exhibit sexual dimorphism, male lions have a large mane around their head that females do not. Another example is how male mallard ducks have a green head and white back in full plumage, whereas the females just stay relatively brown.  In the case of billfish, they exhibit sexual dimorphism in that female billfish grows bigger than male billfish. Typically, male blue marlin often does not reach weights above 150kg or roughly 330 to 350 pounds, whereas females are thought to be able to reach weights above 910kg or over 2000 pounds (ICCAT)! A study on black marlin off of eastern Taiwan illustrated the size of females they sampled reached 302kg or about 660lbs, whereas the heaviest of males they sampled reached 85kg or about 185lbs (Institute of Oceanography, National Taiwan University).

This begs the question, why do females grow so much bigger than males? The main reason for the size difference between females and males is reproduction/reproductive success. The larger the female is, the more eggs she can release to be fertilized by males in the water. This allows for potentially more juveniles to be born in each breeding occurrence. Additionally, as females get bigger, their number of predators began to shrink, which allows them to more safely transport those eggs. In scientific terms, as the female’s size increases, their fecundity (reproductive success, increase in reproduction rate) also increases. Therefore, all of the larger billfish are actually females rather than males. So, in Hemingway’s tale, “The Old Man and the Sea”, the great marlin that Santiago does battle with was a female; despite the fact that Santiago called the marlin ‘he’ throughout the story.


October 2, 2018 – A Crazy Fish story

A story almost as bizarre is that of a soon to be divorced angler who placed his wedding band on the bill of a sailfish he caught off of Fort Lauderdale. After some new bling and quick picture, the fish was released. After two years had passed and his divorce finalized, the man decided to go fishing again. To his surprise, the man reeled in the same sailfish, still sporting his wedding ring around its bill. The fish’s’ bill grew around the ring as depicted below. This must’ve meant that the fish was caught and handled correctly the first time it was caught, to have survived this long.

No word if the angler got back together with his ex following this amazing recapture, but I don’t think this angler is getting his ring back!



September 18, 2018 – What is the longest distance traveled by a billfish that was tagged and recaptured? 

Did you know the longest marlin migration recorded by The Billfish Foundation was a blue marlin, which swam 6,614.1 miles?  Billfish are Highly Migratory Species, which will follow warm ocean currents for hundreds to thousands of miles. The Billfish Foundation works to study the migration as well as understand growth rates, migratory habitat utilization, and post-release survival rates of these amazing fish through our Tag and Release program. A prime example of this program and its awesome results is that of a blue marlin which was initially caught and tagged by Captain Charles Cabell and angler Lisa Hawkins on June 29, 1991, off the Oregon Inlet in North Carolina and recaptured by Captain Dietmann Kossman, on February 5, 1993, in Angola, Africa. That’s a migration spanning 587 days, which mean the fish traveled roughly 11 miles a day.

The second longest migration tracked of a tagged billfish was that of another blue marlin which was caught by Captain Bobby Brown in 1997 in St. Thomas, U.S. Virgin Islands and was later recaptured June 2000 in the Gulf of Guinea. That means the fish traveled 4,197.8 miles over the span of almost three years!


August 31, 2018 – What about billfish anatomy makes them so fast?

Did you know that marlin have interlocking vertebrae, making their body compositions stronger than most fish? Most fish tend to have multiple small vertebras whereas billfish have large vertebras. Marlin and sailfish have twenty-four vertebrae, while swordfish have twenty-six. This skeletal structure allows these fish to have a high degree of lateral flexion – which means the spine is able to bend in high degrees laterally, that is to say sidewards. This assist them with migration patterns of traveling long distances, but also assist in reaching high speeds and accelerating out of the water. Billfish are able to have such a diverse and dynamic movement because of their spines elastic energy and their muscle power output.

Skeleton of Swordfish at the National Museum of Natural History, Washington, DC. Photo credit: New World Encyclopedia

The spine of a billfish is made up of large flat plates that extend across the intervertebral joints, with zygapophyses, which is the interlocking part of the vertebra. Similar to humans’ spines, swordfish have collagen between each vertebrae. Each vertebrae of a billfish has an expanded flanges (or a projecting flat rim) and an anterior Zygapophyse from the previous vertebrae which overlaps each flange.


Photo credit: John H. Hebrank, Mary R. Hebrank, John H. Long,JR, Barbara A. Block. And Stephen A. Wainwright.

The versatility and extensive makeup of their spinal cords gives these fish a kind of elastic energy and the ability to be flexible, enabling their diverse movement abilities and fast speeds. These fish unique body composition makes them fast and relentless predators and also fun to catch recreationally. It also makes their migration patterns interesting to tract. To see the awesome abilities these fish’s bodies have be sure to check out TBFs tag database to follow the movements of past tagged marlin!


July 25, 2017- Hunting Tool or Weapon?

Did you know that in 2010 a single blue marlin cost British Petroleum (BP) 100 million dollars in damages? The same features that make marlin one of the most desired fish to catch- their size, speed, and strength- makes them a very dangerous weapon when combined with their very strong and sharp bill. According to Bloomberg, a blue marlin punctured one of the highly protected, ultra thick main pipelines leading to BP’s Plutonio field storage barge off of the west coast of Angola, preventing 900,000 barrels of oil from being exported for sale and over 100,000 million dollars in losses. Thankfully, very little oil was leaked into the water.

The bill of a blue marlin that punctured this ultra-thick BP oil pipeline, costing BP over 100 million dollars. Photo Credit: Africa Travel Channel


Here are some more strange, gruesome, and unexpected instances where billfish (and swordfish) got wild with their bill.

A commercial fishing trip off the coast off Bermagui, Australia caught a large mako shark (below). Upon inspecting the shark and getting it ready for processing, the crew noticed that it had an object protruding from both sides of its body. It quickly became apparent that it was the bill of a large marlin that had penetrated through the shark just under the dorsal fin and broken off. The bill had started growing barnacles, signifying that the shark had been carrying the burden for quite some time. As far as scientists are aware, billfish and swordfish primarily utilize their bills to kill or stun smaller prey, not to attack larger animals. There have been a surprising amount of instances, however, like this one, that prove these fish aren’t scared to go on the offensive.

A mako shark was lucky to survive being impaled by a large marlin and had lived for quite some time with the bill in its back. Photo Credit: Narooma News Online



Some of the craziest documented attacks have been carried out by swordfish in the deep. In the first picture, a camera catches the instant a small swordfish penetrated a deep diver’s welding gear just above their oxygen tank- Click here to watch the full video. The footage clearly shows the swordfish singling out the diver, charging sword-first at speed. The second photo shows a swordfish lodged into the hull of Alvin, a deep sea Navy submarine. The Alvin is famous for voyage missions to survey hydrothermal vents, decommissioning sunken WWII bombs, and exploring the wreck of the Titanic. According to the Smithsonian, in 1967, Alvin was exploring deep sea corals in 2000 feet of water off the coast of Florida when a swordfish attacked and got stuck to its hull. The swordfish was just under 200 pounds and hit the sub so hard the majority of its head was jammed into a small gap in the sub’s exterior and was brought to the surface.

A deep sea welder is charged and his gear impaled by a swordfish, luckily the sword just missed the oxygen tank. Photo credit: Luis Nascimento

The famous U.S. Navy Submarine, the Alvin, was impaled by a swordfish in 2000 feet of water off the coast of Florida. Photo Credit: Woods Hole Oceanographic Institute

More common instances of billfish impalement are those associated fishing. There have been many instances of billfish impaling boat hulls and transoms, as well as the anglers themselves, sometimes even causing fatalities. In 2008, a young man off the coast of Panama was bringing in what looked to be a tired 600 pound black marlin when it suddenly jumped bill first into the boat. According to the Travel Channel, the marlin’s bill penetrated through the man’s mouth and lodged into his throat. Thankfully, he survived the encounter.

Billfish bone has a density and strength much closer to that of large mammals like horses than it does to most fish bone. This strength is accompanied by a dangerously sharp tip, which is why indigenous peoples around the world used bills and (swordfish) swords as daggers and spears. A 500 pound animal wielding this sort weapon is part of what makes these fish some of the most exciting fish to pursue but also some of the most dangerous. It’s important that anglers, divers, and even submarines never let their guard down around these incredible, powerful fish.

July 18, 2017- From Cat Food to Million Dollar Fish 

Did you know that the bluefin tuna sold for 1.8 million dollars in 2013 at the famous Tsukiji Fish Market in Tokyo would have probably been used for cat food prior to 1970? The tuna was sold in the traditional new year auction that is famous for attracting astonishingly high bids for bluefin. The buyer, Kiyoshi Kimura, has won the coveted tuna auction since 2011; and, although not rivalling 2013’s exorbitant price, has spent around half a million dollars on multiple other fish. Bluefin meat (maguro), more specifically the fatty meat around the belly (toro), is of the most desired meats by sushi chefs and consumers worldwide; but believe it or not, just  50 years ago bluefin meat was considered nearly worthless.

A flesh sample being taken from a bluefin tuna at the Tsukiji Fish Market in Tokyo on the first Saturday of 2013 – It would eventually be sold for 1.8 million dollars

According to the Smithsonian, during the 40’s, 50’s and 60’s, when tuna sportfishing was becoming exceedingly popular, bluefin were sought out by anglers due to their massive size –  measuring larger than 14 ft and weighing more than 1500 pounds. During that period, however, tuna were perceived to have little to no food value for the human consumer. They were often weighed, pictured, and then dumped back into the ocean or ground up for cat and dog food. The drastic turnaround from pet food to million dollar sushi meat began in the 1970’s when a palate shift occurred in Japan. Rather than favoring only light, white meat fish, an increase in beef consumption around the same time shifted Japanese palates toward the dark, rich meat of the bluefin. Once the appreciation for bluefin as a raw delicacy spread to the U.S. and Europe, the bluefin became one of the most sought after fish in the world.

An Atlantic bluefin tuna feeds in the cold waters off of the coast of Canada

Commercial fisherman utilizing large, encircling purse seine nets began targeting the giant schools of North Atlantic bluefin to sell to Japan. Intense overfishing has led to a severe depletion of bluefin stocks worldwide, where populations of the three bluefin species are less than 10 percent of their levels prior to the 1970’s. Although currently considered endangered, a high demand still exists.


A bluefin tuna tagged by Captain Eric Stewart near Cape Cod, Massachusetts migrated 5,000 miles before being re-captured off of the island of Malta in the Mediterranean Sea

On the bright side, over the last 20 years, there have been some strong conservation efforts worldwide trying to halt overfishing. The United States has been a standout leader in conservation efforts, establishing rigid regulations and low catch quotas for both commercial and recreational bluefin fisherman. However, conservation has proven difficult due to the bluefin being a highly migratory fish, often making multiple trans-Atlantic trips in its lifetime. In order to help synchronize efforts, much research is needed in order to understand where these fish migrate, breed, and feed. TBF is proud to collaborate with the Large Pelagics Research Center in their quest to gather this vital data through large scale tagging operations. Hopefully efforts such as this can help in preserving these incredible fish for years to come.

July 11, 2017 – Billfish Eye Heaters

Did you know that billfish, although cold blooded, can heat up their eyes to help them see better? Billfish and tuna are considered some of the ocean’s most athletic apex predators- and they have to be. Picking off small, agile baitfish from a dynamic school in the open ocean requires extreme speed, precision, and agility. Recent science has revealed one of physiological adaptations that gives these fish a major advantage over their prey- an internal eye and brain heating system.

The ability to heat their eyes allows billfish and tuna to react to visual cues much faster than their prey

Fish are ectotherms (cold blooded), meaning that their body heat is heavily dependent on the ambient water temperature. But billfish, swordfish, tuna, and most sharks possess the unique ability of heating their eye muscles and certain parts of their brain. According to a 2005 study, these fish have muscles tucked behind their eyes and adjacent to ocular nerves that vibrate to create heat. By heating their eyes 10-15 degrees celsius above the ambient water temperature, these fish significantly enhance their ability to detect and respond to motion. The same 2005 study found that fish with these heating muscles can pick up and react to visual cues up to ten times faster than fish whose eyes are the same temperature as the surrounding water. This physiological trait is especially important in swordfish, since the temperature in their deepwater habitat can reach near freezing.

The opah, also known as the moonfish, is the only true “warm blooded” fish

The opah is the only truly “warm-blooded” fish. According to National Geographic, opah generate their heat from metabolic activity, just like most other fish. But rather than losing heat to the colder ambient water, they conserve it and circulate the heat throughout their entire body, warming their muscles, heart, and brain.

June 29, 2017 – Billfish Breathing

Did you know that the cone shaped mouth on billfish is designed to do more than just maximize the fish’s hydrodynamic profile? Their large, smooth cone shaped mouths funnel massive amounts of water over their gills as they swim. Billfish and tuna-like species are ram-ventilators, meaning that they breathe by swimming with their mouth open, forcing water through their gills. In contrast, fish like groupers take in water by creating a vacuum in their mouths to “suck” it in, a method called buccal pumping. This allows them to breathe while stationary, which is why you may often find them motionlessly hiding out under a rock outcropping waiting for unaware prey.

Cone shaped mouths help billfish funnel massive amounts of water through their gills

The fact that billfish can only breathe while swimming means that they must constantly be moving at a decent pace to ensure that they receive enough water to their gills. This is why it’s crucial to keep them moving forwards in the water when reviving them. No movement… no oxygen!