Life History of a Fish: Atlantic Blue Marlin

By Scotty Kyle

I am an Atlantic blue marlin. I am probably the most sought after species for extreme sport fishing anywhere in the world. While I support hundreds of companies and thousands of people in sport fishing enterprises, I also provide thousands of tons of food to many countries. My proper scientific name is Makaira nigricans. We first came to sport fishing prominence in the 1920’s and 1930’s in the Caribbean Seas, where we inspired such legendary stories as “The old man and the sea” by Ernest Hemingway. Then our popularity spread to many other tropical areas of the world. There are now sport fishing operations dedicated to seeking my species out in many areas from Hawaii to Portugal, Australia, South Africa, and many other countries in between.   

There are about ten species in my family (the Istiophoridae), but mine is the largest. We  attain a mass of over 820 kilograms (1 810 lbs.) but, I, as a male, will only grow to about a quarter the size of my sisters. I have a very close cousin, the Indo-Pacific blue marlin (Makaira mazara), that inhabits other tropical oceanic areas of the world. Other family members include the black, white and striped marlin as well as sailfish, swordfish and spearfish, all of whom are collectively called “billfishes”.   

We are all roughly the same shape – an elongate body with a spear-like bill in front and a sail-like dorsal fin sticking up sharply on our backs. We are an extremely hydrodynamically shaped fish, to the extent that even the fins on our sides can fold into groves along our bodies to reduce water resistance. This makes us probably the fastest swimmers in the ocean and leads to our ability to travel extremely long distances and also our famous fighting abilities. We are also well known for repeatedly leaping high out of the water when hooked and often successfully throwing hooks or lures from our mouths. 

We are very dark above, either dark blue or even black, but our undersides are much paler. We are also renowned for our ability to change colour rapidly, especially when stressed or captured. Our mouths are big but lack large “shark type” teeth as many small file-like teeth are enough to hold and cut up the food that we eat. We have large round eyes that help us spot and home in on our prey that consists of mostly small or large fish but includes squid or anything else that we come across. Our bill has evolved to assist us, when we attack shoals of small fish and we stun or damage them as we smash into a shoal and then circle back to gulp in the injured fish. We can also use our bills to stun individual larger fish or other prey and, sadly, many sport anglers have recognized this and when we hit a bait or lure they often release the line and fool us into taking the bait. We do not, as has been suggested, use our bills for attack on humans or other predators.

Because we are such a popular species for sport and commerce, we are under extreme fishing pressure in many parts of the world including off the Portuguese coast to which many of us travel each June or July to spawn. The International Union for the Conservation of Nature (IUCN) regard us as “threatened” due to people targeting us where we congregate to spawn, and also in places like the Caribbean Sea, where long-liners from Cuba, Japan and elsewhere kill thousands of us annually. We are also killed as “bycatch” in other fisheries such as various forms of tuna fishing in many parts of the world. On a happier note, however, any billfish caught within 200 kilometers (124 miles) of the USA coastline must, by law, be released. Unfortunately we often fight so well that we are exhausted when we are brought to a boat after a long, strong fight and the delays during photographing and measuring can often result in us succumbing to fatigue. 

In past decades many tropical sport fishing destinations erected “gabions” where large billfish would be hung for the standard photograph of people standing below and beside huge, very dead, billfish. In many areas these have been removed and in most sport fishing competitions billfish may not be “boated” and photographs and measurements of the fish in the water are seen as adequate proof of size and capture. So long as stainless steel hooks or leaders are not used, the line to a deeply embedded hook can be cut and then the hook will usually either be thrown from the mouth or ingested and then digested by our strong stomach juices.  A billfish brought to a boat can often be “resuscitated” by gently holding it beside the boat while moving slowly forward forcing water through the gills. A few extra minutes spent resuscitating a marlin can easily markedly improve its chances of recovery. 

In many areas of the world sport fishers do their best to release a large proportion of billfish but great care is needed to avoid damage and, sadly, often only a small proportion of tagged and released billfish are ever seen again. Despite this low recovery rate of tagged fish, some amazing information has already come to light. One of the longest distance recoveries was from an Atlantic blue marlin tagged off the Delaware coast. It was caught again off the Island of Mauritius, in the Indian Ocean, a distance of over 9 000 kilometers (5 592 miles). Other tagged marlin have been recovered, sometimes many years later, near the site of first capture but this doesn’t necessarily mean the fish spent its whole life in the area – it might return to specific places routinely. Many billfish are being tagged with equipment that can be followed by satellite and, slowly, a much better idea of marlin movements, numbers and distribution is becoming clear. Tagging fish also gives excellent and useful insights into growth and recapture rates which are very important parameters in terms of mankind trying to manage our populations and their fisheries wisely.    

blue marlin and a boat

Because we are such a charismatic and important species, a lot of time and effort has been directed at us, our biology and life history. As adults, we are nearly completely a “blue water” fish. We rarely come close to shore and although we mostly stay in the upper reaches of the oceans we much prefer deep water below us. Apart from some spawning migrations, there is a general movement of many blue marlin from more tropical to cooler areas in summer followed by a return in the fall. Much of our habits and movements still remain unknown but the annual summer migration towards the Portuguese coast by some of us is recognized and heavily exploited.

Female blue marlin can spawn several times per year and each large fish can lay around seven million eggs at each spawning, each egg being about 1 mm in diameter. It is not known what proportion of our species spawns near Portugal or how many other spawning grounds exist but the result must be many millions, or indeed billions, of tiny planktonic blue marlin floating in among and feeding on the other planktonic organisms. Our young drift with the ocean currents for months but they grow very rapidly indeed and larval fish growth of up to 16 mm (0.63 in.) has been recorded in one day. Blue marlin can reach maturity in between two to four years and at sizes of from 35 to 44 kilograms (77-97 lbs.) in males and 47 to 61 kilograms (104-134 lbs.) in females to begin the life cycle again.  Female blue marlin are known to live for at least 27 years while we males live shorter lives and probably die at around 18 years. During this time, as adults, we have few natural enemies except occasionally orcas or large sharks and, of course, man. 

I was born in the warm waters off Portugal over ten years ago and so am now in my prime but also rapidly heading towards the end of my life. As a youngster nearly all my planktonic associates died or were eaten and mortality rates are incredibly high, thank goodness for the amazing fecundity of our ladies. Our species has been hard hit by man’s sport and commercial fishing but it can, or could, rebound if our spawning areas are protected, commercial fishing is controlled and careful catch and release is practiced.  

We are usually solitary animals as adults but I have seen many colleagues dead or dying attached to impersonal long lines, waiting to be hauled in and frozen by commercial fishing operators. I have seen friends take baits only to be hauled to a boat then landed and never seen again. I was one of the very lucky ones to be caught by scientists off Florida, landed quickly and carefully and then a transmitter was attached to me before I was resuscitated and released.  Now, as I move around and undergo my migrations, I send a constant flow of important information back to the scientists.  Hopefully, this information will help the scientists learn more about me and us and, in turn, this information can be used to help improve the future for the Atlantic blue marlin as well as the prospects of the various fisheries. 

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