Un­lock­ing the se­crets of an epic evo­lu­tion­ary suc­cess story

By Scott Ban­nerot

Most of us are some­where be­tween in­fat­u­ated and ob­sessed with blue mar­lin, but un­less you’re a billfish sci­en­tist, you prob­a­bly don’t un­der­stand how these light­ning-fast finely honed eat­ing ma­chines are able to swim thou­sands of miles, pop­u­late the vast trop­i­cal and sub­trop­i­cal oceans of the en­tire world, and de­tect and chase down the fastest, most elu­sive prey species — and to have done it suc­cess­fully for lit­er­ally mil­lions of years. The short an­swer is in­cred­i­bly en­gi­neered anatomy and phys­i­ol­ogy. While all top-level fish hunters re­al­ize that de­tailed knowl­edge about one’s quarry is the key to find­ing it, get­ting the bites and con­vert­ing them into cap­tures, most lack ex­po­sure to many of the cut­ting-edge sci­en­tific ad­vances that can give them an edge. I re­cently re­viewed these dur­ing the course of writ­ing a chap­ter about blue mar­lin bi­ol­ogy and ecol­ogy for Capt. Steve Camp­bell’s out­stand­ing book, Blue Mar­lin Magic. I was for­tu­nate to in­ter­view and work with a num­ber of the top sci­en­tists in the field, and the in­for­ma­tion they gra­ciously shared blew me away.


Pro­fes­sor John Graves, of Vir­ginia In­sti­tute of Marine Sci­ence, shared with me re­sults of DNA and mor­pho­log­i­cal analy­ses he con­ducted with col­leagues. Some of the billfish evo­lu­tion­ary tree we would all have guessed cor­rectly — such as white and striped mar­lin be­ing closely re­lated, as are the four species of spearfishes. All billfish — mar­lin, spearfishes and sail­fish — be­long to the fam­ily Is­tio­phori­dae, with broad­bill sword­fish the sole species of the off­shoot fam­ily Xiphi­idae. Now, which billfish species would you guess is the clos­est ge­netic rel­a­tive of blue mar­lin? I’d have guessed black mar­lin, but the cor­rect an­swer is ac­tu­ally sail­fish.

My next sur­prise came in re­view­ing just how long blues have been around. The branch of the billfish evo­lu­tion­ary tree we know as a blue mar­lin was suf­fi­ciently per­fect that they are the most com­mon billfish iden­ti­fied in fos­sil records dat­ing to the late Miocene epoch. This means they were dom­i­nat­ing their en­vi­ron­ment, world­wide, more than 12 mil­lion years ago. Sea lev­els were much higher than they are to­day. These fos­sils have been found scat­tered in lo­ca­tions as var­ied as Italy, Vir­ginia, North Carolina, Cal­i­for­nia and Mex­ico. To put this in per­spec­tive, we hadn’t even evolved yet — the clos­est things to hu­mans roam­ing the earth were some hu­manoid apes. So the next time you feel hum­bled by a lit-up blue crash­ing your spread, you have ev­ery rea­son to be: They’ve been around for a very long time.


Re­search sci­en­tist Michael Mu­syl pro­vided a fas­ci­nat­ing look at the in­cred­i­ble ca­pa­bil­i­ties we are learn­ing about blue mar­lin through pop-up satel­lite archival tags. Ev­ery­thing these crea­tures do is finely tuned, and the more we un­der­stand, the bet­ter we can adapt our tech­niques to en­counter them more of­ten. The con­tin­u­ous data pro­duced by tagged in­di­vid­u­als tells us their po­si­tion, depth and am­bi­ent wa­ter temperature over con­sid­er­able pe­ri­ods of time. In­ter­est­ingly, some of this data only serves to con­firm what far-flung ar­ti­sanal fish­er­men in­de­pen­dently fig­ured out a long time ago. By the time Ernest Hem­ing­way wrote The Old Man and the Sea, he was aware that the old Cuban han­d­lin­ers would set their baits deep — 300 to 800 feet — and catch blue mar­lin as well as big yel­lowfin and big­eye tuna. Mean­while, Poly­ne­sian hand-lin­ers were do­ing ex­actly the same thing, us­ing rounded vol­canic stones and a spe­cial slip­knot to sink fly­ing fish and other baits, in which a sharp tug on the line re­leases the weight. Mod­ern-day sword­fish and tuna long­lin­ers catch large num­bers of blues by fish­ing dead nat­u­ral baits much deeper than sport an­glers. PSAT re­sults tell us why, and sug­gest

op­tions for think­ing out­side the box.

We now know that blue mar­lin spend much of their time easing along at 1 to 3 knots in warm, tur­bu­lence-mixed wa­ter above the ther­mo­cline, within a temperature range of 72 to 88 de­grees, pre­fer­ring a range be­tween 75 and 81 de­grees. How­ever, dur­ing the day­time, they ex­hibit what Mu­syl calls a “W-pat­tern,” fre­quently div­ing to depths be­tween 500 and 650 feet, and some­times as deep as 2,600 feet. Stom­ach-con­tent analy­ses show these fish are feed­ing on squid and deep­wa­ter fishes (“stuff­ing them­selves” might be a bet­ter de­scrip­tion), then sur­fac­ing to re­pay oxy­gen debts and warm up their mus­cles. Are is­land hand-lin­ers and in­dus­trial long­lin­ers catch­ing blues that are on their way down or per­haps swim­ming back up from these dives? Are there times and lo­ca­tions where large num­bers of blue mar­lin are present but mostly feed­ing down near the ther­mo­cline? Would sport-fish­ing op­er­a­tions be far more ef­fec­tive drift­ing deep baits or pre­sent­ing some­thing com­pletely dif­fer­ent and yet-to-be-de­signed in the deep, such as large, scented soft plas­tics? Some­times re­search raises more ques­tions than it an­swers.

Re­cently, I was off­shore of Is­lam­orada, Florida Keys, out near the con­ti­nen­tal shelf drop-off, aboard my 20-foot SeaCraft. The sea was mir­ror-calm, and I spot­ted the dor­sal and ta­pered tail lobe of a 250-pound-or-so blue mar­lin, mo­tion­less and flushed nearly black. We eased over and swam a

schoolie mahimahi in front of the fish, which slowly sank out of sight in re­sponse. We cir­cled, dropped live and dead baits: noth­ing. I won­der if it had just sur­faced from a deep, cold dive, stuffed with prey, mus­cles oxy­gen-de­prived, chilled to the bone, just try­ing to soak in some warm sun. I could al­most hear the mar­lin say, “Are you kid­ding me?”


Imag­ine a blue mar­lin swim­ming in the cold, inky dark­ness. How do they ef­fec­tively sense prey? For one thing, blue mar­lin have the big­gest eyes in the billfish fam­ily, com­pa­ra­ble to the enor­mous eyes of broad­bill sword­fish. Eye size is pro­por­tional to the ca­pac­ity to gather light and other visual in­for­ma­tion. In ad­di­tion, blues, like other mar­lin, sword­fish and some sharks, have the abil­ity to heat their brains and eyes us­ing a counter-cur­rent ex­change method and spe­cial warmth-pro­duc­ing cells con­tained in tis­sue lo­cated at the base of the cra­nium. The abil­ity to process “frames per sec­ond” is pro­por­tional to heat, which gives these preda­tors a huge ad­van­tage over schools of deeper-dwelling, slower-re­act­ing prey an­i­mals that lack this heater or­gan.


Two other sen­sory ap­pa­ra­tus also aid feed­ing ca­pa­bil­i­ties: the in­ner ear and the lat­eral line sys­tem. The in­ner ear in­cludes tiny otolith bones, which sense lin­ear ac­cel­er­a­tions like sound waves, grav­i­ta­tional forces and body mo­tion, and a canal sys­tem, which re­sponds to an­gu­lar ac­cel­er­a­tions of the head. The com­bi­na­tion con­fers the abil­ity to hear and also to ori­ent in space. A mar­lin’s lat­eral line sys­tem, like the in­ner ear canals, op­er­ates by us­ing sen­sory hair cells that trig­ger a nerve sig­nal to the brain. These chan­nels con­tain­ing them run in net­works down the sides of the body, and in­ter­est­ingly, the pat­tern of the net­work dif­fers be­tween At­lantic and Indo-Pa­cific blue mar­lin pop­u­la­tions. They func­tion as hy­dro­dy­namic re­cep­tors of low-fre­quency wave­lengths from 10 to 200 Hertz be­ing emit­ted within two body lengths of the mar­lin, which could come from prey, preda­tors or even inan­i­mate ob­jects such

as a fish­ing lure. It is highly likely that blues use their lat­eral line sys­tem to as­sist in close-range track­ing and at­tack­ing of prey or lures. The in­ner ear sys­tem, on the other hand, de­tects sen­sa­tions from much greater dis­tances, such as the har­monic pat­terns in hulls and diesel en­gines, and sound-mak­ing knocks or rat­tles from a lure.


Both PSAT and con­ven­tional tags have pro­vided in­cred­i­ble ad­vances in our knowl­edge of move­ments and mi­gra­tions of blue mar­lin, and anatomy and phys­i­ol­ogy stud­ies tell us how they do it. World­wide, cyclic move­ment pat­terns oc­cur in sync with mi­gra­tory food sources or to prime feed­ing ar­eas, fol­lowed by travel to spawn­ing ar­eas in which prey items may be rel­a­tively scarce. It’s in­ter­est­ing that some in­di­vid­u­als make largescale move­ments, while oth­ers might hang around a given area for ex­tended pe­ri­ods of time. One blue tagged off of Delaware was re­cap­tured in the In­dian Ocean, near Mau­ri­tius. An­other moved from the Tas­man Sea off south­east­ern Aus­tralia to the In­dian Ocean off south­east­ern In­dia. An­other in­di­vid­ual tagged near Puerto Rico had its PSAT pop up 120 days later, 4,776 miles away, off­shore of An­gola, Africa. Oth­ers swam from Hawaii to Mex­ico, and to French Poly­ne­sia. Some blues cir­cu­late around the Western Pa­cific, and oth­ers be­tween the Co­ral Sea off of north­east­ern Aus­tralia to the South Pa­cific is­lands. And then there are the home­bod­ies, PSAT-tagged blue mar­lin that stayed in lim­ited ar­eas, such as Hawaii, the Gulf of Mex­ico, the Kuroshio Cur­rent sys­tem and the Caribbean Basin, for ex­tended time pe­ri­ods.

What are the se­crets that en­able these in­cred­i­ble fish to ac­com­plish all of this? Bi­ol­o­gist Nick Weg­ner ex­plained to me that sea­wa­ter is far more vis­cous and oxy­gen-poor than air. Blues uti­lize ram-jet ven­ti­la­tion of sea­wa­ter en­ter­ing the mouth and flow­ing aft through the mil­lions of tiny, leaflike struc­tures called lamel­lae stacked in rows along each gill filament. Blood pumps counter-cur­rent (or for­ward, op­po­site to the wa­ter flow) through these highly vas­cu­lar­ized struc­tures to max­i­mize oxy­gen ex­trac­tion. The ef­fi­ciency of this ar­range­ment is un­equaled in the an­i­mal king­dom.

Blue mar­lin anatomy con­trib­utes enor­mously both to ef­fi­cient dis­tance swim­ming and to burst speeds as high as 72 mph. Elon­gated tail lobes reach undis­turbed wa­ter be­yond the tur­bu­lence cre­ated by the body. The bill may pro­vide a hy­dro­dy­namic ad­van­tage, and cer­tainly, the grooves and de­pres­sions for fold­ing the dor­sal, pec­toral and pelvic fins against the mar­lin’s body help sig­nif­i­cantly. I can re­mem­ber so many times gaz­ing down from the tuna tower of as­sorted boats I was cap­tain­ing and mar­veling at the ef­fort­less swim­ming mo­tion of that fusiform body, head rel­a­tively mo­tion­less as the scythe-like tail swept the body for­ward. Forty miles a day for these fish on a transoceanic

jour­ney? No prob­lem. Ex­plo­sive bursts ex­ceed­ing high­way speed lim­its? Easy.


We know from as­sorted stud­ies that blue mar­lin are flex­i­ble, op­por­tunis­tic feed­ers. Stom­ach analy­ses from around the world of­ten in­di­cate that blues have been feed­ing on pri­mar­ily a sin­gle species, what­ever they are fol­low­ing, or tim­ing their mi­gra­tions with which to co­in­cide. Doc­u­mented ex­am­ples in­clude chub mack­erel off south­ern Por­tu­gal, skip­jack tuna off south­west­ern Ja­pan and bul­let tuna off the Pa­cific coast of Mex­ico. How­ever, the pres­ence of other species in­di­cates flex­i­bil­ity and op­por­tunism, the mar­lin equiv­a­lent of, “Hey, we might be here to eat skip­jack and smaller yel­lowfin, but if we run into a cloud of some­thing else, we won’t turn it down.” Hence, we of­ten find in­di­vid­u­als packed with tiny file­fish, puffers or trig­ger­fish, or loaded up with deep­wa­ter fishes and squid.

I ex­pe­ri­enced the other end of the scale work­ing as a guide for No­mad Sport­fish­ing, a sea­plane fly-in moth­er­ship op­er­a­tion in north­east­ern Aus­tralia, where we put to good use the sci­en­tific fact that blue and black mar­lin have the jaw­bone adap­ta­tions and stom­ach elas­tic­ity and size to con­sume prey up to 10 per­cent of their body weight. We tar­geted school


yel­lowfin, nar­row-barred mack­erel, and dog­tooth tuna from 8 to 50 pounds for both live- and dead­rigged baits, and did not shy away from baits on the larger end of the scale. We reg­u­larly slow-trolled live yel­lowfin weigh­ing 40 pounds and more, and had them slammed by huge fish. We also did things like drill lon­gi­tu­di­nal holes through full-size Boone Lu Lu teasers and rig them with 400-pound stain­less-steel cable and a 12/0 sin­gle Mus­tad 7692 to form what was in ef­fect a giant fat ver­sion of a cedar plug. They got crushed even by smaller blues, par­tic­u­larly around the sunken reefs and atolls out in the Co­ral Sea, such as Kenn, Wreck and Fred­er­ick reefs. And, of course, we all know how much those big Hawai­ian blues like short­bill spearfish.

How do they do it? One key el­e­ment is a cap, called the pre­den­tary bone, that joins the tips of the two lower jaw­bones. Unique to billfish, this bone al­lows the jaws to open much wider than oth­er­wise pos­si­ble without com­ing apart. We ob­served big mar­lin flash­ing in and at­tack­ing whole live tuna, open-mouthed, like preda­tors without bills. The hits were so hard that the bait would have to be stunned just from con­tact with the jaws. None­the­less, there were also in­stances of mar­lin fir­ing in and drilling the baits in the head with their bills. Camp­bell ac­tu­ally has a video of one such in­ci­dent that oc­curred in Tonga.


Sci­en­tist Kerstin Fritsches, a lead­ing au­thor­ity on vi­sion in billfish, en­light­ened me about key as­pects of the way blue mar­lin see their world. The abil­ity to de­tect, track and cap­ture of­ten very fast and elu­sive prey items con­trib­utes heav­ily to their evo­lu­tion­ary suc­cess. Blue mar­lin have eye mus­cles es­sen­tially iden­ti­cal to those of hu­mans, which al­low them to swivel and fo­cus mul­ti­di­rec­tion­ally, yet their eyes are lo­cated on the sides of their head, so their vi­sion is less binoc­u­lar. They track items more of­ten one eye at a time, and they pos­sess high flicker fu­sion fre­quency, which means they can process a very fast rate of frames per sec­ond.

Per­cep­tion of spe­cific dif­fer­ent col­ors re­quires the pos­ses­sion of a pig­ment, housed in a rod or cone, which is stim­u­lated by the spe­cific wave­length of the color, and this sig­nal trans­mits to the giant op­tic lobes of the mar­lin brain. This has all been ex­am­ined and tested with fresh eyes from spec­i­mens brought to the dock. The bot­tom line is that blue mar­lin see es­sen­tially in black and white with the dor­sal por­tion of the retina (the por­tion that looks down, into the dark­ened depths), and they can see as color mostly shades of vi­o­let, blue and green with the ven­tral part of the retina (the part that looks up into the sun­lit lay­ers). Other fish, such as fresh­wa­ter trout, can see the full color spec­trum in a man­ner sim­i­lar to hu­mans. This means a blue mar­lin would see red as black, and var­i­ous other col­ors as per­haps shades of gray. They still see them as dif­fer­ent shades, but not in the way we hu­mans do. Re­mem­ber that they are per­fec­tion in­car­nate in their en­vi­ron­ment, so what­ever they see, and how­ever they see it, is the pin­na­cle of mil­lions of years of evo­lu­tion.

The mo­ment of truth, as a blue mar­lin is re­leased at the boat (above). These fish have evolved over mil­lions of years to be­come one of the ocean’s apex preda­tors. A mar­lin’s pro­por­tion­ally enor­mous eye (left) is heated by spe­cial cells in the brain,...

Blue mar­lin are ambush preda­tors, of­ten ap­pear­ing sud­denly from the white wa­ter to crash a teaser, lure or bait.

The fish’s lat­eral line al­lows it to de­tect small vi­bra­tions at close dis­tances. A mar­lin can ac­tu­ally feel a dredge mov­ing through the wa­ter.

From below, a mar­lin would see these rigged bal­ly­hoo (right) in shades of vi­o­let, blue and green. A pow­er­ful tail gives the fish plenty of horse­power for daz­zling ae­rial dis­plays (below) as a pair of re­moras hangs on for the ride.

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