Fish Facts

Marlin vertical movements

NO gamefish are more spectacular for offshore anglers than those ultimate oceanic apex predators, the marlins. Striped marlin (Kajikia audax), as far as marlin go, are considered only medium sized, growing to a maximum size of around 260 kg. In contrast, black marlin (Istiompax indica) are true heavyweights, with the all tackle world record taken in 1956 off Peru exceeding 700 kg.  Even bigger blacks have been occasionally encountered, some perhaps nearing the size of the biggest of them all, the blue marlin (Makaira nigricans). The latter is confirmed to grow to at least 900 kg, based on commercially landed specimens.

These days all species of marlin are targeted by commercial longlining in many parts of their range. Attempts to manage this activity sustainably have driven concerted scientific efforts in recent years to learn more about marlin biology and movements. Study of the movements of marlins has been done mainly by tagging, firstly using conventional tags which provide information on short term horizontal movements and growth. More recently, however, great strides in knowledge have been gained with increasingly sophisticated electronic tagging equipment that can tell us about vertical movements as well.  

Tagging of billfish in general, and striped marlin in particular, has been challenging for scientists, due to extremely high rates of shedding of conventional tags. This has been demonstrated by the relatively low (0.9-1.3%) tag return rates from the conventional dart tags traditionally used by researchers and recreational game fishers, compared to tag recovery rates exceeding 10% for striped marlin tagged with electronic archival tags surgically implanted into the abdominal cavity. The latter have been recovered up to 7.7 years after tagging, while in that same study (conducted off Baja California) none of the external dart tags implanted at the same time as the electronic tags lasted more than a year.  

Studies using electronic archival tags and “pop up” satellite tags in various parts of the world have generated some very interesting horizontal movement data which has confirmed beyond doubt the “highly migratory” status of marlins. For example, striped marlin can swim from the Coral Sea in early spring to Tasmania by mid-summer, averaging around 50 km per day. One stripey swam from New Zealand to New Caledonia over 2000 km away, before returning to within 400 km of its original tagging location 8 months later. Off the east coast of Africa, striped marlin covered distances of up to 9,187 km through the Indian Ocean over periods ranging up to 183 days, with a mean daily distance travelled of around 48 km (though they averaged less than 2 km per night). Even those enormous distances pale in comparison to the massive migrations undertaken by the black marlin that aggregate to spawn each spring off Cairns.  Straight line track distances of up to 5780 km (average 2146 km) away from the Great Barrier Reef into the central Pacific Ocean in a mere 100 days have been recorded, with total track path distances of up to 13,431 km (average 5016 km) in 180 days, and average daily swimming distances ranging between 21 and 137 km.  A couple of black marlin tagged off Cairns with conventional tags have even swum across the entire Pacific Ocean and been recaptured off Baja California – a straight line distance of over 11,000 km. That’s some swim.

The vertical movements of marlin are equally fascinating, and can only be determined with electronic archival tags. Striped marlin tagged off Baja California are usually captured in warm surface waters between 22 and 27°C, but electronic tags found most of these fish repeatedly dived below the thermocline during the day as deep as 350 meters, where water temperatures were around 10°C. These excursions are thought to be foraging dives where the marlin are targeting squid or mesopelagic fishes schooled up around the base of the thermocline (the so called “deep scattering layer”).  It appears that larger marlin dive deeper, as similar daytime feeding dives below the thermocline (which is usually between 200 and 250 meters deep in tropical seas) have also been recorded in both black marlin (to depths over 600 meters and 7.4°C ), and blue marlin (to depths of 700 meters and temperatures as cold as 5.4°C).  

A recently published study using pop-up satellite tags examined the vertical movements of 34 black marlin and 39 striped marlin caught off Kenya. This was a relatively short term study (the maximum time period after which tags were set to pop up was 6 months), nevertheless some fascinating results were obtained.  The researchers found that the tagged black marlin averaged up to 1.4 km of cumulative vertical diving per day, with a maximum of over 6 km of vertical diving in a particularly active black that also dived as deep as 472 meters.  Striped marlin were equally active, diving down to as deep as 463 meters, and cumulatively averaging up to 2 km of vertical diving per day (maximum 3.5 km).  

Both species of marlin stayed in shallower waters and were less active during night time. All of the deeper diving behaviour occurred during the day, with striped marlin diving more frequently than black marlin, and also spending more time at the water surface in the top 5 meters (50.7% of the time vs. 32.3% in black marlin). This surface dwelling behaviour may have been due to the fact that the striped marlin tended to stay longer in deeper, cooler, oxygen poor water down to 10.2°C, and thus may require a longer time at the surface to “recharge” their oxygen and warm back up.  Around 35% of black marlin showed a crepuscular pattern, diving particularly deep at dusk and dawn to waters as cold as 13°C. Dawn and dusk are useful times for predators to forage, as light levels are changing fast, reducing the visual acuity of their prey. 

The horizontal movements recorded during this particular study were also impressive. One black marlin tagged for 167 days travelled a total of 11,944 km, while the longest ranging striped marlin was the one previously mentioned which swam 9187 km in 183 days. Researchers in this study also recorded “free-jumping” behaviour which occurred immediately prior to premature shedding of pop-up tags in two black marlin. The tag shedding was associated with burst swimming events of up to 18.2 meters per second (65 km/h) as the marlin angled from 20 meters depth to above the water in less than 6 seconds. No wonder it’s hard to keep those tags in there. 

The recent study of the vertical movements of black and striped marlin in the Indian Ocean can be found at https://www.int-res.com/abstracts/meps/v690/p165-183 .  An excellent earlier study of vertical migration of black marlin off Australia’s east coast can also be found at https://royalsocietypublishing.org/doi/epdf/10.1098/rsos.170694   

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