Tracking the Oceans’Cheetahs

Results of tagging project on Kenya’s striped- and black marlin

By Sheena Carnie

THE black marlin is known as one of the fastest fish in the ocean,with sailfish and striped marlin other clear contenders for that title.Whoever takes the prize, black marlin swimming speeds as high as 129km/h have been recorded.But when you’re targeting a specific species, one of the most crucial questions is not how fast they swim,but rather,“What areas do they frequent?”

Charter captains in Kenya wanted to know more about the movements of the black- and striped marlin that inhabit the western Indian Ocean,so a few years ago they contacted the scientists from Marine Megafauna Found-ation (MMF) to formulate a plan.

Together they undertook a tagging research project on black marlin (Istiompax indica) and striped marlin (Kajikia audax) off Kenya between 2015 and 2019.They hoped the results of this project would answer some of the questions surrounding the movements and habits of these highly migratory species.

Marlin fishing boat captains deployed the pop-up archival satellite tags,and the project was overseen by a team of MMF scientists,led by Dr Christoph Rohner.Forty-nine black marlin weighing between approximately 40and 227kg were tagged in this way.Only 34 of the black marlin tags transmitted data that could be used,but they provided a great deal of useful and interesting information.Forty striped marlin weighing between 8- and 100kg were tracked, with nine other tags not reporting data. The reasons tags didn’t transmit usable data included the fish presumably dying as they appeared to stay at a static depth,and tag anchors not properly locking in and the tags thus falling off.This highlights the need to deploy as many satellite tags as possible to ensure good data returns.

The tracked black marlin dispersed widely in the western Indian Ocean, with greatly varied movement patterns. The shortest distance recorded was 30km and the longest 11 944km,with an average of 1818km for the time the tags were attached.

“We found a lot of similarities between the species;they both swim far and fast,covering 40–50 km per day,” said Christoph in an interview with MMF scientist Dr Simon Pierce.

“Most moved northwards from Kenya towards Somalia and Oman,but there was plenty of individual variation too.One of the striped marlin swam all the way over to the Maldives,and a black marlin swam south into the Mozambique Channel.The longest track was 11 944km over 167 days.”

“Striped marlin followed ocean productivity through the year,with high productivity off Kenya during the December to March period when they were caught,before moving north towards the Horn of Africa around the island of Socotra.There was lots of coolwater upwelling there,which means productive surface waters and probably dense prey,”Christoph explained.

“Black marlin were similar during the first half of the year,moving north to the same general area,but they moved back down the coast to Kenya in the second half of the year,”he added.“That return movement is difficult to explain because there’s no particular front,upwelling,or obvious productivity we could detect through those months.Generally, though,both species are going to be

Four individual tracks from among the 34 black marlin tracked between 2015 and 2019. Below: The four-panel seasonal tracks of 39 striped marlin tagged off Kenya.

Density plot of the depth and temperature use of black marlin (n = 34; panels A & C) and striped marlin (n = 39; panels B, D), with the lower panels C and D zooming into the highly-used habitat between the surface and 100m depth and between 20- and 32°C. Basically the warmer the colour, the more time marlin spent in that temperature-depth zone.

moving around to find food,so there’s probably a prey source present that we can’t identify remotely.

“There’s also the possibility of spawning activity,which we haven’t gotten to the bottom of yet.Striped marlin larvae have been found up north,off Oman.No Indian Ocean spawning areas for black marlin have been identified. We think they might be spawning off Mozambique,so we’ll be looking into that for the next phase of the project,” Christoph commented.

Vertical movements of marlin over five days (left) and (right) the average depth every ten minutes of the day, averaged over the whole track. The striped marlin #142292 consistently dived deeper during the day than at night, while the black marlin #159225 dived particularly deep at dusk and dawn.

The tagged black marlin spent most of their time in areas with warm surface temperatures averaging 27.4°C,but there was a lot of vertical movement recorded.The tagged striped marlin also preferred relatively warm sea surface temperatures between 26- and 31°C.

“Striped marlin dived deeper more frequently than black marlin,and also spent more of their time at the water surface,”said Christoph in an abstract from the article published by him and five other scientists in the Journal of Fish Biology.

Tag results showed that most striped marlin had a normal daily dive pattern over their track,while approximately 35% of black marlin dived particularly deep at dusk and dawn.“Squid are an important part of the diet for both species,so they dive a lot.One individual striped marlin moved almost 15km vertically over a single day.Both marlin species dived down to 450–500m on occasion,with the coolest temperature around 10°C,”the scientists reported.

Scientists believe that as apex predators,black marlin are susceptible to overfishing,but because so little was known about their movements,it was difficult to properly assess their conservation status and manage fisheries.This project has provided a great deal of data which can help to inform future decisions regarding this fishery.

“There are two areas of high commercial catches for striped marlin in the region:up north off the Horn of Africa, and off Kenya.Those were the areas our tagged marlin used too.Prior to this work,we didn’t know that it was the same fish moving between both.Now we have shown that this whole region needs to be considered as a single management unit,”Christoph explained.

“The entire Indian Ocean might,in practice,be a single population for both species — our tags were set to pop-off the marlin after six months,so they’re only capturing a snapshot of their movements,and one swam over to the Maldives in that time alone.”

• Movement and habitat use of striped marlin Kajikia audax in the Western Indian Ocean by Christoph A. Rohner, Roy Bealey, Bernerd M. Fulanda and Simon J. Pierce was published in the Journal of Fish Biology. The full article can be accessed at https://doi.org/10.1111/jfb.14508

• Movement ecology of black marlin Istiompax indica in the Western Indian Ocean by Christoph A. Rohner, Roy Bealey, Bernerd M. Fulanda, Jason D. Everett, Anthony J. Richardson and Simon J. Pierce was published in the Journal of Fish Biology. The full article can be accessed at https://doi.org/10.1111/jfb.14809

• Vertical habitat use of black and striped marlin in the Western Indian Ocean by Christoph A. Rohner, Roy Bealey, Bernerd M. Fulanda, Clare E. M. Prebble, Samuel M. Williams and Simon J. Pierce can be accessed at www.intres.com/prepress/m14043.html

TRACKING MARLIN OFF MOZAMBIQUE

Sponsorship sought

MMF now plans to launch a black marlin satellite-tagging program off the Bazaruto Archipelago in September 2022;it will run for three years.In addition to tracking the movements of the marlin,they will also be conducting some genetics work,some dietary analysis work,and using additional methods to confirm if the area is a spawning ground.The MMF scientists will once again be working with local sportfishing charter captains who will do the actual tagging.

“In the Indian Ocean,where sportfishing captures of black marlin exceeding 1 000 pounds are extremely rare and are basically down to a single destination, one wonders where these big females go after their supposed spawning season between September and December of every year in the offshore waters of the Bazaruto Archipelago in Mozambique,”said Capt Duarte Rato,a sportfisher charter captain who operates off Bazaruto.

“And,with no identified proven spawning area in all of the Indian Ocean and only two known for the species in the world’s oceans (both in the Pacific) is it not time to scientifically prove the Bazaruto Archipelago to be one? In the twenty-odd years we have been here we have certainly seen enough evidence for this to be the case,”Duarte commented.

“In an area where these fish are caught in such remarkably shallow water,how interesting would it be to be able to see not only their horizontal movement but also their vertical utilisation of these waters,”he added.“It has been a long road but it seems we finally got the right people onboard with the same passion and enthusiasm to make it a reality.” Duarte and Captain Morgan O’Kennedy of Big Blue will both be working with the MMF scientists in Mozambique.

DONORS NEEDED

The project relies totally on donor funding,so the the public and corporations are invited to sponsor a total or partial tag.To fully sponsor one satellite tag is US$5000. Full sponsors will be able toname their marlin when the tag is deployed,but all sponsors will receive photographs or video of their marlin being tagged,updates about the project,and a sneak peek about the marlin’s progress before everyone else.

For more information contact Duarte Rato (pictured above) on <dudas7mares@gmail.com> or +258 84 639 0466;Morgan O’Kennedy on <ahoy@bigbluevilankulo.com>;or Dr Clare Prebble <clare@marinemega fauna.org>.

For further details visit www.marinemegafauna.orgor find MMF on Instagram,Twitter,Facebook and LinkedIn.