Secretive Sharks of the Open Ocean

Secretive sharks of the open ocean Open ocean sharks face an uncertain future, but new research gives hope, says David Sims. Blue shark. © Jeremy Stafford-Deitsch. lue stretched down as far as watched her disappear into the depths, Box 1. Satellite tracking could be seen, illuminated by knowing we’d be tracking her move- Bshifting shafts of down-welling ments for many months to come. light. Gazing down through this drop That was over 10 years ago and in the ocean I’d expected to see since then our project has provided something, but nothing moved across insights into the secret lives of oce- the void, nor had it for hours. A anic sharks (see Box 2). Based at the container ship, like a skyscraper on its Marine Biological Association (MBA), side, churned past us a few miles away. we commenced the Save Our Seas Then from the deep a shape glanced Foundation funded project on blue across the corner of my eye, there and and shortfin mako sharks in 2006 to We use two types of electronic tag to not there at the same time. Coming satellite track their ocean travels. Very track long-distance movements of back into view, a sleek, torpedo shape little was known about them in deep sharks. The first is an Argos transmitter that emits ultra-high frequency radio with a sinuous movement, waters of their oceanic habitats, such as a sleek, torpedo signals when the tag’s antenna breaks well camouflaged against where juvenile and adult oceanic sharks the sea surface. Polar-orbiting satellites shape with a the inky backdrop: it was spend time, how and when they move relay data to ground stations. Geo- sinuous movement a large blue shark. between preferred areas, and the timing graphic positions of the tag are computed in near-real time and available Now our boat was a hive of activity and location of migratory routes. The to researchers via a web portal. as we made sure lines were secure, project started out very much as a The second is a pop-off satellite- tagging equipment was set and the basic science study. However, it quickly linked archival transmitter (PSAT) that is attached to the shark’s fin and records satellite transmitter tags turned on. became evident to me and my then depth, water temperature and light Within a few minutes we were guiding research student, Nuno Queiroz (now intensity, before releasing from the shark a blue shark alongside and fixing the a PhD with his own shark research at a pre-programmed time, floating to the surface and relaying data to tag onto the triangular dorsal fin so group at the University of Porto) that overpassing Argos receivers. Data are that the little antenna reached above finding sufficient oceanic sharks to tag then processed by researchers to the fin’s tip. The tag would signal the would be one of our major challenges. estimate movement paths and activities. shark’s geographic position each time Absent sharks Tags are small, (ca. 120g) and can be attached rapidly. Tagging procedures it cruised at the surface (Box 1). After The longer we spent at sea in the require institutional and, in the U.K., taking the shark’s length and sex, we western English Channel off Plymouth, governmental training and approval to released this 2 m long female and our first tagging site, the more we reduce the potential of injury to sharks. 06 The Marine Biologist | October 2017 Research digests Oceanic sharks aggregate in preferred space use ‘hotspots’ strongly associated with productive thermal fronts. Satellite tracks of ~100 large sharks totalling 8,000 track days (2006 to 2012) shown: red colour dots, blue shark track locations; orange, shortfin mako; white, tiger shark; grey, hammerhead sharks. A collaboration between MBA and the universities of Porto and Miami. realized that our catch rates were much of oceanic shark catches, making it ing in the blue ocean we decided lower than previously recorded there. difficult for stocks to be assessed accu- from 2007 not only to track shark In the 1970s John Stevens, also work- rately. Today, catches of blue and mako movements by satellite, but also to ing from the MBA, had studied blue sharks remain largely unregulated glob- track vessels from the Global Position- shark ecology. His pioneering work on ally, particularly in international waters ing System (GPS) transmitters many their diet, reproduction and long-range where no management is in place to carry (see Box 3). By doing this we movements informed from mark- help sustain populations for the future. could explore how ranges of sharks recapture tagging provided valuable To shed light on potential overfish- overlapped with ocean-wide fishing new data on this species. In the sum- grounds—were oceanic sharks at risk mers of 2006 and 2007 we repeated Box 2. Oceanic sharks year-round or only in certain seasons? his surveys, but our catch rates were Sharks that spend most of their lives Also, we needed to tag sharks in the in deep waters of the open ocean away about 75% lower than Stevens’. Were from continental landmasses are termed heart of their distribution—the central the numbers of blue sharks visiting the oceanic sharks. The blue shark is one of Atlantic, where the bottom can lie Channel simply lower in more recent the widest ranging sharks, found some 5,000 or 6,000 m below. globally in all temperate and tropical years, perhaps altered by different sea oceans. They can grow to 3.8 m in Shark secrets glimpsed temperatures or prey movements? Or length with females giving birth to The project has managed to tag and had fishing taken its toll on popula- between four and 135 pups per year. As track over 140 blue and mako sharks the most fished oceanic shark they are tions in the intervening three decades? IUCN Red Listed as Near Threatened. and the insights have been fascinat- Overfishing was certainly possible. The shortfin mako is warm-bodied, ing. In the first years we found that Blue and mako sharks together make maintaining body temperature some blue sharks visiting UK waters during o up about 90% of all sharks caught 6-8 C above ambient and thought to be summer months migrated primarily the fastest swimming shark, clocking by open ocean pelagic fisheries (see burst speeds above 30 mph. It can grow from further south off the Azores, Box 3). Catches are high too, having to 4 m long. This species is at particular Iberia and north-west Africa, moving increased greatly in recent years as east- risk from overfishing because growth to north as sea temperatures warmed in sexual maturity is long (18 yrs in ern markets for valuable fins expanded. females) with few pups produced per spring to spend the summer foraging However, there was and continues to female (four to 18 every three years); it is on mackerel and herring schools in be poor monitoring and data reporting IUCN Red listed as globally Vulnerable. northern shelf seas. Tracking more October 2017 | The Marine Biologist 07 Research digests Box 3. Fishing far from land and existing management Pelagic longline fishing vessels are large ships that deploy a monofilament line measures. These hotspots were where extending up to 100 km long. From this surface line over 1,200 baited hooks hang down to depths of between 50 – 300 m. For many years swordfish and tuna were target oceanic sharks spent extensive periods catches with sharks being low value by-catch, however, as target stocks declined and of time each year—for instance, ten markets for sharks fins developed, oceanic sharks have become targeted by-catch. makos were tracked for six months Hundreds of longliners operate in Atlantic, which is the most heavily fished open and remained within an area about ocean in the world. Atlantic longline fishing is up to eight times higher than in the Pacific. Global catches of blue and mako sharks are estimated to be many millions, 800 km in diameter, which sounds however on the high seas catches remain largely unregulated. enormous but actually is very localized in the context of how far the individu- als move during an annual cycle. A key question was: what deter- mined where a hotspot was found? Several months of analysis provided a clue to the common factor. It appeared that hotspots were strongly associated with ocean thermal fronts, bounda- ries between different temperature water masses with high plankton than 180 longline fishing vessels Oceanic shark hotspots biomass. It seemed oceanic sharks it was clear that to reach the UK, The emerging picture of multi- were preferring to spend extended migrating blue sharks had to pass species aggregation areas led us in periods of time in productive areas. through a dense ‘curtain’ of hooks 2014 to collaborate with colleagues in Another discovery related to what stretching along the shelf-edge off the United States who were tracking the sharks were doing in the hotspots. south-west England. We speculated blues and makos in the western North Both blue and mako sharks undertook that increasing fishing pressure from Atlantic—together our blue sharks had to pass very deep dives down growing longline fishing fleets along shark tracks spanned to more than 1,700 the shelf-edge could be one reason the entire ocean. through a dense ‘curtain’ m, sometimes on a why we observed fewer blue sharks in Slowly, our view of of hooks daily basis. What was the Channel than 30 years before. oceanic sharks became a little less mys- the function of deep dives into the Within a couple of years of tag- tified.

Load more