Secretive of the open

Open ocean sharks face an uncertain future, but new research gives hope, says David Sims. Blue . © 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 . 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 . 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- 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 (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 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. Spatial analysis of shark tracks midnight zone where no surface light ging smaller sharks in shelf seas we in relation to satellite remote-sensing reaches? Examining sharks caught by ventured into the centre of the North maps of temperature and productivity commercial longliners in the region Atlantic. Focusing on three tagging for example, identified multi-species gave an answer: stomach contents sites hundreds of kilometres apart, shark ‘hotspots’ in open ocean regions comprised many different species of we found that blue and shortfin mako sharks there were much larger in body size compared to most sharks found near North Atlantic landmasses. Several years of track- ing indicated that we had stumbled across areas where adult males and females of both species aggregated, including pregnant sharks. Like the smaller shelf-edge sharks, these large individuals moved north and west as summer came, and moved back south as waters cooled in autumn. Move- ments were extensive: makos could cross the North Atlantic in a matter of weeks, while adult, possibly preg- nant, female blue and mako sharks Distribution of individual longline deployment from 186 Spanish and Portuguese fishing migrated south to tropical waters, vessels, 2003-2011. Fishing locations occurred predictably in shark space-use hotspots, perhaps to give birth in productive overlapping their tracked range almost entirely. Redder colour in grid cell denotes more areas off Africa and South America. longline deployments.

08 The Marine Biologist | October 2017 Research digests

significant proportion of the space used by oceanic sharks, effectively protect- ing them for the period they remain within the MPA. Key to appropriate designation of MPAs will be know- ing where oceanic shark hotspots are located and the timings and duration of movements to and from the area. Our research has identified two shark hotspots in the North Atlantic called Mako Metropolis and Blue Shark Central, both of which have recently been designated as Hope Spots (see Box 4). As part of our ongoing work we are launching the Shark Hotspots Ocean Challenge (SHOC), Map of the major locations where large sharks co-occur with longliners. Shark space-use hotspots denoted by dotted lines. Redder colour denotes more shark/fishing vessel spatial to double the number of oceanic interactions. shark hotspots found in the Atlantic within three years. It is hoped that by deep-sea cephalopods, including the sharks and longliners included the understanding more about the odysseys enigmatic vampire squid, Vampyro- North Atlantic Current/Labrador Cur- of oceanic sharks there will be a greater teuthis. So shark hotspots seemed to rent convergence zone east of New- chance of sustaining their populations. be areas not only with high surface pro- foundland and the Mid-Atlantic Ridge After all, managing populations effec- duction and prey biomass but also huge southwest of the Azores. Strikingly in tively will be difficult if we don’t know abundances of squids and octopuses these main regions, and subareas within where they are and how and when way below the sunlit surface waters. them, shark/vessel co-occurrence was they move between exploited areas. Tracking sharks to their death spatially and temporally persistent Prof David Sims ([email protected]) As our study progressed, the number between years, which highlighted MBA Director of Research, MBA of tagged sharks caught by commercial how broadly the fishing exploitation Senior Research Fellow, and Professor of Marine Ecology at the longliners grew too. Tags were returned efficiently ‘tracked’ oceanic sharks University of Southampton. to us from many different countries, within their hotspots year-round. The such as Spain, Japan and the United shark/vessel interaction maps enabled Further reading N. Queiroz et al. (2016) Ocean-wide tracking States. Importantly, from satellite track- us to calculate that an individual shark of pelagic sharks reveals extent of overlap with ing we could detect a shark mortality in may be at risk of capture for up to 20 fishing vessel hotspots. Proc. Natl. Acad. Sci. a -independent manner—from days per month, highlighting that blue USA 113: 1582-1587. the different movements of tags when and mako shark hotspots were at risk aboard a vessel compared to when of overfishing (see Further reading). Box 4. Shark Hotspots Ocean Challenge (SHOC) attached to a free-living shark we could Conserving shark hotspots tell when a shark was caught. And In light of this, something worth capture rates seemed high, at around considering in addition to much- 20%, at least several times higher than needed catch limits for oceanic sharks, fishing mortality estimates used in blue and for mako sharks in particular, and mako fishery stock assessments. is the role that high seas Marine The data were pointing towards much Protected Areas (MPAs) could play in higher fishing pressure for oceanic helping to conserve oceanic sharks. sharks than previously realised. But it No-take marine reserves currently was our next analysis that emphasized cover only a few per cent of the ocean’s Working with Mission Blue’s Hope Spots programme, we are launching the oceanic shark vulnerability to fishing. surface. However, in the last few years, SHOC project to discover new oceanic Overlaying simultaneous GPS some very extensive no-take MPAs hotspots in the Atlantic Ocean trackings of the entire Spanish and have been designated around oceanic (www.mission-blue.org/hope-spots/). We are raising funds to purchase new Portuguese longline-vessel fishing fleets islands, for example in the British satellite tags that we’ll deploy on sharks with oceanic shark hotspots showed an Overseas Territory of Pitcairn Island in areas with strong ocean thermal 80% overlap with fished areas. Regions in the Pacific Ocean. MPAs of this size fronts. To learn more visit www.mba.ac.uk/simslab of high overlap between oceanic tagged are those most likely to encompass a

October 2017 | The Marine Biologist 09