Impact of nondegradable marine debris on the ecology and survival outlook of sea turtles

Marine Pollalion Bullctin Bauer, K. M. & Glulz *.'o1~}Holzheim. U. N. (1966). lhmdfiuch der | ;igel Mitteleuropas. VoL L Akad. VerL Ges. Frankfurt. Hrunckhorst. H. (19851. Das heufige Vorkommen tics l]at|t(ilpels Sula bassana bet Itelgoland. Setnfigel 6. 611-62, Coleman. IL C. & Wehle. D. I1. S. (It~84). Plaslie pollution: A worltlwide oceanic problem, l}trks 9. 9 ~ 12. Cramp. S. & Simmons. K. I L L . (1977). Ilandhook ~f the Binls of Eump6 the Middh' F.a~land A'orth A/r/ca. Vol. l. Oxford University Press. Oxford. l)a); R. Ft. (1980). The occurrence and characteristics t~f plaslie poilulion in Alaska's marine birds. M.Sc. Thesis. University of Alnska. Day. R. H . Wehle. D. H. S. & Coleman. I:. C. (l~)HS). Inge.~tion of plastic pollutants by marine birds. In P~ceedings afthe IVorL~hop on the htte and hnpaet of ~htrine l)ehris. 26-29 Nov. 1984. ]-h~noltlhl. l~lawaii (R, S. Shomara & ]1. O, Yoshida. ells.) pp. 344-386. US Dep. Commcr.. NOAA "l~ch. Memo. NMFS. NO~LA-TM-NMFS-SWFC54. Drenckhahn. I). & Kuscherl. | | . 11974). BaL~.~Hpc|--St|Iabassana, h~ I "ogel.vlt Schh.s~vig-th~lxteins. Vol. 1.(R. K. Berndl & D. Drenckhaha eds). Selbstverlag. KeiL G~ilkc. It. (19(|11). Die l/ogeh,~rte Itelgoland (R. IHasius. ed.). Brz~unschweig. Goethe. 1~ (1978). BaBl/ilpel--Sula ha.~saaa. In Die Ifigel Nie~h'L~m'h,~en~ uml de~ Landes Brt.merh Nutur~chutz trod Land.~dmJ~spJh.ge in Niedersachsen. Sonderreihe'B. Heft 2.1 (E Goethe, H. Heckenroth & H. Schumann, eds). Hannov'er. Grimminger. M. (198 I). l)as Vorkommen neun pelagisehcr Vogelarlen bet dcr Forschu~gsplalfform "'Nordsce" im Herbsl 19811. Se¢a¢igel 2. 39-47. Harlwig. E.. Rcineking. B. Schrc); E. & Vauk-Hentzeh. E. ( 19851. Au.'.-

wirku,gcn dcr Nordsee-Vcrmiillung auf Seev6g¢l. Rtd~hen trod I:i~cbc. St.~fJgel6 (Sonderband). 57-02. Itenders~m. J. R. (19831. Encaplure.s and entanglement of tlawaiian Monk Seals with losl and discarded fL~binggear. Abslracts tff the 5lh Biemfial Conference ~m the ltioh~gy tffMarine Mammal~. Kirchh~d~. K. ( 19~2~. W.~sservogeh'erht~tc durch Ok" Fi~chetci an dcr ~chleswig-hol~.l¢inischcn OslscekiJntc. I'ogeh~a'# !113. Hl-St). Klauscwilz. W. 119841. Kunstslofl~: all tier K{iste trod im .Mecr--ein ilkoh,gisches Problem. Natur uml 3 luxeum I 14. 162 - 174. L.3hraer. K. & VatJk. G, (|969). Nabrung~.i;~k~h~gischeth~Icrsuclumgen ;hi iibcrsommernden SilhermSwcn (Larus argenlalus) auf ltelgola|'id im August/Scptcffd~cr 1967. Bonn. ZooL Ihqtr. 20. 110-124. Morilz. 11. & Dierschke. V. (in ms.). Ornilholt~gi...che Beobachlungen in den S~mmermonalcn Mat bis Augus| auf tier Off-slmre-Forschungsplatti~.~rm"Nordsee" in der I)eulxchen Bucht. NeLson. J3. (1978). 7~tw(kmnel. Poyser. |3erkbamsled. Ro~; N.. Thomansun. J.. Anker-Nilr.scn. '1:. I;lalrct|. R.. l:olkc.~tad. A. O, & Runde. O. (1984). Sjofuglprospektet 1979-1984. Viltrapport 35. Trondheim. N o r w a ) ; "¢al1 l:r~lllckcl; J. A. 119851. Plastic b~gcslion in lhc Nl~rlh AIIzIJlliC l:ulnlar. Mar I'ollut. Bull 16. 367-369. Vauk. O. (19721. /)it" l'[ib.t.l Ih'tgolated.s. l~arl2~.~Vcrli|g,l-hnilbutg alltl Berlin. Wallace. N. (19841. Bib|it~graphy i~n Enlimglemcnl. In Pt~iceedings of the |Vorkshol~ on the t.iltt, and htllntCt o f Marble Debri.~. 26-29 Nov. 1984. Homfluhl. Hawaii (R. S. Shomura. & H. O. Yoshida eds.) US I)cp. Commcr.. NOAA Tcch. Memo. NIMI:S. NOAA-TM-NMFSSWFC-54. pp. 259-277. Wehle. D. It. S. & Coleman. 1"~C. (191q3). I'll.slit at sea. Nat. Ill.st. 92. 211-26.

MarlneI~lhaionlhdlelm.VoL18.No.611.pp. 352-356.19/47, Prln~cdin GreatBritain.

0025-32hX/~t7S3.00-HLn0 ~ 1914"/Petgaml;nJt~urnals[.td.

Impact of Nondegradable Marine Debris on the Ecology and Survival Outlook of Sea Turtles ARCHIE CARR

Department of Zoology, Universityof Florida, Gainesville, Florida 32611, USA

Sea turtles of all kinds are peculiarly prone to eat plastic scraps and other buoyant debris and to tangle themseh'es in lines and netting discarded by fishermen, and records of such mishaps have increased markedly in recent years. Advances in our understanding of the developmental ecology of sea turtles shed new light on the impact o f buoyant wastes on the juvenile stages. The initial developmental stages of all species are passed in the open sea. In the case of the loggerhead Caretta this period of pelagic life is likely to include 3 - 5 years of planktonic open-ocean travel, which may involve multiple transatlantic crossings. During this time both the young turtles and their buoyant food are drawn by ads~ection into fronts (eonvergences, tips, driftlines) and the same process also brings in ai~d aligns persistent plastics and lost fishing gear. This effect exacerbates survival problems for sea turtles that are dependent on driftlines for their food supply or shelter. 352

Two recent advances in our understanding of sea turtle ecology have made the survival prospects of the group seem even less favourable than when the species were first recognized as threatened and endangered (Carr, 1986a). One is the dependence of the young stages on convergences--fronts, rips, driftlines, into which they and their food are drawn by downwelling. The other is new evidence that the juvenile pelagic stage may last for a period of 3-5 years, according to the species. During this time of early development the young turtles are passive migrants in driftlines in the surface water of the open sea. Here they come into intimate contact with buoyant debris (Carr, 1984,1986a), and there is massive evidence that entrapment, entanglement, and impact|on of the alimentary canal by ingested plastics have become major threats to sea turtle survival (Balazs,

19851.

For some years a principal research goal of the sea turtle programme of the Caribbean Conservation Cor-

Vt~lunte IX/Number 6ll/June 1987

por~tion at Tortuguero, Costa Rica has been to fill gaps in knowledge of the developmental ecology and migration of marine turtles. Until lately the earliest stages were shrouded in mystery so deep that the period was spoken of as the 'lost year' (Carr, 1984). Eventually, however, investigations showed that post-hatchlings pass their initial period of development as surfacedwelling plankton, in the borders of currents a n d e"ddies into which they are drawn by advection. This same force mobilizes and aligns buoyant pollutants; ~.nd the ecological implications of this have not been given the attention they warrant. The fluid dynamics of advection and diffusion are well understood by physical oceanographers; and the role of currents, gyres, and rings in transporting debris is well documented (Gait, 1985); but the gathering action of downwelling in worsening the effect of pollutants on animals in driftline habitats has had little attention from marine ecologists. Neglect of this fundamental aspect of marine ecology can probably be attributed to the formidable arithmetic of adveetion and diffusion. But differential equations are not needed to understand that the driftlines, slicks, rips, and windrows that form along fronts, large and small, are an essential feature of the surface water of the ocean (Fig. 1). They are the hedgerows of the epipelagic environment. The importance of these zones as marine habitat is well known to pelagic fishermen and to a scattering of marine zoologists (Ashmole, 1971), but the role of downwelling in the ecologic organization of the surface waters of the ocean has been generally overlooked. With the present volume of biologically injurious flotsam in the sea, and the probability that the spread of these materials will increase, it seems appropriate to examine the ecological threat posed by persistent plastic materials in driftlines along which biologic activity is at a maximum.

R o l e o f C o n v e r g e n e e s in t h e L i f e H i s t o r y o f

Sea Turtles In a relatively stable driftline, such as those that form at the borders of major currents and gyres, the ecologic contribution of flotsam is threefold. For one thing, it increases diversity by providing a substrate for the pelagic larvae of sedentary animals. Bryozoa, barnacles, pelagic crabs, and many other species of invertebrates attach themselves to objects floating in the front, including trees and driftwood, weed rafts, and increasingly today, tar clots and plastic objects. Besides the passively mobilized pelagic eggs and planktonic animals, other species come in actively. The first of these are the shelterseekers, which are attracted to any solid floating object. These then become bait for predaceous fishes and sea birds. Commercial fisheries for tuna, billfish, and the other big cursorial species regularly exploit the tendency of their quarry to aggregate at fronts, where they are attracted by the schools of bait species that form there. For centuries artisanal fishermen have known that flotsam attracts fish and that it accumulates in driftlines. Sport fishermen also regularly go out to the rips to fish for cobia, dolphin, and various other species. Hatchlings of sea turtle populations in the range of Sargasso weed usually move directly into an initial habitat in which both food and shelter are provided by the algal mats (Fig. 2). When the young turtles swim seaward, as they always do, they eventually encounter a convergence along the inner edge of either a border current or a longshore eddy. If sargassum occurs in the region, rafts will be strung out in this convergence. Because sargassum has a specialized invertebrate fauna it provides both shelter and a food suppb: Off many nesting shores, however, perhaps off most, sargassum is not present. But even there, downwelling at the fronts

.?

Fig, I Sargassum line off Tortuguero, Costa Pica, at the front between the South Equatorlal Current and Southwest Caribbean Gyre. (Lynn Fowler photo.}

353

Marine Pollution Bulletin

assembles and aligns any organisms that float or are able to control their buoyancy at a given depth, including the maeroplankton that hatchlings feed on. In most cases the pelagic stage of sea turtle development ends prior to maturity and a transition to benthic habitats occurs. Two of the eight named species, however, the lealherbaek and the olive ridley, appear to be mainly epipelagic foragers even after reaching maturity. When Atlantic leatherbacks leave their main nesting shore in northern South America, they follow the edge of the Gulf Stream northward along the US coast, sometimes as far as Cape Cod and beyond. The attraction is the seasonal abundance of jellyfish there. The ol!~er species that characteristically lives in the open sea even after reaching sexual maturity is the olive ridley (Lepidochelys oli~cea). Observations at sea, and recent tag-recoveries, indicate that olive ridleys may migrate between their sites of mass nesting in Mexico and Costa Rica, and feeding grounds as far south as Ecuador, probably foraging on macroplankton in longshore convergences along the way. Mature Atlantic green turtles (Chelonia ng,das) and loggerheads (Catetta caretta) also feed along rips when jellyfish are abundant; but more characteristically the),, like the hawksbill

Fig. 2 Pelagic stage green turtle in massive sargassum raft off the tip of Little Bahama Bank. (Lisa Bibko photo., R/V Cape Florida Cruise,)

(Eretmochelys hnbricata) and Kemp's ridley (Lepidochelys kempO, forage on the bottom. Recent evidence indicates that the pelagic developmental stage is much longer than was once supposed. Pacific loggerheads are nol seen again for at least 4 yr after leaving the nesting shore in Natal (Hughes, 1978). It now seems clear that the development of the Atlantic loggerhead may involve a comparable period, and that during this time the young turtles may make one or more passive trans-Allanlie migralions in the Gulf Stream system (Carr, 1986a, b). The existence of this extended epipelagic period is supported by new age-group data from the Azores. As part of an effort to trace the developmental migrations of American sea turtles, I graphed carapace-length distribution in the three developmental stages of Caretut that occur in American waters. In the histogram (Fig. 3) a conspicuous gap separated the smallest turtles of the benthic subadult class which is abundantly represented along the coasts of northeast Fiorida and Georgia, and the biggest pelagic post-hatchlings and early juveniles that occur in offshore US waters. A little later, I received a set of measurements of little loggerheads from collaborators in the Department of Oceanography, University of the Azores, and the size-range in these makes a trans-Atlantic developmental migration by West Atlantic Caretta seem a virtual certainty. The carapace length.', of all 82 turtles tagged range within the gap in the age-class histogram (Fig. 3). That at leasl some young American loggerheads go through part of their developmenl in the East Atlantic now seems clearly indicated. The little Azores loggerheads were taken on Princesse Alice Bank by tuna fishermen who say the turtles regularly show up there in the summer months. To bring them home within the size-range that would fill the gap in the US data, would require at least two years" additional growth. By the most direct recirculation route back to US waters in the Gulf Stream system they

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354

V~lumc 18/Number 6B/June 1~187

would return far too soon. To consume the time for the necessary growth, either repeated crossings in the main system must be made, o r time must be passed circling in rings and minor eddies along the way. Twenty years ago the latter supposition might have been rejected on the grounds that fronts, rings, and small eddies were too few in the open sew to provide the resources for prolracted transoceanic travel by azl airbreathing, surface-feeding migrant. Recent observations made from the Space Shuttle shows that the ocean teems with fronts of varied origin, size and configuralion (Scully-Power, 1986). A growing body of oceanographic data from the Atlantic (Kerr, 1985) indicates that small eddies, recognizable by their salinity or oxygen content, occur far more often, last longer, and travel farther through the ocean than has been realized. Aboul 11 cold-core rings (enclosing shelf water) and about three warm-core rings (surrounding Sargasso Sea water) may occur at any given time in the North Atlantic--the former south and east of the Gulf Stream, the latter to the northward of the Gulf Stream (Kerr, 1977). Some small eddies are being traced back to dislant places of origin. According to McDowell (1986), shallow eddies detected 800 km southeast of Cape Hatteras had originated 4500 km to the east toward Gibraltar. Others had come from far down to the southeast in the mid-Atlantic Gyre, or 500 km to the east in the Gulf Stream. Sometimes one of these small eddies splits and spawns two new rings that continue separate life (Lindslrom et aL, 1986). It is significant that these newly discovered structures occur in precisely the region where they are needed to account for the growth of the Azores migrants to the size of the West Atlantic subadults. When the geographic distribution of size classes in other species of marine turtles is better known it seems probable that patterns of long distance developmental travel comparable to that of the Atlantic loggerhead ",,,'ill be revealed. This new aspect of the developmental ecology of the species increases the biological interest of the animal, but it obviously adds gravity to the problena of marine pollution. Effects of Debris on Sea Turtles Besides providing useful resources, the convergences may become, in a way, a trap for marine life. In addition to the food and shelter, the animals that come in find themselves in close association with lethal floating objects and toxic substances. Persistent plastics are often the most conspicuous visible flotsam in a rip, and ghost gear and tar pellets are also often present, Juvenile sea turtles are peculiarly vulnerable to the hazards of this new breed of driftline because the fronts are their essential habitat and the only source of the resources that make life possible during their period of pelagic development. The tendency of marine animals to investigate, pry into, or eat floating debris is well known. Many years ago, Dr. E. W. Gudger published a series of short papers reporting cases in which fishes had" been trapped, entangled or otherwise hurtfully involved with

human ~rlifacts of various kinds (Gudger, 1922, 1928, 1929, 1937, 19382, b). In those days plastics were barely known. Fishing gear was made of degradable vegetable fibre, and the agents of injury to marine life were mostly rings or other objects made of perishable metal or rubber. Nevertheless, the Gudger anecdotes made it plain that fishes will eat inedible objects, or poke their heads imo rings, tins and the like. It ha• recently become disturbingly clear that turtles, sea birds, and seals have the same hazardous tendencies. When heavy seas wash the Florida East Coas! in the fall, little loggerheads, dead or moribund, are often thrown ashore, sometimes in great numbers. The stomachs of the dead ones often contain pellets of tar ant] the ubiquitous plastic beads that are delivered to the sea by the millions in industrial waste water. Both the tar pellets and the beads are suggestively similar in size and shape to sargassum floats, and this likeness may account for the turtles" misguided feeding. Plastic beads are ubiquitous in surface waters of the oceans, tn 1985-1986 they were particularly abundant in the West Caribbean Gyre, littel'ing the Tortuguero nesting beach of the green turtle in Costa Rica like little hailstones, The southern arc of this gyre, which transports the "l'ortuguero hatchlings into their pelagic habitat, is increasingly cluttered with plastic debris of every conceivable kind, and of untraceable origin. The special susceptibility of sea turtles to injury by ingesting such material, and by entanglement in ghost nets and nondegradable cordage, was thoroughly documenled by Balazs (1985). His list of such casualties was compiled from a widely scattered literature, from extensive interviews and correspondence, and from broad personal experience. He documents 79 cases in which the guts of turtles were loaded down with synthetic scrap including fishing lines and nets, plastic bags, beads, boltles, vinyl films, and tar balls. The gut of one turtle found dead contained a sheet of heavy plastic measuring 3 × 4 m. Balazs also re¥orded 60 cases of entanglement involving monofilament line, rope, netting, cloth debris, tar, and even such strange forms of entrapment as that shown in Fig. 4. His paper leaves no doubt that persistent plastics and discarded or lost nylon fishing gear must be added to tar, liquid petroleum, and toxic chemical wastes in any assessment of the impact of pollution on marine vertebrates (Balazs, 1985). The marked tendency of leatherbacks to ingest

Fig, 4 Hawaiian grecnturllewith plastic collar. (George Balazsphoto.)

355

Marine Pollution Butfetin s h e e t s a n d bags o f plastic film has b e e n a t t r i b u t e d to m i s i d e n t i f i c a t i o n o f the translucent films a s jellyfish. T h a i t h e s p e c i e s is a generally i n d i s c r i m i n a t e feeder, h o w e v e r , is s u g g e s l e d by the ingestion o f a 180 m length o f h e a v y m o n o f i l a m e n t line by a N e w Y o r k l e a t h e r b a e k (Sadove, 1980). T h e vulnerability o f y o u n g sea turtles to t h e s e materials is particularly serious. D u r i n g o u r early investigations o f the qost-year' puzzle, s a r g a s s u m rafts w e r e ultimately s h o w n to b e an i m p o r t a n t habitat for t h e hatchlings o f turtle colonies n e s t i n g in certain areas. H o w e v e r , t h e r e r e m a i n e d the p r o b l e m o f a c c o u n t i n g for the e x i s t e n c e o f p o p u l a t i o n s b r e e d i n g w h e r e n o algal rafts drift by. F r o m the b e g i n n i n g it s e e m e d clear that in t e m p e r a t e a n d tropical regions pelagic m a c r o p l a n k t o n w a s too thinly d i s t r i b u t e d to p r o v i d e a d e q u a t e f o o d for an air b r e a t h i n g animal restricted to s u r f a c e w a t e r a n d nol a b l e to filter food. A f t e r p o n d e r i n g o c e a n o g r a p h i c p u b l i c a t i o n s for a while, I realized that this d r a w b a c k is r e m o v e d by the p r o c e s s e s o f a d v e e t i o n a n d d o w n welling, w h i c h align b u o y a n t m a t e r i a l - - s a r g a s s u m m a t s a n d a n y t h i n g else that f l o a t s - - a l o n g c o n v e r g e n c e s . That tb;s has not b e e n cried from the r o o f t o p s reflects a cu, ious c o m p a r t m e n t a l i z a t i o n o f the marine., sciences, with biologists not taking stock o f the ecologic i m p o r t a n t e o f fronts, a n d with physical o c e a n o g r a p h e r s m a k i n g little effort to s p r e a d the c o n c e p t . T h e sea turtles o f t h e w o r l d , already suffering from over-exploitation, elimination o f b r e e d i n g g r o u n d , a n d incidental t r a w l e r catch, are n o w s e c r e t e face a n e w threat in the g r o w i n g b u r d e n o f n o n d e g t a d a b l e d e b r i s in the b o r d e r s o f c u r r e n t s a n d e d d i e s in their epipelagic habitat. O n e r e a s o n for the scant a t t e n t i o n the driftline habitats have received in m a r i n e e c o l o g y is the fact that until lately m o s t c o n v e r g e n c e s have b e e n c o n s p i c u o u s o n l y to t h e o c e a n o g r a p h e r s o r tuna f i s h e r m e n w h o run transects to d e t e c t s u d d e n c h a n g e s in salinity, density, o x y g e n , etc, To the casual o b s e r v e r it is o n l y w h e n t h e r e is flotsam in the line o r a fog-wall along it that a c o n v e r g e n c e is r e c o g n i z e d , o r w h e n an a b r u p t c h a n g e in the c o l o u r o r clarity o f the w a t e r o c c u r s , o r a b a n d o f little w a v e s glitters in tl~e s u n l i g h t - - o r w h e n fish, p o r p o i s e s , sea snakes, sea birds, whales, w h a l e sharks, o r m a n t a rays string out a l o n g the front. Today, h o w e v e r , the c o n v e r g e n c e s are e m d r g i n g into p u b l i c ' v i e w . W h e n styrof o a m trash g a t h e r s in o n e it c a n b e seen by a n y b o d y from the d e c k o f a s h i p o r w i n d o w o f an airplane. T h e a c c u m u l a t i o n o f floating refuse in t h e o c e a n has b e e n n o t e d a n d d e p l o r e d for several d e c a d e s . F o r a while it was s e e n as mainly a n a e s t h e t i c violation, a s y m p t o m o f h u m a n untidiness. Today, h o w e v e r , n o n d e g r a d a b l e d e b r i s is clearly a p r o g r e s s i v e d i s r u p t i o n o f the e c o l o g i c o r g a n i z a t i o n o f m a r i n e systems. With g r o w i n g f r e q u e n c y the fronts n o w e m e r g e as white lanes o f s t y r o f o a m (Carr, 1986a). T h e m o s t c o n s p i c u o u s intrus i o n o f t h e invasion is in the L a n g m u i r b a n d s , t h e fields o f roughly parallel jagged lanes that o f t e n string out a h e a d o f a s t e a d y wind. But t h e b o r d e r s o f t h e m a j o r c u r r e n t s a n d their pyres are also o f t e n white with plastic. O n o u r last trip a c r o s s t h e G u l f S t r e a m s e a r c h i n g for little pelagic turtles, the f r o n t off Little B a h a m a B a n k was m a r k e d b y a c o n t i n u o u s line o f plastic s c r a p s o f u n t r a c e a b l e p r o v e n a n c e . 356

In their p a p e r o n o c e a n fronts C r o m w e l l & Reid (1956) gave a s the basic definition o f a c o n v e r g e n c e , "a b a n d a l o n g the sea s u r f a c e a c r o s s which t h e d e n s i t y o f t h e w a t e r c h a n g e s abruptly'. To this I w o u l d add, ' . . . a n d a lane along which little turtles, their food supply, a n d t h e b u o y a n t pollutants o f the seas are all b r o u g h t in together" For data and helpful advice I am indebted to Mr. Larry Ogren and Mr.

George Balazs. Dr. Jeanne Mortimer has rendered indispensable help

in coping with the suggestions of editorial referees. Ashmolc, N. i~ (1971). Sea bird ecology and the marine environment. In Avian Biolog3; VoL 1 (1). S. Farmer & J. R. King. cds). pp. 223286. Academic Press, New York. Ralazs, G. (1985). Impact of ocean debris on marine turtles: entanglemeat ,and ingestion. In Proceedings of the Bbrkshop on the Fate attd ltnpoct of Marine Debris, 27-29 November 1984, Honolulu, Hawaii (R. S. Shomura & H. O. Yoshida, cds), pp. 387-429. US Dept. Cornmere., NOAA Tceh. Memo. NMFS, NOAA-TM-NMFS-S~,VFS-54. Cart, A. (1984}. Mystery of the missing :,'ear. The Sciences 24, 44-49. Cart, A. (1986a). Rips, FADS and little loggerheads. BioScience 26, 92- 100.

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