Centre de Recherches et dEnseignement sur les SystmecUniversit Pierre et Marie Curie-Paris 06, UMR 7208 (C43 rue Cuvier, CP26, 75231 Paris cedex 05, France
a r t i c l e i n f o
Article history:Received 6 November 2009Received in revised form 4 June 2010Accepted 9 June 2010Available online 6 July 2010
coasts, undergoing during this phase a severe mortality. Lepto-cephali are thought to metamorphose into glass eels at their arrivalon the continental shelf. Later they colonize coastal and inlandwaters. After the settlement they become sedentary yellow eel.Their subsequent growth period lasts 38 years in males and 815 years in females of the temperate European eel (Feunteun,
areas. Their sexual maturation occurs during this migration. Littleis known about this second oceanic migration except that it occursin the rst 1000 m in open ocean, which makes surveys difcultand expensive (Tesch, 1979). However, new tools as pop-up aredeveloped to follow individually eel adults, and this range ofmigration depths seems to be conrmed (Jellyman and Tsukamoto,2005; Aarestrup et al., 2009). Nevertheless, most of the keys tounderstand eel life history are related to reproduction and recruit-ment. Consequently, a particular attention has to be given to themarine phase. Indeed, larval dispersal by oceanic currents
* Corresponding author.E-mail addresses: email@example.com (S. Pous), firstname.lastname@example.org (E. Feunteun),
Progress in Oceanography 86 (2010) 396413
Contents lists availab
email@example.com (C. Ellien).spawning area. They all allow migration to each recruitment sites consistent with duration estimatedfrom otolith microstructure analyses. Nevertheless, there is substantial variability on intra-seasonal tointerannual timescale in simulated migration durations and arrival success, with specic amplitude toeach recruitment site and spawning location.
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Eels (Anguilla genus) are highly migratory diadromous species,i.e. a part of their life cycle takes place in fresh water and the otherpart in sea water. Spawning takes place at depths over 400 m inwarm oceans (Tsukamoto, 1992). After hatching, leptocephali aredriven by the oceanic currents towards the continental (or islands)
2002), but this kind of information is not known for tropical eel.Moreover, duration of the growth period varies strongly, amongspecies and within species, according to environmental parameterssuch as food availability, water quality and temperature (Jellyman,1997; Tesch, 2003). After this growth period, a second metamor-phosis occurs: yellow eels change into silver eels and migratethousands of kilometres to go back to their oceanic spawning0079-6611/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.pocean.2010.06.002s Ctiers, 38 rue du Port Blanc, 35800 Dinard cedex, FranceNRS-UPMC-MNHN-IRD), Dpartement Milieux et Peuplements Aquatiques, Musum national dHistoire naturelle,
a b s t r a c t
In the South-Western Indian Ocean (SWIO), four eel species of the genus Anguilla (i.e. Anguilla bicolorbicolor, Anguilla nebulosa labiata, Anguilla marmorata andAnguilla mossambica) were identied, while theirrespective oceanic spawning area remained unknown. Based on collected larvae, glass eel captures andhydrodynamical conditions, previous studies raised the hypothesis that the eel spawning area mightbe common to all of those freshwater eel species, and located East of Madagascar. An original modelingapproach, based on backward simulations, is developed to assess how the ocean circulation in the SWIOdetermines the location of the spawning areas and whether a common spawning area for each recruit-ment site where glass eels were found is possible. We use a hydrodynamical model, which reproducesrealistically the 3D open ocean circulation in the region, associated with a Lagrangian model that calcu-lates the possible migration pathways of larvae, represented by passive particles. Some biological param-eters, provided by previous otolith microstructures analysis, are taken into account to constrain oursimulations. Results suggest the existence of a common spawning area located between 13S and 19Sand westwards of 60.5E, although these boundaries vary on the interannual timescale. Salinity frontswere reported beside the boundaries, reinforcing this assumption. We explore the impact of hydrody-namic conditions on recruitment and migration durations from three specic regions within the commonExprimentation et Approches Numrique, (LOCEAN), CC100, 4 Place Jussieu, 75252 Paris cedex 05, FrancebMusum national dHistoire naturelle, UMR 7208 (CNRS-UPMC-MNHN-IRD), Dpartement Milieux et Peuplements Aquatiques,Investigation of tropical eel spawning areInuence of the oceanic circulation
S. Pous a,*, E. Feunteun b, C. Ellien c
aMusum national dHistoire naturelle, UMR 7159 (CNRS-IRD-UPMC-MNHN), Laboratoi
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journal homepage: www.ll rights reserved.in the South-Western Indian Ocean:
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eanodetermines at least partly, the location of recruitment and theabundance of recruits. The long larval duration of leptocephalimakes the precise location of their spawning areas important sincespecic current or temperature conditions could affect their sur-vival and then, their recruitment success (Tsukamoto, 1992; Cas-tonguay et al., 1994; Dsaunay and Gurault, 1997). Then theleptocephalus stage needs to be better understood. Indeed, theduration of the oceanic larval migration is subject of controversy,particularly for Atlantic eels (Bonhommeau et al., 2008; McCleave,2008).
The analysis of geographical trends of leptocephalus size distri-bution have been almost the only way to discover the eel spawn-ing locations (Schmidt, 1923; Jespersen, 1942; Tsukamoto, 1992;Kimura et al., 2006). In the Atlantic and Pacic oceans, the distri-bution of the youngest eel larvae suggests that spawning takesplace in specic areas that may facilitate mating and place larvaein the appropriate landwards-owing currents (Kimura et al.,1999, 2001; Tsukamoto et al., 2002; Tsukamoto, 2006; Friedlandet al., 2007). It has been suggested that maturing European (Angu-illa anguilla), American (Anguilla rostrata) and Japanese (Anguillajaponica) silver eels make long migrations to spawn in thesubtropical gyres of the North Atlantic or North Pacic oceans(McCleave et al., 1987; Tsukamoto, 1992). In the case of the twoAtlantic eel species, the surface expression of the 22.5 C isotherm,located near the frontal zone in the Sargasso Sea, forms the north-ern boundary of the spawning area (McCleave, 1993). Tempera-ture fronts in the Sargasso Sea may act as cues that help adulteels to locate the spawning area (Friedland et al., 2007). Jetsassociated with these fronts appear to transport a variety of lepto-cephali species eastwards (Miller and McCleave, 1994). Therefore,changes in the latitude or intensity of these fronts (and the asso-ciated jets) may affect both the spawning location and the subse-quent transport of the leptocephali to continental habitats, andmay partly explain the decline in eel recruitment recorded sincethe early 1980s (Dekker, 2003; Friedland et al., 2007). In the Paci-c Ocean, the young Japanese eel leptocephali are mostly foundwithin the margins of the North Equatorial Current, owing wes-terly towards the Japanese coasts, and at a depth ranging between50 and 150 m. Their spawning location corresponds to a salinityfront (Tsukamoto, 1992; Kimura et al., 2001). The reduced salinityin the frontal zone, or some associated features such as odour,may provide migrating eel adults with a cue which triggers cessa-tion of migration and initiation of spawning. As previously statedfor the Atlantic eels and the thermal front, changes in the locationof the salinity front may contribute to the decline in Japanese eelrecruitment observed in the recent decades (Kimura et al., 2001;Kimura and Tsukamoto, 2006; Friedland et al., 2007). The key sim-ilarity in Atlantic and Pacic oceans is the position of the spawn-ing areas connected to major currents, the Gulf Stream for theAtlantic Ocean and the NEC with its northern bifurcation (i.e.Kuroshio Current) for the Pacic Ocean, which can drive the off-springs to their growth habitats and thus complete the recruit-ment of eels (Tsukamoto, 1992).
In the South-Western Indian Ocean (SWIO), four species of An-guilla have been identied: Anguilla bicolor bicolor,Anguilla nebulosalabiata, Anguilla marmorata and Anguilla mossambica (Ege, 1939;Robinet et al., 2007). The anthropogenic impact on those tropicaleel stocks is less documented than for the Atlantic and Pacic eelspecies (Moriarty and Dekker, 1997; Dekker, 2003). Nevertheless,recent observations explain the lack of large eels in most islandsof the Indian Ocean as a consequence of traditional sheries(Robinet et al., 2007). Indeed, those eel stocks are facing a growinginterest from the international markets in Madagascar and South
S. Pous et al. / Progress in OcAfrica (Robinet et al., 2008). Of all shes that breed in the westernIndian Ocean, the genus Anguilla has probably the highest eco-nomic value per unit weight of sh, and for decades the world-wide demand exceeds the supply (Jackson, 1976).
The eel spawning area in the SWIO has not been discovered yet,despite oceanographic cruises aiming at sampling eel larvae. How-ever, Jespersen (1942), following his observations on small lepto-cephali distribution and hydrographic conditions in this region,suggested that spawning area should be shared by all those fresh-water eel species and located East of Madagascar. Since then, noleptocephalus has been caught. Glass eels or riverine yellow eelsof those four species were sampled in estuaries and rivers of LaRunion Island, Mauritius Island, Mayotte Island, and on the east-ern coast of Madagascar between 2000 and 2006, highlighting thatthe composition of eel communities seems contrasted between thedifferent islands of this region (Robinet et al., 2008), as Ege (1939)previously suggested. This contrasted distribution may be due: (1)to different migratory routes followed by the eel larvae before theirestuarine recruitment (Robinet et al., 2003, 2008; Rveillac et al.,2008); (2) to different spawning areas or periods among species:A. marmorata and A. mossambica would share the same spawningarea located between Madagascar and the Mascarene ridge, withdifferent spawning periods, while A. bicolor bicolor might spawnfrom an area located westwards and closer to La Runion Island(Robinet et al., 2003); (3) to different behavioral traits, or intrinsicmetabolism (Robinet et al., 2003, 2008; Rveillac et al., 2008).These hypotheses are mainly based on otolith microstructure anal-yses, which allow to assess the leptocephalus stage duration, theperiod of recruitment as well as the spawning period (Robinet etal., 2003; Rveillac et al., 2008, 2009). However, eld data beingsparse, these hypotheses need to be confronted to modelingapproach.
Hydrodynamic modeling provides quantitative methods ofapplying physical oceanographic information to the question oflarval dispersal (Tremblay et al., 1994; Young et al., 1998; Ellienet al., 2004). These models can provide a synoptic view of larvaldistribution for a large range of geographic, hydrodynamic and cli-matic conditions at different spatial and temporal scales. They canalso be used as a tool to determine the relative inuence of variouscomponents of the water circulation and biological factors on lar-val dispersal (Barnay et al., 2003; Ellien et al., 2004). Lagrangianmodels have already been used to explain the distribution of theeels and their larvae and to discuss the inuence of oceanographicpatterns on larval dispersal and recruitment in the Pacic (Kimuraet al., 1999; Kim et al., 2007) and Atlantic Ocean (Kettle and Haines,2006; Bonhommeau et al., 2009a,b).
In this study, we use a state-of-the-art hydrodynamical model,which realistically reproduces the 3D circulation in the region,associated with a Lagrangian model that reproduces the possiblemigration pathways of larvae. The biological inputs, such as migra-tion durations, together with spawning and recruitment periods,have been deduced from the published results on otolith micro-structure analyses from all eel species. Hence simulations repro-duce migration of eel larvae of the SWIO with no distinction ofthe species.
The aim of this study is to understand how the ocean circulationin the SWIO constraints the location of spawning areas for tropicaleels: (1) can there be a common spawning area to all freshwatereel species sampled in their different recruitment sites? (2) Whatare the leptocephali possible migration routes to the recruitmentsites? (3) What is the variability in the recruitment and migrationduration induced by the environmental conditions? As a rst stepto answer these questions, we consider a very simple parameteri-sation of the larvae behavior in order to focus on the impact ofenvironmental conditions alone. We nally discuss the effect of
graphy 86 (2010) 396413 397more complex behavior (vertical diurnal migration and mortalityscenarios).
2. Data, models and methods
Our results are based on simulations of the oceanic circulationin the SWIO, from a state-of-the-art hydrodynamical model, usedto investigate migration pathways of eel larvae, taking into accountthe few observed biological parameters. In this section, we de-scribe the general features of the ocean circulation in the SWIO, re-view the available observations referring to eel early life traits,present the hydrodynamical and Lagrangian models and nallyintroduce the suite of experiments used in the present study.
2.1.1. Circulation and hydrology in the SWIOThis study is located in the SWIO, between 5N and 27S, 34E
and 75E (Fig. 1). We focus on the eastern coast of Madagascar,the island of Mayotte and the Mascarene Plateau including
398 S. Pous et al. / Progress in OceanoFig. 1. Chart of the southwestern part of the Indian Ocean. Gray lines indicateisobaths 200 m (shaded), 1000 m and 2000 m from the bathymetry of DRAKKARMauritius and La Runion islands. The Mascarene Plateau is char-acterized by a series of shallow banks and shoals separated by dee-per ridges, encompassing over 2000 km from The Seychelles at thenorthern end, to Mauritius at the southern end.
Circulation in the SWIO mostly consists of the South EquatorialCurrent (hereafter SEC), which is the northern boundary of the ba-sin-wide anticyclonic subtropical gyre (Stramma and Lutjeharms,1997). The SEC is a broad ow, heading westward, located between10S and 20S. New et al. (2007) showed that when the SEC passesacross theMascarenePlateaunear 60E, it splits into twocores form-ing a northern core between 10S and 14S (passing north of Saya deMalha Bank and between Saya de Malha and Nazareth Banks) and asouthern core between 17S and 20S (passing between CargadosCarajos Bank and Mauritius). When reaching M...