•2 different-season surveys for leptocephali were conducted in the southwest Pacific.•Mesopelagic larvae were abundant, shallow-water eel larvae were diverse in the west.•Anguillids and probably ...conger eels spawned in the South Equatorial Current.•The Chesterfield Is.–New Caledonia area was diverse for shallow water eel larvae.•S. Pacific hydrographic structure varied latitudinally with west and east currents.
Although juvenile anguillid eels live in freshwater/estuarine habitats, and marine eels live in diverse ocean environments ranging from shallow-to-deep continental shelf areas and around islands to deep-benthic habitats and deeper meso- and bathy- pelagic zones, the larvae (leptocephali) of all species mix together in the ocean surface layer. All types of eel habitats are present in the western South Pacific (WSP), so it is a unique region for studying long-lived leptocephali, especially because the westward flowing South Equatorial Current (SEC) and several countercurrents pass through many different WSP island groups and deep waters where both anguillid and marine eels live and spawn. Large mouth-opening IKMT sampling surveys for leptocephali were conducted in the southwest Pacific extending to French Polynesia in Jan-Mar 2013 (99 tows, 78 stations, 1052 larvae) and Jul-Sept 2016 (187 tows, 111 stations, 3976 larvae) that collected about 152 species of 18 anguilliform and elopomorph families. The larvae of mesopelagic serrivomerid eels were the most abundant taxa in all oceanic areas, and they were particularly abundant at northern SEC or equatorial latitudes. Australian and New Zealand anguillid eels had spawned in the western SEC areas, as previously detected, and the larvae of tropical anguillids were also only caught in western areas. The larvae of the mesopelagic nemichthyid and derichthyid eels were also widely distributed at lower abundances and with more patchy distributions, but larvae of Eurypharyngidae, Cyematidae, and Monognathidae were rare. Shallow-water eel larvae were most abundant west of New Caledonia near the banks of the Chesterfield Islands, or near other island-groups, but they were rare in the 2 easternmost 2016 transects passing by both sides of Tahiti. Some conger eels were suggested to have spawned in offshore areas in the western region. Congrid Ariosoma and various shallow-water or slope eels had spawned in the region near the Chesterfield Islands or near New Caledonia where current jets can transport larvae westward, and eastward countercurrents exist. Some taxa of larvae of coastal species (muraenesocids, and elopomorphs) were extremely rare, all non-mesopelagic eel larvae were rare in the far-eastern transects, but the New Caledonia region with large shelf areas appears to be a high biodiversity region for marine eels, as it is for reef/shore fish in general.
Malgré l’intérêt croissant pour la cartographie des fonds marins, seul un quart d’entre eux a pu être levé fidèlement à une résolution de l’ordre du km (30 arc-secondes). Ceci s’explique par les ...lourds coûts engendrés par les campagnes par bateau (sonar) et/ou par avion (lidar). Ainsi, la bathymétrie dérivée de satellite connaît un essor considérable depuis deux décennies. En fer de lance, l’imagerie multispectrale à très haute résolution spatiale de Pléiades Neo dispose de 6 bandes (4 visibles, 1 red edge, et 1 infrarouge) pourvues d’une résolution spatiale de 1,2 m, surclassant ainsi l’imagerie multispectrale de Pléiades-1 dotée de 4 bandes (3 visibles et 1 infrarouge) à 2 m. En s’appuyant sur un jeu de données Pléiades Neo 4 acquis au-dessus des eaux modérément turbides de la baie de Saint-Malo, ce travail a permis de quantifier les contributions des bandes deep blue et red edge à la prédiction de la bathymétrie lidar en regard de la nature de la modélisation, et de l’architecture du réseau neuronal. Premièrement, la modélisation semi-analytique (transfert radiatif) basée sur les transformées de ratio, testées individuellement et linéairement: le remplacement de la bande bleu par la bande deep blue a diminué la régression linéaire standard (bleu-vert-rouge-infrarouge, R2=0,36) de 11,1%, mais le remplacement de la bande rouge par red edge, a produit le même score. Deuxièmement, la modélisation semi-analytique basée sur la transformée de ratio Pléiades-1 versus la combinaison des 15 transformées Pléiades Neo : augmentations respectives de 52,8% de la régression linéaire (R2Pléiades-1=0,36, et R2Pléiades Neo=0,55), et de 36,2% de la régression non-linéaire par réseau de neurones à deux couches cachées à trois neurones (R2Pléiades-1=0,58, et R2Pléiades Neo=0,79). Finalement, la modélisation empirique basée sur la bonification spectrale de la simulation de Pléiades-1 par deep blue et red edge a généré un gain maximal de 6% pour la régression linéaire (R2Pléiades-1=0,50, et R2Pléiades Neo=0,53) et de 1,4% pour la régression non-linéaire par réseau de neurones à deux couches cachées à trois neurones (R2Pléiades-1=0,73, et R2Pléiades Neo=0,74).
•The spawning areas anguillid eel species in the South Pacific are mostly unknown.•An extensive 2016 net sampling/CTD survey was conducted including five long transects.•Small larvae of three species ...were collected at similar latitudes of the SEC in the west.•Spawning appeared to occur at three hydrographically similar locations.•Multiple spawning locations would facilitate recruitment to the numerous island areas.
Seven South Pacific anguillid eel species live from New Guinea to French Polynesia, but their spawning areas and life histories are mostly unknown despite previous sampling surveys. A July–October 2016 research cruise was conducted to study the spawning areas and times, and larval distributions of South Pacific anguillid eels, which included a short 155°E station-line northeast of New Guinea and five long transects (5–25°S, 160°E–140°W) crossing the South Equatorial (SEC) and other currents. This survey collected nearly 4000 anguilliform leptocephali at 179 stations using an Isaacs-Kidd Midwater Trawl accompanied by 104 CTD casts. Based on morphometric observations and DNA sequencing, 74 anguillid leptocephali were collected, which in the southern areas included 29 larvae of six species: Anguilla bicolor pacifica, A. marmorata, A. australis, A. reinhardtii, A. megastoma,and A. obscura (all anguillid species of the region were caught except A. dieffenbachii). Small A. australis (9.0–16.8 mm) and A. reinhardtii (12.4, 12.5 mm) leptocephali were collected south of the Solomon Islands, other A. australis (10.8–12.0 mm) larvae were caught northwest of Fiji along with an A. obscura (20.0 mm) larva, and an A. marmorata (7.8 mm) larva was collected near Samoa. Considering collection sites, larval ages from otolith analysis, and westward SEC drift, multiple spawning locations occurred from south of the Solomon Islands and the Fiji area (16–20 days old larvae) to near Samoa (19 days old larva) during June and July in areas where high-salinity Subtropical Underwater (STUW, ~150 m depth) and the warm, low-salinity surface Fresh Pool were present. Five long hydrographic sections showed the strong Fresh Pool in the west and the STUW formation area in the east.
•Western South Pacific microbial biomass/function and stable isotopes were examined.•Primary producers, whatever the depth, were N-P co-limited.•All zones were based on a heterotrophic planktonic ...functioning.•Stable isotope signatures of leptocephali, POM and other components of the food web were analyzed.•POM signatures varied with depth and latitude, influencing all taxonomic groups.•The presence of Trichodesmium may contribute to low southern δ15N values.
The ecology of leptocephali remains poorly known but they appear to feed on marine snow that can vary spatially and temporally according to the food web dynamics. This study provided new information about the position of leptocephali within the functional structure of microbial plankton and other food web components of the western South Pacific (WSP) region at a large geographic scale including the New Caledonia, Fiji, and Samoa islands. The hydrographic structure varied with latitude, and nutrient levels were generally low but somewhat variable. Stable isotopic signatures were examined in relation to the 3 current zones of the eastward flowing South Equatorial Countercurrent (SECC; north), the westward South Equatorial Current (SEC; mid-latitudes), and the eastward South Tropical Countercurrent (STCC; south), and all zones were found to be primarily based on a heterotrophic planktonic functioning that were co-limited by inorganic nitrogen and phosphorus, with biomasses of planktonic groups varying with depth. Isotopic signatures of leptocephali were compared to the signatures of other mesozoplankton, micronekton, and Trichodesmium components of the food web, and in relation to the signatures of particulate organic matter (POM) that varied among the 3 collection depths. The isotopic signatures of six taxa of leptocephali, other taxonomic groups and POM showed interesting variability according to latitude and among some stations. The presence of Trichodesmium at the surface in the STCC zone influenced the isotopic signatures of POM and thus the signatures of leptocephali. The signatures of leptocephali were therefore linked with the overall food web and were consistent with the larvae feeding on marine snow components of POM. The two apparent groups of leptocephali with different isotopic signatures that have also been observed in other oceanic areas may be explained by feeding behavior at different depths or on different types of marine snow.
The unique semi-enclosed Indian Ocean basin includes large Mascarene Plateau banks, offshore coral-reef islands, seasonal equatorial current jets, and cross-basin westward South Equatorial Current ...(SEC) flow, making it interesting for studying long larval-duration eel larvae (leptocephali) and regional eel biodiversity. Three surveys for leptocephali (in 2003, 2006, 2010) included sampling west of the Mascarene Plateau (west), a major survey and other stations off Sumatra and Java (east), and 2 cross-basin transects across the SEC. The highest numbers of leptocephali species were observed along Sumatra (2003: ~143 species; 2006: 72 species) and south of Java (2010: 69), with intermediate numbers being collected in the western Indian Ocean (2006: 71; 2010: 53) compared to low numbers in the hydrographically variable offshore zones (2006, 2010: 3–27). The larger continental shelf areas along Sumatra including the Mentawai Islands provide more coral reef and other habitats for species such as congrid, muraenid, ophichthid, and chlopsid eels compared to the Mascarene Plateau banks. Some larvae in these areas get transported offshore, but the majority of offshore larvae were of Nemichthyidae and Serrivomeridae mesopelagic eels that were spawning across the basin. Habitat differences between the southern Mascarene Plateau and Sumatra and southern Indonesia along the edge of the high biodiversity Coral Triangle likely explain the higher biodiversity of eel larvae observed along the western side of the basin, which for the Congridae and Ophichthidae included more species than observed previously within the central Indonesian Seas. In addition to local spawning, seasonal currents likely transport larger larvae towards Sumatra from the north or west and larvae may enter the basin from the Indonesian Throughflow in the east, but it is unknown if equatorial jets or the SEC can transport larvae across the whole basin.
The American eel (Anguilla rostrata) has long been regarded as a panmictic fish and has been confirmed as such in the northern part of its range. In this paper, we tested for the first time whether ...panmixia extends to the tropical range of the species. To do so, we first assembled a reference genome (975 Mbp, 19 chromosomes) combining long (PacBio and Nanopore and short (Illumina paired‐end) reads technologies to support both this study and future research. To test for population structure, we estimated genotype likelihoods from low‐coverage whole‐genome sequencing of 460 American eels, collected at 21 sampling sites (in seven geographic regions) ranging from Canada to Trinidad and Tobago. We estimated genetic distance between regions, performed ADMIXTURE‐like clustering analysis and multivariate analysis, and found no evidence of population structure, thus confirming that panmixia extends to the tropical range of the species. In addition, two genomic regions with putative inversions were observed, both geographically widespread and present at similar frequencies in all regions. We discuss the implications of lack of genetic population structure for the species. Our results are key for the future genomic research in the American eel and the implementation of conservation measures throughout its geographic range. Additionally, our results can be applied to fisheries management and aquaculture of the species.
Connectivity in aquatic systems is often related to abundance and permeability of physical barriers, such as dams, which delay or impede movements of biota with important consequences for aquatic ...biodiversity. Water quality may, however, also control connectivity between essential habitats. In macrotidal estuaries, Estuarine Turbidity Maxima (ETM) have a strong impact on water quality because of the low oxygen concentration occurring as a response to the related high bacterial and low photosynthetic activities. In this study, we assess Allis shad estuarine spawning migration in 2011 and 2012 in the Loire River (France) where the ETM occurs at spring and summer. Using an acoustic telemetry array, we show that trans-estuarine migration is inhibited during hypoxic episodes in the middle part of the estuary. Shad tends to stay in downstream areas, and even at sea, where oxygen conditions are more suitable. Trans-estuarine migration occurs hastily during neap tide when the ETM decreases, both in terms of spatial extent and intensity, inducing a shift in a set of covariates including dissolved oxygen, which increases, and suspended matter, which decreases. In the context of climate warming, ETM are expected to increase with probable adverse implications for shad migration success and doubtless other diadromous populations.
European eel is thought to be a symbol of the effects of global change on aquatic biodiversity. The species has persisted for millions of years and faced drastic environmental fluctuations thanks to ...its phenotypic plasticity. However, the species has recently declined to historically low levels under synergistic human pressures. Sublethal chemical contamination has been shown to alter reproductive capacity, but the impacts and required actions are not fully addressed by conservation plans. This paper proposes a modelling approach to quantify the effects of sublethal contamination by anthropogenic pollutants on the expression of life history traits and related fitness of the critically endangered European eel. Material and Methods: We sampled female silver eels from eight different catchments across Europe previously shown to be representative of the spectrum of environmental variability and contamination. We measured 11 fitness-related life history traits within four main categories: fecundity, adaptability and plasticity, migratory readiness, and spawning potential. We used machine learning in models to explore the phenotypic reaction (expression of these life history traits) according to geographical parameters, parasite burdens (the introduced nematode Anguillicoloides crassus) and anthropogenic contaminants (persistent organic pollutants (POPs) in muscular tissue and trace elements (TEs) in gonads, livers and muscles). Finally, we simulated, the effects of two management scenarios—contamination reduction and contamination increase—on the fecundity and recruitment. Results: Contamination in our sampling was shown to have a stronger control on life history traits than do geographic and environmental factors that are currently described in the literature. We modelled different contamination scenarios to assess the benefit of mitigation: these scenarios suggest that reducing pollutants concentrations to the lowest values that occurred in our sampling design would double the fecundity of eels compared to the current situation. Discussion: Remediation of contamination could represent a viable management option for increasing the resilience of eel populations, with much more effects than solely reducing fishing mortality. More broadly, our work provides an innovative way for quantitative assessment of the reaction norms of species’ biological traits and related fecundity to contamination by organic and inorganic pollutions thus opening new management and conservation pathways to revert the erosion of biodiversity.
Because of their unusual life cycle, American and European eels (Anguilla rostrata Lesueur and A. anguilla L.), are particularly exposed to pollutant effects. Because silver eels fast when they leave ...the freshwater system, the transoceanic migration forces them to constitute energy reserves in the form of muscle lipids, that are needed for successful spawning. Using species biological data, toxicological and ecotoxicological information, hypotheses are given to assess the contribution of pollution from freshwater sources to the recorded decline in the American and European eels fisheries since the 1980s. This paper first describes the lipid storage problems and the relative migratory capacities. Then several studies on the accumulation of xenobiotics in various anatomical compartments, on the biological half-lives of these compounds, and on their sublethal toxicity, are reviewed. During migration, lipid mobilization returns persistent lipophilic pollutants back into circulation, these being concentrated particularly in gonads at the crucial time of gametogenesis. Extrapolation of toxicological analysis (individual physiology) to the population level (spawning success) suggests that the quality of future spawners leaving freshwaters is one of the prime factors for the conservation of this threatened species.
•Stable isotope study on leptocephali (12 taxa) from near the Mascarene Plateau.•Other organisms and POM also isotopically analyzed, with FlowCam observations of POM.•Isotopic signatures revealed two ...groups of leptocephali separated by maximum larval size.•Signatures only consistent with leptocephali feeding on POM.•Flow over Mascarene Plateau may influence variability of POM composition.
Leptocephali are the poorly-understood transparent larvae of elopomorph fishes that live in the ocean surface layer throughout the world’s tropical and subtropical oceans. Their feeding ecology has been difficult to understand because they appear to primarily feed on particulate organic material (POM), which contains few identifiable objects, and there have been few studies on their diets or trophic positions. This study presents the first comparative results on the stable isotope ratios of 7 families of leptocephali in relation to the ratios of 30 taxa of other marine animals and POM samples. The carbon and nitrogen stable isotope ratios were analyzed using 50 specimens of leptocephali, 354 specimens of mesozooplankton, cephalopods, fishes, and POM collected west of the Mascarene Plateau in the western Indian Ocean. Nitrogen and carbon isotopic ratio analyses indicated that the 12 taxa of DNA barcoded leptocephali (⩾15 species) could be separated into 2 groups of species with either higher (Group 1: 9 taxa of 7 families, 25–91mm) or lower (Group 2: 3 taxa of 2 families, 43–275mm) δ15N ranges. Group 2 exclusively included species that reach much larger sizes of >150–200mm (Nemichthys and Avocettina, 3 species of Ariosoma-type), whereas Group 1 included Anguilla bicolor bicolor, Serrivomeridae, Muraenidae, Congridae (3 species), Chlopsidae, Ophichthidae (2 species), and Thalassenchelys. Differences in feeding depths, the types of POM ingested by preference or because of different jaw morphology, or the transport of larvae from other regions with different isotopic signatures are possible reasons for the differences between the two groups. The isotopic signatures of 14 taxa of copepods had higher but slightly overlapping δ15N and δ13C signatures compared to leptocephali and most crustaceans and other mezozooplankton, cephalopods and mesopelagic fish taxa had even higher values. The δ15N and δ13C signatures and composition of POM were variable spatially and with depth and may have been influenced by particulates originating from the shallow banks of the Mascarene Plateau. The two apparent isotopic groups of leptocephali should be examined in relation to their consumption of POM, which can include various proportions of prokaryotes, phytoplankton, protozoans, discarded appendicularian houses and other materials, by conducting further studies in different regions and using a variety of techniques.
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