Effects of pre‐treatments on bulk stable isotope ratios in fish samples : a cautionary note for studies comparisons

RATIONALE : Stable isotope analysis (SIA) has revolutionised ecological studies over the past thirty years. One of the major fields where SIA is applied in the marine environment is related to the definition of ecosystem structure and function. With marine top predators such as sharks, SIA is a method of choice because tissue samples can be collected without the sacrifice of the animal. In elasmobranch research, the influence of compounds such as urea, trimethylamine oxide and lipids must be considered when using stable isotopes as ecological markers. Currently, a range of pre‐treatments are used to chemically remove these molecules prior to SIA. METHODS : This study investigated the impact of eleven commonly used pre‐treatments on carbon and nitrogen contents and C:N atomic ratio, as well as carbon and nitrogen SI ratios in elasmobranch tissues and its prey, measured by isotope ratio mass spectrometry. Three tissues were tested: blood and muscle of the ragged‐tooth shark Carcharias taurus, and muscle of one teleost species, the Cape knifejaw Oplegnathus conwayi. RESULTS : Compared with untreated samples, no trend or generalisation could be highlighted with the influence of pre‐treatments being species‐, tissue‐ and chemical‐element‐dependent. For the δ13C and δ15N values, differences among pre‐treatments were as high as 3‰, therefore potentially leading to erroneous ecological interpretation. CONCLUSIONS : The chemical properties of compounds (e.g. urea, lipids) combined with the polarity of solutions (e.g. water, solvents) explained a large part of these observations. This study highlights that pre‐treatments need to be considered especially when comparing carbon and nitrogen stable isotope ratios between studies. The results of this study provide a call to all stable isotope researchers to make a concerted effort to standardise pre‐treatment methods. This is crucial as global reviews are becoming increasingly more informative.Supplementary material: Table S1. Post‐hoc pairwise comparisons after ANOVAs for repeated analyses conducted on ragged‐tooth shark muscle samples. Significant results are highlighted. Table S2. Post‐hoc pairwise comparisons after ANOVAs for repeated analyses conducted on Cape knifejaw muscle samples. Significant results are highlighted. Table S3. Post‐hoc pairwise comparisons after ANOVAs for repeated analyses conducted on ragged‐tooth shark blood samples. Significant results are highlighted.Field work was funded by the African Coelacanth Ecosystem Programme of the National Research Foundation (Grant No. 81879).http://wileyonlinelibrary.com/journal/rcm2020-02-15hj2019Mammal Research Institut

Geographic variation in the trophic ecology of an avian rocky shore predator, the African black oystercatcher, along the southern African coastline

International audienceThe reflection of baseline isotopic signals along marine food chains up to higher trophic levels has been widely used in the study of oceanic top predators but rarely for intertidal predators. We investigated variation in the δ13C and δ15N ratios of a sedentary, rocky shore predator, the African black oystercatcher Haematopus moquini, over ~2000 km of the southern African coastline, which is characterized by strong biogeographic patterns in primary productivity and intertidal communities. Blood and feathers from breeding adults and chicks and muscle tissues from primary prey items (mussels and limpets) were sampled between southern Namibia and the southeast coast of South Africa. 15N enrichment was observed between the southeast and west coasts in oystercatcher tissues and their prey, mirroring an isotope shift between the oligotrophic Agulhas Current on the east coast and the eutrophic Benguela upwelling system on the west coast. Oystercatcher blood showed δ13C values that varied between those of the carbon-depleted mussels and the carbon-enriched limpets along the coastline, which reflected changes in the proportion of grazers and filter feeders in the oystercatcher diet across the sampling range. The geographic shift in diet, dominated by mussels on the west coast and composed of mixed proportions of mussels and limpets on the southeast coasts, strongly reflected regionally high abundances of the invasive Mediterranean mussel Mytilus galloprovincialis. Finally, isotope signatures of blood and feathers displayed a strong correlation throughout the study area, indicating seasonal stability in environmental conditions and feeding habits of the adults. There were, however, local discrepancies on the south coast that indicated movement of adults occurred outside the breeding season possibly in response to a lower abundance of food in this region. Overall, the results indicate that the influence of regional oceanic conditions on the base of the food web can penetrate to the predator level, but that local effects can be incorporated within this pattern

Aquatic birds have middle ears adapted to amphibious lifestyles

Birds exhibit wide variation in their use of aquatic environments, on a spectrum from entirely terrestrial, through amphibious, to highly aquatic. Although there are limited empirical data on hearing sensitivity of birds underwater, mounting evidence indicates that diving birds detect and respond to sound underwater, suggesting that some modifications of the ear may assist foraging or other behaviors below the surface. In air, the tympanic middle ear acts as an impedance matcher that increases sound pressure and decreases sound vibration velocity between the outside air and the inner ear. Underwater, the impedance-matching task is reversed and the ear is exposed to high hydrostatic pressures. Using micro- and nano-CT (computerized tomography) scans of bird ears in 127 species across 26 taxonomic orders, we measured a suite of morphological traits of importance to aerial and aquatic hearing to test predictions relating to impedance-matching in birds with distinct aquatic lifestyles, while accounting for allometry and phylogeny. Birds that engage in underwater pursuit and deep diving showed the greatest differences in ear structure relative to terrestrial species. In these heavily modified ears, the size of the input areas of both the tympanic membrane and the columella footplate of the middle ear were reduced. Underwater pursuit and diving birds also typically had a shorter extrastapedius, a reduced cranial air volume and connectivity and several modifications in line with reversals of low-to-high impedance-matching. The results confirm adaptations of the middle ear to aquatic lifestyles in multiple independent bird lineages, likely facilitating hearing underwater and baroprotection, while potentially constraining the sensitivity of aerial hearing

Biomarqueurs lipidiques, réseau trophique pélagique et écologie alimentaire des oiseaux de mer Procellariiformes

PARIS-BIUSJ-Thèses (751052125) / SudocBANYULS/MER-Observ.Océanol. (660162201) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Lipids from stomach oil of procellariiform seabirds document the importance of myctophid fish in the Southern Ocean

International audienceWe investigated the relative importance of myctophid fish and Antarctic krill in the diet of adult flying seabirds of the Southern Ocean. The main prey of short-tailed shearwaters Puffinus tenuirostris (P. ten.), white-chinned petrels Procellaria aequinoctialis (P. aeq.), blue petrels Halobaena caerulea (H. cae.), thin-billed prions Pachyptila belcheri (P. bel.), and Antarctic prions Pachyptila desolata (P. des.) were mostly deduced from the lipid analysis of adult stomach oils. More than 97% of the 125 analyzed oils mainly consisted of wax esters (WEs) and triacylglycerols (TAGs) (.70% of total lipids). WE fatty alcohol (FAlc), WE fatty acid (FA), and TAG-FA profiles clearly segregated P. aeq. from P. ten., with smaller, but still significant, differences among the three other petrel species. P. aeq. and P. ten. therefore preyed on distinct prey species, whereas H. cae., P. bel., and P. des. had a more similar diet, but still with some prey differences. Comparisons between FAlc and FA patterns of oils with those of potential prey species showed that .93% of FAlc and FA patterns of oil WEs had a high probability of resemblance with the myctophid signatures, and similar results were obtained with the TAG fractions. Almost no stomach oil fit the lipid patterns of subantarctic and Antarctic euphausiids, including those of the WE-rich Thysanoessa macrura and the TAG-rich Antarctic krill Euphausia superba. This study thus demonstrates for the first time the importance of myctophids in the nutrition of adult flying seabirds breeding in subantarctic islands and foraging in Antarctic waters during the austral summer

Roseate Terns Sterna dougallii on the southeast coast of South Africa: information on moult and migratory status

Determining the migratory status of seabird populations is crucial for addressing conservation concerns. The Roseate Tern Sterna dougallii is considered endangered in South Africa in view of its small breeding population and the threats to eggs and chicks. Earlier works based on capture-recapture data suggest that this population adopts a partial migratory strategy. Updated capture-recapture data were combined with new data on moult and stable isotopes of scapular feathers to determine whether subpopulations could be identified in the South African breeding population. Moult data on 404 individuals failed to identify subpopulations, but two groups were identifiable with the stable isotopes: one group of four individuals exhibiting low carbon and nitrogen stable isotope ratios and a second group (comprising the remaining 152 individuals) with significantly higher ratios. The isotope data suggest that birds from the two groups moulted in different areas. Comparisons with published studies showed that the 152 individuals had likely moulted their scapulars in South African waters, but the moulting area for the remaining four individuals could not be ascertained due to the absence of detailed isoscapes in the southern part of the Mozambique Channel. Further work investigating the genetic differentiation between South African and Madagascan Roseate Terns may bring new insight into the migratory behaviour of the South African population. Sternes de Dougall Sterna dougallii sur la côte sud-est de l’Afrique du Sud: informations sur la mue et le statut de migrationDéterminer le statut de migration des populations d’oiseaux marins est crucial pour les problèmes de conservation. La Sterne de Dougall Sterna dougallii figure sur la liste des espèces en voie de disparition en Afrique du Sud, compte tenu de sa petite population reproductrice et des menaces pesant sur ses pontes et ses poussins. Des travaux antérieurs basés sur des données de capture-recapture suggèrent que cette population adopte une stratégie de migration partielle. Des données de capture-recapture actualisées ont été combinées avec des données d’isotopes stables et de mue pour déterminer si des sous-populations pouvaient être identifiées dans la population reproductrice sud-africaine. Les données de mue recueillies sur 404 individus n’ont pas permis d’identifier de sous-populations, mais deux groupes étaient identifiables avec les isotopes stables: un groupe de quatre individus présentant des ratios d’isotopes stables à faible teneur en carbone et en azote et un second groupe comprenant les 152 individus présentant des ratios beaucoup plus élevés. Les données isotopiques suggèrent que les oiseaux des deux groupes ont mué dans des zones différentes. Une comparaison avec de précédentes études publiées a montré que les 152 individus avaient probablement mué leurs scapulaires dans les eaux sud-africaines. La zone de mue des quatre individus restants n’a pas pu être déterminée en raison de l’absence d’empreintes isotopiques «isoscapes» détaillées dans le sud du chenal du Mozambique. Des travaux ultérieurs, notamment la différenciation génétique entre les sternes de Dougall sud-africaines et malgaches, pourraient apporter de nouvelles informations sur le comportement migratoire de la population sud-africaine. Keywords: Algoa Bay, coastal seabird, feathers, plumage characteristics, ringing data, stable isotopes

Seabird year-round and historical feeding ecology: blood and feather δ13C and δ15N values document foraging plasticity of small sympatric petrels

International audienceThe foraging ecology of small seabirds remains poorly understood because of the difficulty of studying them at sea. Here, the extent to which 3 sympatric seabirds (blue petrel, thinbilled prion and common diving petrel) alter their foraging ecology across the annual cycle was investigated using stable isotopes. δ13C and δ15N values were used as proxies of the birds' foraging habitat and diet, respectively, and were measured in 3 tissues (plasma, blood cells and feathers) that record trophic information at different time scales. Long-term temporal changes were investigated by measuring feather isotopic values from museum specimens. The study was conducted at the subantarctic Kerguelen Islands and emphasizes 4 main features. (1) The 3 species highlight a strong connection between subantarctic and Antarctic pelagic ecosystems, because they all foraged in Antarctic waters at some stages of the annual cycle. (2) Foraging niches are stagedependent, with petrels shifting their feeding grounds during reproduction either from oceanic to productive coastal waters (common diving petrel) or from subantarctic to high-Antarctic waters where they fed primarily on crustaceans (blue petrel and thin-billed prion). (3) The common diving petrel segregated from the surface-feeders blue petrel and thin-billed prion by a coastal habitat and lower trophic level prey, while the blue petrel segregated from the thin-billed prion by foraging further south and including more fish in its diet. (4) Feather δ13C and δ15N values from historical and recent specimens of thin-billed prion depicted a pronounced temporal shift to higher latitudes in its main moulting ground, where it feeds on higher trophic level prey. The study contributes to growing evidence that seabirds exhibit considerable foraging plasticity and sheds new light on their flexibility at different time scales (from intra-seasonal to decadal)

Trophic relationships of white-chinned petrels from Crozet Islands: combined stomach oil and conventional dietary analyses

International audienceThe diet of white-chinned petrels Procellaria aequinoctialis breeding at the Crozet Archipelago (southern Indian Ocean) was studied using two complementary methods: lipid analysis of stomach oils as trophic markers together with the conventional dietary approach (i.e., stomach content analysis). Objectives were (1) to investigate the adult diet when they feed for themselves by analyzing stomach oil lipids, and (2) to compare the lipid signature of chick and adult oils. Stomach oils mainly consisted of triacylglycerols (TAG), diacylglycerol-ethers (DAGE) and wax esters (WE) (66, 14 and 11%, respectively). The dietary origin of TAG and WE was evaluated by linear discriminant analyses with fatty acid and fatty alcohol fractions. Analyses evidenced that stomach oils did not originate from Antarctic krill, but instead from myctophid Wsh, thus demonstrating the importance of mesopelagic Wsh in the nutrition of adult petrels. This result was consistent with the identiWcation of digested remains of myctophids recovered from adult stomach contents after long foraging trips. Large amounts of a rare lipid class, DAGE (up to 76% of total lipids), were identiWed in two stomach oils, together with fresh remains of the squid Gonatus antarcticus (99% by mass), suggesting that DAGE could have the potential to be trophic markers of cephalopods. Moreover, six oils probably originated from Patagonian toothWsh, thus conWrming strong interactions between white-chinned petrels and Wsheries. Comparison between chick and adult stomach oils indicated no major diVerences in their biochemical composition suggesting an identical dietary origin of oils, mainly myctophids. Both adult and chick oils can therefore be used to determine the feeding ecology of adult birds when they feed far away from their breeding grounds. Finally, food analysis of chick samples and adult samples collected after short and long trips indicated diVerent foraging grounds during the two kinds of trips, and also between long trips performed in subtropical and Antarctic waters

Lipid composition of stomach oil in a procellariiform seabird Puffinus tenuirostris: implications for food web studies

International audienceSome procellariiform seabirds use a dual strategy for provisioning their chicks by alternating between short and long foraging trips (LT). Trophic relationships of adult birds are unknown when they feed for themselves during LT because digestion processes preclude direct prey determination. Since stomach contents collected after LT contain oil of dietary origin, we tested the use of oil lipids as prey trophic markers using the Tasmanian short-tailed shearwater Puffinus tenuirostris as a model seabird. The intra-specific variability of stomach oils was investigated through lipid class composition, and their fatty acid and fatty alcohol profiles. Oils mainly consisted of wax esters (WE) and triacylglycerols (TAG) (49 to 86 and 7 to 41%, respectively). Major fatty acids of TAG were in a decreasing order 18:1n-9, 16:0, 16:1n-7, 14:0, 20:5n-3 and 22:6n-3. The WE-fatty acid profiles were dominated by 18:1n-9 and 16:1n-7 while fatty alcohol profiles were dominated by 16:0. Fatty alcohol and fatty acid patterns were tested as possible descriptors of ingested prey (derived from literature data) through multivariate discriminant analyses. Comparisons of the WE fatty alcohol patterns showed a close association with the alcohol structure of 3 myctophid fish species namely Krefftichthys anderssoni, Gymnoscopelus braueri and Electrona antarctica; these results were corroborated by WE fatty acid analysis. Comparison of TAG fatty acid patterns showed the highest similarity between oils and the digestive gland of the myctophid-eater squid Moroteuthis ingens in association with the myctophid Electrona carlsbergi. Hence, biochemical analysis of both WE and TAG strongly suggested that adult short-tailed shearwaters mainly prey upon Antarctic/sub- Antarctic myctophids when they feed for themselves, thus emphasizing the role of these oceanic mesopelagic fish in the marine ecosystem of the Southern Ocean