Isotopic niche variability in macroconsumers of the East Scotia Ridge (Southern Ocean) hydrothermal vents: What more can we learn from an ellipse?

Aspects of between-individual trophic niche width can be explored through the isotopic niche concept. In many cases isotopic variability can be influenced by the scale of sampling and biological characteristics including body size or sex. Sample size-corrected (SEAc) and Bayesian (SEAb) standard ellipse areas and generalised least squares (GLS) models were used to explore the spatial variability of δ13C and δ15N in Kiwa tyleri (decapod), Gigantopelta chessoia (peltospirid gastropod) and Vulcanolepas scotiaensis (stalked barnacle) collected from 3 hydrothermal vent field sites (E2, E9N and E9S) on the East Scotia Ridge (ESR), Southern Ocean. SEAb only revealed spatial differences in isotopic niche area in male K. tyleri. However, the parameters used to draw the SEAc, eccentricity (E) and angle of the major SEAc axis to the x-axis (θ), indicated spatial differences in the relationships between δ13C and δ15N in all 3 species. The GLS models indicated that there were spatial differences in isotope-length trends, which were related to E and θ of the SEAc. This indicated that E and θ were potentially driven by underlying trophic and biological processes that varied with body size. Examination of the isotopic niches using standard ellipse areas and their parameters in conjunction with length-based analyses provided a means by which a proportion of the isotopic variability within each species could be described. We suggest that the parameters E and θ offer additional ecological insight that has so far been overlooked in isotopic niche studies

Assessment of fish trophic status and relationships by stable isotope data in the coral reef lagoon of New Caledonia, southwest Pacific

This study examines the trophic status and relationships of coral reef fish in the lagoon of New Caledonia, southwest pacific. The feeding habits of 34 fish species collected at three contrasted sites were first described using a compilation of gut contents observations and data from the literature. The carbon and nitrogen isotope signatures of these fish and of some of their potential ultimate food sources were also determined at each site. Despite some spatial variations in the isotopic signatures of most food sources and fish trophic groups, the overall trophic structures of fish assemblages were similar at the three sites. Stable isotope data were then used to re-assign fish species to trophic groups based on the δ15N signatures of fish and their food sources. Herbivorous fish species were clearly distinguished from the other trophic groups by their lower δ15N signatures, consistent with an estimated trophic position of ~2 for all species examined. Scaridae were however characterized by relatively higher δ13C and lower δ15N, which is probably linked with the role of detritus in their diet. The estimated trophic positions of planktivorous fish species were consistent with their gut contents-based classification. Conversely, the isotopic signatures of carnivorous and piscivorous fish species largely overlapped, and their estimated trophic positions were much lower than expected. This suggests that these species feed over a broader range of trophic levels and food sources than implied by the gut contents observations, and indicates that their diet is partly omnivorous. Finally, the relationships between body mass and the isotopic signatures of four fish species were significant for at least one isotopic ratio for each species. Since ontogenetic variations and omnivorous diets are difficult to assess with gut contents data only, stable isotopes revealed essential in estimating the actual trophic status and relationships characterizing the fish species under study

Lag effects in the impacts of mass coral bleaching on coral reef fish, fisheries, and ecosystems

Recent episodes of coral bleaching have led to wide-scale loss of reef corals and raised concerns over the effectiveness of existing conservation and management efforts. The 1998 bleaching event was most severe in the western Indian Ocean, where coral declined by up to 90% in some locations. Using fisheries-independent data, we assessed the long-term impacts of this event on fishery target species in the Seychelles, the overall size structure of the fish assemblage, and the effectiveness of two marine protected areas (MPAs) in protecting fish communities. The biomass of fished species above the size retained in fish traps changed little between 1994 and 2005, indicating no current effect on fishery yields. Biomass remained higher in MPAs, indicating they were effective in protecting fish stocks. Nevertheless, the size structure of the fish communities, as described with size-spectra analysis, changed in both fished areas and MPAs, with a decline in smaller fish (<30 cm) and an increase in larger fish (>45 cm). We believe this represents a time-lag response to a reduction in reef structural complexity brought about because fishes are being lost through natural mortality and fishing, and are not being replaced by juveniles. This effect is expected to be greater in terms of fisheries productivity and, because congruent patterns are observed for herbivores, suggests that MPAs do not offer coral reefs long-term resilience to bleaching events. Corallivores and planktivores declined strikingly in abundance, particularly in MPAs, and this decline was associated with a similar pattern of decline in their preferred corals. We suggest that climate-mediated disturbances, such as coral bleaching, be at the fore of conservation planning for coral reefs.\u

Can the age of the tropical species be determined by otolith measurement?: a study using Pristipomoides filamentosus (Pisces: Lutjanidae) from the Mahe Plateau, Seychelles

The sagittal otoliths of bluespot jobfish (Pristipomoides flamentosus) from the Mahe Plateau, Seychelles, were examined for growth rings using light microscopy. Banding with putative annual and monthly frequency were observed. Consistent age estimates were derived from each of the two patterns. The resulting length-at-age data were use t estimate the parameters K and t sub(0), viz: K=0.33, t sub(0) = 0.16 for males and K = 0.36, t sub(0) = 0.06 for females (using von Bertalanffy plots). Possible causes of the banding are discussed

Spatial variation in the 15N and 13C stable isotope composition of plants, invertebrates and fishes on Mediterranean reefs: Implications for the study of trophic pathways

δ15N and δ13C were determined for plants, invertebrates and fishes collected from 3 sites on the southern coast of the Mediterranean island of Mallorca, Spain. The sites were separated by distances of 1250 to 3750 m. The mean δ15N of plants was 1.1 to 4.1‰, benthic invertebrates 5.9 to 6.9‰, planktonic invertebrates 5.5 to 5.8‰ and fishes 84 to 13.8‰. δ15N became enriched with increasing trophic level. The mean δ13C of plants was -11.4 to -16.3‰, benthic invertebrates -14.8 to -16.8‰, planktonic invertebrates -19.3 to -19.8‰ and fishes -16.1 to -19.2‰. There were significant differences in the isotopic composition of individual species within the plant, invertebrate or fish groupings at each site and there were significant differences in the isotopic composition of the same species at different sites. Depleted 13C was associated with benthic food chains and enriched 13C with planktonic chains. The data suggest that benthic food chains are important to the rocky reel associated fishes studied, as might be expected in a nutrient poor system where planktonic production is relatively low. However, the variance in δ13C composition between sites was such that the relative significance of the 2 pathways could not be determined. 15N measurements indicated that some of the fish species studied had adopted different feeding strategies at different sites and, as a result, individuals of the same species could sometimes be assigned to different trophic groups at different sites. The data suggest that these fishes exhibit plasticity in their feeding strategies and this may provide them with greater adaptive flexibility to respond to site-specific changes in food availability. Moreover, the data provide empirical support for current theories of food web dynamics which suggest that trophic 'levels' are dynamic rather than fixed and that 'multichannel omnivory' is an important feature of food webs.Peer Reviewe