|Accueil||<< 1 2 3 4 5 6 7 8 9 10 >> [11–20]|
BORDIER, C., SARAUX, C., VIBLANC, V. A., GACHOT-NEVEU, H., BEAUGEY, M., LE MAHO, Y., et al. (2014). Inter-Annual Variability of Fledgling Sex Ratio in King Penguins. Plos One, 9(12), 1–17.
Résumé: As the number of breeding pairs depends on the adult sex ratio in a monogamous species with biparental care, investigating sex-ratio variability in natural populations is essential to understand population dynamics. Using 10 years of data (20002009) in a seasonally monogamous seabird, the king penguin (Aptenodytes patagonicus), we investigated the annual sex ratio at fledging, and the potential environmental causes for its variation. Over more than 4000 birds, the annual sex ratio at fledging was highly variable (ranging from 44.4% to 58.3% of males), and on average slightly biased towards males (51.6%). Yearly variation in sex-ratio bias was neither related to density within the colony, nor to global or local oceanographic conditions known to affect both the productivity and accessibility of penguin foraging areas. However, rising sea surface temperature coincided with an increase in fledging sex-ratio variability. Fledging sex ratio was also correlated with difference in body condition between male and female fledglings. When more males were produced in a given year, their body condition was higher (and reciprocally), suggesting that parents might adopt a sex-biased allocation strategy depending on yearly environmental conditions and/or that the effect of environmental parameters on chick condition and survival may be sex-dependent. The initial bias in sex ratio observed at the juvenile stage tended to return to 1:1 equilibrium upon first breeding attempts, as would be expected from Fisher's classic theory of offspring sex-ratio variation.
NIEBLAS, A. - E., DRUSHKA, K., REYGONDEAU, G., ROSSI, V., DEMARCQ, H., DUBROCA, L., et al. (2014). Defining Mediterranean and Black Sea Biogeochemical Subprovinces and Synthetic Ocean Indicators Using Mesoscale Oceanographic Features. Plos One, 9(10), 1–13.
Résumé: The Mediterranean and Black Seas are semi-enclosed basins characterized by high environmental variability and growing anthropogenic pressure. This has led to an increasing need for a bioregionalization of the oceanic environment at local and regional scales that can be used for managerial applications as a geographical reference. We aim to identify biogeochemical subprovinces within this domain, and develop synthetic indices of the key oceanographic dynamics of each subprovince to quantify baselines from which to assess variability and change. To do this, we compile a data set of 101 months (2002-2010) of a variety of both “classical'' (i.e., sea surface temperature, surface chlorophyll-a, and bathymetry) and ”mesoscale'' (i.e., eddy kinetic energy, finite-size Lyapunov exponents, and surface frontal gradients) ocean features that we use to characterize the surface ocean variability. We employ a k-means clustering algorithm to objectively define biogeochemical subprovinces based on classical features, and, for the first time, on mesoscale features, and on a combination of both classical and mesoscale features. Principal components analysis is then performed on the oceanographic variables to define integrative indices to monitor the environmental changes within each resultant subprovince at monthly resolutions. Using both the classical and mesoscale features, we find five biogeochemical subprovinces for the Mediterranean and Black Seas. Interestingly, the use of mesoscale variables contributes highly in the delineation of the open ocean. The first axis of the principal component analysis is explained primarily by classical ocean features and the second axis is explained by mesoscale features. Biogeochemical subprovinces identified by the present study can be useful within the European management framework as an objective geographical framework of the Mediterranean and Black Seas, and the synthetic ocean indicators developed here can be used to monitor variability and long-term change.
Lezama-Ochoa, A., Irigoien, X., Chaigneau, A., Quiroz, Z., Lebourges Dhaussy, A., & Bertrand, A. (2014). Acoustic reveal the presence of Macrozooplankton biocline in the Bay of Biscay in response to hydrological conditions and predator-prey relationships. PLoS One, 9(2).
Résumé: Bifrequency acoustic data, hydrological measurements and satellite data were used to study the vertical distribution of
macrozooplankton in the Bay of Biscay in relation to the hydrological conditions and fish distribution during spring 2009.
The most noticeable result was the observation of a &8216;biocline&8217; during the day i.e., the interface where zooplankton biomass
changes more rapidly with depth than it does in the layers above or below. The biocline separated the surface layer, almost
devoid of macrozooplankton, from the macrozooplankton-rich deeper layers. It is a specific vertical feature which ties in
with the classic diel vertical migration pattern. Spatiotemporal correlations between macrozooplankton and environmental
variables (photic depth, thermohaline vertical structure, stratification index and chlorophyll-a) indicate that no single factor
explains the macrozooplankton vertical distribution. Rather a set of factors, the respective influence of which varies from
region to region depending on the habitat characteristics and the progress of the spring stratification, jointly influence the
distribution. In this context, the macrozooplankton biocline is potentially a biophysical response to the search for a
particular depth range where light attenuation, thermohaline vertical structure and stratification conditions together
provide a suitable alternative to the need for expending energy in reaching deeper water without the risk of being eaten.
SARAUX, C., FROMENTIN, J. - M., BIGOT, J. - L., BOURDEIX, J. - H., MORFIN, M., ROOS, D., et al. (2014). Spatial Structure and Distribution of Small Pelagic Fish in the Northwestern Mediterranean Sea. Plos One, 9(11), 1–12.
Résumé: Understanding the ecological and anthropogenic drivers of population dynamics requires detailed studies on habitat selection and spatial distribution. Although small pelagic fish aggregate in large shoals and usually exhibit important spatial structure, their dynamics in time and space remain unpredictable and challenging. In the Gulf of Lions (north-western Mediterranean), sardine and anchovy biomasses have declined over the past 5 years causing an important fishery crisis while sprat abundance rose. Applying geostatistical tools on scientific acoustic surveys conducted in the Gulf of Lions, we investigated anchovy, sardine and sprat spatial distributions and structures over 10 years. Our results show that sardines and sprats were more coastal than anchovies. The spatial structure of the three species was fairly stable over time according to variogram outputs, while year-to-year variations in kriged maps highlighted substantial changes in their location. Support for the McCall's basin hypothesis (covariation of both population density and presence area with biomass) was found only in sprats, the most variable of the three species. An innovative method to investigate species collocation at different scales revealed that globally the three species strongly overlap. Although species often co-occurred in terms of presence/absence, their biomass density differed at local scale, suggesting potential interspecific avoidance or different sensitivity to local environmental characteristics. Persistent favourable areas were finally detected, but their environmental characteristics remain to be determined.
Travers-Trolet, M., Shin, Y. - J., Shannon, L. J., Moloney, C. L., & Field, J. G. (2014). Combined fishing and climate forcing in the southern Benguela upwelling ecosystem : an end-to-end modelling approach reveals dampened effects. Plos One, 9(4).
Résumé: The effects of climate and fishing on marine ecosystems have usually been studied separately, but their interactions make ecosystem dynamics difficult to understand and predict. Of particular interest to management, the potential synergism or antagonism between fishing pressure and climate forcing is analysed in this paper, using an end-to-end ecosystem model of the southern Benguela ecosystem, built from coupling hydrodynamic, biogeochemical and multispecies fish models (ROMS-N(2)P(2)Z(2)D(2)-OSMOSE). Scenarios of different intensities of upwelling-favourable wind stress combined with scenarios of fishing top-predator fish were tested. Analyses of isolated drivers show that the bottom-up effect of the climate forcing propagates up the food chain whereas the top-down effect of fishing cascades down to zooplankton in unfavourable environmental conditions but dampens before it reaches phytoplankton. When considering both climate and fishing drivers together, it appears that top-down control dominates the link between top-predator fish and forage fish, whereas interactions between the lower trophic levels are dominated by bottom-up control. The forage fish functional group appears to be a central component of this ecosystem, being the meeting point of two opposite trophic controls. The set of combined scenarios shows that fishing pressure and upwelling-favourable wind stress have mostly dampened effects on fish populations, compared to predictions from the separate effects of the stressors. Dampened effects result in biomass accumulation at the top predator fish level but a depletion of biomass at the forage fish level. This should draw our attention to the evolution of this functional group, which appears as both structurally important in the trophic functioning of the ecosystem, and very sensitive to climate and fishing pressures. In particular, diagnoses considering fishing pressure only might be more optimistic than those that consider combined effects of fishing and environmental variability.