Gironde River Research Articles

On the role of wind and tides in shaping the Gironde River plume (Bay of Biscay)

We study the Gironde River plume in the Bay of Biscay (North-East Atlantic) using a high resolution modelling approach of the estuary-mouth-shelf continuum. The Gironde River plume flows over a gently sloping shelf where semi-diurnal tides reach an amplitude of 1.3–1.5 m on average; the Gironde River discharge undergoes a significant seasonal variability with floods (>1000 m3/s) in winter and spring. From two years of simulation (2011–12), we document the behavior of the plume under varying forcing conditions. We then discuss the dynamical mechanisms at play, from the estimate of some mechanical energy budget terms and using sensitivity runs: run with no tides, run with filtered atmospheric forcing (no variability at time scales smaller than 30 days) and run with no river runoff. We first show that the plume's properties are determined by the processes occurring in a transition area at the estuary outlet between the Royan and Cordouan sections. The outflow passes through a deep and narrow channel both because of bathymetric constraints and tides. Indeed, tides generate a residual circulation made of anticyclonic and cyclonic vortices with diameters of 5–12 km. Tides also induce intense mixing that strongly modifies the stratification between the two sections. The mean conditions lead to classify the plume as a narrow outflow which is a favorable condition for a bulge to be formed according to the literature. We indeed observe the recurrent formation of a bulge with sharp salinity gradients and a narrow coastal current. The bulge develops in moderate to high discharge conditions, during the spring to neap tides transition and when winds are weak. When the intensity of tides and wind increases, the bulge breaks down and the plume spreads offshore under upwelling wind conditions or along the coast (both in northward and southward directions) under strong eastward wind conditions. Tides are the main driver at the estuary outlet (with a bottom friction term larger by two orders of magnitude than over the adjacent shelf) but winds and atmospheric pressure are the main forcing over the inner shelf. These results evidence the need to model accurately the whole estuary-shelf continuum when studying the fate of riverine waters over the shelf.

Non-Linear Processes in the Gironde River Plume (North-East Atlantic): Instabilities and Mixing

Instability and mixing are ubiquitous processes in river plumes but their small spatial and temporal scales often limit their observation and analysis. We investigate flow instability and mixing processes in the Gironde river plume (Bay of Biscay, North-East Atlantic ocean) in response to air-sea fluxes, tidal currents, and winds. High-resolution numerical simulations are conducted in March (average river discharge) and in August (low discharge) to explore such processes. Two areas of the Gironde river plume (the bulge and the coastal current) experience different instabilities: barotropic, baroclinic, symmetric, and/or vertical shear instabilities. Energy conversion terms reveal the coexistence of barotropic and baroclinic instabilities in the bulge and in the coastal current during both months. These instabilities are intensified over the whole domain in August and over the inner-shelf in March. The Hoskins criterion indicates that symmetric instability exists in most parts of the plume during both periods. The evolution of the Gironde plume with the summer stratification, tidal currents and winds favors its development. During both seasons, ageostrophic flow and large Rossby numbers characterize rapidly-growing and small-scale frontal baroclinic and symmetric instabilities. The transition between these instabilities is investigated with an EKE decomposition on the modes of instability. In the frontal region of the plume, during both months, symmetric instabilities grow first followed by baroclinic and mixed ones, during wind bursts and/or high discharge events. In contrast, when the wind is weak or relaxing, baroclinic instabilities grow first followed by symmetric and then mixed ones. Their growth periods range from a few hours to a few days. Mixing at the ocean surface is analyzed via Potential Vorticity (PV) fluxes. The net injection of PV at the ocean surface occurs at submesoscale buoyant fronts of the Gironde plume during both months. Vertical mixing at these fronts has similar magnitude as the wind-driven and surface buoyancy fluxes. During both months, the frontal region of the plume is restratified during wind relaxation events and/or high river discharge events through frontogenetic processes. Conversely, wind bursts destratify the frontal plume interior through non-conservative PV fluxes.

Instabilities and vertical mixing in river plumes: application to the Bay of Biscay

In the Bay of Biscay (north-east Atlantic), long-living eddies and the frontal activity that they induce substantially contribute to mesoscale and submesoscale dynamics. Tides and river plumes also contribute to frontal activity. Biological productivity is sensitive to river plume fronts and to external forcings (tides and wind). Considering the importance of river plumes, we study here the structure, stability and vertical mixing processes in such river plumes (similar to those generated by the Gironde river). Restratification budget is considered here for evaluating stirring (frontogenetic/frontolytic) or vertical mixing (parametrised here from Ertel potential vorticity mixing) processes. Using high-resolution idealised numerical simulations, we analyse the evolution of the bulge and of the coastal part of this plume and we conduct sensitivity experiments to the river discharge, to southwesterly winds and to M2 tides. The bulge and the coastal current are stable (unstable) in case of moderate (high) river discharge, due to mixed barotropic/baroclinic instabilities. In the unstable case, near surface symmetric and vertical shear instabilities develop in the coastal current and in the core of the bulge where the Rossby number is large. When southwesterly winds blow, the river plume is squeezed near the coast by Ekman transport. The river plume is then subject to frontal symmetric, baroclinic, barotropic and vertical shear instabilities in the coastal part, north of the estuary (its far field). Conversely, in the presence of M2 tides, the river plume is barotropically, baroclinically and symmetrically unstable in its near field. Interior vertical mixing is induced by advective (stirring) and frontogenetic processes. Frontogenesis is dominant in the far-field (in the presence of southwesterlies) or in the near-field (when M2 tide is active). Frontogenesis is important in the far-field region in unforced river plumes (both with moderate and high river discharges). Potential vorticity is eroded in the far-field when southwesterlies blow. This is primarily due to the frictional processes which are dominant at the surface. This study has identified the instabilities which affect a river plume in different cases, and the local turbulent processes which alter the stratification.

Early back-calculated size-at-age of Atlantic yellow eels sampled along ecological gradients in the Gironde and St. Lawrence hydrographical systems

An international sampling program investigating the causes of the decline of American eels (Anguilla rostrata) and European eels (Anguilla anguilla) in the St. Lawrence (Canada) and Gironde (France) rivers systems provided the opportunity to compare early growth of eels of each species among habitats using back-calculated size-at-age from 1 to 5 years old. Our study supports previous studies showing that the early back-calculated lengths of A. anguilla were higher in downstream and more saline Gironde system habitats than in the upstream fluvial section and provides a new indication that length at 1 year old is twice higher than that reported 10 years earlier. However, our data contradict the current paradigm for A. rostrata by providing evidence that early size-at-age of eels from the upstream St. Lawrence system, most distant from the spawning area, exceeds those of eels sampled downstream, at less distant sites in the estuarine section. Overall, these observations are consistent with the hypothesis that the observed spatial variations in early growth rate of yellow eels sampled in the St. Lawrence and Gironde systems are the consequence of processes occurring in the first year of age, such as genetic selection and (or) genetically dependent habitat choice.

Multi-temporal dynamics of suspended particulate matter in a macro-tidal river Plume (the Gironde) as observed by satellite data

The Gironde River plume area is unique in terms of Suspended Particulate Matter (SPM) dynamics. Multiple factors contribute to the variations of SPM at multiple time scales, from river outputs to wind stress, currents and tidal cycles. The formation and evolution of the Maximum Turbidity Zone (MTZ) inside the estuary also plays a significant role. Thus, detailed analyses and monitoring of the region is important for better understanding the mechanisms governing the turbid plume dynamics, for proper future management and monitoring of SPM export from the estuary to the coastal ocean. In this study we use an unprecedented volume of satellite data to capture and better understand the dynamics of the river plume. We combine two types of satellite information in order to achieve these goals: data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensors. The integrated information allows accounting for multiple time scales, i.e. from seasonal to diurnal cycles. We show and parameterize the overall effects of river discharge rates over the plume extension. Seasonal variations are also analyzed and an overall relationship between river discharge rates and plume magnitude is computed. For the first time, we clearly observe and explain the diurnal cycle of SPM dynamics in the river plume. Despite the limited capabilities of the SEVIRI sensor, geostationary data was successfully used to derive such information and results similar to in-situ datasets were obtained. The same patterns are observed, with significant increase in SPM plume during spring/ebb tide periods. Results from our study can be further used to refine sediment transport models and to gain a better perspective on the ecological implications of the sediment output in the continental shelf area.

Loire and Gironde turbid plumes: Characterization and influence on thermohaline properties

Knowledge and predictability of turbid river plumes is of great importance because they modulate the properties of the seawater adjacent to river mouths. The Loire and Gironde Rivers form the most important plumes in the Bay of Biscay, as they provide >75% of total runoff. The development of the turbid plume under the influence of its main drivers was analyzed using Moderate Resolution Imaging Spectroradiometer satellite data from the period 2003–2015. River discharge was found to be the main driver, followed by wind, which also had an important effect in modulating the turbid plume during periods of high river discharge. Seaward and upwelling favorable winds enhanced the dispersion of plumes on seawater, whereas landward and downwelling favorable winds limited mixing with the adjacent ocean water. The maximum extension of the turbid plume was reached under landward winds. In addition, the spatio-temporal evolution of the East Atlantic pattern and the North Atlantic Oscillation was observed to affect the dynamics of plumes: positive values of both indices favored a greater extension of the plume. Thermohaline properties differed inside and outside the area affected by both rivers. In particular, these rivers maintain winter stratification inside the turbid plume, which results in a different warming ratio when compared with the adjacent ocean.

Using ClustOfVar to Construct Quality of Life Indicators for Vulnerability Assessment Municipality Trajectories in Southwest France from 1999 to 2009

Climate change is increasingly accepted as a major worldwide issue, bringing with it a host of long-term consequences for both human beings and ecosystems. To more effectively limit the damage caused by natural hazards and global change, it is essential that we gain a greater understanding of the complex question of social vulnerability—a subject that has been widely discussed in the literature. This paper examines the use of a conceptual “human wellbeing” framework to analyse vulnerability. It also proposes an innovative statistical method (ClustOfVar) to capture the multidimensional nature of that vulnerability. Using our approach, it is possible to construct composite indicators of residents’ living conditions at municipality level. To test our methodology, we carried out a comprehensive evaluation of the development of residents’ quality of life for two specific years (1999 and 2009) in areas close to the Garonne and Gironde rivers in southwest France. The results reveal different municipality trajectories in terms of quality of life profiles. This study helps to understand the multivariate characteristics of communities with higher social vulnerability.

Analysis of SMOS sea surface salinity data using DINEOF

Abstract An analysis of daily Sea Surface Salinity (SSS) at 0.15 ° × 0.15° spatial resolution from the Soil Moisture and Ocean Salinity (SMOS) satellite mission using DINEOF (Data Interpolating Empirical Orthogonal Functions) is presented. DINEOF allows reconstructing missing data using a truncated EOF basis, while reducing the amount of noise and errors in geophysical datasets. This work represents a first application of DINEOF to SMOS SSS. Results show that a reduction of the error and the amount of noise is obtained in the DINEOF SSS data compared to the initial SMOS SSS data. Errors associated to the edge of the swath are detected in 2 EOFs and effectively removed from the final data, avoiding removing the data at the edges of the swath in the initial dataset. The final dataset presents a centered root mean square error of 0.2 in open waters when comparing with thermosalinograph data at their original spatial and temporal resolution. Constant biases present near land masses, large scale biases and latitudinal biases cannot be corrected with DINEOF because persistent signals are retained in high order EOFs, and therefore these need to be corrected separately. The signature of the Douro and Gironde rivers is detected in the DINEOF SSS. The minimum SSS observed in the Gironde plume corresponds to a flood event in June 2013, and the shape and size of the Douro river shows a good agreement with chlorophyll-a satellite data. These examples show the capacity of DINEOF to remove noise and provide a full SSS dataset at a high temporal and spatial resolution with reduced error, and the possibility to retrieve physical signals in zones with high initial errors.

Modulation of sea surface temperature warming in the Bay of Biscay by Loire and Gironde Rivers

AbstractThe influence of Loire and Gironde River discharges over the sea surface temperature (SST) in the eastern Bay of Biscay (0.6º–36.6ºW, 44.2º–47.8ºW) was analyzed by means of two complementary databases (MODIS and OISST1/4). The area influenced by river plume showed a different SST when compared with the adjacent oceanic area for the months when the plume attains its highest extension (December, January, and February). Ocean was observed to warm at a rate of approximately 0.3ºC dec−1 while temperature at the area influenced by the rivers cooled at a rate of −0.15ºC dec−1 over the period 1982–2014. The mere presence of a freshwater layer is able to modulate the warming observed at adjacent ocean locations since the coastal area is isolated from the rest of the Bay. This nearshore strip is the only part of the Bay where changes in SST depend on North Atlantic Oscillation (NAO) but not on North Atlantic SST represented by the Atlantic Multidecadal Oscillation (AMO). These different cooling‐warming trends are even more patent over the last years (2002–2014) under atmospheric favorable conditions for plume enhancement. River runoff increased at a rate on the order of 120 m3s−1dec−1 over that period and southwesterly winds, which favor the confinement of the plume, showed a positive and significant trend both in duration and intensity. Thus, the coastal strip has been observed to cool at a rate of −0.5°C dec−1.

Characterization of Time-Varying Regimes in Remote Sensing Time Series: Application to the Forecasting of Satellite-Derived Suspended Matter Concentrations

The spatial and temporal coverage of satellites provides data that are particularly well suited for the analysis and characterization of space–time-varying geophysical relationships. The latent-class models aim here to identify time-varying regimes within a dataset. This is of particular interest for geophysical processes driven by the seasonal variability. As a case example, we study the daily concentration of mineral suspended particulate matters estimated from satellite-derived datasets, in coastal waters adjacent to the French Gironde river mouth. We forecast this high-resolution dataset using environmental data (wave height, wind strength and direction, tides, and river outflow) and four latent-regime models: homogeneous and nonhomogeneous Markov-switching models, with and without an autoregressive term (i.e., the mineral suspended matter concentration observed the day before). Using a validation dataset, significant improvements are observed with the multiregime models compared to a classical multiregression and a state-of-the-art nonlinear model [support vector regression (SVR) model]. The best results are reported for a mixture of three regimes for the autoregressive model using nonhomogeneous transitions. With the autoregressive models, we obtain at day+1 for the mixture model forecasting performances of 93% of the explained variance, compared to 83% for a standard linear model and 85% using an SVR. These improvements are more important for the nonautoregressive models. These results stress the potential of the identification of geophysical regimes to improve the forecasting. We also show that nonhomogeneous transition probabilities and estimated autoregressive terms improve forecasting performances when observation data is lacking for short-time period of 1–15 days.

Effect of river runoff on sea level from in-situ measurements and numerical models in the Bay of Biscay

Daily time series of in-situ tide gauge records and river runoff data were analysed to investigate the contribution of river discharge storm events to sea level in the Bay of Biscay. Three main river systems were considered for this study, representing cases of small (Nervión, 1900km2), medium (Adour, 16,880km2) and large (Gironde, 84,811km2) watershed basins. Typical storms correspond to water discharge rates of 150, 700, and 1100m3s−1 from the Nervión, Adour, and Gironde rivers, respectively. The effect of these events on daily mean sea level was evaluated using two different approaches: (1) through the analysis of time series of tide gauges placed within the river mouths; and (2) through numerical simulations using the ROMS model (Regional Ocean Modeling System). The three selected tide gauges are located at Bilbao (Spain) for the Nervión river, Boucau-Bayonne (France) for the Adour river and Port-Bloc (France) for the Gironde river. River runoff extreme events were more notable at the two tide gauges located within the largest rivers, namely, Adour and Gironde, where approximately 13% and 53.6% of the pressure-adjusted sea-level variance was explained by those events. The results obtained from the ROMS simulations suggest that the main effect of river discharge storm events occurs as a response to the input of lower salinity water. This plume of overlaying less dense water would produce a sea-level increase around the river mouth and along the coast, suggesting the generation of a coastal density current through the balance between the Coriolis force and the cross-shore pressure gradient. The main area of influence of the selected river discharges was confined to the river mouth for the Nervión, but extended up to approximately 28km (33km) offshore for the Adour (Gironde) river, before turning northward to flow as a density current along the coast.

Modeling individual growth trajectories of the female European eel in relation to temperature and habitat-use history in the Gironde River, France

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 19:185-193 (2013) - DOI: https://doi.org/10.3354/ab00526 Modeling individual growth trajectories of the female European eel in relation to temperature and habitat-use history in the Gironde River, France Kazuki Yokouchi1,2,3,*, Françoise Daverat1 1IRSTEA, Unité EPBX, 50 avenue de Verdun, 33612 Cestas cedex, France 2Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8564, Japan 3Present address: Institute for East China Sea Research, Nagasaki University, 1551-7, Tairamachi, Nagasaki 851-2213, Japan *Email: kazukiy30@gmail.com ABSTRACT: Although the average growth rate of European eel is known to be generally dependent on temperature and habitat at the population scale, large variation in growth among eels in a single river basin can also be widely observed. In this study, individual growth trajectories of female European eels in the Gironde River basin, France, were modeled with habitat-use histories and thermal variations in relation to regional climatic events such as the heat wave in 2003 observed in SW Europe. A mixed-effects model was used to take into account the repeated measures of individual growth of 173 female eels in 1993 to 2003, obtained from otolith structure and chemistry. Temperature effect was tested using a proxy of efficient temperature for growth between 2 thermal thresholds in a year. The models with thresholds for growth between 12 and 15°C (lower) and 18°C (higher) were selected considering Akaike’s information criterion. Individual annual growth estimates were higher in the estuary habitat than in the river habitat. Estimated growth was higher when eels shifted habitats between freshwater and brackish water or seawater. The years 1999 and 2001 had, respectively, the lowest and highest temperature proxy value, showing that a longer growing season (moderate summer/warm winter) had a positive effect on growth. The 2003 heat wave had a negative effect on eel growth, reflected by the lower temperature proxy value. The same approach could be used in the future to estimate the effect of global climate change on the growth and distribution of eels at local and regional scales. KEY WORDS: Anguilla anguilla · Inter-annual temperature variation · Habitat-use history · Mixed-effects model Full text in pdf format PreviousNextCite this article as: Yokouchi K, Daverat F (2013) Modeling individual growth trajectories of the female European eel in relation to temperature and habitat-use history in the Gironde River, France. Aquat Biol 19:185-193. https://doi.org/10.3354/ab00526 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 19, No. 2. Online publication date: October 08, 2013 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2013 Inter-Research.

Modelling the spatio-temporal distribution of age-1 Bay of Biscay anchovy (<i>Engraulis encrasicolus</i>) at spawning time

To efficiently manage and maintain fish stock abundance and age structure, it is necessary to understand variability in its spatial distribution. The main objective of this study was to describe the demographic structure of the Bay of Biscay anchovy during the main spawning period based on adult samples from 15 daily egg production method surveys. The proportion of age-1 individuals was modelled using generalized additive models with a binomial distribution and a logit link in relation to geographical and environmental covariates. The possibility of obtaining different models depending on the average age-1 proportion (low or high) was also explored. In general, age-1 individuals were found in shallow waters close to the coast, especially associated with the Gironde and Adour river plumes, whereas older individuals were prevalent on the shelf break and in oceanic waters. However, in years with a high age-1 proportion the younger individuals were also dominant in oceanic waters. These results could be used for management purposes, such as defining protected areas for particular age groups.

Development of a German Action Plan for the restoration of the European sturgeon Acipenser sturio L. - implementing international commitments on a national scale

Summary Since the European sturgeon (Acipenser sturio) is one of the most highly endangered fish species worldwide with only one population known to still exist in the Gironde River, France, it is attempted to facilitate in-situ and ex-situ protection of the species through coordinated efforts within the former range. To increase the collaborative approach in safeguarding and restoration of the species’ population the Bern Convention has adopted an Action Plan (AP) at the Standing Committee meeting in November 2007. Based on this agreement, the member states to the Bern Convention are requested to develop national APs addressing the critical issues for its conservation. This paper discusses in detail the prerequisites for the development of such a national AP, thus focusing on the complex structures in the German Federal and Regional governance system, the stakeholder involvement required, as well as the dynamics of a process unprecedented in any fish species under this convention. For the national AP the governmental agencies for nature conservation and for fisheries, the regional and national stakeholders such as commercial and recreational fisheries, inland navigation administration, port authorities, river catchment management units, recreational water users and environmental conservation NGO’s have to be brought together, overcoming major barriers to allow a constructive dialogue leading towards a common problem analysis. As a result, the German national AP is intended to provide a common basis for the future protection attempts, outlining the main focal points for necessary activities. As such it also serves as the framework for the management plans for the river systems considered suitable for the attempts to remediate populations of the European sturgeon in German and other European Rivers in a joint effort with the French partners.

Colonisation tactics of three temperate catadromous species, eel Anguilla anguilla, mullet Liza ramada and flounder Plathychtys flesus, revealed by Bayesian multielemental otolith microchemistry approach

Daverat F, Martin J, Fablet R, Pécheyran C. Colonisation tactics of three temperate catadromous species, eel Anguilla anguilla, mullet Liza ramada and flounder Plathychtys flesus, revealed by Bayesian multielemental otolith microchemistry approach. Ecology of Freshwater Fish 2011: 20: 42–51. © 2010 John Wiley & Sons A/SAbstract – The colonisation of Gironde (SW France) river catchment by juvenile, eel, Anguilla anguilla, flounder Platychtys flesus and thinlipp mullet Liza ramada was investigated comparatively using Sr:Ca and Ba:Ca composition of otolith. The relation between Sr, Ba and habitat was investigated based on Sr and Ba water composition sampled each month along the estuary‐river gradient. A total of 50 mullets, 30 eels and 47 flounders were collected in the Gironde river catchment. Analysis was performed with a Femtosecond LA‐ICPMS along a trajectory from the core to the edge. Sr and Ba water concentrations discriminated three habitats within the Gironde system, the lower estuary, the upper estuary and the freshwater sites. A signal processing method based on Gaussian hidden Markov models was applied to the multielemental life‐history data. The linear model used to allocate a Gironde habitat to coupled Sr, Ca values was parameterised with seasonal patterns and magnitude of Sr and Ba water values in the different habitats. The results showed that the three species used three different habitats and they had a large diversity of habitat use patterns with resident and nomadic tactics. Resident tactics were less frequent than nomadic tactics that suggested individual fish used two or more habitats. Mullet used a wider range of habitats in the lower part of the estuary than eel and flounder and switched habitats more frequently. Flounders tended to colonise initially freshwater, and then estuarine habitats later in life while mullets used the entire range of available catchment habitats throughout their life.

Growth and movement patterns of early juvenile European anchovy (Engraulis encrasicolus L.) in the Bay of Biscay based on otolith microstructure and chemistry

AbstractVarious hypotheses have been put forward to explain the mechanisms in the Bay of Biscay that result in a good recruitment of European anchovy. Anchovy larvae from the spawning area in the Gironde River plume are advected towards off‐shelf waters, where juveniles are commonly observed. Otolith microstructural and chemical analysis were combined to assess the importance of this off‐shelf transport and to determine the relative contribution of these areas for anchovy survival. Chemical analysis of otoliths showed that anchovy juveniles in the Bay of Biscay can be divided into two groups: a group that drifts towards off‐shelf waters early in their life and returns later, and a group that remains in the low salinity waters of the coastal area. The first group presents significantly faster growth rates (0.88 mm day−1) than those remaining in the coastal waters (0.32 mm day−1). This may be due to off‐shelf waters being warmer in spring/summer, and to the fact that the lower food concentration is compensated for by higher prey visibility. Furthermore, the group of juveniles that drifted off the spawning area and had faster growth rates represents 99% of the juvenile population. These findings support the hypothesis that anchovy in the Bay of Biscay may use off‐shelf waters as a spatio‐temporal loophole, suggesting that transport off the shelf may be favourable for recruitment.

Esturgeon européen… le retour?

Trente-cinq annees de recherches ont ete necessaires pour parvenir a eviter la disparition du plus grand poisson migrateur de l’ouest de l’Europe, l’esturgeon europeen, Acipenser sturio, et parvenir, a partir de jeunes specimens eleves en station experimentale, a obtenir des alevins permettant la realisation d’un programme de restauration de la derniere population de Gironde, avant d’esperer restaurer la presence de l’espece sur son aire d’origine, en creant d’autres populations sauvages. Elles auront permis incidemment de creer une production originale de caviar d’elevage a partir d’une autre espece, permettant de limiter les prelevements sur les stocks sauvages d’especes d’esturgeon les plus menacees.

Past distributions of the European freshwater eel from archaeological and palaeontological evidence

The spatial distribution of the European freshwater eel ( Anguilla anguilla) was very different in historic and prehistoric times in comparison to the present. A database of the spatial and temporal distribution of eel remains in archaeological and palaeontological sites is presented and used to assess the spatial distribution of populations from the height of the last glacial maximum. The results show that the eel was absent from northern Europe until about 11 000 years ago. The reason was probably a southerly displacement of the Gulf Stream carrying the larval migration from the Sargasso Sea. However, additional factors preventing eel populations in northern Europe may have also been the colder temperatures in the Arctic tundra landscape that existed at the time and the extreme distance to the European Atlantic coast along the Channel River. The archaeological record shows that eels were absent from the Baltic Sea until about 6700 cal BC, but there is some indication of an earlier presence during the Yoldia Sea stage at the beginning of the Holocene. Only in southern Europe south of the Gironde river basin were eel populations maintained through the last glaciation. The species may have survived the last glaciation in a relatively restricted area in the Mediterranean and the Atlantic coast of western Europe. Published palaeontological and genetic information gives important insights into climatic, geologic, and tectonic events on longer time scales. The oldest subfossil remains from Pleistocene sediments in northern Europe are approaching the age of the estimated genetic divergence of the European and American eel populations, and hence the species identity of the oldest subfossil remains may be ambiguous.

Genetic Identification of Western Mediterranean Sturgeons and its Implication for Conservation

To date, the only native sturgeon species in Western Europe was believed to be Acipenser sturio. However, this species is currently restricted to the Gironde River (Southern France), and it poses serious difficulties for rearing in captivity and for using in recovery programme. Furthermore, it has been questioned whether A. sturio is in fact the only species within the rivers of Western Europe, as A. naccarii, a species previously considered endemic of the Adriatic region, has been reported from the Iberian Peninsula in recent years. Here, we have used forensic techniques to obtain DNA from several museum specimens of sturgeons caught in the Spanish Guadalquivir River and in other European rivers. We analysed DNA sequences from these specimens for five genetic markers (three nuclear and two mitochondrial regions), which were subsequently compared with sequences obtained from A. sturioand A. naccarii. Our study demonstrates that A. naccarii coexists withA. sturio, from the Adriatic Sea to the Iberian Peninsula, a finding that could be taken into account in future sturgeon recovery programmes in Western Europe.

Relationships between frontal structures and zooplankton communities along a cross-shelf transect in the Bay of Biscay (1995 to 2003)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 284:65-75 (2004) - doi:10.3354/meps284065 Relationships between frontal structures and zooplankton communities along a cross-shelf transect in the Bay of Biscay (1995 to 2003) Aitor Albaina*, Xabier Irigoien Marine Research Division, AZTI Foundation, Herrera Kaia Portualde, z/g 20110, Pasaia (Gipuzkoa), Spain *Email: aalbaina@pas.azti.es ABSTRACT: A high-resolution cross-shelf transect in front of the mouth of the Gironde estuary (45.62°N, and from 1.36 to 4.27°W, with 3 nautical miles between stations) is carried out once a year during the spring stratification period. In this study, we analysed the zooplankton samples in 1995, 1997, 1999, 2001 and 2003 in order to elucidate the effects that mesoscale structures exert over the zooplankton community. The Gironde river plume and a shelf-break front due to internal wave generation over the slope are the main frontal structures described along the transect. Their position and magnitude determine zooplankton community changes both qualitatively and quantitatively. Two peaks of zooplankton abundance and distinct zooplankton assemblages were found to be related to the river plume and the frontal structure associated with the shelf-break. However, the distribution of biomass was more homogenous, suggesting that other factors apart from enhancement of primary production related to the location of frontal structures are involved in the determination of the pattern of secondary production along the transect. KEY WORDS: Fronts · Zooplankton · Mesoscale structures · Bay of Biscay · Internal waves · Gironde river plume · Shelf-break Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 284. Online publication date: December 21, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.