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Etude de l'impact de l'environnement biophysique sur les premiers stades de vie de Sardinella aurita dans le système d'upwelling sénégalo-mauritanien : modélisation saisonnière et interannuelle / Impact of biophysical environment on Sardinella aurita early life stage within the Senegalese-mauritanian upwelling system : seasonal and interannual modellingMbaye, Baye Cheikh 04 March 2015 (has links)
Le système d’upwelling Nord-Ouest africain au large du Maroc, de la Mauritanie et du Sénégal est le système le plus productif au monde en terme de biomasse planctonique. La zone sénégalo-mauritanienne, située dans la partie Sud du système, soutient une importante pêcherie de Sardinella aurita, l’espèce de petit poisson pélagique la plus abondante. Dans ces systèmes la dynamique des populations de petit poisson pélagique comme la sardinelle est très liée à la variabilité de l’upwelling. Ainsi le succès du recrutement des juvéniles de poisson dépend fortement de la circulation locale où les larves peuvent être, soit retenues dans les zones de nourriceries côtières favorables à leur survie, soit dispersées vers la large où elles sont soumises à la prédation et au manque de nourriture. Dans un contexte de gestion de cette ressource partagée par plusieurs pays, comprendre les facteurs qui contrôlent les stocks des petits pélagiques le long des côtes, est un enjeu crucial pour les pêcheries de ces régions.Le premier objectif de ce travail de thèse est d’utiliser pour la première fois dans le système sénégalo-mauritanien une approche de modélisation individu centré (IBM) visant à étudier les premiers stades de vies (oeufs et larves) de la sardinelle, et à comprendre l’influence des paramètres environnementaux sur la dynamique des populations. L’évolution des individus dans leur environnement est étudiée à partir de simulations océaniques régionales dont les sorties sont utilisées comme forçages du modèle IBM.Les résultats obtenus renseignent sur la rétention larvaire en fonction des lieux et dates de ponte. L’impact de la migration verticale et de l’effet de seuil de températures létales sur la survie des larves a également été évalué. Nous montrons que la stratégie de ponte de Sardinella aurita résulte de la combinaison de deux facteurs : (1) une rétention élevée associée à la circulation locale et (2) à la disponibilité en nourriture. Le modèle révèle également une importante connectivité larvaire entre les différentes sous-zones de ponte et le Sud du système. Ceci souligne l’importance d’une gestion concertée du stock de la sardinelle à l’échelle régionale.Le second objectif de la thèse est d’étudier la variabilité interannuelle de l’habitat de ponte de la sardinelle à l’intérieur de ces zones de rétention larvaire. Un modèle couplé physique biogéochimie (ROMS-PISCES) est utilisé pour évaluer l’habitat de ponte de la sardinelle. Le volume potentiel de l’habitat de ponte (PHV) est défini comme une fonction de la température, de la salinité et de la profondeur. Nous avons également utilisé le PHV, la biomasse de plancton, le succès de la rétention et de la survie des larves pour étudier la variabilité interannuelle de l’abondance de sardinelle et les années de fort recrutement estimées à partir des campagnes acoustiques d’évaluation de stock menées durant la période 1996-2006. Nos résultats nous suggèrent que les périodes de fort volume d’habitat et de faible mortalité larvaire pourraient expliquer les périodes de biomasses importantes pendant la période 1996-1999. Cette étude nous a aussi permis de proposer des hypothèses permettant d’expliquer la période de fort recrutement observée en 1998-1999. / The North West african upwelling system off Morocco, Mauritania and and Senegal is the most productive system in the world in term of plankton biomass. South of this system, the senegalese-mauritanian zone sustains important Sardinella aurita fisheries, the main small pelagic fish. In these systems the population dynamics of small pelagic fish such as sardinella is closely linked to the variability of the upwelling. Thus the success of the recruitment of juvenile fish depends heavily on local circulation where larvae can either be retained in areas of coastal nurseries suitable for survival or dispersed into the sea where they are subject to predation and lack of food. In a management context of this shared resource by several countries, understand the factors that control the stocks of small pelagic fish along the coast, is a critical issue for fisheries in these regions.Our results provide information on larval retention based on places and dates of spawning. The impact of vertical migration and lethal temperature threshold effect on larval survival was also evaluated. We show that Sardinella aurita spawning strategy is the combination of two factors : (1) a high retention associated with local circulation and (2) the availabilityof food. The model also reveals an important larval connectivity between different spawning sub-areas and south of the system. This highlights the importance of concerted management of the stock of sardinella on a regional scale.The second aim of the thesis is to study the variability of the spawning habitat of sardinella within the larval retention areas. A coupled physical biogeochemistry (ROMS-PISCES) is used to assess the spawning habitat of sardinella. The potential spawning habitat volume (PHV) is defined as a function of temperature, salinity and depth. We also used thePHV, plankton biomass, the success of retention and larvae survival to study the variability of the abundance of sardinella and years of strong recruitment estimated from acoustic surveys assessment stock carried out during the period 1996-2006.Our results suggest to us that periods of high volume of habitat and low larval mortality could explain the periods of major biomass during the period 1996-1999. This study also allowed us to propose hypotheses to explain the period of strong recruitment observed in 1998-1999.
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On the spatial and temporal variability of upwelling in the southern Caribbean Sea and its influence on the ecology of phytoplankton and of the Spanish sardine (Sardinella aurita)Rueda-Roa, Digna Tibisay 01 January 2012 (has links)
The Southern Caribbean Sea experiences a strong upwelling process along the coast from about 61°W to 75.5°W and 10-13°N. In this dissertation three aspects of this upwelling system are examined: (A) A mid-year secondary upwelling that was previously observed in the southeastern Caribbean Sea between June-July, when land based stations show a decrease in wind speed. The presence and effects of this upwelling along the whole southern Caribbean upwelling system were evaluated, as well as the relative forcing contribution of alongshore winds (Ekman Transport, ET) and wind-curl (Ekman Pumping, EP). (B) Stronger upwelling occurs in two particular regions, namely the eastern (63-65°W) and western (70-73°W) upwelling areas. However, the eastern area has higher fish biomass than the western area (78% and 18%, respectively, of the total small pelagic biomass of the southern Caribbean upwelling system). The upwelling dynamics along the southern Caribbean margin was studied to understand those regional variations on fish biomass. (C) The most important fishery in the eastern upwelling area off Venezuela is the Spanish sardine (Sardinella aurita). The sardine artisanal fishery is protected and only takes place up to ~10 km offshore. The effects of the upwelling cycle on the spatial distribution of S. aurita were studied. The main sources of data were satellite observations of sea surface temperature (SST), chlorophyll-a (Chl) and wind (ET and EP), in situ observations from the CARIACO Ocean Time-Series program, sardine biomass from 8 hydroacoustics surveys (1995-1998), and temperature profiles from the World Ocean Atlas 2005 used to calculate the depth of the Subtropical Underwater core (traced by the 22°C isotherm). The most important results of the study were as follows:
(A) The entire upwelling system has a mid-year upwelling event between June-August, besides the primary upwelling process of December-April. This secondary event is short-lived (~5 weeks) and ~1.5°C warmer than the primary upwelling. Together, both upwelling events lead to about 8 months of cooler waters (-3, averaged from the coast to 100 km offshore) in the region. Satellite nearshore wind (~25 km offshore) remained high in the eastern upwelling area (> 6 m s-1) and had a maximum in the western area (~10 m s-1) producing high offshore ET during the mid-year upwelling (vertical transport of 2.4 - 3.8 m3 s-1 per meter of coastline, for the eastern and western areas, respectively). Total coastal upwelling transport was mainly caused by ET (~90%). However, at a regional scale, there was intensification of the wind curl during June as well; as a result open-sea upwelling due to EP causes isopycnal shoaling of deeper waters enhancing the coastal upwelling.
(B) The eastern and western upwelling areas had upwelling favorable winds all year round. Minimum / maximum offshore ET (from weekly climatologies) were 1.52 / 4.36 m3 s-1 per meter, for the western upwelling area; and 1.23 / 2.63 m3 s-1 per meter, for the eastern area. The eastern and western upwelling areas showed important variations in their upwelling dynamics. Annual averages in the eastern area showed moderate wind speeds (6.12 m s-1), shallow 22°C isotherm (85 m), cool SSTs (25.24°C), and phytoplankton biomass of 1.65 mg m-3. The western area has on average stronger wind speeds (8.23 m s-1) but a deeper 22°C isotherm (115 m), leading to slightly warmer SSTs (25.53°C) and slightly lower phytoplankton biomass (1.15 mg m-3). We hypothesize that the factors that most inhibits fish production in the western upwelling area are the high level of wind-induced turbulence and the strong offshore ET.
(C) Hydroacoustics values of Sardinella aurita biomass (sAsardine) and the number of small pelagics schools collected in the eastern upwelling region off northeast Venezuela were compared with environmental variables (satellite products of SST, SST gradients, and Chl -for the last two cruises-) and spatial variables (distance to upwelling foci and longitude-latitude). These data were examined using Generalized Additive Models. During the strongest upwelling season (February-March) sAsardine was widely distributed in the cooler, Chl rich upwelling plumes over the wide (~70km) continental shelf. During the weakest upwelling season (September-October) sAsardine was collocated with the higher Chl (1-3 mg m-3) found within the first 10 km from the upwelling foci; this increases Spanish sardine availability (and possibly the catchability) for the artisanal fishery. These results imply that during prolonged periods of weak upwelling the environmentally stressed (due to food scarceness) Spanish sardine population would be closer to the coast and more available to the fishery, which could easily turn into overfishing. After two consecutive years of weak upwelling (2004-2005) Spanish sardine fishery crashed and as of 2011 has not recovered to previous yield; however during 2004 a historical capture peak occurred. We hypothesize that this Spanish sardine collapse was caused by a combination of sustained stressful environmental conditions and of overfishing, due to the increased catchability of the stock caused by aggregation of the fish in the cooler coastal upwelling cells during the anomalous warm upwelling season.
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