Project topics and materials
Get Complete Project Material File(s) Now! »
Study area and fish sampl ing
Juveniles of European sea bass were sampled by fyke nets (4 mm mesh, 20 m long and 1.80 m height) during ebb tide in creeks of 5 salt marshes in the Normand Breton Gulf (French coast of the English Channel) from October to November 2011 and in a seagrass bed from a distant site, Brest (i.e Dinan Anse precisely), in June 2012 (n = 5) (Figure 1). Figure 1: Location of the sampling sites in the Normand Breton Golf (south-western English Channel). Fish samples are localised by circles and triangle (Brest), and water samples for salt marshes (circle), fresh water (diamond), sea surface waters (square). The Lessay is the northern site and is characterized by a heavily sheep grazed salt marsh and dominated by a monospecific Elytrigia maritima lawn (Table 1). The Mont Saint Michel Bay (MSMB) receives the output water of 3 rivers covering a drainage of 2700 km2. Due to high tidal ranges of up to 15 m, and huge sediment accretion the tidal flats cover 240 km2 including 200 km2 of mudflats and 40 km2 of salt marshes which are among the biggest in Europe (Lefeuvre et al., 2000). About 2/3 of the salt marsh is sheep grazed and therefore covered by Puccinellia maritima monospecific assemblages and 1/3 is ungrazed and used to be covered by a succession of communities ranging from P. maritima in the low marsh, Atriplex portulacoides in the middle marsh and Elytrigia sp. in the high marsh. However, to eutrophication Elytrigia sp. has invaded up to about 73 % of the salt marsh (Valéry et al., 2004). The Rance ria is strongly modified by a tidal plant built in 1963. The estuary covers 5.29 km2 including 10% of salt marshes dominated by Elytrigia sp. and A. portulacoides according to the distance to the sea and the elevation. A recent proliferation of Ulva spp. occurred over recent years due to eutrophication. The catchment area of the Rance covers a drainage of 1400 km2. The Arguenon is a lowly modified estuary covering about 1.2 km2 including c.a. 0.55 km2 of A. portulacoides and Atriplex sp. salt marshes. The catchment of the Arguenon river covers c.a. 601 km2. The Saint Brieuc Bay has a tidal flats cover 27 km2 with 25 km2 of mudflats and 1.12 km2 of salt marshes. The Bay is drained by three rivers Gouët, Urne and Gouessant. The Yffiniac Anse is a 11.4 km2 natural reserve since 1998. The catchment area of Urne is subject to low cattle grazing and covers a drainage of 108 km2 in the Yffiniac Anse. This bay has a high tidal range of 13 m and the salt marshes are flooded when the level is above 10.90 m. The vegetation where the fishes are captured is dominated by A. portulacoides (Table 1).
Microchemistry analysis and otoli thometry
The sagittal otoliths were extracted and washed three times in an ultra-pure water bath (milliQ 0.0055 μS). After the remaining tissues were removed under a binocular, otoliths were dried and stored in 1.5-mL plastic Eppendorf tubes. The left otolith extracted from each fish was embedded in araldite resin 2020 (Huntsman) with the sulcus acusticus downward. They were grounded in the sagittal plane up to the core with ultra-pure water and sandpaper with grains gradually decreasing from 2400 μm to 1200 μm, 9 μm, and 3 μm. Finally, the otoliths were rinsed with ultra-pure water and air-dried. Otolith microchemical composition was assessed using a high repetition rate 257 nm femtosecond laser ablation (Lambda 3, Nexeya, France) inductively coupled with plasma mass spectrometry (Element XR Thermo Scientific) (LA-ICPMS). In order to improve spatial resolution and sensitivity, an elongated beam shape (10 x 30 μm) parallel to the growing rings was simulated by using the special feature of this laser allowing the combination of high repetition rate (up to 100 kHz) with a fast and precise laser beam movement driven by 2D galvanometric scanner. The laser was operated here at 1000 Hz with a pulse energy of 18mJ and a beam diameter of 20 μm. The galvanometric scanner was set to move the laser beam according to a permanent 20-μm-wide, back-and-forth movement (at a speed of 1 mm/s), resulting in a virtual 10 × 30 μm laser beam perpendicular to the posterior axis. Combined with this back-and-forth movement, the sample was continuously moved along the posterior axis from the nucleus to the edge of the otolith at a speed of 5 μm/s, resulting in an uninterrupted ablation on the grounded surface (Figure 2). Taking into account the ICPMS acquisition rate set to 1.6 s, each data point along the transect corresponded to roughly 8 μm. In order to prevent a blast effect on the nucleus, the ablation was started 200 μm before the nucleus. The ablation depth was evaluated between 5 and 10 μm. In order to prevent any contamination, a fast and soft laser pre-cleaning of the otolith surface was carried out prior each ablation along the selected transect. The laser parameters used for this pre-cleaning (4 μJ, stage speed 50 μm/s) were adjusted so that the ablation depth was kept within the range of 1-2 μm.
Mult i -elemental signature of sites of capture
The capture fingerprint representing the 2-3 last days of life permitted to compare different sites imprints (Table 2). Signature of Brest was significantly distinct from the others. MSMB (Bay of Mont St Michel) was also significantly different of all sites, except from that of Arguenon, which appeared not significantly different neither from Rance nor St Brieuc. Lessay was distinct to Rance, MSMB and Brest. Saint Brieuc and Arguenon sites were both less dissimilar. Note that the dicriminant elements are very variable according to the pairs of sites comparison.
Life histories of sea bass juveniles
A strong correlation appeared between the water Sr/Ba ratio and the water salinity from the study sites and completed with additional water samplings (Figure 3a). The Sr/Ba ratio followed an exponential curve according to the salinity (Sr/Ba = 7.23e0.141Salinity, r2 = 0.87, n = 11, Figure 3a). Additionally, a correlation was assessed between the water salinity and the Sr/Ba ratio of otoliths. The Sr/Ba ratio followed an exponential curve according to the salinity (Sr/Ba = 63.84e0.087Salinity, r2 = 0.61, n = 85, Figure 3b). The salinities in salt marshes of Rance (24 psu) and Arguenon (9 psu) were not included in the relation because they were related to a rainy event (lower salinities than expected) and did not reflect the last days before the fish capture and consequently they did not match with the signatures of capture. Otolith Sr/Ba ratios were compared with salinity and classified in 4 groups, with the exponential relationship, corresponding to 4 salinity thresholds. Halin group (superior to 34 psu) corresponded to Sr/Ba values superior to 1217, euhalin group (34-30 psu) included 1217- 860 Sr/Ba values, polyhalin group (30-18 psu) was between 860-304 Sr/Ba values and mesohalin group (18-5 psu) contained values between 304 and 0 Sr/Ba.
Table of contents :
INTRODUCTION GENERALE
1. Problématique et objectif principal
2. Les modèles biologiques
3. Objectifs scientifiques et organisation de la thèse
PARTIE 1: LE CAS DE LA COMMUNAUTE DE LANÇON
I. Contexte de l’étude
II. Article 1 : Life history of the Small Sandeel, A. tobianus, infered from otolith microchemistry. A methodological approach.
1. Introduction
2. Materials and methods
3. Results
4. Discussion
Conclusions and perspectives
III. Article 2 : Contrasted life histories of three sympatric sandeels cross validated by otolith microchemistry, stable isotopes and functional traits
1. Introduction
2. Material and methods
3. Results
4. Discussion
Conclusion
PARTIE 2 : LE CAS DU BAR EUROPEEN, DICENTRARCHUS LABRAX
I. Contexte de l’étude
II. Article 3 : Diversity of life histories of juvenile European sea bass, Dicentrarchus labrax, revealed by the microchemistry of their otolith.
1. Introduction
2. Materials and methods
3. Results
4. Discussion
Conclusion
III. Article 4 : Variability of feeding and life traits to young European sea bass in contrasted nursery habitats of the western Channel
1. Introduction
2. Material and methods
3. Results
4. Discussion
Conclusion
DISCUSSION GENERALE
Histoire de vie inter-espèce à l’échelle des communautés de lançon
Variabilité d’histoire de vie intra-espèce au sein du bar européen
Relations entre histoires de vie et connectivité des HEE
BIBLIOGRAPHIE
