Engraulis encrasicolus (Linnaeus, 1758)egg and larval distribution in the eastern coast of Tunisia in relation to environmental characteristics

An ichthyoplankton survey was conducted on the eastern coast of Tunisia from the 10th to 18th August 2004 with the investigation of 68 stations. The sampling period corresponded to stratified summer conditions, with average SST 26.97°C. The thermocline was found between 25 and 50 m depth. The eastern coast of Tunisia was influenced by the water exchanges between the eastern and western Mediterranean. Anchovy eggs and larvae were a major component of the ichthyoplankton, representing 26 and 19% of the total mean fish egg and larval abundances, respectively. Their mean abundances were 108 eggs.10 m-2 and 112 larvae.10 m-2. The spawning areas were mainly located off the 100 m isobaths, with the maximum abundance occurring near the shelf break, as opposed to the inshore waters where anchovy eggs and larvae were practically absent. Depth was the main factor controlling the distribution of anchovy eggs and larvae. Larval transport from the spawning areas is made evident following southward to south-eastward directionsPublicado

Distribution of tuna larvae in Tunisian east coasts and its environmental scenario

Early life stages of tuna species were investigated along the Tunisian east coast during the summer of 2008 by means of a larval survey conducted on board the R/V HANNIBAL. In this study we define the spawning grounds of tuna fishes in Tunisian waters and relate the tuna larvae spatial distribution to the hydrographic features and other environmental parameters. A total of 282 larvae of Atlantic bluefin tuna (Thunnus thynnus, Linnaeus, 1758), 902 of bullet tuna (Auxis rochei, Risso, 1810) and 19 of little tuna (Euthynnus alletteratus, Raf, 1810), were identified. High larval concentrations of bluefin tuna were located offshore (mean = 200 ± 18 m), at sea surface temperatures ranging between 23.7 to 25.3°C, whereas the highest concentrations of bullet and little tuna were located in shallower waters (mean = 135 ± 75 m), at sea surface temperature ranging between 25.1 to 25.9°C. According to their preference for waters with surface salinities ranging between 37.1 and 37.8, spawning of these species seems to occur mainly in mixed waters. Stations where tuna larvae were collected showed oxygen concentrations between 6.55 and 6.85 mg l-1, as well as low turbidity and chlorophyll a values, from 0.2 to 0.5 NTU a d less than 1 mg m-3, respectively. These three environmental factors were weakly correlated with tuna larvae abundancePublicado

Influence of summer conditions on the larval fish assemblage in the eastern coast of Tunisia (Ionian Sea, Southern Mediterranean)

Publicado

Linking surface hydrodynamics to planktonic ecosystem: the case study of the ichthyoplanktonic assemblages in the Central Mediterranean Sea

Oceanographic processes play a key role in influencing the structure of the marine planktonic ecosystems. Taking advantage of the quasi-simulta-neous collection of a large ichthyoplanktonic dataset in different regions of the Central Mediterranean Sea (Italian/Maltese, Tunisian and Libyan waters), this study aimed at the identification of the main environ-mental drivers that control the structure of the larval fish assemblages. Spatial distribution and taxa com-position were related to physical forcings (geostrophic currents and wind stress) and environmental condi-tions (bottom depth, temperature, salinity, chlorophyll-a concentration). ANOSIM and SIMPER identified contribution of fish taxa to the average Bray–Curtis dissimilarity among regions. In Italian and Libyan waters, two assemblages (neritic and oceanic) were identified, while a mixed assemblage characterized only some stations. Two neritic and one oceanic assemblages were discriminated in Tunisian waters. Random Forest classification model high-lighted the essential role of the bathymetry, while Lagrangian simulations evidenced the action of the hydrodynamics in mixing neritic and oceanic assem-blages in the Italian/Maltese and partially in Libyan waters. These findings highlighted the importance of the multidisciplinary approach and shed light on the potential value of the ichthyoplanktonic surveys for the assessment of the state of the marine ecosystem and the conservation of the fishery resources