Engraulis encrasicolus
is of great economic importance in the Mediterranean. However, little is known about its stock structure. Otolith shape analysis has been successfully used for fish stock ...identification. In this study, the stock structure of anchovy caught off the open sea and the coastal area of the Gulf of Tunis, lagoon of Bizerte and Lake of Ichkeul were investigated using otolith shape. Otolith shape was determined by Fourier analysis and then compared among specimens sampled from different areas with forward stepwise canonical discriminant analysis. Significant differences in otolith shape between the open sea and inshore anchovy groups were detected. Otolith shape of anchovy collected in the Lake of Ichkeul was distinct from the other groups. This finding suggests a clear discreteness of the open sea and the continental groups. The data highlighted the potential for using otolith shape analysis for anchovy stock identification, as well as the role of oceanographic features in determining stock separation. These findings will have major implications for anchovy fisheries management in Tunisia. By using a precautionary approach and considering the three areas as separate stocks, fisheries management strategies should be adjusted to achieve optimum sustainable production from each stock and to avoid decreases in genetic variety.
We investigated at the single cell level during 16months (June 2012 to September 2013) the temporal distribution of picophytoplankton (picoeukaryotes, Synechococcus and Prochlorococcus) communities ...in two contrasted ecosystems: the Bay of Bizerte characterised by an oligotrophic regime typical of the Mediterranean Sea and the Bizerte Lagoon that exhibits a mesotrophic/eutrophic state. We aimed at depicting seasonal variations and quantifying the relationships between the environmental factors and the structure and abundance of picophytoplankton communities. Results showed that picophytoplankton groups were able to grow under a wide range of environmental conditions varying seasonally, although their abundances and contributions to the total chlorophyll biomass significantly varied and showed importance in the Bay of Bizerte. Synechococcus was the most abundant group reaching 225∗103cells·cm−3 in the Bay and 278∗103cells·cm−3 in the lagoon. This group was present all over the year in both ecosystems. Structural equation model results pointed out a different configuration regarding the picophytoplankton environmental drivers. The complexity of the configuration, i.e. number of significant links within the system, decreased under enhanced eutrophication conditions. The less exposure to anthropogenic stress, i.e. in the Bay of Bizerte, highlight a larger role of nutrient and hydrological conditions on the seasonal variations of picophytoplankton, whereas a negative effect of eutrophication on picophytoplankton communities was unveiled in the Bizerte Lagoon. We stress that such influence may be exacerbated under expected scenarios of Mediterranean warming conditions and nutrient release in coastal ecosystems.
•Picophytoplankton was able to grow under a wide range of environmental conditions varying seasonally.•Picophytoplankton variability was more marked in the bay than in the lagoon.•Picophytoplankton decreased in the lagoon under enhanced anthropogenic stress.•Synechococcus, the most abundant group of the community, was frequently present in both ecosystems during the monitoring.
•Alexandrium pseudogonyaulax RCs were highly represented over Bizerte lagoon.•Correlation between cyst densities and silt was found but this link was not strict.•Environmental factors and cyst ...densities were highly spatially autocorrelated.•Hydrodynamics is the most important factor affecting cysts dispersal.
This study investigated the spatial distribution of Alexandrium pseudogonyaulax resting cysts in recently-deposited sediment of Bizerte lagoon (South-Western Mediterranean, Tunisia). This lagoon is the subject of many anthropogenic impacts, such as holding important fishing and aquaculture activities. A. pseudogonyaulax has been shown to produce Goniodomin A, which is a biologically-active compound. We showed that this dinoflagellate produces two types of resting cysts, which could be distinguished by the presence or the absence of a paratabulate wall. The average cyst density across the whole lagoon was rather high, reaching 639cystsg−1 of dry sediment (DS). Cyst densities varied widely among the sampled stations, with the highest density of 1685cystg−1DS being recorded at station 51 near a mussel farm. With respect to sediment characteristics, the highest cyst densities were found within silty sediments with high water content values. The distribution of A. pseudongoyaulax cysts in Bizerte lagoon appears to be related to hydrodynamic factors.
A specimen of dusky spinefoot Siganus luridus (Rüppel, 1829) was recorded in the peri-Mediterranean lagoon of Bizerte in northern Tunisia. This record constitutes the northernmost range of the ...species in Tunisian waters and the westernmost range on North African coasts. Siganus luridus is the second record known to date of a teleost Lessepsian species in a restricted brackish area. We describe the specimen, include morphometric measurements and meristic counts, and discuss the distribution of this herbivorous species in the Lagoon of Bizerte, in Tunisian waters and in the Mediterranean Sea.
A specimen of dusky spinefoot Siganus luridus (Rüppel, 1829) was recorded in the peri-Mediterranean Lagoon of Bizerte located in northern Tunisia. This record constitutes the northernmost extension ...range of the species in the Tunisian waters, and the second record of a teleost Lessepsian species, known to date in a restricted brackish area. The specimen is described including morphometric measurements and meristic counts. Additionally, the distribution of S. Luridus, herbivorous species, in the Lagoon of Bizerte, the Tunisian waters in the area and in the Mediterranean Sea is commented and discussed.
Coastal areas, in general, are affected more and more by anthropogenic activities. Benthic macrofauna assemblages react to this disturbance and constitute, then, an effective biological tool to study ...the degree of contamination of the biotope. For this study three coastal sites, more or less exposed to anthropogenic activities but differing according to their opening to the sea, were studied. Sediments were sampled aboard a research vessel and using a 0.1 m2 Van Veen grab. Results show that Tunis bay and Dkhila coast, which are more open to the sea, appear more balanced on the trophic plan, and the biodiversity state is more satisfactory. However, Bizerte lagoon is closed, except for a limited communication with the sea via a narrow ship canal. So, the ecosystem presents some eutrophication signs and a great trophic imbalance in which carnivores largely dominate the benthic assemblages. In this situation, the herbivore chain is substituted by a microbial chain and only some opportunistic species can survive in these anoxic sediments related to the strong fluctuations of the environment factors during the year.
Common torpedo Torpedo torpedo (Linnaeus, 1758) is widely distributed in the Mediterranean Sea, especially in southern and northern regions. The species is known throughout the Tunisian coast and ...entered brackish areas, such as the northeastern Lagoon of Bizerte, where it is commonly captured. Investigations conducted in this latter area showed that males and females sexually matured between 234–285 mm and 245–280 mm total length (TL), respectively. The largest male and female were 403 mm and 435 mm TL, respectively and weighed 721.5 and 1273.5 g, respectively. There was a not significant relationship total mass versus TL between males and females. Conversely, the relationship of liver mass versus TL significantly differed between sexes. The diameter of the largest yolky oocytes ranged between 43 and 47 mm (mean 45.08 ± 0.98)}, while the mass ranged between 29.5 and 37.4 g (mean 31.79 ± 2.20). Near term embryos ranged from 19.34 to 31.28 mm TL (mean: 24.60 ± 4.12) and weighed from 3.9 to 9.0 g (mean 5.97 ± 1.41). Ovarian fecundity ranged from 4 to 36 (mean = 11.60 ± 5.96). Uterine fecundity or litter size ranged from 3 to 13 (mean = 6.36 ± 2.67). Both fecundities showed a positive relationship with TL of females. A chemical balance of development based on mean dry masses of yolky oocytes and near-term embryos was 0.75 and suggested that T. torpedo could be considered as a pure lecithotrophic species. Hepatosomatic index (HSI) and gonadosomatic index (GSI) calculated for females presented significant higher values than those of males. HSI values of males significantly differed differences between juveniles, sub-adults and adults, and in females, HSI values were not significantly different between juveniles and sub-adults and significantly different between sub-adults and adults. The values of male GSI increased with TL of specimens; additionally, they showed significant differences between juveniles and subadults, but not between sub-adults and adults. Monthly mean values of adult male and female HSI significantly varied throughout the year. Monthly mean of females GSI showed significant changes especially from January to April, due to a high vitellogenic activity, GSI decreased from May to August, during this period all yolky oocytes were ovulated, then GSI increased progressively from September. The reproductive cycle lasted one year approximately and gestation period 4 months, while near-term females were captured in August. Vitellogenesis did not proceed in parallel with embryonic development: during ovarian vitellogenic activity, the uteri were empty and in an appearent resting phase, conversely during gestation, the ovaries did not exhibit an appearent activity, by contrast other specimens exhibited a vitellogenic activity during gestation. With special regard to the latter case, it remains difficult to state if these small yolky oocytes were not ovulated, or if they were new oocytes produced immediately after ovulation, both hypothesis remaining suitable.
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