Optimization of fishing for stock enhancement of Rutilis rutilus and Scardinius erythropththalmus in forage fish-deficient Tunisian reservoirs.

Twenty specially designed fishing nets were used to catch broodstock of the forage fish roach and rudd (Rutilus rutilus and Scardinius erythrophthalmus). The monofilament gill nets were of different mesh sizes (18, 22, 24, and 26 mm) but of identical dimensions (4 × 50 m). The fish, caught in Sidi Salem reservoir in 2014, amounted to a total of 8901 roach and rudd caught at 41 different times; 3335 broodstock forage fish were transferred to 11 forage fish-deficient Tunisian reservoirs. The best yields (0.464 fish/m2) were obtained with fishing gear fitted with 18-mm mesh size; however, fish mortality with this net was very high, exceeding 83.36%. The greatest efficiency was obtained using nets of 22- and 24-mm mesh with a number of fish per unit effort (NPUE) of 0.181 and 0.173 fish/m2, and a mortality of 54.34 and 53.85%, respectively. Gill nets of 26-mm mesh size were inappropriate (lowest yield 0.129 fish/m2). Length of rudd and roach at first maturity and Gonadosomatic index (GSI) were measured, indicating that all transferred fish were mature. Transfer techniques were improved by holding the captured fish for 5 days before release into the host reservoir.

Characterization of fish assemblages and population structure of freshwater fish in two Tunisian reservoirs: implications for fishery management.

To monitor and assess the state of Tunisian freshwater fisheries, two surveys were undertaken at Ghezala and Lahjar reservoirs. Samples were taken in April and May 2013, a period when the fish catchability is high. The selected reservoirs have different surface areas and bathymetries. Using multi-mesh gill nets (EN 14575 amended) designed for sampling fish in lakes, standard fishing methods were applied to estimate species composition, abundance, biomass, and size distribution. Four species were caught in the two reservoirs: barbel, mullet, pike-perch, and roach. Fish abundance showed significant change according to sampling sites, depth strata, and the different mesh sizes used. From the reservoir to the tributary, it was concluded that fish biomass distribution was governed by depth and was most abundant in the upper water layers. Species size distribution differed significantly between the two reservoirs, exceeding the length at first maturity. Species composition and abundance were greater in Lahjar reservoir than in Ghezala. Both reservoirs require support actions to improve fish productivity.

Trace element accumulation and elutriate toxicity in surface sediment in northern Tunisia (Tunis Gulf, southern Mediterranean).

Metal concentrations in sediments were investigated in the Gulf of Tunis, Tunisia, in relation to anthropic activities along the Mejerda River and Ghar El Melh Lagoon, with effluents discharged into the gulf. Distribution of grain size showed that the silty fraction is dominant with 53%, while sand and clay averages are 34 and 12% respectively. Zn concentration increased in the vicinity of the Mejerda River while Pb was at its highest levels at the outlet of Ghar El Mehl Lagoon. Sediment elutriate toxicity, as measured by oyster embryo bioassays, ranged from 10 to 45% abnormalities after 24h, but no relation was found between metal concentration and sediment toxicity. The AVS fraction that represents monosulfide concentrations in the sediment was higher in the central part of the gulf than in the coastal zone. The results reveal the influence of AVS, TOC and grain size on metal speciation and sediment toxicity.

Assessing trace metal pollution through high spatial resolution of surface sediments along the Tunis Gulf coast (southwestern Mediterranean).

Tunis Gulf (northern Tunisia, Mediterranean Sea) is of great economic importance due to its abundant fish resources. Rising urbanization and industrial development in the surrounding area have resulted in an increase in untreated effluents and domestic waste discharged into the gulf via its tributary streams. Metal (Cd, Pb, Hg, Cu, Zn, Fe, and Mn) and major element (Mg, Ca, Na, and K) concentrations were measured in the grain fine fraction <63 µm by atomic absorption spectrophotometry. Results showed varying spatial distribution patterns for metals, indicating complex origins and controlling factors such as anthropogenic activities. Sediment metal concentrations are ranked as follows: Fe > Mg > Zn > Mn > Pb > Cu > Cd > Hg. Metals tend to be concentrated in proximity to source points, suggesting that the mineral enrichment elements come from sewage of coastal towns and pollution from industrial dumps and located along local rivers, lagoons, and on the gulf shore itself. This study showed that trace metal and major element concentrations in surface sediments along the Tunis Gulf shores were lower than those found in other coastal areas of the Mediterranean Sea.

Assessing pollution in a Mediterranean lagoon using acid volatile sulfides and estimations of simultaneously extracted metals.

Bizerte Lagoon is a southern Mediterranean semi-enclosed lagoon with a maximum depth of 12 m. After assessing sediment quality, the authors report on the physicochemical characteristics of the lagoon's surface sediment using SEM (simultaneously extracted metals) and AVS (acid volatile sulfides) as proxies. Biogeochemical tools are used to investigate the environmental disturbance at the water-sediment interface by means of SEM and AVS to seek conclusions concerning the study area's pollution status. Results confirm accumulation of trace elements in sediment. The use of the SEM-AVS model with organic matter in sediment (ƒOC) confirms possible bioavailability of accumulated trace elements, especially Zn, in the southern part of the lagoon, with organic matter playing an important role in SEM excess correction to affirm a nontoxic total metal sediment state. Individual trace element toxicity is dependent on the bioavailable fraction of SEMMetal on sediment, as is the influence of lagoon inflow from southern water sources on element bioavailability. Appropriate management strategies are highly recommended to mitigate any potential harmful effects on health from this heavy-metal-based pollution.