Othman Jarboui

Reproductive cycle and size at first sexual maturity of Hexaplex trunculus (Gastropoda: Muricidae) in the Gulf of Gabès (southern Tunisia)

Abstract The reproductive cycle and size at first sexual maturity of the banded murex (Hexaplex trunculus) from the tidal areas of the Gulf of Gabès (southern Tunisia) were investigated from June 2007 to May 2008 through macroscopic analysis of the gonads and monthly variations in the general condition (K) and gonadosomatic indices (GSI). This muricid gastropod is gonochoristic (separate sexes throughout life) and lacks external sexual dimorphism. The overall sex-ratio of the population was significantly unbalanced, with a predominance of females (M:F = 1:1.6). The monthly variation in the gonad maturation stages, K and GSI revealed that H. trunculus has a yearly reproductive cycle, with both sexes showing a long period of gonadal activity and slightly asynchronous gametogenic cycle. Gametogenesis occurred during most of the year (11 out of 12 months examined) and almost every gametogenic stage was observed throughout the study period. Ripe males were found mainly from November to January, preceding the main period of gametic emission (between January and May). Ripe females occurred mainly from February to April, preceding the main spawning season (between April and May), apparently triggered by the increasing daylength and seawater temperature during this period (from 13.2 to 14.2 h and from 19.3 to 23.4°C, respectively). Both sexes displayed a relatively short resting phase (males: June—July; females: June—September), coincident with the highest daylength and seawater temperature during summer in the Gulf of Gabès. The first sexual maturity was reached at a smaller size in males (SL50 = 36.9 mm SL) than in females (SL50 = 50 50 40.3 mm SL). The information gathered in this study allowed preliminary management measures for the fishery to be proposed, including a closed season during the main spawning season (April—May) and the establishment of a minimum landing size (of at least 40 mm SL) for H. trunculus from the Gulf of Gabès.

Appraisal of the usefulness of operculum growth marks for ageing Hexaplex trunculus (Gastropoda: Muricidae): Comparison between surface striae and adventitious layers

Abstract This study estimated the age of Hexaplex trunculus (Gastropoda: Muricidae) by analysing operculum growth marks. Surface striae and adventitious layers were counted in opercula of H. trunculus from the Ria Formosa Lagoon (southern Portugal) and from the Bizerte Lagoon (northern Tunisia), and in aquacultured individuals used for validation of the ageing technique. Operculum growth features were also compared between H. trunculus populations under contrasting environmental conditions (namely seawater temperature). Despite high inter-individual variability, all H. trunculus populations displayed a clear increasing trend in the number of striae and layers with increasing specimen size. The operculum marginal growth revealed that opercular deposition is not annual. Instead, a few growth marks are formed in the operculum each year, which was confirmed with reared specimens. In both ageing methods, considerable variation in the number of counts at a determined shell length indicates that operculum growth marks should be interpreted cautiously because they might not correspond accurately with individual age. Because of the subjectivity inherent to either method, counting striae and layers in opercula only provides rough age estimates of H. trunculus.

Food and feeding habits of Caranx crysos from the Gulf of Gabès (Tunisia)

The diet of blue runner Caranx crysos (Carangidae) in the Gulf of Gabes (Tunisia, Mediterranean) is described from analysis of stomach contents (N = 1668 fish). The majority of samples were obtained from commercial purse seine and gill-net catches. The index of vacuity (%VI) was relatively high (58.7%) and differed significantly across months. Blue runner is an opportunistic predator that consumes mostly pelagic organisms, with benthic prey representing only a small proportion of the diet. The diet was quantified using the frequency of occurrence (%F), numerical abundance (%N), weight (%W) and the index of relative importance (IRI and %IRI) for each prey taxa. The most important prey categories were teleosts (%IRI = 83.4) and crustaceans (%IRI = 16.6), with molluscs only observed occasionally (%IRI < 0.1). Fish were also the dominant food items in both terms of weight (89.60%) and frequency of occurrence (82.44%). In terms of numerical abundance, crustaceans were the most abundant prey (78.07%). Ontogenetic and seasonal differences in the diet were observed, although there was no difference between the diets of males and females.

Biology of little tunny Euthynnus alletteratus in the Gulf of Gabes, Southern Tunisia (Central Mediterranean Sea)

Se describen algunos parametros biologicos de la bacoreta Euthynnus alletteratus en el Golfo de Gabes (sur de Tunez), tales como la proporcion de sexos, el Indice Gonado-Somatico (IGS), el Indice Hepato-Somatico (IHS), la longitud de primera madurez sexual, y la relacion longitud-peso. Se analizo un total de 397 specimenes (244 hembras y 153 machos) entre los 36,7 y 97,8 cm de longitud, con un promedio 52,29 ± 5,87 cm. Las muestras se recolectaron mensualmente, entre enero de 2008 y diciembre de 2009. El 61% de los ejemplares fueron hembras. El periodo de desove se extiende de junio a septiembre, calculado a partir de los altos valores de IGS. Las relaciones longitud-peso revelaron que la bacoreta presenta una alometria negativa. La longitud promedio de primera madurez de las muestras tomadas en el Golfo de Gabes fue de 44,8 cm de longitud de horquilla para las hembras y 42,8 cm para los machos.

First occurrence of the yellow roughneck shrimp, Rimapenaeus similis (Smith, 1885) (Crustacea: Decapoda: Penaeidae) in the Mediterranean Sea (Tunisian waters)

During an experimental trawl survey carried out by the R/V “Hannibal” in June 2006 several specimens of the Western-Atlantic penaeid shrimp Rimapenaeus similis (Smith, 1885) were caught in the Gulf of Gabes (southern Tunisia, Central Mediterranean). This represents the first recording of Rimapenaeus similis in Tunisian waters and for the Mediterranean Sea. Biological information on the sampled population is also included.

Diet composition and food habits of Caranx rhonchus (Carangidae) from the Gulf of Gabes (central Mediterranean)

From June 2004 to May 2006, 1040 Caranx rhonchus (Carangidae) ranging from 6.5 to 30.6 cm in total length (TL), fished in the Gulf of Gabes (south Tunisia, central Mediterranean), were used to investigate feeding habits of the species. This study was based mainly on the evolution of the index of relative importance (IRI) with respect to sex, fish size and season. Among 1040 stomachs examined, 595 were empty (%VI = 57.2). The vacuity index (%VI) differed significantly across months and declined slightly with season. A total of 13 species representing 12 different families belonging to four major groups: teleosts, crustaceans, molluscs and annelids, were identified in stomach contents of Caranx rhonchus. Caranx rhonchus is primarily an opportunistic piscivorous fish with a diet characterized by a wide range of species. Teleosts occurred in the majority of stomachs, with a total of the relative importance (%IRI) of 52.7%. The anchovy Engraulis encrasicolus were the most important teleost species (%IRI = 11.2), while crustaceans were the second most important food category consumed (47.1%). There were no significant differences in the diet between males and females, but significant differences were found with season and among size-classes.

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-simultaneous 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 environmental drivers that control the structure of the larval fish assemblages. Spatial distribution and taxa composition were related to physical forcings (geostrophic currents and wind stress) and environmental conditions (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 highlighted the essential role of the bathymetry, while Lagrangian simulations evidenced the action of the hydrodynamics in mixing neritic and oceanic assemblages 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.

Distribution, abundance and population structure of Hexaplex trunculus and Bolinus brandaris (Gastropoda: Muricidae) in offshore areas of the Gulf of Gabès, southern Tunisia

The banded murex Hexaplex trunculus and purple dye murex Bolinus brandaris are frequently caught as bycatch by bottom trawlers operating in the Gulf of Gabès, southern Tunisia. Although these muricid gastropods might importantly constitute an additional source of income for fishermen as well as help to diversify local shellfish production, information on their catches is relatively scarce. Hence, the present study aimed to assess the status of local populations of H. trunculus and B. brandaris, including data on their spatial and depth distributions, abundance, biomass, population structure and mortality. Fishing surveys using bottom trawls were performed between May 2006 and June 2007 in offshore areas of the Gulf of Gabès. In total, 198 fishing hauls were analysed, corresponding to a total fishing duration of 333.3 hours and a total prospecting area of 1 534 ha at depths of 20–86 m. Altogether, 3 026 H. trunculus and 3 599 B. brandaris of a broad size range (22.9−91.1 and 22.4−97.0 mm shell length, respectively) were sampled during the study period. GIS-produced maps were employed to determine the species’ spatial and bathymetric distributions by extrapolating their estimated fishing yield (bycatch per unit effort), abundance, biomass and mortality in relation to the total area surveyed in the gulf. The results provide valuable baseline information that may be applied to management proposals for regulating this untargeted resource, ultimately assisting sustainable exploitation and long-term preservation of this alternative fishing resource in the offshore areas of the Gulf of Gabès.

Exploitation status and stock assessment by pseudo-cohort analysis of the bluefish, Pomatomus saltatrix , in the Gulf of Gabes (Tunisia)

The bluefish, Pomatomus saltatrix, is quite abundant in the Gulf of Gabes, off the south-eastern coast of Tunisia. It is commercially exploited by artisanal gears and trawlers all year round, and by purse seine nets from May to August (bluefish fishery season). Catches of bluefish, in the period 1996-2004 fluctuated between 365.6 t and 1240.1 t with an annual average of 805 t. This fluctuation, partially due to the migratory nature of the species, could be also attributed to the fleet activities that sometimes changed at the mercy of the operators. The analysis of the virtual population of bluefish in the Gulf of Gabes showed that, in 2002, the stock that had just recovered rightly after an excessive fishing in 1996-1997 was again subject to a fishing effort that passed its capacity (E=0.71; E>0.5). The biomass (B) estimated to be ∼2178.9 t only tolerated the extraction of 713.4 t (more or less one-third of B). However, the three fleets in use removed 1029.1 t with a yield per recruit (Y/R) of 70.5 g. As a result of this over-fishing, the actual stock of bluefish in the Gulf of Gabes was characterized by individuals having a mean total length of 17.88 cm, a size which is much lower than the one at first sexual maturity (23.5 cm). The turnover (D/B) being of 75.23%, it did not allow the reconstitution of the stock. The total removals (∼1639 t) due to both natural mortality (M=0.28) and fishing mortality (F=0.675) had to be compensated especially by individual growth (1534.2 t; 93.6%) because of the low weight of the recruits.

Spawning habitats of two small pelagic species (Engraulis encrasicolus and Sardinella aurita) in the Gulf of Gabès (Central Mediterranean)

Spawning habitats of anchovy Engraulis encrasicolus and sardinella Sardinella aurita were investigated in the Gulf of Gabès by means of two surveys carried out during the summer of 2005 and 2009. The spatial patterns of early developmental stages were analyzed through Lloyd’s patchiness index, single quotient analyses, and principal component analyses. The results showed that Gulf of Gabès is an important area for anchovy and sardinella spawning. Within this area, anchovy and sardinella showed preference for the warmest waters to spawn. The main spawning areas for anchovy were located in the inner parts of the Gulf, and a secondary spawning area was observed offshore. Contrastingly, the main spawning ground of sardinella was found in the center of the Gulf. In both years, early larvae of anchovy showed a greater degree of aggregation than that of sardinella. The developmental stages of both species showed low spatial overlap, indicating that the spawning habitats of these species are spatially differentiated in the Gulf of Gabès.