Acta Adriatica: International Journal of Marine Sciences, Dec 1, 2006
An abnormal partially-albino embryo with atypical characteristics was removed from a pregnant fem... more An abnormal partially-albino embryo with atypical characteristics was removed from a pregnant female sandbar shark, Carcharhinus plumbeus, caught in the Gulf of Gabès in southern Tunisian, central Mediterranean. The specimen is described and atypical characteristics and albinism in shark species is reviewed.
... DOI: 10.1080/17451000.2011.619546 Hasna Kadri a ... of connective tissue, including the neura... more ... DOI: 10.1080/17451000.2011.619546 Hasna Kadri a ... of connective tissue, including the neural and haemal arches, soaked in a 5% sodium hypochlorite solution until the remnants material could be removed effectively, subsequently washed in running water and air-dried. ...
Two species of spurdog of the genus Squalus occur in the Gulf of Gabès (southern Tunisia, central... more Two species of spurdog of the genus Squalus occur in the Gulf of Gabès (southern Tunisia, central Mediterranean): the longnose spurdog Squalus blainvillei (Risso, 1827) and a short-snout spurdog of the Squalus megalopscubensis group. Morphometric and meristic data as well as a genetic analyses (DNA inter-simple sequence repeat markers and molecular barcoding methods) support the assignation of this short-snout spurdog to Squalus megalops (Macleay, 1881). Squalus megalops occurs commonly in temperate and tropical Australian waters, and is also thought to occur in the eastern Atlantic, southern Indian Ocean and western North Pacific although these records need to be confirmed. Our study confirms that it occurs in the Mediterranean Sea. Populations of both S. blainvillei and S. megalops are described based on Tunisian material.
The food habits of rough ray Raja radula were investigated, with respect to sex, size and season,... more The food habits of rough ray Raja radula were investigated, with respect to sex, size and season, based on stomach content analyses of specimens collected from commercial fisheries, monthly during 2007 in the Gulf of Gabès (Central Mediterranean Sea). Of 950 stomachs examined, 183 were empty (19.26%). The overall diet of R. radula mainly consisted of crustacean decapods, teleosts and molluscs. Polycheates, sipunculids and echinoderms were minor dietary components. No differences were observed in the major preys consumed between the sexes, but ontogenetic and seasonal changes were found. The dietary trend with size shifted from crustaceans to teleosts. The consumptions of minor preys were independent from both size and season. Quantitative analysis and graphical methods indicated the rough ray to be a specialized feeder.
The authors have compiled published information on taxonomy, distribution, status, statistics, fi... more The authors have compiled published information on taxonomy, distribution, status, statistics, fisheries, bycatch, biologic, and ecologic parameters mainly on food and feeding habits and reproductive biology of elasmobranchs along the Tunisian coasts. This bibliographic analysis shows that cartilaginous species, including sharks and rays are by far the most endangered group of marine fish, with 63 species, about 53% of all are critically endangered, endangered, or vulnerable. Overfishing, fishing practices, and habitat degradation are leading to dramatic declines of these species. Biologic parameters concern a few species primarily in the Gulf of Gabes. Therefore, recommendations to fill gaps in order to protect and manage elasmobranchs stocks are proposed in this chapter.
Cartilaginous fishes are a very ancient group; Sharks have lived on Earth for about 500 million y... more Cartilaginous fishes are a very ancient group; Sharks have lived on Earth for about 500 million years, since long before the dinosaurs. They have been able to survive and overcome five mass extinctions since their appearance. They play in fact a key role in maintaining the balance of marine ecosystems. More than 97% of the shark population has disappeared in the last two centuries. Currently, a high percentage is threatened, many are data-deficient. Many menaces face sharks, but fishing pressure seems to be most critical. This chapter focuses on the current status of cartilaginous fishes and progress on conservation measures and actions taken mainly through regional plans.
Summary Small-scale fisheries are generally promoted as a sustainable alternative to large-scale ... more Summary Small-scale fisheries are generally promoted as a sustainable alternative to large-scale industrial fisheries. However, there is recent growing evidence that small-scale fisheries may be the largest threat to marine species of conservation concern. The objective of this study was to evaluate the potential impact of the trammel net fishery on elasmobranchs in the Gulf of Gabes, Southern Tunisia. Data are based on 191 shrimp trammel net set (40 mm stretched mesh size) surveys conducted aboard commercial fishing vessels from May to July 2009. Five species of the small coastal elasmobranchs (Mustelus mustelus (Linnaeus, 1758), Mustelus punctulatus Risso 1827, Dasyatis pastinaca (Linnaeus, 1758), Dasyatis marmorata (Steindachner, 1892) and Torpedo torpedo (Linnaeus, 1758)) and two species from the large coastal shark (Carcharhinus plumbeus (Nardo, 1827) and Carcharhinus brevipinna (Muller & Henle, 1839)) were recognized as by-catch in this fishery. Elasmobranch by-catch was dominated by sharks (90.3%), smoothhound sharks Mustelus sp. being by far the most important (88.9%) and reflecting their abundance in the area; 58% of the sets caught at least one specimen, with 4.8 ± 1.3 caught per set. Captures were composed essentially of neonate and juvenile sharks, while the batoids were dominated by mature individuals. This study shows that shrimp trammel nets represent a considerable source of mortality for early life stages of elasmobranch species in the Gulf of Gabes. Additionally, there was a high density of neonates and small juvenile M. mustelus in the Sfax zone, suggesting that these nearshore waters are a nursery grounds for smoothhound sharks. Further research should focus on the incidents of by-catch and evaluate the potential solutions to allow trammel net fisheries to coexist alongside the elasmobranch species.
Abstract The thornback ray Raja clavata is one of the most abundant bycatch species of trawl fish... more Abstract The thornback ray Raja clavata is one of the most abundant bycatch species of trawl fishing in the waters of the Gulf of Gabès, but little is known about its life history in this area. The present study provides the first data on age and growth of this species in Tunisian waters, as well as additional data on its reproduction. The age of individuals was estimated by counting growth bands on central vertebral sections. Marginal increment and edge analysis supported the hypothesis of annual deposition of growth bands, with the opaque band formed during autumn. The parameters estimated from von Bertalanffy and Gompertz growth functions showed differences between sexes. The oldest thornback ray female was aged 15 years (104 cm TL), whereas the oldest male was 12 years (89 cm TL). Variations in the gonadosomatic and hepatosomatic indices of both sexes and the presence of vitellogenic activity in mature females during the year indicated that the species cannot be considered to be a continuous spawner. Males were found to mature between 55 and 75 cm TL, whereas females matured between 72.4 and 85 cm TL. The TL at which 50% of the population reached maturity was 67.44 and 81.42 cm for males and females, respectively. Males and female thornback rays mature, respectively, at approximately 5.3 and 7 years old.
This study explores the status of exploited shark populations in the Gulf of Gabes in relation to... more This study explores the status of exploited shark populations in the Gulf of Gabes in relation to fishing pressure, based on data pertaining to shark longline fishery, collected during two periods. Catch data from 48 and 96 pelagic longline sets collected, respectively, during 2007/2008 and 2016/2017, were used to compare species composition and catch rates between periods. Four species were recorded during 2007/2008, while 11 additional species of pelagic and demersal elasmobranchs along with teleosts were fished during 2016/2017. Elasmobranch species dominated the catches with 94% and 99.3% in terms of specimens number, respectively, during 2016/2017 and 2007/2008. The sandbar shark, Carcharhinus plumbeus, was the predominant species in catch (>84% of all fishes) during both periods. Two other species of sharks, Carcharhinus brevipinna and Isurus oxyrinchus, were relatively common, while the other species were a minor component. Despite the increase in terms of diversity pertaining to species captured during 2016/2017, the nominal catch rates of elasmobranch and C. plumbeus decreased by 39.17% and 42.21%, respectively, suggesting populations' declines. In addition, size distribution analyses revealed that this fishery may opportunistically operate on breeding and nursery areas, which affect main species over most of their life stages. The mortality and hooking location proportions were species-specific, with some species having significant percentages of live specimens at the time of haulback, providing an opportunity to release the sensitive life stages. Given the described state of shark exploitations in the Gulf of Gabes, urgent and efficient fishery regulation measures and conservation strategies, together with further investigations, are required to allow changing the unregulated shark longline fishery to a sustainable one.
Aquatic Conservation-marine and Freshwater Ecosystems, Nov 24, 2019
1. In order to increase their catches, longliners targeting sharks in the Gulf of Gabes, Tunisia,... more 1. In order to increase their catches, longliners targeting sharks in the Gulf of Gabes, Tunisia, have increased the number of hooks per basket and have included new bait types instead of mackerel (Scomber scombrus). This paper reports the effects of these changes on catch composition, catch rates, size distribution, life-stage captures, and mortality at haulback. 2. Data from 48 and 96 longline sets, carried out during the shark fishing seasons of 2007 and 2008, with two hooks per basket, and 2016 and 2017, with five hooks per basket, respectively, were examined to assess the effect of gear change. Moreover, the effects of bait types were inspected based on 33 fishing sets using whole mackerel, 19 using salema (Sarpa salpa) halves, and 27 using pieces of stingray (Dasyatis spp.), sampled during 2016 and 2017. 3. The species composition indicated that longliners expand the vertical distribution of their hooks to operate from the surface to the bottom. The catches with both longline designs were dominated by sandbar shark (Carcharhinus plumbeus). Despite gear changes, the catch rate of C. plumbeus decreased by 42.21%, suggesting population decline. The size composition indicated a shift towards larger specimens, mainly adult females, which could jeopardize the shark population. Moreover, the mortality of the main species at haulback seems to be affected by the design of the longline. 4. The bait used, mainly pieces of stingray, significantly increased the catch rate of sandbar shark, which suggests a greater attractiveness of the new bait; however, fish size and mortality rates at haulback were unchanged in response to bait variation. 5. The fishery operates in shark nursery grounds, which exposes these fish (principally C. plumbeus) to considerable exploitation pressure. The new fishing practices intensified the pressure and thus the risk of a rapid depletion of populations. To preserve the shark species in the area, the use of new gear and attractive bait should be banned.
Four species of stingrays of the genus Dasyatis occur in the Gulf of Gabès, but the taxonomic sta... more Four species of stingrays of the genus Dasyatis occur in the Gulf of Gabès, but the taxonomic status of one of them, D. tortonesei has been questioned by several authors, and synonymized with D. pastinaca by some. The present study, using morphological comparisons and genetic analyses, demonstrates that there is a closely related species to D. pastinaca in the Gulf of Gabès, but distinguishable by several characters. This species is provisionally identified as Dasyatis cf. tortonesei, pending a more extensive comparative study including specimens from other regions of the Mediterranean Sea and eastern North Atlantic Ocean.
Journal of the Marine Biological Association of the United Kingdom, Sep 10, 2013
In Tunisia, elasmobranch landings have decreased substantially in recent years. Generally, specie... more In Tunisia, elasmobranch landings have decreased substantially in recent years. Generally, species-specific information is largely unavailable for artisanal fisheries, but it is essential to increase knowledge and to ensure proper management of these species. This study analysed elasmobranch catches with longline fishery in the Gulf of Gabès. In total, 21 and 20 pelagic and bottom longline fishing trips were conducted, respectively, from July to September in 2007 and 2008. A total of eight elasmobranch species were caught: four batoids and four sharks. Pelagic longline captures were hooked externally while a high number of individuals captured with bottom longline were hooked internally. Discards due essentially to low commercial value and size represented 7.6% of total number of elasmobranch specimens caught. Longline landings in the Gulf of Gabès were principally composed of sandbar shark, Carcharhinus plumbeus, representing, respectively, 94.14% and 21.17% in number of pelagic and bottom longline captures. The importance of rhinobatids and Mustelus capture with bottom longline reflect their abundance in this area compared to other Mediterranean zones. Juveniles, including neonates with umbilical scars, dominated carcharhnids specimens, while smoothhound and guitarfish captures were dominated by mature individuals. Mitigation measures based on gear modifications, size limits and delineated nursery areas in the Gulf of Gabès should be developed to protect these vulnerable species.
Elasmobranchs are an important catch component in Tunisian artisanal fisheries. Generally, specie... more Elasmobranchs are an important catch component in Tunisian artisanal fisheries. Generally, species-specific information is largely unavailable for artisanal fisheries; an increase in knowledge thereof is essential to ensure proper management of these species. The study analyzed the gillnet fishery elasmobranch catches in the Gulf of Gabe`s, whereby 45 fishing trips were conducted from April to June 2007 and 2008 for the capture of a total of six species: three Rajiformes and three Carchariniformes. Rhinobatos cemiculus was the most important species (52%; 4.588 ind km À2 net per day). Among shark species, Mustelus mustelus was the most important (66.8%; 2.21 ind km À2 net per day). Discards represented 6.87% of total catch in weight and 14.28% in number of specimens caught. Size composition of captures varied by species, but usually mature, mainly gravid females were abundant. Further investigations are needed to obtain more information on such fragile species for the development of protective measures.
ABSTRACT The diet of the smooth-hound shark, Mustelus mustelus, from the Gulf of Gabès (southern ... more ABSTRACT The diet of the smooth-hound shark, Mustelus mustelus, from the Gulf of Gabès (southern Tunisia, central Mediterranean Sea) was investigated with respect to fish size and season. Stomach contents were analyzed from 540 specimens with total lengths ranging from 34 to 158.5 cm. Of the total number of stomachs examined, 63 were empty (11.67%). Smooth-hound shark fed mainly on crustaceans, fishes and cephalopods. Sipunculids, polychaetes and echinoderms were occasional preys. No differences were found between the diets of males and females. Ontogenetic changes in diet of M. mustelus were apparent, with crustaceans forming a greater proportion of the diet of smaller sharks. Both teleosts and molluscs increased in importance with increasing shark size. Consumptions of polycheates, sipunculids and echinoderms were not related to predator size. Prey diversity also increased with size, with large and mobile prey species found more commonly in the diet of larger sharks. The limited overlap in the dietary compositions of juveniles, subadults and adults suggests the possibility of resource partitioning. Seasonality in food habits was in accordance with the dynamics of the predator and the prey species.
Regime alimentaire du requin gris Carcharhinus plumbeus dans le golfe de Gabes, Tunisie. La nourr... more Regime alimentaire du requin gris Carcharhinus plumbeus dans le golfe de Gabes, Tunisie. La nourriture et le comportement alimentaire du requin gris Carcharhinus plumbeus capture dans le golfe de Gabes sont etudies a partir de l'examen de 130 contenus stomacaux. C. plumbeus se nourrit surtout de poissons (osteichtyens et chondrichtyens). Les cephalopodes et les crustaces constituent respectivement des proies secondaires et accessoires. C. plumbeus est une espece opportuniste et son regime alimentaire depend des proies disponibles dans l'environnement. Des changements dans le regime alimentaire ont ete observes et ils sont lies a la taille des specimens.
Acta Adriatica: International Journal of Marine Sciences, Dec 1, 2006
An abnormal partially-albino embryo with atypical characteristics was removed from a pregnant fem... more An abnormal partially-albino embryo with atypical characteristics was removed from a pregnant female sandbar shark, Carcharhinus plumbeus, caught in the Gulf of Gabès in southern Tunisian, central Mediterranean. The specimen is described and atypical characteristics and albinism in shark species is reviewed.
... DOI: 10.1080/17451000.2011.619546 Hasna Kadri a ... of connective tissue, including the neura... more ... DOI: 10.1080/17451000.2011.619546 Hasna Kadri a ... of connective tissue, including the neural and haemal arches, soaked in a 5% sodium hypochlorite solution until the remnants material could be removed effectively, subsequently washed in running water and air-dried. ...
Two species of spurdog of the genus Squalus occur in the Gulf of Gabès (southern Tunisia, central... more Two species of spurdog of the genus Squalus occur in the Gulf of Gabès (southern Tunisia, central Mediterranean): the longnose spurdog Squalus blainvillei (Risso, 1827) and a short-snout spurdog of the Squalus megalopscubensis group. Morphometric and meristic data as well as a genetic analyses (DNA inter-simple sequence repeat markers and molecular barcoding methods) support the assignation of this short-snout spurdog to Squalus megalops (Macleay, 1881). Squalus megalops occurs commonly in temperate and tropical Australian waters, and is also thought to occur in the eastern Atlantic, southern Indian Ocean and western North Pacific although these records need to be confirmed. Our study confirms that it occurs in the Mediterranean Sea. Populations of both S. blainvillei and S. megalops are described based on Tunisian material.
The food habits of rough ray Raja radula were investigated, with respect to sex, size and season,... more The food habits of rough ray Raja radula were investigated, with respect to sex, size and season, based on stomach content analyses of specimens collected from commercial fisheries, monthly during 2007 in the Gulf of Gabès (Central Mediterranean Sea). Of 950 stomachs examined, 183 were empty (19.26%). The overall diet of R. radula mainly consisted of crustacean decapods, teleosts and molluscs. Polycheates, sipunculids and echinoderms were minor dietary components. No differences were observed in the major preys consumed between the sexes, but ontogenetic and seasonal changes were found. The dietary trend with size shifted from crustaceans to teleosts. The consumptions of minor preys were independent from both size and season. Quantitative analysis and graphical methods indicated the rough ray to be a specialized feeder.
The authors have compiled published information on taxonomy, distribution, status, statistics, fi... more The authors have compiled published information on taxonomy, distribution, status, statistics, fisheries, bycatch, biologic, and ecologic parameters mainly on food and feeding habits and reproductive biology of elasmobranchs along the Tunisian coasts. This bibliographic analysis shows that cartilaginous species, including sharks and rays are by far the most endangered group of marine fish, with 63 species, about 53% of all are critically endangered, endangered, or vulnerable. Overfishing, fishing practices, and habitat degradation are leading to dramatic declines of these species. Biologic parameters concern a few species primarily in the Gulf of Gabes. Therefore, recommendations to fill gaps in order to protect and manage elasmobranchs stocks are proposed in this chapter.
Cartilaginous fishes are a very ancient group; Sharks have lived on Earth for about 500 million y... more Cartilaginous fishes are a very ancient group; Sharks have lived on Earth for about 500 million years, since long before the dinosaurs. They have been able to survive and overcome five mass extinctions since their appearance. They play in fact a key role in maintaining the balance of marine ecosystems. More than 97% of the shark population has disappeared in the last two centuries. Currently, a high percentage is threatened, many are data-deficient. Many menaces face sharks, but fishing pressure seems to be most critical. This chapter focuses on the current status of cartilaginous fishes and progress on conservation measures and actions taken mainly through regional plans.
Summary Small-scale fisheries are generally promoted as a sustainable alternative to large-scale ... more Summary Small-scale fisheries are generally promoted as a sustainable alternative to large-scale industrial fisheries. However, there is recent growing evidence that small-scale fisheries may be the largest threat to marine species of conservation concern. The objective of this study was to evaluate the potential impact of the trammel net fishery on elasmobranchs in the Gulf of Gabes, Southern Tunisia. Data are based on 191 shrimp trammel net set (40 mm stretched mesh size) surveys conducted aboard commercial fishing vessels from May to July 2009. Five species of the small coastal elasmobranchs (Mustelus mustelus (Linnaeus, 1758), Mustelus punctulatus Risso 1827, Dasyatis pastinaca (Linnaeus, 1758), Dasyatis marmorata (Steindachner, 1892) and Torpedo torpedo (Linnaeus, 1758)) and two species from the large coastal shark (Carcharhinus plumbeus (Nardo, 1827) and Carcharhinus brevipinna (Muller & Henle, 1839)) were recognized as by-catch in this fishery. Elasmobranch by-catch was dominated by sharks (90.3%), smoothhound sharks Mustelus sp. being by far the most important (88.9%) and reflecting their abundance in the area; 58% of the sets caught at least one specimen, with 4.8 ± 1.3 caught per set. Captures were composed essentially of neonate and juvenile sharks, while the batoids were dominated by mature individuals. This study shows that shrimp trammel nets represent a considerable source of mortality for early life stages of elasmobranch species in the Gulf of Gabes. Additionally, there was a high density of neonates and small juvenile M. mustelus in the Sfax zone, suggesting that these nearshore waters are a nursery grounds for smoothhound sharks. Further research should focus on the incidents of by-catch and evaluate the potential solutions to allow trammel net fisheries to coexist alongside the elasmobranch species.
Abstract The thornback ray Raja clavata is one of the most abundant bycatch species of trawl fish... more Abstract The thornback ray Raja clavata is one of the most abundant bycatch species of trawl fishing in the waters of the Gulf of Gabès, but little is known about its life history in this area. The present study provides the first data on age and growth of this species in Tunisian waters, as well as additional data on its reproduction. The age of individuals was estimated by counting growth bands on central vertebral sections. Marginal increment and edge analysis supported the hypothesis of annual deposition of growth bands, with the opaque band formed during autumn. The parameters estimated from von Bertalanffy and Gompertz growth functions showed differences between sexes. The oldest thornback ray female was aged 15 years (104 cm TL), whereas the oldest male was 12 years (89 cm TL). Variations in the gonadosomatic and hepatosomatic indices of both sexes and the presence of vitellogenic activity in mature females during the year indicated that the species cannot be considered to be a continuous spawner. Males were found to mature between 55 and 75 cm TL, whereas females matured between 72.4 and 85 cm TL. The TL at which 50% of the population reached maturity was 67.44 and 81.42 cm for males and females, respectively. Males and female thornback rays mature, respectively, at approximately 5.3 and 7 years old.
This study explores the status of exploited shark populations in the Gulf of Gabes in relation to... more This study explores the status of exploited shark populations in the Gulf of Gabes in relation to fishing pressure, based on data pertaining to shark longline fishery, collected during two periods. Catch data from 48 and 96 pelagic longline sets collected, respectively, during 2007/2008 and 2016/2017, were used to compare species composition and catch rates between periods. Four species were recorded during 2007/2008, while 11 additional species of pelagic and demersal elasmobranchs along with teleosts were fished during 2016/2017. Elasmobranch species dominated the catches with 94% and 99.3% in terms of specimens number, respectively, during 2016/2017 and 2007/2008. The sandbar shark, Carcharhinus plumbeus, was the predominant species in catch (>84% of all fishes) during both periods. Two other species of sharks, Carcharhinus brevipinna and Isurus oxyrinchus, were relatively common, while the other species were a minor component. Despite the increase in terms of diversity pertaining to species captured during 2016/2017, the nominal catch rates of elasmobranch and C. plumbeus decreased by 39.17% and 42.21%, respectively, suggesting populations' declines. In addition, size distribution analyses revealed that this fishery may opportunistically operate on breeding and nursery areas, which affect main species over most of their life stages. The mortality and hooking location proportions were species-specific, with some species having significant percentages of live specimens at the time of haulback, providing an opportunity to release the sensitive life stages. Given the described state of shark exploitations in the Gulf of Gabes, urgent and efficient fishery regulation measures and conservation strategies, together with further investigations, are required to allow changing the unregulated shark longline fishery to a sustainable one.
Aquatic Conservation-marine and Freshwater Ecosystems, Nov 24, 2019
1. In order to increase their catches, longliners targeting sharks in the Gulf of Gabes, Tunisia,... more 1. In order to increase their catches, longliners targeting sharks in the Gulf of Gabes, Tunisia, have increased the number of hooks per basket and have included new bait types instead of mackerel (Scomber scombrus). This paper reports the effects of these changes on catch composition, catch rates, size distribution, life-stage captures, and mortality at haulback. 2. Data from 48 and 96 longline sets, carried out during the shark fishing seasons of 2007 and 2008, with two hooks per basket, and 2016 and 2017, with five hooks per basket, respectively, were examined to assess the effect of gear change. Moreover, the effects of bait types were inspected based on 33 fishing sets using whole mackerel, 19 using salema (Sarpa salpa) halves, and 27 using pieces of stingray (Dasyatis spp.), sampled during 2016 and 2017. 3. The species composition indicated that longliners expand the vertical distribution of their hooks to operate from the surface to the bottom. The catches with both longline designs were dominated by sandbar shark (Carcharhinus plumbeus). Despite gear changes, the catch rate of C. plumbeus decreased by 42.21%, suggesting population decline. The size composition indicated a shift towards larger specimens, mainly adult females, which could jeopardize the shark population. Moreover, the mortality of the main species at haulback seems to be affected by the design of the longline. 4. The bait used, mainly pieces of stingray, significantly increased the catch rate of sandbar shark, which suggests a greater attractiveness of the new bait; however, fish size and mortality rates at haulback were unchanged in response to bait variation. 5. The fishery operates in shark nursery grounds, which exposes these fish (principally C. plumbeus) to considerable exploitation pressure. The new fishing practices intensified the pressure and thus the risk of a rapid depletion of populations. To preserve the shark species in the area, the use of new gear and attractive bait should be banned.
Four species of stingrays of the genus Dasyatis occur in the Gulf of Gabès, but the taxonomic sta... more Four species of stingrays of the genus Dasyatis occur in the Gulf of Gabès, but the taxonomic status of one of them, D. tortonesei has been questioned by several authors, and synonymized with D. pastinaca by some. The present study, using morphological comparisons and genetic analyses, demonstrates that there is a closely related species to D. pastinaca in the Gulf of Gabès, but distinguishable by several characters. This species is provisionally identified as Dasyatis cf. tortonesei, pending a more extensive comparative study including specimens from other regions of the Mediterranean Sea and eastern North Atlantic Ocean.
Journal of the Marine Biological Association of the United Kingdom, Sep 10, 2013
In Tunisia, elasmobranch landings have decreased substantially in recent years. Generally, specie... more In Tunisia, elasmobranch landings have decreased substantially in recent years. Generally, species-specific information is largely unavailable for artisanal fisheries, but it is essential to increase knowledge and to ensure proper management of these species. This study analysed elasmobranch catches with longline fishery in the Gulf of Gabès. In total, 21 and 20 pelagic and bottom longline fishing trips were conducted, respectively, from July to September in 2007 and 2008. A total of eight elasmobranch species were caught: four batoids and four sharks. Pelagic longline captures were hooked externally while a high number of individuals captured with bottom longline were hooked internally. Discards due essentially to low commercial value and size represented 7.6% of total number of elasmobranch specimens caught. Longline landings in the Gulf of Gabès were principally composed of sandbar shark, Carcharhinus plumbeus, representing, respectively, 94.14% and 21.17% in number of pelagic and bottom longline captures. The importance of rhinobatids and Mustelus capture with bottom longline reflect their abundance in this area compared to other Mediterranean zones. Juveniles, including neonates with umbilical scars, dominated carcharhnids specimens, while smoothhound and guitarfish captures were dominated by mature individuals. Mitigation measures based on gear modifications, size limits and delineated nursery areas in the Gulf of Gabès should be developed to protect these vulnerable species.
Elasmobranchs are an important catch component in Tunisian artisanal fisheries. Generally, specie... more Elasmobranchs are an important catch component in Tunisian artisanal fisheries. Generally, species-specific information is largely unavailable for artisanal fisheries; an increase in knowledge thereof is essential to ensure proper management of these species. The study analyzed the gillnet fishery elasmobranch catches in the Gulf of Gabe`s, whereby 45 fishing trips were conducted from April to June 2007 and 2008 for the capture of a total of six species: three Rajiformes and three Carchariniformes. Rhinobatos cemiculus was the most important species (52%; 4.588 ind km À2 net per day). Among shark species, Mustelus mustelus was the most important (66.8%; 2.21 ind km À2 net per day). Discards represented 6.87% of total catch in weight and 14.28% in number of specimens caught. Size composition of captures varied by species, but usually mature, mainly gravid females were abundant. Further investigations are needed to obtain more information on such fragile species for the development of protective measures.
ABSTRACT The diet of the smooth-hound shark, Mustelus mustelus, from the Gulf of Gabès (southern ... more ABSTRACT The diet of the smooth-hound shark, Mustelus mustelus, from the Gulf of Gabès (southern Tunisia, central Mediterranean Sea) was investigated with respect to fish size and season. Stomach contents were analyzed from 540 specimens with total lengths ranging from 34 to 158.5 cm. Of the total number of stomachs examined, 63 were empty (11.67%). Smooth-hound shark fed mainly on crustaceans, fishes and cephalopods. Sipunculids, polychaetes and echinoderms were occasional preys. No differences were found between the diets of males and females. Ontogenetic changes in diet of M. mustelus were apparent, with crustaceans forming a greater proportion of the diet of smaller sharks. Both teleosts and molluscs increased in importance with increasing shark size. Consumptions of polycheates, sipunculids and echinoderms were not related to predator size. Prey diversity also increased with size, with large and mobile prey species found more commonly in the diet of larger sharks. The limited overlap in the dietary compositions of juveniles, subadults and adults suggests the possibility of resource partitioning. Seasonality in food habits was in accordance with the dynamics of the predator and the prey species.
Regime alimentaire du requin gris Carcharhinus plumbeus dans le golfe de Gabes, Tunisie. La nourr... more Regime alimentaire du requin gris Carcharhinus plumbeus dans le golfe de Gabes, Tunisie. La nourriture et le comportement alimentaire du requin gris Carcharhinus plumbeus capture dans le golfe de Gabes sont etudies a partir de l'examen de 130 contenus stomacaux. C. plumbeus se nourrit surtout de poissons (osteichtyens et chondrichtyens). Les cephalopodes et les crustaces constituent respectivement des proies secondaires et accessoires. C. plumbeus est une espece opportuniste et son regime alimentaire depend des proies disponibles dans l'environnement. Des changements dans le regime alimentaire ont ete observes et ils sont lies a la taille des specimens.
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