Age and growth of the dusky grouper Epinephelus marginatus (Lowe 1834) in an exploited population of the western Mediterranean Sea

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Reproductive biology of Klein’s sole, Synapturichthys kleinii (Actinopterygii: Pleuronectiformes: Soleidae), off Tunisian coast (central Mediterranean)

Background. Klein’s sole, Synapturichthys kleinii (Risso, 1827), is a rare fish, included in the IUCN Red List as Data Deficient, and its biology is poorly known. Biological investigations of S. kleinii have been surprisingly rare in Tunisian waters. In the presently reported study, we investigated the reproductive period, size at first sexual maturity, and fecundity of S. kleinii along the Tunisian waters, with the aim of achieving the first complete and comprehensive description of the reproduction of this fish in the Mediterranean Sea. Materials and methods. A total of 294 specimens of S. kleinii were sampled over two years, off the Tunisian coasts. Sex ratio was estimated through the seasons of the year and the size classes. The stage of maturity was determined macroscopically only for females. Monthly gonadosomatic index (GSI), monthly hepatosomatic index (HSI), and condition factor (K) were calculated for males and females of S. kleinii. Size at first maturity was estimated only for females during the spawning season. Eighteen mature females at spawning stage were collected, weighed, and fixed in 4% formaldehyde solution for fecundity estimation. Results. Annual sex ratio showed significant differences between males and females. In specimens exceeding 27 cm in total length females outnumbered males significantly. The Klein’s sole reproduction period started in October and ended in January. The gonadosomatic index reached highest values in November for females and males. Females attained sexual maturity at 20.53 cm total body length (TL). Total fecundity ranged from 3647 to 23 174 eggs. We found a low correlation coefficient between total fecundity and total length, but a high correlation coefficient between eviscerated weight and ovary weight. Conclusion. Our results constitute a preliminary baseline for monitoring changes in the biological indexes linked to the reproduction cycle and length at first sexual maturity of S. kleinii. The presently acquired data will enable biologists to assess the status of this fish and develop culture technology in natural waters and will be useful for the fishery biologists and conservation biologists, for successful development, management, production, and ultimate conservation of this favoured Tunisian food fish

Gonadal cycle of the dwarf oyster Ostreola stentina from the south of the Gulf of Hammamet on the eastern coast of Tunisia

The reproductive cycle of the dwarf oyster Ostreola stentina (Payraudeau 1826) at Monastir in the south of the Gulf of Hammamet on the east coast of Tunisia was studied using condition indices and histological techniques. The species was shown to be a protandric hermaphrodite. Whereas males dominated the small shell length categories (12.45–34.99 mm), females (21.00–47.16 mm) and hermaphroditic individuals (30.00–57.95 mm) become more predominant with increasing size, suggesting that individuals may undergo protandric development. The overall male to female ratio was 1.0:1.7. Gametogenic development begins in February–March for females and in September for males, and spawning occurs during April and July for females and December and July for males. There was a clear relationship between the gonadal condition index and gametogenic stage for both sexes. Values increased from zero in their early stage of development and peaked during ripe and spawning stages, before decreasing in the spent stage. However, the meat condition index did not show any statistical differences between gametogenic stages. Keywords: condition indices, gametogenic cycle, histology, protandrous hermaphroditismAfrican Journal of Marine Science 2012, 34(4): 537–54

Age, growth, mortality, longevity and reproductive biology of the white skate, Rostroraja alba (Chondrichthyes : Rajidae) of the Gulf of Gabes (Southern Tunisia, Central Mediterranean)

The Age, growth, mortality, longevity and reproductive parameters were studied for Rostroraja alba from the Gulf of Gabes (Southern Tunisia, central Mediterranean Sea), collected monthly during 2007 from commercial fisheries. 43.75% of the composition was females and 56.25% males. A total of 140 females (44.5 - 160 cm TL) and 180 males (39.5 - 150 cm TL) were examined to study the reproductive cycle. A subsample of 115 specimens (65 females and 50 males, ranging from 39.5 to 160 cm TL) was used for age and growth estimation derived from vertebral centra. The oldest female was 35 years and 160 cm TL, whereas the oldest male was 32 years and 150 cm TL. The von Bertalanffy growth parameters were L infinity= 177.6 +/- 3.61 cm, k= 0.06 +/- 3.08 year(-1) and t(0)= -1.28 +/- 1.97 10(-1) year for females and L infinity= 199.6 +/- 3.66 cm, k= 0.04 +/- 1.9610(-3) year(-1) and t(0)= -1.47 +/- 0.35 10(-1) year for males. Growth was not significantly different between sexes. The maturity size (TL 50%) was 119.3 cm (94.20 cm DW) for males and 129.4 cm (95.77 cm DW) for females. The maturity age (A50%) was estimated to 19.69 and 23.47 years respectively for males and females. Size and age maturity were not significantly different between sexes

Age, growth, longevity, natural mortality and maturity of the longnose spurdog, Squalus blainvillei (Chondrichthyes: Squalidae), in the Gulf of Gabes (Central Mediterranean Sea)

Age, growth, longevity, natural mortality and size and age at maturity of the longnose spurdog, Squalus blainvillei, in the Gulf of Gabes (Tunisia, Central Mediterranean Sea) are reported. Marginal increment suggested annual band formation. Age estimates ranged from 3 to 15 and 4 to 19 years for males and females, respectively. Von Bertalanffy growth parameters derived from length at age data were: L-infinity = 105.7 cm, K = 0.11year(-1), t(0) = -1.12 year for females and L-infinity = 91.1 cm, K = 0.14 year(-1), t(0) = -1.42 year for males. Males and females reached maturity at 52.3 cm and 62.5 cm TL corresponding to 4.97 and 7.44 years, respectively. Longevity estimates were 24.75 and 31.50 years for males and females, respectively. Based on life history parameters, natural mortality estimation M ranged from 0.16 to 0.33. Ce travail constitue une contribution à l'étude de l'âge, la croissance, la longévité, la mortalité naturelle et la taille et l'âge à la maturité de l'aiguillat coq Squalus blainvillei dans le golfe de Gabès (Tunisie, Méditerranée centrale). L'analyse de l'accroissement marginal en fonction des mois a suggéré la formation annuelle de bandes d'arrêt de croissance sur la deuxième épine dorsale. Les estimations d'âge varient de 3 à 15 ans pour les mâles et de 4 à 19 ans pour les femelles. Nous avons ajusté le modèle de Von Bertalanffy et nous avons, par conséquent, déterminé les paramètres L_ = 105,7 cm, K = 0,11 an-1, to = -1,12 ans pour les femelles et L_ = 91,1 cm, K = 0,14 an-1, to = -1,42 ans pour les mâles. Les mâles et les femelles ont atteint la maturité, respectivement à une longueur totale de 52,3 et 62,5 cm à un âge de 4,97 et 7,44 ans. La longévité a été estimée à 24,75 et 31,50 ans respectivement pour les mâles et les femelles, la mortalité naturelle M a été estimée entre 0,16 à 0,33

Age, growth, sexual maturity and reproduction of the thornback ray, Raja clavata (L.), of the Gulf of Gabes (south-central Mediterranean Sea)

The thornback ray Raja clavata is one of the most abundant bycatch species of trawl fishing in the waters of the Gulf of Gabes, 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

Biochemical adaptation of phytoplankton to salinity and nutrient gradients in a coastal solar saltern, Tunisia

International audienceThe distribution of protein and carbohydrate concentrations of the particulate matter (size fraction: 0.45–160 mm) was studied, from 22 January 2003 to 02 December 2003, in three ponds of increasing salinity in the Sfax solar saltern (Tunisia). The coupling of N/P: DIN (DIN ¼ NO2 þ NO3 þ NH4þ) to DIP (DIP¼ PO4 3 ) with P/C: protein/carbohydrates ratios along salinity gradient allowed the discrimination of three types of ecosystems. Pond A1 (mean salinity: 45.0 5.4) having marine characteristics showed enhanced P/C ratios during a diatom bloom. N/P and P/C ratios were closely coupled throughout the sampling period, suggesting that the nutritional status is important in determining the seasonal change in the phytoplankton community in pond A1. In pond A16 (mean salinity: 78.7 8.8), despite the high nitrate load, P/C ratios were overall lower than in pond A1. This may be explained by the fact that dinoflagellates, which were the most abundant phytoplankton in pond A16 might be strict heterotrophs and/or mixotrophs, and so they may have not contributed strongly to anabolic processes. Also, N/P and P/C ratios were uncoupled, suggesting that cells in pond A16 were stressed due to the increased salinity caused by water evaporation, and so cells synthesized reserve products such as carbohydrates. In pond M2 (mean salinity: 189.0 13.8), P/C levels were higher than those recorded in either pond A1 or A16. N/P and P/C were more coupled than in pond A16. Species in the hypersaline pond seemed paradoxally less stressed than in pond A16, suggesting that salt-tolerant extremophile species overcome hypersaline constraints and react metabolically by synthesizing carbohydrates and proteins

Distribution of ciliates in relation to environmental factors along the coastline of the Gulf of Gabes, Tunisia

International audienceWe studied the seasonal distribution of the ciliate community coupled with environmental factors along the coast at three stations sampled (from March 2006 to February 2007) in the Gulf of Gabes (Tunisia, Eastern Mediterranean Sea). A total of 56 species belonging to 11 orders, were identified. Harbor of Gabes station was more diversified (45 species) than both Tabia (26 species) and Karboub (31 species) stations. The ciliate assemblage was numerically dominated by Spirotrichea in Tabia (82% of the total abundance), in the Harbor of Gabes (86% of the total abundance), whereas, in Karboub, Spirotrichea represented only 40% of the total abundance. The unexpected lower quantitative importance of Spirotrichea in Karboub station was apparently the result of the high salt concentration found in water samples throughout the study, probably originating from the saline area surrounding Karboub station, known as Sabkha. The distribution of species in the nearshore of the Gulf of Gabes seemed most likely influenced by the combined effects of temperature, salinity and hydrographic conditions

Age, growth and reproduction of Raja miraletus (Linnaeus, 1758) (Chondrichthyes: Rajidae) of the Gulf of Gabes (Tunisia, Central Mediterranean Sea)

Age and growth of the brown ray, Raja miraletus, was estimated from ring counts on vertebral sections from 95 females (13.5-56.0 cm total length (TL)) and 85 males (13.5-58.0 cm TL) collected from the Gulf of Gabes (Central Mediterranean, Tunisia) during 2007. Marginal increment and edge analyses suggest annual band pair formation. The oldest female and male brown rays were 9 and 7 years old, respectively. Von Bertalanffy growth parameters are L8 = 69.20 cm TL, k = 0.18 year(-1) and t(0) = -0.11 years for females, and L8 = 67 cm TL, k = 0.22 year(-1) and t(0) = -1.01 years for males. Longevity estimates were 19.25 and 15.75 years for females and males, respectively. Based on life history parameters, natural mortality estimation M was in the range 0.37-0.47 for females and 0.60-0.61 for males. Males were found to mature between 28 and 43 cm TL and L-50% was estimated at 34.31 cm TL (21.91 cm DW). Females mature between 33 and 48 cm TL and reached L-50% at 41.8 cm (27.10 cm DW). Age at sexual maturity (A(50%)) was estimated at 2.7 years for males and 4.41 years for females. Females and males have an annual reproductive cycle. Mating occurred mostly through spring and summer