The Norwegian skate Dipturus nidarosiensis (Storm, 1881) has only recently been recorded in the western Mediterranean Sea along the coast of southern Sardinia, off Algeria and the Alboran Sea. The present study confirmed the presence of the species in the Central Mediterranean Sea by identifying morphometric, morphological features and molecular markers. Biological sampling was conducted from 2010 to 2016 on eight specimens collected through commercial landings, offshore observer programmes and scientific surveys in Adriatic and Ionian waters at depths between 320 and 720 m. The total lengths of the specimens (juveniles and adults) ranged from 268 to 1,422 mm, and their body weights ranged from 44.5 to 12,540.0 g. They showed morphometric features that corresponded to those of Norwegian skates in the Northeast Atlantic and the Western Mediterranean. In previous analyses, molecular data were obtained by mitochondrial COI sequences. The haplotype network showed the occurrence of a common haplotype (Hap_1) shared by the individuals from areas in the North Atlantic, Sardinian, Algerian and Spanish Mediterranean Sea areas but not South Africa. The occurrence of individuals in different stages of life (i.e., juveniles, sub-adults and adults) and sexual development (immature and mature) suggested the presence of a species with a permanent reproductive allocation in the deep waters of the Mediterranean, which was exposed to a low level of fishing exploitation. Indeed, the deep depth distribution of the species could be the reason for the absence of information about this species in onshore or offshore fishery data collection programmes and scientific surveys.
The oocyte maturation and spawning pattern of European hake Merluccius merluccius, one of the most important species in the Mediterranean fishery, were investigated using data collected in a scientific trawl survey (Mediterranean International Trawl Survey, MEDITS) as well as commercial landing data collected in several geographical sub-areas (GSAs): the Southern and Central Tyrrhenian Sea (GSA10), Sardinia (GSA11), the Southern Adriatic Sea (GSA18) and the Western Ionian Sea (GSA19). The lack of the validation of macroscopic maturity scales is one impediment to obtaining reliable estimates of the reproductive parameters used to assess a fish stock. The MEDITS maturity scale was validated for female European hake by histological analysis of the oocyte diameter frequency distributions and according to the gonadosomatic index. The oocyte diameter frequency distributions and histological analysis confirmed that the ovarian organisation of hake was asynchronous, including a batch spawning pattern and indeterminate fecundity. Females in spawning condition were recorded throughout the year in all GSAs although the main peaks of spawning activity were in winter and in late summer and early autumn. The L 50 estimated for GSA10, GSA11, GSA18, GSA19 were 33.03 cm, 30.03 cm, 31.95 cm and 32.95 cm, respectively. Moreover, no significant trends in the annual estimate of L 50 from 2007 to 2015 in any of the study areas were observed. Mean batch fecundity and relative fecundity (360,687 eggs/fish, 281.6 eggsg −1 in GSA10, 361,311 eggs/fish, and 262.2 eggs g −1 in GSA18) were higher than those reported in other Mediterranean areas.
Raja clavata is the most widespread and landed skate species in the Mediterranean Basin. Despite its diffusion and economic importance, several aspects of its life history, such as age and growth, are poorly understood. This study evaluated the species’ growth in the South Adriatic Sea (Central Mediterranean Sea) and for the first time attempted an age validation through a tagging experiment. Thin sectioning of vertebral centra proved to be a more accurate preparation method in terms of age estimation precision and reproducibility than whole vertebral centrum staining (cobalt nitrate and ammonium sulfide technique). Marginal analysis showed a clear seasonal pattern, confirming the hypothesis of a single annulus deposition per year. A total of 291 vertebral centra were sampled and used for age estimation purposes. The oldest female was estimated to be 12 years old [total length (TL) = 89 cm], while the oldest male was aged 8 years (TL = 79.9 cm). Females were also found to be characterized by a slightly wider longevity range (ωL = 11.5, ωU = 16.8 years) than males (ωL = 7.8, ωU = 11.2 years). The von Bertalanffy growth curve fit the age and length data more accurately than the Gompertz and logistic models. Eighty-three thornback rays were tagged and released, of which two were recaptured. In both recaptured specimens, oxytetracycline marks were clearly visible. The band deposition after oxytetracycline injection and growth during the freedom period (about 1 year) were consistent with the age estimation method and criteria used and with the obtained growth results. Thus, the analysis of the vertebral centra extracted from the two recaptured specimens confirmed the hypothesis of the deposition of a single annulus per year and in general the age estimation criteria used in this study.
Maturity is one of the most important biological parameters used in stock assessment programmes. Indeed, the macroscopic stage of gonadal development is an essential feature in estimating the maturity ogive and spawning stock biomass. It is also useful for determining the spawning season of a species and for monitoring long-term changes in the spawning cycle as well as for many other research needs related to the biology of fish. Despite the efforts made during the last few decades to standardize maturity stage data among all the researchers, marine biologists continue without a manageable and unique maturity scale to describe the reproductive development of fish gonads. For these reasons, the objectives of this article are 1) to investigate the biological parameters of five selected key species (Mullus barbatus, Galeus melastomus, Etmopterus spinax, Aristeus antennatus, Loligo vulgaris) with different reproductive strategies following the macroscopic maturity scales based on the same univocal criteria adopted in the MEDITS programme and described in this work; and 2) to compare them among some Mediterranean geographical sub-areas (GSAs) involved in the programme. Neither spawning periods nor size at first maturity (SFM50) for the studied species showed particular differences in the investigated GSAs.
An integrative approach based on morphological and genetic analyses was undertaken for the first time to confirm the species identification of Mediterranean samples belonging to the genus Tremoctopus. Sequences of two mtDNA genes (cytochrome c oxidase subunit (COI) and 16S) were generated for the first time from Mediterranean samples. Both the similarity-based identifications and tree-based methods indicated that three females can be identified as Tremoctopus violaceus sensu stricto in agreement with their morphological classifications. All Mediterranean sequences clustered with the sequences of Tremoctopus violaceus from the Gulf of Mexico and were clearly differentiated from the sequences attributed to T. gracilis and T. robsoni. The chromatic pattern of the web and some features of gill filaments, arms formula, stylets, radulae, beaks, and stomach contents were given for all the samples; 105,758, 20,140, and 11,237 oocytes were estimated in the mature, immature, and developing samples, respectively. The presence of four spermatangia inside the cavity of the maturing female suggested the ability of this species to mate before reaching full maturity with more partners. Age investigation using beaks, performed for the first time in T. violaceus and within the genus gave results consistent with the different sizes and maturity conditions of the samples.
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