Sergio Ragonese

Growth, mortality and yield-per-recruit of the deep-water shrimp Aristeus antennatus (Crustacea-Aristeidae) of the Strait of Sicily (Mediterranean Sea)

Abstract The MIX and Compleat ELEFAN methods were used to analyse length-frequency distributions of females of the deep- water shrimp Aristeus antennatus in order to estimate growth, mortality and yield-per-recruit. Length data were gathered in four seasonal experimental trawl surveys made in the Strait of Sicily (Mediterranean Sea). Up to four annual cohorts were distinguishable. Modal lengths were integrated and the parameters of the von Bertalanffy growth function estimated by a weighted non-linear regression, yielding an asymptotic carapace length of 69.1 mm and an annual Brodys coefficient K = 0.532, with a location parameter t 0 = 0 (line through the origin). The consistency of these estimates was evaluated by comparison with the corresponding parameters obtained applying ELEFAN-I and producing a confidence interval by bootstrap resampling; this last procedure revealed the presence of two possible solutions corresponding to a ‘slow’ and a ‘fast’ growth hypothesis. The growth estimates derived from the modal progression analysis in any case fall inside the confidence ellipse of the bivariate plot of the asymptotic lengths and growth coefficients derived from the bootstrapped data sets. The total annual mortality was estimated as Z = 1.1, following Heinckes approximation; two values of annual natural mortality, M l = 0.5 and M h = 0.8, were used afterwards. As a negative allometric length-weight relationship ( b Y R curves suggests that the current Y R can be improved by a moderate increase, from 1 to 1.5 years, in the age of first capture; this could be obtained by increasing the mesh size of the cod-end from the present 18 mm to 28 mm, without any economic loss, even short term.

Abundances of demersal sharks and chimaera from 1994-2009 scientific surveys in the central Mediterranean Sea.

Bibliographic and data gathered in scientific bottom trawl surveys carried out off the Southern Coasts of Sicily (Mediterranean Sea), from 1994 to 2009 and between a depth of 10 and 800 m, were analysed in order to prepare a checklist of demersal sharks and chimaera, which are species sensitive to fisheries exploitation. Out of the 27 previously reported demersal shark and chimaera taxa in the Mediterranean, only 23 were found in literature and 20 sampled during the surveys in the investigated area. Among the species sampled in the surveys, only 2 ubiquitous ( Squalus blainville and Scyliorhinus canicula ) and 3 deep-water ( Chimaera monstrosa , Centrophorus granulosus and Galeus melastomus ) species showed a wide geographical distribution with a consistent abundance. Excluding the rare (such as Oxynotus centrina ) or uncommon shark (e.g. Squalus acanthias ), the estimated frequencies of occurrence and abundance indexes show a possible risk of local extinction for the almost exclusively (e.g. angelshark, Squatina spp.) or preferential (e.g. Scyliorhinus stellaris ) neritic species.

Stock identification of Rajaclavata L. (Chondrichthyes, Rajidae) in two contiguous areas of the Mediterranean

The abundance, occurrence, biological features, growth and mortality parameters of the thornback ray, Raja clavata L. 1758 (Rajformes, Rajidae), coming from two contiguous areas of the Strait of Sicily (Central Mediterranean Sea) were analysed in order to discriminate the stocks. The two investigated areas include bottoms off southern Sicily (I-G16) and bottoms further at large (O-G16). The density indexes, biomass indexes and occurrences were substantially lower in I-G16 than O-G16. On the contrary, the biological traits were more similar, although significant differences were detected in the sex ratio, in the median total length of all specimens, in the length frequency distribution, in the estimated maximum length and in the female length-weight slope parameter. The median length of fully mature specimens did not significantly differ between the two areas. The von Bertalanffy’s asymptotic length and the ratio of total mortality/von Bertalanffy’s curvature parameter suggest a total mortality higher in the I-G16 than the O-G16 populations. This article supports the hypothesis that the investigated populations, in spite of a similar original life cycle, have evolved in two distinct unit stocks as a consequence of the different fishing efforts and the low-intermingling rate between the two areas.

Spatial distribution of Decapoda in the Strait of Sicily (Central Mediterranean Sea) based on a trawl survey

The spatial representation of decapod crustaceans (Crustacea, Malacostraca) of the Strait of Sicily (Central Mediterranean Sea) is presented for the first time, together with comments on their vertical and horizontal distributions. The most relevant trawlable species were analysed using quantitative data gathered in 260 daylight lasting hauls, which covered a large portion (about 100 000 km2) of the investigated area. The index of local abundance, standardized to 1 km2, both in weight (Biomass Index), number (Density Index) and mean body weight were obtained for each species and for all species combined. A spatial representation of the Density Index was produced for all decapod crustaceans combined and for the most frequent species. In particular, the indices were transferred under a GIS system and spatial maps produced by applying the program ArcView™ 9.0. Different interpolators were tested and the most exact interpolator (IDW Inverse Distance Weighting) was chosen. Both historical (i.e., doubts about the presence or identification of the species) and spatial (i.e., differential distribution among different fishing areas) heterogeneity were detected and commented upon. Concerning the trawlable bottoms, very few species (such as Parapenaeus longirostris (Lucas, 1846), Aristaeomorpha foliacea (Risso, 1827), Aristeus antennatus (Risso, 1816), Nephrops norvegicus (Linnaeus, 1758) and Plesionika martia (A. Milne-Edwards, 1883)) are consistently and largely represented in the catches. A high spatial heterogeneity, likely reflecting the complex interaction between fishing pattern, hydrological features and bottom typology, was made evident.

Evidence of trawling impact on Hoplostethus mediterraneus in the central–eastern Mediterranean Sea

The silver roughy, Hoplostethus mediterraneus is a benthopelagic cosmopolitan fish regularly caught as by-catch of the deep-water crustacean trawl fishery (CTF) in the central–eastern Mediterranean. Monthly samples of silver roughy were sampled from the catches of four commercial trawlers in 2004. Each trawler operated in different fishing grounds (FGs), located off Northern Tunisia, South of Sicily, Malta Islands and in South Levant, for which different exploitation levels are reported. The overall length–frequency distribution (LFD) was constructed, and fishing impact indices (length as percentage of LFD, optimum and maximum length, percentage of mega-spawners and total mortality/von Bertalanffy curvature ratio) were calculated. In spite of an overall acceptable status (juveniles, matures and mega-spawners were present in the catch), sampling data revealed significant differences in LFD shape and status indices between FGs. Those FGs traditionally considered more exploited (Northern Tunisia and South of Sicily) showed a dominance of juveniles, a rarefaction of mega-spawners, a reduction in maximum and asymptotic length and a higher Z/K ratio. Considering the general homogeneity of Mediterranean deep-water habitats, the pelagic dispersal of eggs and the poor swimming capabilities of silver roughy, the present results indicated that deep-water trawling may induce a slow and subtle, although significant, erosion of the older, late maturing and slow growing component of the stocks in the Mediterranean (so-called longevity-overfishing).