Marco L. Bianchini

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.

Effect of codend mesh size on the performance of the deep-water bottom trawl used in the red shrimp fishery in the Strait of Sicily (Mediterranean Sea)

The influence of different codend nets mounting diamond-shaped (16, 20, 24, 28 and 45 mm, mesh side) in the performance of the commercial bottom trawl presently in use in the Strait of Sicily (Central Mediterranean Sea) to catch deep-water red shrimps, Aristaeomorpha foliacea and Aristeus antennatus, is analyzed. In two surveys carried out in fishing grounds in spring and summer 1993, 93 daylight hauls (of 3 hours each) based on the covered codend (cover of 14 mm mesh side) method were carried out. The composition and quantity of the combined, retained and escaped catch for the most relevant species and mixed categories of the different gear configurations (codends + cover) were analyzed and compared by applying both univariate (ANOVA) and multivariate (clustering and multidimensional scaling) techniques. The reduction of by-catch, the economic loss and the economic efficiency were used as indexes of the retention performance of the different codends. The overall catch was 6398 t (23 kg/h); Aristaeomorpha foliacea accounts for most of the catch in number, weight (48% of the total) and value (87% of the total value). Bony fishes, cartilaginous fishes and other crustaceans are the by-catch, representing, respectively, 26%, 16% and 9% of the catch in weight, the rest belonging to cephalopods. The use of the different codends does not influence the overall performance of the gear. No saturation was detected, with the escapement ratio uninfluenced by the amount retained. Considering the retention characteristics, results indicate that the 16 mm and 20 mm mesh codends are not selective, whereas the 24/14 and 28/14 configurations produce increased escaping ability. The 45 mm mesh sieves out 79% of specimens, indicating that no masking effect was induced by the cover. Regarding economic efficiency, the 20/14 configuration provides values comparable to those obtained with the 24/14, whereas the 16/14 configuration is as efficient as the 28/14. The lowest efficiency was obtained using the 45/14 configuration. Multidimensional scaling and clustering techniques (performed on 20, 24 and 28 mm codends) allow to discriminate the escapement ability of the 28/14 gear configuration only. Present results support the introduction of the 28 mm mesh codend in the deep-water red shrimp fisheries in the view of a precautionary management of the resources.

The potential reproductive contribution of Mediterranean migrating eels to the Anguilla anguilla stock

The European eel is a highly migratory fish. After the reproduction in the Sargasso Sea early larval-stages start a passive ocean migration towards European and Mediterranean continental waters. After several years as yellow eels, mature adults change to silver stage and then start their return trip. The trajectory of their backward migration is unknown, because of low probability of capturing migrating individuals, having this capture never been reported in the Mediterranean. Recently, 8 silver eels were collected in the Strait of Sicily. Using literature information about possible individual route and speed, their geographical position was projected up to the spawning site during reproductive season. Despite using optimal and continuous migration swimming speed, none of the specimens may have been able to reach the Sargasso Sea in time for mating. Subsequently, to identify putative Mediterranean areas from which eels could have been reaching the spawning grounds on time, a backward scenario was postulated using the previous scientific assumptions. Our results suggests that just a small quota of Mediterranean silver males successfully reaches the Sargasso area, and only females from the westernmost and central parts of the basin could be able to fruitfully pond their eggs during the supposed spawning period.

TRAWL MESH SELECTIVITY AND BODY ENGAGEMENT PATTERN IN THE RED SHRIMP ARISTAEOMORPHA FOLIACEA (RISSO, 1827) (CRUSTACEA : DECAPODA)

The paper presents selectivity data and utilizes an elliptic surface index (maximum height and width of the folded body), in order to explain the selectivity differences between sexes and to investigate the body engagement pattern of the red shrimp Aristaeomorpha foliacea. The results show that males are retained more, with significant differences, and demonstrates a previously unknown morphological difference between sexes (folded males are significantly higher than females of the same carapace length, while the latter are larger, but not significantly, than males of similar size). Nevertheless, in practice the overall escapement rate should not change dramatically with sex, since the resulting ‘elliptic surface’ is not statistically different.

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.