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- Influence of tow duration and tooth length on the number of damaged razor clams Ensis siliquaPublication . Gaspar, Miguel; Castro, Margarida; Monteiro, C. C.The incidence of shell damage due to dredging was studied in the bivalve Ensis siliqua off Lagos, on the south coast of Portugal. Three tow durations (1, 3 and 5 min) and 2 tooth lengths (30 and 40 cm) were investigated. Both factors affected the proportion of damaged individuals. The increase of tooth length results in lower proportions of damaged razor clams. An increase in tow duration increased total numbers caught but also increased the proportion of damaged clams. It is suggested that dredges with 40 cm teeth and tows of 1 min duration should be used in this fishery, although experiments should be undertaken in order to evaluate the environmental and ecological impact of dredges.
- Growth studies on Norway lobster, Nephrops norvegicus (L.), in different areas of the Mediterranean Sea and the adjacent AtlanticPublication . Mytilineou, C.; Castro, Margarida; Gancho, P.; Fourtouni, A.A comparative study of the growth of Nephrops norvegicus among different areas in the Mediterranean Sea and the adjacent Atlantic was conducted. MIX and Bhattacharya's length-based methods were used for age determination. Both methods were used for all the studied areas. For the estimation of the growth parameters two non-linear methods, based on the results of the length frequency analysis, were used; the Gauss-Newton method, implemented by the SAS program, was applied using the results of the MIX and the FISHPARM program using the results of the Bhattacharya's method. The identification of the age groups and their mean lengths-at-age as well as the estimation of the growth parameters proved to be difficult. A question regarding the adequacy of the von Bertalanffy model was also posed. Remarkable differences were obvious between sexes in the number of identified age groups and their mean lengths-at-age as well as in their growth parameters in all areas. The comparison of the results obtained for the studied areas showed differences, which could not be considered very important except in the case of the Nephrops population of the Alboran Sea, which was characterised by a high growth rate. All other areas seemed to be close; among them the populations from Euboikos Gulf and Catalan Sea being the most different.
- An alternative methodology for fitting selectivity curves to pre-defined distributionsPublication . Erzini, Karim; Castro, MargaridaA non-linear least-squares methodology for simultaneously estimating parameters of selectivity curves with a pre-defined functional form, across size classes and mesh sizes, using catch size frequency distributions, was developed based on the model of Kirkwood and Walker [Kirkwood, G.P., Walker, T.L, 1986. Gill net selectivities for gummy shark, Mustelus antarcticus Gunther, taken in south-eastern Australian waters. Aust. J. Mar. Freshw. Res. 37, 689-697] and [Wulff, A., 1986. Mathematical model for selectivity of gill nets. Arch. Fish Wiss. 37, 101-106]. Observed catches of fish of size class I in mesh m are modeled as a function of the estimated numbers of fish of that size class in the population and the corresponding selectivities. A comparison was made with the maximum likelihood methodology of [Kirkwood, G.P., Walker, T.I., 1986. Gill net selectivities for gummy shark, Mustelus antarcticus Gunther, taken in south-eastern Australian waters. Aust. J. Mar. Freshw. Res. 37, 689-697] and [Wulff, A., 1986. Mathematical model for selectivity of gill nets. Arch. Fish Wiss; 37, 101-106], using simulated catch data with known selectivity curve parameters, and two published data sets. The estimated parameters and selectivity curves were generally consistent for both methods, with smaller standard errors for parameters estimated by non-linear least-squares. The proposed methodology is a useful and accessible alternative which can be used to model selectivity in situations where the parameters of a pre-defined model can be assumed to be functions of gear size; facilitating statistical evaluation of different models and of goodness of fit. (C) 1998 Elsevier Science B.V.
- Effect of tooth spacing and mesh size on the catch of the Portuguese clam and razor clam dredgePublication . Gaspar, Miguel; Castro, Margarida; Monteiro, C. C.Experiments on the selectivity of the clam and razor clam dredges were carried out during July 1995, on the south coast of Portugal. The effects of mesh size and tooth spacing were investigated for both the clam and razor clam dredges. Four different mesh sizes (25, 35, 40, and 50 mm) and three different tooth spacings (15, 20, and 25 mm for the clams Spisula solida and Venus striatula, and 10, 15, and 20 mm for the razor clam Ensis siliqua) were used. The results indicated that the tooth spacing did not have an effect on the selectivity. The best mesh sizes were 40 mm for the clam fishery (S. solida and V. striatula) and 66 for the razor clam (E. siliqua). These mesh sizes will result in considerable reduction in the numbers of small- and medium-sized individuals in the catch. (C) 1999 International Council for the Exploration of the Sea.
- Measures of dispersion as constraints for length-frequency analysisPublication . Erzini, Karim; Castro, Margarida
- Comparison of several populations of Norway lobster, Nephrops norvegicus (L.), from the Mediterranean and adjacent Atlantic. A biometrics studyPublication . Castro, Margarida; Gancho, Paula; Henriques, PaulaSix populations of Nephrops norvgicus were compared using canonical variate analysis on morphometric characters. The areas where the samples were obtained were the south coast of Portugal (Algarve) in the Atlantic and five areas in the Mediterranean: the Catalan Sea, the Ligurian Sea, the Tyrrhenian Sea, the Adriatic Sea and the Gulf of Euboikos. Each sample consisted of 50 males with sizes chosen to belong as much as possible to the carapace length range of 30-35 mm. Two systems of variables were used: body measurements (74 variables) and counts of spine rows on the carapace (23 variables). Several criteria were used to select experimental units and variables to include in the statistical analysis: a) homogeneous groups for all areas (using as indicators the mean and the variance of the carapace length), b) normal distribution of the original variables within each group, c) homogeneity of variance between groups, d) variables with few missing values. The conjunction of all these criteria led to a reduction of individuals and variables to be included in the analysis. In the end, 27 variables representing body measurements and groups of 22 to 43 individuals for each area were kept. The first two canonical correlation coefficients were highly significant (p<0.0001) with the corresponding variates explaining 81% of the variation. There were no single pairs of populations showing complete separation, although the degree of overlap was different when different pairs were compared. The 3 populations in the West and Central Mediterranean, Catalan Sea, Ligurian Sea and Tyrrhenian Sea showed the highest levels of similarity. The population from the Atlantic showed greatest distances overall, followed by the population from the Euboikos Gulf, representing the Eastern extreme of the geographical range.