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- Development of indices for nonsacrificial sexing of imposex-affected Hexaplex (Trunculariopsis) Trunculus (Gastropoda : Muricidae)Publication . Vasconcelos, Paulo; Gaspar, Miguel; Castro, MargaridaThe muricid Hexaplex (Trunculariopsis) trunculus lacks external sexual dimorphism and is highly affected by imposex, which further complicates its sexual identification. In this context, the aim of this study was to develop sexual indices based on the dimensions of male and imposex-affected female penis, which could constitute a useful, simple and nonsacrificial tool for sexing live specimens of T. trunculus. The adoption of sexual indices consisting of penis dimensions of both sexes relative to individual size revealed a high accuracy in the sexual identification of sacrificed specimens (> 95% correct sexing). Additionally, multivariate discriminant analysis allowed correct sexing of 98.8% of the original 1053 sacrificed individuals, with accurate sexual identification being higher for males (99.5%) than for imposex-affected females (97.7%). An anaesthetization experiment was performed to investigate the effects produced by the anaesthetic (MgCl2) on penis measurements, to test this nonsacrificial approach and to validate the previously developed sexual indices. The anaesthetic provoked an expected enlargement in penis dimensions but, despite this side effect, the sexual indices developed for sacrificed specimens were still highly successful in sexing anaesthetized T. trunculus (generally more than 95% correct sexual identification). The practical application and some limitations of developing and employing this kind of index for the sexual identification of T. trunculus and other imposex-affected gastropod species are discussed.
- 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.
- Aspects of the biology and fishery of the European spiny lobster (Palinurus elephas) from the SouthWest coast of PortugalPublication . Galhardo, Amelia Cristina; Serafim, Paula; Castro, MargaridaThe biology and fishery of the lobster, Palinurus elephas from the SW coast of Portugal was studied during two distinct periods 10 years apart in 1993-1994 (March 93 to March 94) and during 2003 (May to July). The landings at the port of Sagres, representing half of the catch of the country for this species, were sampled twice a week. The ovigerous season extends from September to March, with an individual incubation period of five months. Considering the ovigerous condition as an indicator of maturity in females, 50% of the females were mature at carapace length of 110 mm. Females below this size represent 95% of the population and account for 41% of the egg production. Females above 50% maturity, representing only 5% of the population, provide 59% of the eggs, showing the importance of larger individuals in the reproduction of this species. Most biological parameters estimated are within the range of values reported for this species in other areas with the exception that in our study the total length was smaller in relation to carapace length, so that females of a given carapace length carried fewer eggs. Parameters were estimated for the following relationships: total length and carapace length, weight and carapace length, weight and total length, maturity at size (carapace length) and fecundity at size (carapace length). It is suggested that a set of measures including catch control, effort control, gear control and protected areas are combined to allow the recovery of this resource.
- 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.
- The artisanal fishery for muricid gastropods (banded murex and purple dye murex) in the Ria Formosa lagoon (Algarve coast, southern Portugal)Publication . Vasconcelos, Paulo; Carvalho, Susana; Castro, Margarida; Gaspar, MiguelThe artisanal fishery for muricid gastropods in the Ria Formosa lagoon (Algarve coast, southern Portugal) is a locally important fishing activity because the banded murex (Hexaplex trunculus) and the purple dye murex (Bolinus brandaris) are greatly appreciated seafoods with high commercial value in the Portuguese seafood market. An integrated study was implemented to monitor the muricid gastropod fishery with the artisanal fishing gear ("wallet-line") through monthly experimental fishing operations carried out during one year. The aim Was to describe the fishing operations and fishing gear, to estimate the fishing yield, to characterise the target species catch composition, and to identify by-catch species and discards. The "wallet-line" is neither a species-specific nor a size-selective fishing gear, because the catches comprised a variety of by-catch species and a high proportion of commercially under-sized target species. The vast majority of the by-catch is discarded immediately on board, so mortality is presumably negligible. The CPUE of both target species and by-catch species decreased during consecutive fishing days, mainly due to declining bait attraction. Considering the overall information gathered on this fishing activity, some management measures are proposed for this artisanal fishery, which might ultimately contribute to the long-term Sustainable exploitation of the fishing resource.
- Dynamics of black spot sea bream (Pagellus bogaraveo) mean length: evaluating the influence of life history parameters, recruitment, size selectivity and exploitation ratesPublication . Erzini, Karim; Salgado, M.; Castro, MargaridaStochastic simulations were used to evaluate the influence of recruitment pattern (log-normal, decreasing), size selectivity (normal, logistic model) and fishing mortality pattern (abrupt, continuous increase in fishing mortality) on the evolution of mean length and the dispersion of mean length for a relatively long-lived deep-water species, the black spot sea bream (Pagellus bogaraveo). An abrupt increase in fishing mortality resulted in mean size decreasing and stabilizing at a lower level while a steady increase in fishing mortality caused the continuous decrease in mean size that has been reported for many long-lived species. Decrease in mean size was greatest for logistic model simulations and for cases where fish were susceptible to capture at a small size. Logistic selectivity, with decreasing recruitment and increasing fishing mortality over time, resulted in mean length and variability in mean length trends similar to that observed for the Strait of Gibraltar fishery. Furthermore, it was found with the declining recruitment that moderate increases in fishing mortality can result in significant decreases in mean length. Given the importance of mean size as an indicator of the state of a resource, these simulations are a useful alternative or complement to standard fisheries assessment methods, helping to provide information on exploitation patterns and rates that can be used for conservation and management.
- 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
- Discards of the Algarve (southern Portugal) crustacean trawl fisheryPublication . Monteiro, P.; Araujo, A.; Erzini, Karim; Castro, MargaridaThe crustacean trawl fishery off the Algarve coast (southern Portugal) takes place on the lower continental shelf and upper continental slope at depths between 150 and 600 m. This is a multi-species fishery targeting the shrimps Parapenaeus longirostris and Aristeus antennatus and the Norway lobster Nephrops norvegicus, with the latter two species the most important in the landings. The fishery is characterised by significant by-catch and discarding of a large number of species. As part of a study on the fate of trawl fishery discards, this component of the study focused on the quantification of the by-catch and discards of crustacean trawlers. Sampling took place on board seven commercial trawlers from June 1998 to October 1999. Data was collected from 48 tows in 22 fishing trips. The observers collected all of the catch that was discarded by the crew during the sorting operation and samples were taken to the laboratory for identification, weighing and measuring. The quantities of target species were recorded along with the presence of retained by-catch. Commercially valuable species that were retained included Plesiopenaeus edwardsianus, Aristeomorpha foliacea, Plesionika sp., and the fishes Lophius piscatorius and Merluccius merluccius. However, most of the species had no or little commercial value and were almost always discarded to the sea (90%). A total of 91 species were identified, 47 vertebrates and 44 invertebrates corresponding to 65 families. The Teleostei (78% and 68%) were the dominant group, both in number and weight. The species Micromesistius poutassou (34%), Gadiculus argenteus (10%) and Hoplostethus mediterraneus (8%) were the most important in weight. Ten species represented more than 82% of all discards in weight. Gadiculus argenteus (29%), Hoplostethus mediterraneus (21%) and Nezumia sclerorhynchus (10%) accounted for 60% of all discards in numbers. Data on the landed species composition is also presented.