Browsing by Author "Castro, Margarida"
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- 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.
- 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.
- Comparing trawl and creel fishing for Norway lobster (Nephrops norvegicus): biological and economic considerationsPublication . Leocadio, Ana Maria; Whitmarsh, David; Castro, MargaridaThis study compares the fishing activity and landings of the trawl and creel fisheries for Norway lobster (Nephrops norvegicus (L.)) off the Portuguese coast, and evaluates the financial viability of two vessels typical of each fleet. Crustacean trawlers are part of an industrial fleet that, besides Nephrops, targets deep water shrimps. Creels are used by a multi-gear, multi-target artisanal fleet, fishing only in areas unavailable to trawlers and, when catching Nephrops, set specifically to target this species. Trawlers have in recent years contributed with 85% of the landings in weight, but only 74% in value (2005-2009 average). Despite smaller landings, the Nephrops creel fishery provides individuals of larger size and in better condition, thereby obtaining higher unit prices. Economic viability was also higher for the creel vessel, with trawling being only viable if major costs (such as labor and fuel) are covered by the revenue from other target species (e. g., the rose shrimp). At present, Nephrops populations on the South and SW coast are subject to intense fishing and to a recovery plan. The possibility of reallocation of some of the fishing effort directed at Nephrops from trawlers to creels is discussed in terms of the conservation of the resource and economic return.
- 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.
- 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.
- 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.
- 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.
- Effects of reallocating fishing effort from trawling to creels in a Norway lobster fisheryPublication . Eichert, Moritz; Campos, Aida; Fonseca, Paulo; Lopes, Pedro; Marques, Lino; Castro, MargaridaThe consequences of reallocating fishing effort from trawling to creels in an area off the West coast of Portugal for the operation of the trawl fleet, as well as on the population dynamics and the global catch value for a high valued species, the Norway lobster (Nephrops norvegicus) are evaluated. The results suggest that only small portions of the area of interest are used by the trawl fleet, opening the possibility for a trawl ban without major disruption of the trawling activity. Economic benefits are expected, since predictions two scenarios tested, heavy and moderate exploitation, indicate no loss of catch value with much lower operational costs. Social benefits to local communities can also be expected since a much lower investment is needed for creel fishing, making this fishery attractive to small local companies already operating in the region. The proposed ban is consistent with the ecosystem approach to fisheries management and the Common Fisheries Policy (CFP) where the transition towards more responsible fishing practices is encouraged.
- Fate of discards from deep water crustacean trawl fishery off the south coast of PortugalPublication . Castro, Margarida; Araújo, Artur; Monteiro, PedroTrawling for crustaceans takes place on the south coast of Portugal at depths between 200 and 800 m. Large amounts of discards are released back to sea, reaching the bottom in the general area where trawling occurs. The objective of this work was to study the time taken for decomposition of the discards, to identify the most important scavenging species involved, and to evaluate the impact on the species targeted by the fishery. We conducted a series of 22 trials, using traps baited with samples of the discards. The time of immersion varied between 1 and 40 h. The amount of tissue removed from the discards was evaluated on a qualitative scale of 1–3, and quantitatively. The species caught in the traps were identified. Considerable consumption of the bait had occurred after 5 h of immersion. After 24 h of immersion only fish bones were left; past 40 h, consumption was complete. The conger eel (Conger conger) was the most common fish species present in the traps. Two species, the amphipod Scopelocheirus hopei and the isopod Natatolana (Cirolana) borealis, were identified as most important for the recycling of organic matter in the system. The stomach contents of a sample of species caught in trawls were analysed for the presence of small scavengers. Preliminary results show that S. hopei and N. (C.) borealis are part of the diet of some of the target species of this fishery, such as the Norway lobster, Nephrops norvegicus and the rose shrimp Parapenaeus longirostris.
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