Fish conservation in South America is a pressing issue. The biodiversity of fishes, just as with all other groups of plants and animals, is far from fully known. Continuing habitat loss may result in biodiversity losses before full species diversity is known. In this review, the main river basins of South America (Magdalena, Orinoco, Amazon and Paraná-La Plata system), together with key aquatic habitats (mangrove-fringed estuaries of the tropical humid, tropical semi-arid and subtropical regions) are analysed in terms of their characteristics and main concerns. Habitat loss was the main concern identified for all South American ecosystems. It may be caused by damming of rivers, deforestation, water pollution, mining, poor agricultural practice or inadequate management practice. Habitat loss has a direct consequence, which is a decrease in the availability of living resources, a serious social and economic issue, especially for South American nations which are all developing countries. The introduction of exotic species and overfishing were also identified as widespread across the continent and its main freshwater, coastal and marine ecosystems. Finally, suggestions are made to find ways to overcome these problems. The main suggestion is a change of paradigm and a new design for conservation actions, starting with integrated research and aiming at the co-ordinated and harmonized management of the main transboundary waters of the continent. The actions would be focused on habitat conservation and social rescue of the less well-off populations of indigenous and non-indigenous peoples. Energy and freshwater demands will also have to be rescaled in order to control habitat loss.

The main objective of this study was to develop a statistical model for accurate estimates of relative growth. The method was based on identifying patterns of the residuals obtained from the Huxley's allometric equation. Three different approaches were applied: (1) growth with variable proportionality and constant allometry coefficient, (2) growth with constant proportionality and variable allometry coefficient and (3) distinct growth phases in which proportionality and allometry coefficients remained constant. The proposed statistical models were applied to the relationship of the otolith size and fish size of whitemouth croaker Micropogonias furnieri. The best fit was obtained when using approach (3). A change in the growth parameters was associated with the attainment of sexual maturity.

Spatial distribution and population structure of Ocypode quadrata (Fabricius, 1787) were studied at Pinhal beach, Rio Grande do Sul, from November/91 to December/93. Analysis of frequency distributions of the ghost crab population was based on indirect census, with burrow diameter measurement. It was identified the presence of two age-groups, with the possible presence of a third one. The maximal life span was estimated to be about 3 years. Burrow diameter/carapace width, lenght/width and weight/width relationships were estimated from samples obtained by excavating burrows and individual capture by traps. These relationship were ajusted by the following equations, respectively: DT = 0,6648 + 1,0013.L; C = 0,77.L 1,02 ; and P = 0,0004.L 3,0876 (DT is burrow diameter; C is the carapace lenght; L is the carapace width; and P is the weight). Spatial distribution analysis was performed based on distance and altitude to water line. These crabs dig burrows preferentialy in the central large range of the beach, although having possibility of migration at adverses conditions. Burrow shape also was analysed. Key words: Ocypode quadrata, ghost-crab, population structure, burrows. RESUMOAnalisou-se a distribuição espacial e a estrutura da população do caranguejo-fantasma Ocypode quadrata (Fabricius, 1787), habitante da praia de Pinhal, no litoral do Rio Grande do Sul, no período de novembro de 91 a dezembro de 93. Para a análise da distribuição etária, utilizou-se o censo indireto, baseando-se em medidas aleatórias dos diâmetros das tocas. Constatou-se a existência de 2 grupos etários distintos, com possibilidade de um terceiro grupo. A longevidade foi estimada em até 3 anos. Para as relações diâmetro de toca/largura do cefalotórax, comprimento/largura e peso/largura, foram tomadas amostras de animais, utilizando-se técnicas de escavação e uso de armadilhas específicas. As equações que descrevem estas relações são, respectivamente, DT = 0,6648 + 1,0013.L; C = 0,77.L 1,02 ; e P = 0,0004.L 3,0876 (DT é o diâmetro da toca; C é o comprimento da carapaça; L é a largura da carapaça; e P é o peso). A análise da distribuição espacial foi realizada através do mapeamento das tocas, baseando-se na distância e no desnível em relação à linha d'água. Constatou-se existir uma preferência pela construção das tocas na grande área central da faixa de praia, podendo haver, no entanto, migração da população em condições adversas. Analisou-se também a morfologia das tocas encontradas, estabelecendo-se padrões para o local. Palavras-chave: Ocypode quadrata, caranguejo-fantasma, estrutura populacional, tocas..

Correct estimates of size at first maturity (L 50 ) are useful for fish stock management and different methods have been proposed. In this study we propose the use of a modified logistic to estimate L 50 , including a variable asymptote (A). We also test the use of the Gonadossomatic Index (GSI) as a flag to establish the reproductive status of each female fish. The method is demonstrated using GSI data for four Neotropical fish species: Characiformes: Astyanax fasciatus (n = 473) and Oligosarcus robustus (n = 297); Siluriformes: Loricariichthys anus (n = 427) and Trachelyopterus lucenai (n = 195). The results were stable enough to propose this method be used for other fish species with different life histories and with a relatively unconstrained sampling programs. Nevertheless, a wide scale test program is desirable to identify any possible bias in this approach.Estimativas precisas do tamanho de primeira maturação (L 50 ) são úteis para a gestão de estoques naturais, existindo diversos métodos disponíveis na literatura. No presente estudo é proposto um método alternativo através de uma equação logística modificada, incluindo-se uma assíntota variável (A). Testou-se igualmente o uso do Índice Gonadossomático (GSI) como indicativo do status reprodutivo para fêmeas de peixes. O método é demonstrado com dados de quatro espécies de peixes Neotropicais: Characiformes: Astyanax fasciatus (n = 473) e Oligosarcus robustus (n = 297); Siluriformes: Loricariichthys anus (n = 427) e Trachelyopterus lucenai (n = 195). Os resultados obtidos foram suficientemente estáveis para propor-se o emprego do método para outras espécies de peixes com diferentes histórias de vida, mesmo que em programas de amostragem limitados. Sugere-se, entretanto, avaliação da metodologia em larga escala como forma de identificar-se possíveis desvios da abordagem proposta.

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