Aspects of the biology of, and fishery for, the Hottentot, Pachymetopon blochii (Val.) (Sparidae), in the Western and Southwestern Cape

Data extracted from historic records have been used together with information collected during a survey of major fishing centres in the west and southwest Cape, from Port Nolloth to Struis Bay, to document the development of the handline fishery, with special reference to the fishery for hottentot Pachymetopon blochii (Val.). The fishing communities are described and social conditions are compared with those reported in earlier surveys. Changes in the fishing vessels and tackle used in the handline fishery are described and the economic status of the fishery evaluated. The distribution and availability of P. blochii are assessed in relation to the various tackle types and catch methods in different areas and the contribution by this species to the total annual linefish catch is estimated. Total fishing effort expended in the commercial linefishery is used to calculate catch-per-unit-effort indices for the hottentot fishery and the seasonal and long-term trends in these indices are evaluated in relation to past and present constraints on the marketing of this species

Population dynamics and potential yield of three species of giant winkles in the Western Cape, South Africa

Of the southern African intertidal and shallow subtidal trochid and turbinid gastropods, Turbo sarmaticus, T. cidaris and Oxystele sinensis, are the most abundant large species, and therefore obvious targets for a winkle fishery. T. sarmaticus is harvested by recreational snorkel divers, and an application has been made for a permit to experimentally harvest the other two species commercially. This study involves four major aspects, namely morphometrics and flesh yield, shore-based abundance estimates, subtidal abundance estimates, and an estimate of the potential yield for each of the three species off the South-Western Cape, South Africa. Abundance and biomass of the winkles increased from west to east, highest densities being recorded in the low intertidal regions to 2 m depth. Low biomasses of the three species were recorded west of Cape Hangklip and are unlikely to support a fishery there. East of Cape Hangklip, invasion of rock lobsters Jasus lalandii has had a considerable impact on T. cidaris and O. sinensis populations. Consequently, a boat-based fishery with an estimated annual total allowable catch of 75.5 tons for T. cidaris, with a bycatch of 9.9 tons for O. sinensis, can only be considered in the easternmost fisheries-management area between Kleinbaai and Quoin Point. Because previous studies on the harvesting potential of T. sarmaticus off the Western Cape concluded that it was not commercially sustainable, a total allowable catch for that species was not considered. Management options deserving considerations before the commencement of a giant winkle fishery are discussed. Keywords: commercial exploitation, Oxystele sinensis, stock assessment, Turbo cidaris, Turbo sarmaticus, yieldAfrican Journal of Marine Science 2002, 24: 161–18

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Un refractómetro es un instrumento óptico utilizado para medir el índice de refracción de una sustancia, y por tanto el efecto de la refracción de la luz en ese medio. Los refractómetros más habituales se basan en dos principios: los refractómetros de ángulo crítico y los refractómetros de desplazamiento de imagen. Dentro de los primeros se deben destacar los de Abbe y Pulfrich. Carl Pulfrich en 1895 se unió a la fábrica de Carl Zeiss y realizó mejoras al refractómetro de Abbe, incluyendo un sistema de prismas y lentes, una fuente de luz homogénea, un micrómetro para medir el desplazamiento con alta precisión, un telescopio de observación con retículo, una escala graduada en un disco rotativo para leer el índice de refracción o el ángulo medido, y un compensador de dispersión que corrige la dispersión cromática. Este diseño se convirtió en estándar en muchos laboratorios. Carl Zeiss Optical Works mostró dos refractómetros de cristal, uno grande y otro pequeño, en la Exposición Colombina celebrada en Chicago en 1893.Tamaño (altoxlargoxancho):35.2x19x13 cmEl funcionamiento básico de un refractómetro implica la medición del ángulo de refracción cuando la luz pasa de un medio a otro. Para realizar la medida se introduce el líquido a examinar en el recipiente cilíndrico o se deposita la muestra sobre el prisma que tiene la superficie pulida. Se hace incidir, dirigido hacia la base de la muestra, un haz de luz monocromática concentrado mediante la lente y prácticamente rasante. Observando a través del ocular se puede verificar la medida del ángulo límite para determinar el índice de refracción del líquido. El refractómetro tiene diversas aplicaciones en la industria (como, por ejemplo, medir la concentración de líquidos o de azúcar en jugos y jarabes, verificar la pureza de productos químicos y farmacéuticos) y laboratorios científicos de química y biología donde se emplea para identificar compuestos y analizar mezclas, medir salinidad y solutos, siendo crucial en acuicultura y medio ambiente

Predation by West Coast rock lobsters (Jasus lalandii) on two species of winkle (Oxystele sinensis and Turbo cidaris)

Rock lobsters, Jasus lalandii, may play an important role in structuring the communities on shallow reef ecosystems and have recently increased in density in the area between Cape Hangklip and Danger Point on the southwest coast of South Africa. This has probably resulted in increased predation on natural populations of smooth turban shells (Turbo cidaris) and pink-lipped topshells (Oxystete sinensis), both of which are proposed targets for small-scale commercial fisheries. This study examines the size selectivity, prey species preferences and consumption rates of rock lobsters feeding on these two winkle species. Rock lobsters showed a strong preference for small O. sinensis, and were limited to taking a maximum (or critical) size of prey, which increased linearly with rock-lobster size. By contrast, rock lobsters of all size classes tested were able to crack and consume a full size range of T. cidaris, and no size preferences were evident. When the two gastropods were offered simultaneously, rock lobsters preferentially consumed the thinner shelled O. sinensis.Consumption rates increased linearly with rock-lobster size. Population consumption rates indicate that rock lobsters will have a profound effect on winkle stocks in the area, even if winkles constituted less than 1% of the diet. This in turn suggests that commercial exploitation of these stocks is unlikely to remain sustainable

Seasonality of bivalve larvae within a high Arctic fjord

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