4 resultados para Crescent

em Aquatic Commons


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Shore whaling along North America’s California and Baja California coasts during 1854–99 was ancillary to the offshore and alongshore American whale fishery, which had begun in the North Pacific in the early 1800’s and was flourishing by the 1840’s. From its inception at Monterey, Calif., in the mid 1850’s, the shore fishery, involving open boats deployed from land to catch and tow whales for processing, eventually spread from Monterey south to San Diego and Baja California and north to Crescent City near the California–Oregon border. It had declined to a relict industry by the 1880’s, although sporadic efforts continued into the early 20th century. The main target species were gray whales, Eschrichtius robustus, and humpback whales, Megaptera novaeangliae, with the valuable North Pacific right whale, Eubalaena japonica, also pursued opportunistically. Catch data are grossly incomplete for most stations; no logbooks were kept for these operations as they were for high-seas whaling voyages. Even when good information is available on catch levels, usually as number of whales landed or quantity of oil produced, it is rarely broken down by species. Therefore, we devised methods for extrapolation, interpolation, pro rationing, correction, and informed judgment to produce time series of catches. The resulting estimates of landings from 1854 to 1899 are 3,150 (SE = 112) gray whales and 1,637 (SE = 62) humpback whales. The numbers landed should be multiplied by 1.2 to account for hunting loss (i.e. whales harpooned or shot but not recovered and processed).

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The continental shelf adjacent to the Mississippi River is a highly productive system, often referred to as the fertile fisheries crescent. This productivity is attributed to the effects of the river, especially nutrient delivery. In the later decades of the 2oth century, though, changes in the system were becoming evident. Nutrient loads were seen to be increasing and reports of hypoxia were becoming more frequent. During most recent summers, a broad area (up to 20,000 krn2) of near bottom, inner shelf waters immediately west of the Mississippi River delta becomes hypoxic (dissolved oxygen concentrations less than 2 mgll). In 1990, the Coastal Ocean Program of the National Oceanic and Atmospheric Administration initiated the Nutrient Enhanced Coastal Ocean Productivity (NECOP) study of this area to test the hypothesis that anthropogenic nutrient addition to the coastal ocean has contributed to coastal eutrophication with a significant impact on water quality. Three major goals of the study were to determine the degree to which coastal productivity in the region is enhanced by terrestrial nutrient input, to determine the impact of enhanced productivity on water quality, and to determine the fate of fixed carbon and its impact on living marine resources. The study involved 49 federal and academic scientists from 14 institutions and cost $9.7 million. Field work proceeded from 1990 through 1993 and analysis through 1996, although some analyses continue to this day. The Mississippi River system delivers, on average, 19,000 m3/s of water to the northern Gulf of Mexico. The major flood of the river system occurs in spring following snow melt in the upper drainage basin. This water reaches the Gulf of Mexico through the Mississippi River birdfoot delta and through the delta of the Atchafalaya River. Much of this water flows westward along the coast as a highly stratified coastal current, the Louisiana Coastal Current, isolated from the bottom by a strong halocline and from mid-shelf waters by a strong salinity front. This stratification maintains dissolved and particulate matter from the rivers, as well as recycled material, in a well-defined flow over the inner shelf. It also inhibits the downward mixing of oxygenated surface waters from the surface layer to the near bottom waters. This highly stratified flow is readily identifiable by its surface turbidity, as it carries much of the fine material delivered with the river discharge and resuspended by nearshore wave activity. A second significant contribution to the turbidity of the surface waters is due to phytoplankton in these waters. This turbidity reduces the solar radiation penetrating to depth through the water column. These two aspects of the coastal current, isolation of the inner shelf surface waters and maintenance of a turbid surface layer, precondition the waters for the development of near bottom summer hypoxia.

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The author presents a study on the land shells of the Paracel Islands, group of islands, reefs, banks and other maritime features in the South China Sea. The archipelago includes about 130 small coral islands and reefs, most grouped into the northeast Amphitrite Group or the western Crescent Group. It is approximately equidistant from the coastlines of China and Vietnam.

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This paper deals with the development of crescent-shaped reefs in the China Sea under control of northwest and southeast monsoon winds alternating in direction seasonally. The transport of sediment derived from windward side to leeward around reef ends has been studied. A description of the analogy of these large structures with individual coral colonies developing in similar manner under current action as micro atolls has given.