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This paper describes the age structure, growth, diet and aspects of gonadal development in the cobbler, Cnidoglanis macrocephalus (Valenciennes), in the large Swan estuary in south-western Australia between August 1982 and June 1984. Analysis of otolith annuli showed that while the 0+ to 3+ age classes were regularly represented in monthly samples, the 4+ and more particularly the 5+ and 6+ were much less abundant. The weighted means for the back calculated lengths at the end of the first to fourth years of life were 181 mm (≡ 26 g), 314 mm (≡ 156 g), 418 mm (≡ 410 g) and 518 mm (≡ 833 g) respectively. The mean length at the end of the second year of life was similar to the minimum legal size for capture by commercial fishermen (320 mm). The von Bertlanffy growth curve calculated from the back calculated lengths was Lt = 917 [1 − e−0·20(t + 0·11)]. The relative weight of mulluscs, crustaceans and polychaetes in the intestine varied markedly between small and large fish, apparently reflecting differences in the size of these prey. The large mean diameter of mature eggs (
) was correlated with a low mean absolute fecundity (2078). Trends shown by egg size, gonadosomatic index and time of appearance of spent females indicate that spawning takes place between October and December. The attainment of sexual maturity is both age- and size-dependent. Although sexually maturing and occasionally spent fish were found in the lower estuary, meristic values, commercial catch statistics and other data indicate that the cobbler found in the Swan estuary are part of a population which typically spawns at sea. 相似文献
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South Florida's Miami‐Dade agricultural area is located between two protected natural areas, the Biscayne and Everglades National Parks, subject to the costliest environmental restoration project in history. Agriculture, an important economic activity in the region, competes for land and water resources with the restoration efforts and Miami's urban sprawl. The objective of this study, understanding water quality interactions between agricultural land use and the shallow regional aquifer, is critical to the reduction of agriculture's potentially negative impacts. A study was conducted in a 4‐ha square field containing 0·9 ha of corn surrounded by fallow land. The crop rows were oriented NW–SE along the dominant groundwater flow in the area. A network of 18 monitoring wells was distributed across the field. Shallow groundwater nitrate–nitrogen concentration [N‐NO3?] was analyzed on samples collected from the wells biweekly for 3 years. Detailed hydrological (water table elevation [WTE] at each well, groundwater flow direction [GwFD], rainfall) and crop (irrigation, fertilization, calendar) data were also recorded in situ. Flow direction is locally affected by seasonal regional drainage through canal management exercised by the local water authority. The data set was analyzed by dynamic factor analysis (DFA), a specialized time series statistical technique only recently applied in hydrology. In a first step, the observed nitrate variation was successfully described by five common trends representing the unexplained variability. By including the measured hydrological series as explanatory variables the trends were reduced to only three. The analysis yields a quantification of the effects of hydrological factors over local groundwater nitrate concentration. Furthermore, a spatial structure across the field, matching land use, was found in the five remaining common trends whereby the groundwater [N‐NO3?] in wells within the corn rows could be generally separated from those in fallow land NW and SE of the crop strip. Fertilization, masked by soil/water/plant‐delayed processes, had no discernible effect on groundwater nitrate levels. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury’s exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury’s exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K. 相似文献
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