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821.
822.
This paper examines the optimal tax to achieve maximum economic yield (MEY) exploitation in a rational expectations model of a competitive open-access fishery. To analyze the dynamic evolution of resource use a structural model which explains the relationship between the firm and the industry is presented. The unregulated equilibrium is contrasted with the potential MEY. Conditions under which the unregulated equilibrium will be MEY are explored. In addition, a tax is devised which will cause non-MEY competitive exploitation to become MEY when the tax is implemented. 相似文献
823.
Biodiversity and Ecological Redundancy 总被引:53,自引:0,他引:53
BRIAN H. WALKER 《Conservation biology》1992,6(1):18-23
824.
DENNIS D. COX LAWRENCE H. COX KATHERINE B. ENSOR 《Environmental and Ecological Statistics》1997,4(3):219-233
In this paper we summarize research issues for spatial environmental sampling stemming from a NISS/USEPA workshop held on 21-22 September 1994 at Chapel Hill, NC. 相似文献
825.
Do Habitat Corridors Provide Connectivity? 总被引:33,自引:1,他引:33
826.
The Ria Formosa lagoon, Portugal, receives organic matter from many natural and anthropogenic sources. This paper presents an overview of the contribution these sources made through the use of lipid biomarkers. Sixty-one surface sediment samples were collected and analysed for a range of lipids. Sewage materials were confined to regions within 2 km of discharge points; phytoplankton biomarkers (sterols and fatty acids) suggest production was greatest in the inner parts of the lagoon that also have greatest inorganic nutrient enrichment; terrestrial organic matter was present in relatively low concentrations as shown by both sterol and fatty alcohols; bacteria were widespread although the Sulphate Reducing Bacteria (SRBs, shown by the presence of 3-OH fatty acids) were located near but not adjacent to sewage discharge points. 相似文献
827.
L. Axelsson 《Marine Biology》1988,97(2):287-294
An automatically operated method for high precision measurements of steady-state photosynthesis by macroalgae was developed. Changes in pH and oxygen content of seawater passing the algae in a flowthrough system, could be measured with extremely high accuracy over very long periods of time. The method is especially suitable for measurements on flowthrough systems with high rates of water exchanges (i.e. short retention time), and can be used to study exchange processes for marine plants, animals and small ecosystems. Since the same measuring unit is used for several flowthrough chambers, the method is very suitable for comparisons between different species, or between differently pretreated specimens of the same species (e.g. in toxicological studies). The method was used to study the ratio: [oxygen production] to [CO2+H+ uptake] at different light intensities for several macroalgae belonging to different systematic groups and from different habitats. At lower photosynthetic rates this ratio was similar for all of the algae studied (1.17±0.02). For brown algae of the fucacean family, the ratio increased by 0.08 units at higher photosynthetic rates. This increase was thought to be related to the crassulacean acid metabolism (CAM)-like strategies connected to these algae. For all other algae studied, the ratio remained constant or decreased slightly (at most by 0.04 units) at higher photosynthetic rates. The relations between the abovementioned ratio and the photosynthetic quotient are discussed on a theoretical basis. 相似文献
828.
H. J. Cranfield 《Marine Biology》1973,22(3):211-223
The use during settlement of secretions of the 9 different types of gland in the foot of the pediveliger of Ostrea edulis L. was examined experimentally. During settlement, there is a progressive decrease in the crawling speed of the larva and a strengthening of its bond with the substratum. Both changes are related to (a) a change from ciliary to muscular pedal locomotion; (b) the increasing acidity and viscosity of the mucoid secretions used; (c) the secretion of a byssus thread by the glands of the byssus duet; (d) changes in the form and composition of this byssus. At the termination of settlement, the entire contents of 2 types of gland are discharged. Their secretions give rise to the cement which permanently fixes the larva to the substratum. The 9 types of gland are utilised sequentially in a manner closely related to the 6 phases recognised in settlement behaviour. 相似文献
829.
Behavioral aspects of sperm competition in cuttlefish, Sepia officinalis (Sepioidea: Cephalopoda) 总被引:6,自引:0,他引:6
Sexual selection studies in cephalopods indicate that sperm competition is a central feature of their mating systems, yet
this has not been studied experimentally in any detail. In 1998 we staged 20 matings of the cuttlefish, Sepia officinalis L., in the laboratory. Males rapidly initiated mating in the “head-to-head” position, with no apparent courtship. Mating
lasted an average of 10 min (range 7 to 14 min). For the first 6 min (on average 63% of the mating duration), the male flushed
strong jets of water directly at the female's buccal membrane, which sometimes resulted in the expulsion of parts of spermatangia
placed there in recent matings. Then, in a single discrete movement that lasted an average of only 14 s, the male's modified
fourth left arm – the hectocotylus – wrapped around a single large bundle of spermatophores (ca. 150 to 300) and transferred
them to the female's buccal membrane. For the remainder of the mating (average 3 min, range 1.5 to 5.0 min), the hectocotylus
repeatedly broke the spermatophores open, and manipulated them, so that sperm were released and many spermatangia were attached
along the ventral buccal membrane, near the paired seminal receptacles. Approximately 140 spermatangia were attached in rows
3 to 5 deep around the ventral buccal membrane in a single mating; the rest were usually discarded during mating. Histology
revealed that each of the seminal receptacles consists of a series of sperm storage bulbs connected by a central duct, which
leads to a single pore at the surface of the buccal membrane. Baseline data on sperm motility were obtained, but the mechanism
by which sperm enter the seminal receptacle remains unknown. Females seemed to initiate termination of mating, then males
guarded their mates temporarily. These results, combined with other recent laboratory experiments, provide evidence that sperm
competition may be a major feature of the mating system of S. officinalis.
Received: 4 January 1999 / Accepted: 18 May 1999 相似文献
830.
Ryden JC Whitehead DC Lockyer DR Thompson RB Skinner JH Garwood EA 《Environmental pollution (Barking, Essex : 1987)》1987,48(3):173-184
Emissions of ammonia were measured from livestock excreta and fertilisers applied to grass swards, from grazed paddocks, from decomposing grass herbage and from an animal house containing dairy cows. Emissions from urine, dung, slurry and fertilisers were determined using a system of wind tunnels with each tunnel covering an area of 1 m(2). Emissions from grazed swards were determined using a micrometeorological mass balance method. From the results of these measurements, together with other published information, an inventory for ammonia emissions has been calculated for grassland and livestock production systems over the UK as a whole. It is estimated that emissions from grassland and cattle and sheep production amount to about 230 kt NH(3)-N annually, while emissions from pig and poultry production amount to about 40 kt and 80 kt NH(3)-N, respectively. 相似文献