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1.
The study focuses on ichthyoplankton populations in the southern Angola Current, the Angola-Benguela Front and the coastal upwelling area of the northern and central Benguela Current. The horizontal and vertical distributions of eggs and larvae of sardine Sardinops sagax, anchovy Engraulis encrasicolus and horse mackerel Trachurus trachurus capensis were examined in relation to distribution patterns of temperature, salinity and dissolved oxygen. Samples were collected during February–March 2002 using a Bongo net and a multiple opening/closing net in five depth layers (0–30m, 30–60m, 60–100m, 100–140m and 140–220m). Eggs and larvae were restricted mainly to the shelf areas in the Angola-Benguela Font and southern Angola Current waters. Their vertical distributions in the Angola-Benguela Front and northern Benguela upwelling region showed a sharp decline in abundance at about 60m depth. This decline appeared to be correlated to the distribution of dissolved oxygen in the water column It is hypothesised that low oxygen concentrations have a strong impact on the development and survival of the early life-cycle stages of fish, and that recruitment of sardine and other pelagic species relies more on the upward extension of the oxygen minimum layer than was previously thought.  相似文献   

2.
This paper gives an overview of the main living marine resources of Namibia. It focuses on the scientific research conducted during the past decade as input to the management of these resources. The distribution and habitats of the most important harvested species and the main seabird populations are briefly described and discussed. The life histories of the major exploited species are summarized, with emphasis on spatial and temporal spawning patterns, dispersal of early life stages, migration patterns of recruits and adults, and diet, the latter particularly as it relates to potential competition between species. A number of commercially important species, such as the hake Merluccius capensis and M. paradoxus, deep-sea red crab Chaceon maritae, West Coast rock lobster Jasus lalandii, skipjack tuna Katsuwonus pelamis, southern albacore Thunnus alalunga and to a lesser extent Cape horse mackerel Trachurus capensis, southern African sardine Sardinops sagax and Cape anchovy Engraulis capensis, are distributed across national boundaries, requiring regional cooperation in research and management. The history and current status of the major fisheries is discussed. Over the past 30–40 years total annual catches have declined from a peak of around 2 million tons in the late 1960s to less than a million tons in the 1990s. This decline has been due, mainly, to a collapse in the sardine stock in the late 1960s and 1970s, and a reduction in the catches of hake and horse mackerel under a conservative management strategy in the past decade. Changes in the abundance and distribution of commercially important species, as determined by acoustic and trawl surveys and catch-based analytical methods, are presented. The effect of major environmental anomalies on the distribution and abundance of the resources in recent years is discussed. The most dramatic anomaly in recent years was the wide-scale advection of low-oxygen water into the northern Benguela from the Angola Dome in 1994, and the subsequent Benguela Niño of 1995, which appear to have severely impacted the Namibian sardine population and many other resources. The present socio-economic value of the Namibian fishing industry is given together with the broad policy, legislation and formal structures for managing the living marine resources.  相似文献   

3.
Despite a human presence in the Benguela region for at least one million years, exploitation of marine resources by European seafarers only began in earnest in the 1400s. Ecopath with Ecosim was used to construct and compare mass-balanced foodweb models of the southern Benguela ecosystem, representing the following eras of human influence: aboriginal (10 000 BP–1651), pre-industrial (1652–1909), industrial (1910–1974) and post-industrial (1975–present). Biomass at higher trophic levels (TLs) decreased over the periods examined, whereas that of sardine and anchovy increased in the early 2000s, reflected by the decline in weighted TL of the community (excluding plankton). Fishing became an important predatory impact, taking over consumption of small pelagics and horse mackerel from declined natural predators such as hake. Harvesting of apex predators such as seals and seabirds during the pre-industrial era meant that the mean TL of the catch declined markedly between the pre-industrial (1900) and industrial (1960) models. Biomass removals by fishing have increased substantially over time. Total biomass, consumption, respiration, production and throughput decreased from the pristine model to 1960 and then increased again in the 2000s, probably influenced by the abnormally high small pelagic biomass in the early 2000s. Three additional alternate scenarios were examined for each of the retrospective models, in particular to explore the effects of removing large fish and forage fish from the system. Although biomasses and consumption of various groups in these scenarios differed from base models, indicators such as TL of the community and piscivore groups, and the diversity indices, were not altered much, suggesting that outputs from such retrospective models in the form of derived, relative indicators, may be more robust than comparisons of absolute flows, although the latter provide supplementary inferences. Although South African fisheries have certainly impacted ecosystem structure since their commencement, these effects are in addition to natural (specifically environmental) forcing that has always been influencing the system. Fishing stress at the ecosystem level and the collapse of small pelagic stocks may lead to a shift toward a bottom-up trophic control mechanism becoming the dominant driver of ecosystem dynamics, increasing the impact of environmental events including climate change. It is thus possible that pristine systems were not as severely affected by environmental anomalies as are modern systems.  相似文献   

4.
The diet of sardine Sardinops sagax in the southern Benguela was investigated by microscopic examination of stomach contents. The relative dietary importance of prey size and prey type was assessed by calculating the carbon content of prey items. Sardine is an omnivorous clupeoid, ingesting both phytoplankton and zooplankton, with the relative importance of these two food types varying both spatially and temporally. Stomach contents were numerically dominated by small prey items, principally dinoflagellates, followed by crustacean eggs, cyclopoid copepods, calanoid copepods and diatoms. Virtually all prey items ingested by sardine were <1.2 mm maximum dimension, the particle size below which sardine only filter-feed. Despite the numerical dominance by phytoplankton, zooplankton contributed the major portion to sardine dietary carbon, small calanoid and cyclopoid copepods, anchovy eggs and crustacean eggs being the primary prey types. These results indicate that, like anchovy Engraulis capensis, sardine in the southern Benguela are primarily zoophagous, and contrast with earlier dietary studies on sardine in the region. However, the two species appear to partition their prey on the basis of size; sardine consume small zooplankton, whereas anchovy consume large zooplankton. This difference has been observed in other upwelling ecosystems where the two genera co-exist and is likely to contribute to the regime shifts observed between sardine and anchovy.  相似文献   

5.
The results of detailed morphological, experimental, field and modelling studies on various aspects of the trophic ecology of sardine Sardinops sagax and anchovy Engraulis encrasicolus in the Benguela ecosystem are synthesised, and differences in the trophodynamics of these two species are highlighted. Anchovy possess a relatively coarse branchial apparatus; feed predominantly by particulate-feeding and maximise their net energetic gain by using this feeding mode; show higher weight-standardised clearance rates than do sardine for prey >580μm; are most efficient at assimilating nitrogen from zooplankton and excrete <50% of ingested nitrogen; feed inefficiently on phytoplankton and derive the bulk of their dietary input from larger zooplankton; and maximise their scope for growth on mesozooplankton. In contrast, sardine possess a relatively fine branchial apparatus; feed predominantly by filter-feeding and maximise their net energetic gain by using this feeding mode; show higher weightstandardised clearance rates than do anchovy for prey <580μm; are most efficient at assimilating nitrogen from zooplankton but excrete >50% of ingested nitrogen; are able to feed on phytoplankton but derive the bulk of their dietary input from smaller zooplankton; and maximise their scope for growth on microzooplankton. These differences provide compelling evidence that anchovy and sardine are trophically distinct, and indicate that the two species show resource partitioning based on zooplankton size. The implications of these trophic differences for ecosystem functioning are discussed, and support the hypothesis that species alternations between anchovy and sardine, both in the southern Benguela and in other upwelling ecosystems, are likely to be trophodynamically mediated.  相似文献   

6.
Many top predators in the Benguela ecosystem feed on prey species targeted by commercial fisheries. Their roles as indicators of the state of exploited prey resources, as competitors with commercial fisheries for resources, and as susceptible to impact from commercial fishing on those resources are briefly considered. Trends in the occurrence of anchovy Engraulis capensis and pilchard Sardinops ocellatus in the diet of Cape gannets Morus capensis off South Africa's west coast are related significantly to survey estimates of the abundance of these fish species, and they provide useful confirmation of those estimates. In the 1980s, anchovy decreased in the diet of Cape gannets, but pilchard increased. In both the northern and southern portions of the Benguela system, groundfish were thought to eat most (66–73%) of the total quantity of cephalopods and vertebrates consumed by predators and man in the 1980s. South African fur seals Arctocephalus pusillus pusillus, predatory pelagic fish and man removed roughly equal amounts, with squids, seabirds and cetaceans having a smaller impact. In the 1980s, man and seals removed about two million tons live mass more than in 1930. Indices of the rate of natural mortality of anchovy and pilchard attributable to Cape gannets are not related to biomass of the prey species. That for anchovy was high in 1989 when a poor anchovy year-class was formed. Decreased abundance of anchovy led to poor breeding by Cape cormorants Phalacrocorax capensis in 1989 and 1990. A model linking the Cape cormorant population with anchovy is used to explore the impact of possible exploitation strategies for anchovy on Cape cormorants.  相似文献   

7.
There was a marked decrease in abundance of anchovy Engraulis capensis off South Africa between November 1988 and November 1990 caused by formation of poor year-classes in 1989 and 1990. The percentage of anchovy in the diet of Cape gannets Morus capensis indicated that the most marked decrease in anchovy biomass was between June and July 1989. A filament from the Benguela upwelling front was in the process of being entrained by a passing ring of Agulhas Current water at that time. The ring extracted a large volume of frontal water over a period of 2–3 months, prevented its return to the shelf region and possibly removed anchovy larvae and pre-recruits from the Benguela system. It may have played a role in depressing the 1989 anchovy year-class, and hence the 1989 recruit biomass.  相似文献   

8.
The Benguela, California, Humboldt and Canary currents and the coastal waters of Japan support large stocks of sardine or pilchard Sardinops spp. and Sardina sp., anchovy Engraulis spp., horse mackerel Trachurus spp. and chub mackerel Scomber japonicus, with hake Merluccius spp. also abundant in each of the systems except off Japan. In these systems many of the more numerous fish, birds and mammals are opportunistic feeders having catholic diets. As a result species often have overlapping diets, and when a particular food item is plentiful it may be consumed by a wide variety of organisms at higher trophic levels. Frequently the performances of predator populations have been related to the availability of forage species, but predators have not always been influenced similarly by gross changes in the systems. Predators with the most generalized feeding strategies appear best buffered against collapses of particular prey resources. Although seabirds are generally opportunistic feeders, limitations in their foraging range may make them more susceptible than most other predators to system perturbations. Opportunistic feeding by species at lower trophic levels suggests that they could be advantaged by an increased food supply, and it may not be unreasonable to interpret species replacements in catches as reflecting shifts in the dominance of species. Overall catches from the systems have usually been more stable than the catches of individual species, which have shown wide fluctuations. Shifts in the dominance of species in the catches have often been sustained over a number of years. Generalized feeding creates the possibility that more than one species may benefit from energy made available by the collapse of an abundant resource, and there is evidence that species more abundant in former years have in some instances been partially replaced by a number of organisms. The likelihood of a particular organism becoming the main replacing species will probably be influenced by the degree to which it is utilized during and after the collapse of the originally dominant resource.  相似文献   

9.
We synthesise and update results from the suite of biophysical, larval-dispersal models developed in the Benguela Current ecosystem. Biophysical models of larval dispersal use outputs of physical hydrodynamic models as inputs to individual-based models in which biological processes acting during the larval life are included. In the Benguela, such models were first applied to simulate the dispersal of anchovy Engraulis encrasicolus and sardine Sardinops sagax ichthyoplankton, and more recently of the early life stages of chokka-squid Loligo reynaudii and Cape hakes Merluccius spp. We identify how the models have helped advance understanding of key processes for these species. We then discuss which aspects of the early life of marine species in the Benguela Current ecosystem are still not well understood and could benefit from new modelling studies.  相似文献   

10.
The productivities of the Benguela, Peru and California Current upwelling systems and the trophodynamics of the associated hake stocks are compared. Within these large systems, upwelling rate, primary production and fish production are all positively correlated, as would be expected on trophodynamic grounds. Existing measurements suggest that the Benguela and the Peru Current systems are 2–4 times more productive than the California Current in terms of primary production and support 2–10 times more fish per square kilometre. The Benguela and Peru Current systems are 2–3 times more efficient in production of fish and harvestable yield to man as the California Current. The California Current appears to be less efficient primarily because a greater fraction of the annual primary production is transported offshore from the major upwelling sites by the prevailing winds and meandering structure of the currents. The strong seasonal variability in upwelling off California probably also results in poor trophodynamic coupling between the herbivores and phytoplankton. Consequently, less of the phytoplankton is consumed and, potentially, more can be exported out of the system. Hake (Merluccius spp.) are opportunistic, apex predators of euphausiids and pelagic and demersal fish in the three upwelling systems. The striking dominance of euphausiids in the diet of hake in the California Current probably reflects the lesser abundance of demersal fish in that highly advective region.  相似文献   

11.
Recent research developments on the ecology, dynamics and trophic position of copepods in the Benguela ecosystem are synthesized. Attention is focused on herbivorous species of the southern Benguela and how they cope with the physical and biological variability characteristic of this upwelling region. Copepods constitute on average approximately half of the total zooplankton carbon and. are most abundant during the upwelling season. They are able to maintain large population densities within local coastal upwelling areas by combining ontogenetically based vertical migration behaviour with features of the current system. Some species have developed finely tuned strategies to overcome periods of starvation between upwelling bouts by storing lipid reserves or by entering temporary developmental arrest. In situ measurements of production rates of local species are among the highest recorded for copepods. Despite an apparent excess of food, copepods exert only limited impact on the phytoplankton, removing on average <25 per cent of that available daily. Indirect estimates of carbon flux indicate that 11–25 per cent of copepod daily ration is used for egestion of faecal pellets. Copepods are the preferred prey of a wide variety of invertebrate and vertebrate predators. Large copepods in particular are important in the diet of commercially exploited pelagic fish. Localized areas of low abundance of copepods have been found in association with high densities of anchovy during peak spawning and recruitment periods. Copepods may therefore constitute a central limiting factor for pelagic fish production in the southern Benguela.  相似文献   

12.
Cape fur seals Arctocephalus pusillus pusillus were harvested to extirpation on the Robberg Peninsula, Plettenberg Bay, on the south-east coast of South Africa, between the 17th and early 20th centuries. Seals returned to Robberg in small numbers during the early 1990s and their numbers subsequently increased. We studied the diet of this increasing population using faecal (scat) sampling to determine: the species composition and size of prey in the diet of Cape fur seals at Robberg; to explore temporal variation in the diet; and to investigate the potential for competition between seals and the fisheries around Plettenberg Bay. Of the 445 scats collected, 90% contained hard prey remains and 15 teleost prey species were represented in the 3 127 otoliths that could be identified. The seals’ most important prey species in terms of numerical abundance, frequency of occurrence and mass in the diet, were anchovy Engraulis encrasicolus, sardine Sardinops sagax, horse mackerel Trachurus capensis, sand tongue-fish Cynoglossus capensis and shallow-water hake Merluccius capensis (in decreasing order of importance for numerical abundance). The proportion of anchovy in the diet increased during the study period (2003–2008), whereas the proportion of sardine decreased. The estimated average annual consumption of sardine by seals was higher than the average annual catch made by purse-seine fisheries in this area, suggesting resource competition between seals and purse-seiners, especially in the light of continuing growth of seal numbers in the area. However, direct competition between seals and linefisheries appeared to be minimal. Scat sampling of Cape fur seals holds potential to serve a useful and cost effective indicator of temporal changes in sardine abundance.  相似文献   

13.
We developed delta generalised additive models (GAMs) to predict the spatial distribution of different size classes of South African hakes, Merluccius capensis and M. paradoxus, using demersal trawl survey data and geographical (latitude and longitude) and environmental features (depth, temperature, bottom dissolved oxygen and sediment type). Our approach consists of fitting, for each hake size class, two independent models, a binomial GAM and a quasi-Poisson GAM, whose predictions are then combined using the delta method. Delta GAMs were validated using an iterative cross-validation procedure, and their predictions were then employed to produce distribution maps for the southern Benguela. Delta GAM predictions confirmed existing knowledge about the spatial distribution patterns of South African hakes, and brought new insights into the factors influencing the presence/absence and abundance of these species. Our GAM approach can be used to produce distribution maps for spatially explicit ecosystem models of the southern Benguela in a rigorous and objective way. Ecosystem models are critical features of the ecosystem approach to fisheries, and distribution maps constructed using our GAM approach will enable a reliable allocation of species biomasses in spatially explicit ecosystem models, which will increase trust in the spatial overlaps and, therefore, the trophic interactions predicted by these models.  相似文献   

14.
Literature on trophic relationships in the Benguela ecosystem has stressed the importance of cephalopods as prey of groundfish. The groundfish community of the shelf and upper slope of southern Africa is dominated by the Cape hakes, and the results presented (1984–1991) confirm that both species of hake are important predators of cephalopods, especially taking into consideration the abundance of hake in the ecosystem. However, geographic, seasonal and species variability are evident in the patterns observed. The main prey species are Sepia spp. (predominantly Sepia australis), Loligo vulgaris reynaudii, Todaropsis eblanae and Lycoteuthis ?diadema. The last-named is an important food organism for fish. Its systematic status needs revision, however. Qualitative results of studies of cephalopod predation are also provided for kingklip and monkfish.  相似文献   

15.
A regime shift is considered to be a sudden shift in structure and functioning of a marine ecosystem, affecting several living components and resulting in an alternate state. According to this definition, regime shifts differ from species replacement or alternation of species at similar trophic levels, whereby the ecosystem is not necessarily significantly altered in terms of its structure and function; only its species composition changes. This paper provides an overview of regime shifts, species replacements and alternations that have been observed in the northern and southern Benguela ecosystems over the past few decades. Bottom-up control, initiating and sustaining regime shifts or species replacements via environmental forcing, is documented for both the southern and the northern Benguela ecosystems. Fishing (a case of top-down control) appears to have played an important role in regime shift processes in the Namibian ecosystem. Very low biomass levels of exploited fish stocks associated with less efficient energy transfer in the northern Benguela are indicative of a regime shift. Very high biomass levels have been reached in the southern Benguela in the 2000s. However the alternation between sardine and anchovy that has been observed in the southern Benguela over the last two decades appears not to have had major effects on the overall functioning of the ecosystem. The consequences of regime shifts for exploitation are highlighted, suggesting that fisheries managers should move towards a more effective ecosystem approach to fisheries.  相似文献   

16.
Seasonal and short-term variability of environmental parameters influence the spawning strategies of fish species. In this study, the spawning strategies and the transport of early stages of the two Cape hake species off South Africa were investigated. Distribution of eggs and larvae of Merluccius paradoxus and M. capensis was analysed in order to derive more detailed and species-specific information on spawning season, spawning location, and transport of early stages. Samples were collected during three pilot surveys between January and October 2007 and during an extensive survey in September/October 2008 in the southern Benguela upwelling system off South Africa. Eggs and larvae of M. paradoxus were found in greater numbers than those of M. capensis during all surveys. Highest abundances were found from September to October, indicating one spawning peak for M. paradoxus during late austral winter to spring. The western Agulhas Bank was identified as the primary spawning ground, and smaller spawning events occurred on the West Coast. Larvae of both species were mainly distributed in subsurface waters between 25 and 100 m. More than 50% of all larvae caught had a total length between 3 and 4 mm and size increased significantly with decreasing latitude. Merluccius capensis were found closer inshore than M. paradoxus, indicating that early stages of the two species followed separate drift routes. We assume that this distribution pattern most likely evolved from differences in spawning location and phenology. The spawning strategies of M. paradoxus and M. capensis are well adapted to a time-frame of optimal transport conditions favourable for larval survival in the highly variable environment of the southern Benguela upwelling system, but the peak spawning of the two species is separated in time and space.  相似文献   

17.
Ecosystem-based management of marine fisheries requires the use of simulation modelling to investigate the system-level impact of candidate fisheries management strategies. However, testing of fundamental assumptions such as system structure or process formulations is rarely done. In this study, we compare the output of three different ecosystem models (Atlantis, Ecopath with Ecosim, and OSMOSE) applied to the same ecosystem (the southern Benguela), to explore which ecosystem effects of fishing are most sensitive to model uncertainty. We subjected the models to two contrasting fishing pressure scenarios, applying high fishing pressure to either small pelagic fish or to adult hake. We compared the resulting model behaviour at a system level, and also at the level of model groups. We analysed the outputs in terms of various commonly used ecosystem indicators, and found some similarities in the overall behaviour of the models, despite major differences in model formulation and assumptions. Direction of change in system-level indicators was consistent for all models under the hake pressure scenario, although discrepancies emerged under the small-pelagic-fish scenario. Studying biomass response of individual model groups was key to understanding more integrated system-level metrics. All three models are based on existing knowledge of the system, and the convergence of model results increases confidence in the robustness of the model outputs. Points of divergence in the model results suggest important areas of future study. The use of feeding guilds to provide indicators for fish species at an aggregated level was explored, and proved to be an interesting alternative to aggregation by trophic level.  相似文献   

18.
Regime shifts in the Humboldt Current ecosystem   总被引：4，自引：0，他引：4  
Of the four major eastern boundary currents, the Humboldt Current (HC) stands out because it is extremely productive, dominated by anchovy dynamics and subject to frequent direct environmental perturbations of the El Niño Southern Oscillation (ENSO). The long-term dynamics of the HC ecosystem are controlled by shifts between alternating anchovy and sardine regimes that restructure the entire ecosystem from phytoplankton to the top predators. These regime shifts are caused by lasting periods of warm or cold temperature anomalies related to the approach or retreat of warm subtropical oceanic waters to the coast of Peru and Chile. Phases with mainly negative temperature anomalies parallel anchovy regimes (1950–1970; 1985 to the present) and the rather warm period from 1970 to 1985 was characterized by sardine dominance. The transition periods (turning points) from one regime to the other were 1968–1970 and 1984–1986. Like an El Nino, the warm periods drastically change trophic relationships in the entire HC ecosystem, exposing the Peruvian anchovy to a multitude of adverse conditions. Positive temperature anomalies off Peru drive the anchovy population close to the coast as the coastal upwelling cells usually offer the coolest environment, thereby substantially decreasing the extent of the areas of anchovy distribution and spawning. This enhances the effects of negative density-dependent processes such as egg and larval cannibalism and dramatically increases its catchability. Increased spatial overlap between anchovies and the warmer water preferring sardines intensifies anchovy egg mortality further as sardines feed heavily on anchovy eggs.Food sources for juvenile and adult anchovies which prey on a mixed diet of phyto- and zooplankton are drastically reduced because of decreased plankton production due to restricted upwelling in warm years, as demonstrated by lower zooplankton and phytoplankton volumes and the diminution of the fraction of large copepods, their main food source.Horse mackerel and mackerel, the main predators of anchovy, increase predation pressure on juvenile and adult anchovies due to extended invasion into the anchovy habitat in warmer years. In contrast to these periods of warm and cold temperature anomalies on the decadal scale, ENSO events do not play an important role for long-term anchovy dynamics, as the anchovy can recover even from strong ENSO events within 1–2 years. Consequently, the strong 1972–1973 ENSO event (in combination with overfishing) was not the cause of the famous crash of the Peruvian anchovy fishery in the 1970s.  相似文献   

19.
Three fish species, anchovy, hake and lanternfish, occupy different habitats in the neritic zone of the Benguela Current ecosystem. Compatibility between the predicted environmental variability in each habitat and the collection of life history traits found in each species is examined with simulation models. Variability in early-stage survival is introduced into the models by means of both random and non-random ("red") spectra. Traits exhibited by lanternfish are shown to be inefficient in filtering out random variability and the species therefore cannot prevent collapse in a non-randomly variable environment. By comparison, the traits found in anchovy and hake filter out part of both random and non-random variability in survival and allow a fairly stable population size to be maintained. It is argued that, as life-history traits are adapted to the spectrum of variability associated with the habitat, only anomalies with respect to this spectrum are likely to have a large influence on stock size.  相似文献   

20.
Time-series of condition factor (CF) and gonadosomatic index (GSI) were generated using general linear models (GLM) for sardine Sardinops sagax stocks in the northern and southern Benguela ecosystems over the period 1984–1999. During this period the biomass of sardine in the northern Benguela remained at relatively low levels of <500 000 tons, whereas that of southern Benguela sardine increased 40-fold to 1.3 million tons. The GLMs explained 27 and 45% of the observed variation in CF, and 32 and 28% of the observed variation in GSI, for sardine in the northern and southern Benguela subsystems respectively. Whereas the sardine CF in the northern Benguela remained stable over time, that for the southern Benguela stock declined steadily during the study period. Sardine CF showed a seasonal cycle in the southern but not in the northern Benguela. Time-series of GSI showed high interannual variability but no trends in either subsystem, and the seasonal pattern was similar for both stocks. The lack of coherence between the CF time-series for sardine in the two subsystems further suggests that sardine stocks in the northern and southern Benguela subsystems are independent.  相似文献   

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