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1.
Samples collected during four cruises on board R.S. Africana were used to study the trophic ecology and feeding behaviour of Engraulis capensis in the southern Benguela region. Previous work had indicated that this species was a non-selective filter-feeding omnivore, diatoms comprising the bulk of the diet. The results of the present study reveal that anchovies selectively feed on mesozooplankton, especially calanoid copepods and euphausiids. Investigation of the feeding behaviour of the species indicates that raptorial feeding is dominant over filter-feeding and that prey appears to be selected primarily on the basis of size.  相似文献   

2.
本研究基于2013年夏季“大洋一号”船大洋第30航次西南印度洋海区科学考察走航线路,对西南印度洋21°S到38°S海域表层浮游动物群落进行连续采样调查,研究了该海域表层中型浮游动物群落结构和物种多样性及空间分布格局。结果显示,研究海域表层中型浮游动物群落组成包括:桡足类、磷虾类、端足类、十足类、介形类、毛颚动物、被囊类、异足类、翼足类、刺胞动物及多毛类等11大类,总计50属69种;优势类群为桡足类(69%)和磷虾类(27%)。多样性指数随经度和纬度的变化特征:H′、D多样性指数及J均匀度指数随纬度的升高均呈下降趋势;在经度梯度上,几种多样性指数也大致呈现出东高西低的趋势。聚类分析表明,研究区可大致以36°S附近为界划分为南部和北部两大类群,分别以北部桡足类(平均粒径小),南部磷虾类(平均粒径大)Euphausia属为优势群落。群落相似性结果反映出,南部类群与南极克罗克海峡和南极长城湾水域群落,北部类群与西北印度洋海域群落的联通性不高。  相似文献   

3.
The purpose of the study is to analyze the state of the Barents Sea euphausiids populations in the warm period (2000–2005) based on the study of their structure dynamics and distribution under the influence of abiotic and biotic factors. For estimation of their aggregations in the bottom layer, the traditional method was used with the help of the modified egg net (0.2 m2 opening area, 564 μm mesh size). The net is used for collecting euphausiids in the autumn–winter period when their activity is reduced, which results in high-catch efficiency. The findings confirmed the major formation patterns of the euphausiids species composition associated with climate change in the Arctic basin. As before, in the warm years, one can see a clear-cut differentiation of space distribution of the dominant euphausiids Thysanoessa genus with localization of the more thermophilic Thysanoessa inermis in the north-west Barents Sea and Thysanoessa raschii in the east. The major euphausiids aggregations are formed of these species. In 2004, the first data of euphausiids distribution in the northern Barents Sea (77–79°N) were obtained, and demonstrated extremely high concentrations of T. inermis in this area, with the biomass as high as 1.7–2.4 g m−2 in terms of dry weight. These data have improved our knowledge of the distribution and euphausiids abundance during periods of elevated sea-water temperatures in the Barents Sea. The oceanic Atlantic species were found to increase in abundance due to elevated advection to the Barents Sea during the study period. Thus, after nearly a 30-year-long absence of the moderate subtropical Nematoscelis megalops in the Barents Sea, they were found again in 2003–2005. However in comparison with 1960, the north-east border of its distribution considerably shifted to 73°50′N 50°22′E. The portion of Meganyctiphanes norvegica also varied considerably—from 10% to 20% of the total euphausiids population in the warm 1950s–1960s almost to complete disappearing in 1970–1990s. The peak of this species’ occurrence (18–26%) took place in the beginning of warm period (1999–2000) after a succession of cold years. The subsequent reduction of the relative abundance of M. norvegica to 7% might have been mostly caused by fish predation during a period of low population densities of capelin. This high predation pressure may therefore have been mediated both by other pelagic fishes (i.e. herring, blue whiting, polar cod) but also by demersal fishes such as cod and haddock. Similar sharp fluctuations in the capelin stock (the major consumer of euphausiids) created marked perturbations in the food web in the Barents Sea in the middle 1980s and the early 1990s.  相似文献   

4.
Abstract. The waters surrounding the Pribilof Islands are an important nursery ground for juvenile walleye pollock (Theragra chalcogramma), an important forage fish in the pelagic food web of the productive Bering Sea shelf region. The diet of juvenile pollock was studied in two consecutive years along a transect line crossing from a well‐mixed coastal domain, through a frontal region to stratified water farther offshore. Variability in stomach fullness was high and evidence for increased feeding intensity in the front was weak. Prey diversity and prey size generally increased with increasing fish size, shifting from predominantly small copepods to larger, more evasive prey items such as euphausiids, crab megalopae and fish. The diet of the fish reflected changes in the relative abundance of copepods and euphausiids in the prey fields between years. Juvenile pollock showed increased feeding rates at dusk, and stomach fullness as well as prey condition were generally lowest just before sunrise; however, the proportion of euphausiids increased in the diet of pollock caught at night, suggesting that some food was also ingested during darkness. Juvenile pollock and their euphausiid prey both vertically migrated above the thermocline at night, although each had a different daytime depth.  相似文献   

5.
The shrimp spawn in autumn, and the females carry their eggs as out roe until spring when the larvae hatch. Within a period of 2 months the shrimp larvae settle to the bottom. It has been claimed that the year-class strength probably is determined during the larval phase. Today's assessment and forecast of the shrimp stock productivity and potential fishing yields are weak. This is partly due to poor knowledge on population dynamics from hatching until the shrimp are caught in the fishery at the age of 3 or 4 years. We, therefore, here identify the most important abiotic and biotic factors that affect recruitment in addition to spawning stock biomass. Since 1995, a net attached to the underbelly of the survey trawl used at the annual cruise in the Barents Sea has caught juvenile shrimp. The abundance of settled shrimp larvae varies in time and space. The recruitment to the fishery has been quite stable with the exception of the 1996 year-class, which was observed as 1-year-olds but has not been registered since. The temporal pattern of the three youngest year-classes is studied in relation to abiotic factors such as sea temperature, ice index and North Atlantic Oscillation, as well as biotic factors such as spawning stock biomass and presence of copepods, euphausiids and predating cod. Recruitment indices and factors identified by the Spearmann correlation to be significantly correlated with recruitment were used as input in a principal component analysis (PCA) and a generalized additive model (GAM) was applied. Abundance of 1-year-old shrimp is positively correlated to spawning stock biomass the previous year and to temperature of the previous winter, and negatively correlated with the number of 1-year-old cod. Two-year-old shrimp show significant correlation with temperature, whereas there is a strong negative correlation with euphausiids. Three-year-old shrimp are significantly correlated with the number of 2-year-old shrimp the previous year but negatively correlated to temperature at sampling time. This is probably due to less overlap with the main predator cod when cold. Ricker functions indicate an increased density-dependent mortality with age. When predicting the recruitment of shrimp to the fishery, the spawning stock biomass, the abundance of cod and euphausiids, as well as the temperature should be included.  相似文献   

6.
云纹石斑鱼与褐石斑鱼形态比较研究   总被引：3，自引：0，他引：3  
对在分类上存在混乱的云纹石斑鱼(Epinephelus moara)和褐石斑鱼(E.bruneus)的外部形态和骨骼系统进行了比较研究,结果表明:两种石斑鱼的体形和体表横带等外部形态特征非常相似,但体表斑纹不同;Fisher逐步判别法分析得出两者在可量性状,如体长/肛长、背鳍基底长/臀鳍基底长和头长/眼径,可数性状,如背鳍鳍条数、鳃耙数和幽门盲囊数等方面的差异达到极显著水平(p<0.001);两者骨骼系统均由脑颅、咽颅、脊柱、肋骨和附肢骨骼组成,主轴和附肢骨骼的形态结构基本相同,但在脑颅的眶前骨和眶后骨、咽颅的前鳃盖骨和尾舌骨、附肢骨骼的髓棘间骨以及尾下骨与尾鳍鳍条连接方式等方面存在明显差异。上述形态与骨骼差异可作为两种石斑鱼种间分类的依据。研究结果确认云纹石斑鱼和褐石斑鱼为两个不同的物种。  相似文献   

7.
Between May 1968 and April 1970 a survey was made of the distribution, vertical range, and seasonal occurrence of euphausiids in Cook Strait.

The euphausiid fauna reflects the predominantly subtropical origin of the coastal currents and, of the 11 species present, only Euphausia vallentini has pronounced subantarctic affinities. Nyctiphanes australis is the dominant species, with Euphausia lucens and Thysanoessa gregaria present in lesser numbers. New records included E. recurva, E. spinifera, and Stylocheiron elongatum, and the identity of Southland Current Nematoscelis euphausiids as N. megalops was confirmed. This assemblage of species, lacking a dominant subantarctic or subtropical element, may best be considered as representing a transitional fauna inhabiting a broad Subtropical Convergence Region.  相似文献   

8.
Zooplankton was sampled through eight depth intervals above about 500 m along a transect of the eastern tropical Pacific (ETP), 23°N to 3°S, encompassing four environments. (1) The California Current—ETP transition off Baja California and the mouth of the Gulf of California is inhabited by California Current species at their southern limits, and by the galatheid ‘red crab’ Pleuroncodes planipes together with euphausiids (e.g. Euphausia eximia) of an abundance-based recurrent group of species, distinguished using the criterion of > x abundance (Numbers under unit area of sea surface) at common localities, adapted to productive zones marginal to the O2-deficient part of the ETP. Tropical species appear here where water with surface temperature > 26°C and [O2] of < 0.1 ml l?1 beneath a shoaling thermocline replaces the upwelling environment off Baja California. (1) The zone 22° to 10°N harbors euphausiids of two groups: the vertically migrating tropical species (e.g. Euphausia diomedeae) which tolerate intense O2-deficiency at their daytime depths and enter the oxygenated mixed layer at night, and non-migrating Stylocheiron species which have vertical ranges extending up into the mixed layer. Nevertheless, most of these ‘ETP-adapted’ species are denser farther south, in the north equatorial countercurrent, but three ETP endemics (e.g. E. distinguenda), all vertical migrators belonging in one subgeneric division of Euphausia, are densest in the O2-deficient regions. (3) The zone of the North Equatorial Countercurrent maintains high densities of three groups: the widely-ranging, ETP-adapted tropical species, the four common Stylocheiron species which, while recurring in abundance at the same localities, differ in depth and the mesopelagic tropical-subtropical species, not tolerant of O2-deficiency, which occur here in easterly tongues of range. (4) At the equator (93°W), easterly ranging species (e.g. E. paragibba) and westerly Nyctiphanes simplex appear to migrate between equatorial currents which differ in direction with depth, thereby maintaining their narrow ranges along the equator. The ‘marginal proliferators’ such as E. eximia, prominent off Baja California, are again abundant here, availing of the equatorial divergence for high productivity and of the oppositely-directed currents for geographical stability.A second recurrent grouping of species, based on presence of their larvae at common localities, yielded groups also distinguishable by whether the larvae lived within or beneath the mixed layer.Ontogenetic strengthening of vertical migration capability is demonstrated by many species, with older larvae, juveniles and adults showing ranges, respectively, increasing from a few meters to up to 400 m. The pattern is the same in O2-deficient regions as elsewhere.Regional distribution of euphausiid volume (wet displacement biomass) tended to agree with zooplankton volume, with maxima at the equator, 8°N, and at some localities off Baja California and the Gulf of California where red crab volume peaked. The depth at which euphausiid volume is equal in amount day and night, across which vertical migration takes place, is designated the equilibrium depth (EqD) for euphausiid volume. EqD for euphausiids generally agreed with EqD for zooplankton volume, indicating that euphausiids play a role in determining depth of EqD for zooplankton volume. Euphausiids comprised 13% (x) of zooplankton volume. 80% (x) of euphausiid volume migrated across EqD, the value showing no significant regional differences. 37% (x) of zooplankton volume engaged in such migration, but in the region south of 14°N encompassing the broad O2-deficient zone, the value was 26%, which compares with 18% previously determined for biomass transferring in a comparable way between epiplankton and planktostad in the same region.  相似文献   

9.
Abalone feed in South Africa is mainly produced from locally sourced grains and has soy or fishmeal as a protein source. Contamination with fungal species associated with grain-based animal feed is, therefore, a predictable risk. Various Fusarium, Penicillium and Aspergillus species are known to produce mycotoxins when colonising grains. The aim of this study was to investigate the occurrence of toxigenic fungal species in the ingredients of compound abalone feeds as well as in final abalone-feed products. In this study, 248 fungal isolates were obtained from feed ingredients and compound abalone feeds collected from three different South African abalone farms and one feed supplier. Morphological and multigene phylogenetic analyses were used to identify the fungi isolated. Fusarium, Penicillium and Aspergillus species were consistently isolated. The presence of these species does not always result in mycotoxin contamination.  相似文献   

10.
The spatial and temporal characteristics of trophic structure of fish communities in the southern Huanghai Sea were examined based on the data sampled from bottom trawl surveys conducted during the autumn of 2000 and the spring of 2001. Hierarchical agglomerative cluster method and bootstrap randomization were used to identify significant trophic groups for each fish assemblage in the southern Huanghai Sea. A total of six major trophic groups were identified within this system, which classified predators based upon location in the water column or prey size （ i. e. , benthic to pelagic predators or fish to small invertebrate prey predators）. The similarity level used to identify significant trophic groups in each assemblage ranged from 24% to 34%. Although planktivores were the dominant trophic group in each assemblage （60% - 79% ）, there were spatial and temporal variations in the trophic structure, which reflected the differences in the abundance and availability of dominant preys. Simplified food webs were constructed to evaluate the most important trophic relationships between the dominant prey taxa and the fishes in each assemblage within this system. Although there were some differences in the key prey species among different food webs, pelagic prey items （mainly euphausiids and copepods） represent the most important energetic link between primary producers and higher trophic level predators. The trophic level for most fishes was between 3 and d, and the weighted mean trophic level for each assemblage ranged from 3.3 to 3.4. Compared with previous study in the mid-1980s, there was an obvious downward trend in the trophic level for most fish species, which resulted mainly from the fluctuation in key prey species in the Huanghai Sea. The decrease in the importance of Japanese anchovy seems to be offset by other abundant prey species such as Euphausia pacifica and copepods （ mainly Calanus sinicus ） .  相似文献   

11.
The diet of at least 28 species of mesopelagic fish from the Pacific coast of Hokkaido was examined. The dominant family was the Gonostomatidae (42%) which was represented by five species. The most abundant species wasCyclothone atraria which together with the other species of this genus preyed predominantly on copepods. Euphausiids and copepods were dominant in the diet ofGonostoma gracile. The next most abundant family was the Myctophidae (32%) which was represented by seven species. The dominant species,Stenobrachius nannochir, preyed mainly on copepods. Copepods were also the dominant food item of the other myctophids except forLampanyctus jordani which fed mainly on euphausiids. The other important family was the Bathylagidae (21%).Leuroglossus schmidti was the dominant species and its diet was more diverse with ostracods, copepods, molluscs and larvaceans being the most important food items.Bathylagus ochotensis had a similar diet. Copepods were the most important food items for all but a few species and their occurrence in the fish stomachs was related to the known vertical distribution of both predators and prey. Ostracods and euphausiids were also important prey items, the latter especially in large fish species. Molluscs and larvaceans were restricted to the two species of the family Bathylagidae.  相似文献   

12.
Cadmium and zinc concentrations were determined in 10 species of pelagic zooplankton collected in the northern North Pacific, the Bering Sea, the Okhotsk Sea and off Hokkaid?, during the summers of 1974–1976. The mean cadmium contents in euphausiids, copepods, and amphipods were 1.16μg/g dry wt (range: 0.36–2.17μg/g), 6.63μg/g (1.66–14.55μg/g), and 8.28μg/g (2.83–14.50μg/g), respectively. Cadmium in euphausiids was significantly lower in concentration than in the other two crustacean groups. The difference in cadmium levels among the crustacean groups is discussed on the basis of feeding habits and physiological characteristics of the zooplankton. Zinc levels in the crustaceans were similar, with values around 100μg/g (range: 59–195μg/g). Both cadmium and zinc concentrations in euphausiids taken from coastal waters off Hokkaid? were significantly higher than those from the Bering Sea and the northern North Pacific. This trend may reflect coastal pollution rather than characteristics of the species. Contribution No. 117 from the Research Institute of North Pacific Fisheries, Faculty of Fisheries, Hokkaido University  相似文献   

13.
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   

14.
Attempts are made to consolidate current information on estimates of standing stock, production and consumption of meso- and macrozooplankton from the shelf region of the west and south coasts of southern Africa for inclusion in a network analysis of carbon flow in the Benguela system. The meso- and macrozooplankton communities consist chiefly of copepods and euphausiids respectively. Although geographic and seasonal differences in standing stock are reasonably well described for the inner shelf (<200 m), knowledge of production and rate processes affecting standing stock is limited, and extrapolation to the whole shelf area (<500 m) provides only a crude appraisal of the real situation. Despite the uncertainties, direct measurements have improved on estimates previously inferred. It seems likely that grazing by meso- and macrozooplankton plays a minor role in phytoplankton losses in the Benguela system. However, it would appear that predation by macrozooplankton, particularly under swarming conditions, has an appreciable impact on mesozooplankton biomass.  相似文献   

15.
The species composition of euphausiids was investigated in relationship to the hydrographic conditions in the North Atlantic cold-core rings (CCR) and adjacent waters to elucidate species succession in evolving water masses. Using data, dating back to the 1970s, from as many CCRs as possible and selecting typical cases where no major physical perturbations occurred, a general pattern of euphausiid succession and change in vertical distribution in rings with time was obtained. This pattern was related to the general distribution of euphausiids in the northwestern North Atlantic Ocean, aiming at providing basic information on probable response of North Atlantic marine ecosystem to global warming. Of the 34 euphausiid species identified, 5 were cold-water species, 17 were warm-water species, 6 were wide-ranging warm-water species, 1 was transitional, 4 were cosmopolitan and the remaining was Thysanoessa parva. Among cold-water species, Euphausia krohni and Nematoscelis megalops were dominant in CCRs. E. krohni became rare in rings older than 6 months, whereas N. megalops survived longer, being abundant in some rings of 9 months or older, by staying within its preferred temperature range as the CCR elevated isotherms sank to depths where they are normally found in the Sargasso Sea and because it is an omnivore–carnivore. Among warm-water species, epipelagic species appeared first in rings, corresponding to the physical change occurring most rapidly in the surface layers. Mesopelagic species appeared later. Cold-water species made up 65–85% of the total euphausiid population in number in younger rings (1–5 months old), while warm-water species contributed only 2–7%. Wide-ranging warm-water species made up about up to one fourth of the total in rings 5 and 7 months old. Warm-water species, mainly E. brevis, increased in older rings (9 months old or older) and made up 50% of the total in the oldest ring. The contribution of cold-water species decreased to 14% in older rings. T. parva made up 26–38% of the total in rings 6 months or older. CCR populations can be characterized by high species number, but intermediate evenness between the Slope Water and northern Sargasso Sea. In CCRs, only a limited number of species were dominant even if there were more species present in rings as old as 9–12 months than in the northern Sargasso Sea. In rings older than 9 months, euphausiids showed two peaks in their vertical distribution: a shallow daytime peak at about 400 m and a nighttime peak in the upper 100 m consisting of warm-water species (mainly E. brevis) and a deeper persistent peak at 800 m or deeper consisting of the species N. megalops and T. parva. This shallow peak in CCRs is shallower than that in the surrounding northern Sargasso Sea, and the deep peak is rarely observed in these waters.  相似文献   

16.
A considerable number ofEuphausia similis G. O. Sars were found to be infected with a parasite in the samples collected in the summer of 1969 in Suruga Bay. The same species was also parasitized by an ellobiopsid,Thalassomyces fagei (Boschma), but it was rare in occurrence. The former parasite is slightly oval in shape with the longer diameter ranging between 1.0 and 1.5 mm, average diameter 1.35 mm. It infests cardiac region, rarely gastric region, of the host. It has been preliminarily identified by Drs.T. Oshima andT. Shimazu of the Shinshu University as the progenetic metacercaria of a trematode species very closely related toPseudopecoelus japonicus; its adult was originally reported to infest several bathypelagic fishes and the encysted stage was also found in jack mackerel from Suruga Bay byYamaguti (1938). The occurrence of the metacercaria inE. similis from the area may be new to science, althoughG. O. Sars (1885) has reported the immature forms ofDistomum filiferum G. O. Sars inNematoscelis megalops G. O. Sars andThysanoessa gregaria G. O. Sars and the crustaceans of wide variety have been proved to serve as the second intermediate host of various trematodes. The euphausiids infected with the metacercaria were concentrated in the innermost part of the bay. This might suggest the presence of the euphausiid population which is confined to the geographical area for some period. The metacercaria may be used as a “biological tag” to trace the migratory range of the host when the life history of the parasite is elucidated. The finding ofT. fagei in the area may be the first record of occurrence of the species in the western North Pacific, andE. similis is now recorded as the fourteenth known host euphausiid of the ellobiopsid.  相似文献   

17.
The Northern Humboldt Current system (NHCS) produces more fish per unit area than any other region in the world. Although the system produces enough macrozooplankton to sustain its high production of forage fish, the paucity of information on macrozooplankton hampers research into the system. In this study, we estimated the biomass of the epipelagic crustacean macrozooplankton from the NHCS during both austral summer and spring 2005. To do this, we developed a bi-frequency acoustic method and extracted high-resolution information on the biomass and the patterns of distribution of crustacean macrozooplankton, fish and other marine compartments. We found that, although macrozooplankton comprises a number of distinct organisms, the euphausiids were the zooplankton group that better fitted the patterns from independent net sampling zooplankton data. Also, the similarities between the nocturnal patterns of size and biomass macrozooplankton distribution from this study and the known patterns of euphausiids, in particular Euphausia mucronata, suggest that euphausiids were the main constituent of the estimated nocturnal acoustic macrozooplankton biomass even if other organisms such as large copepods may have contributed considerably to the macrozooplankton biomass. The total macrozooplankton biomass was estimated to about 105 g m−2, i.e., two to five times more than previous estimates. This direct biomass estimation of macrozooplankton is in agreement with the new findings in trophic ecology indicating that forage fish consume mainly macrozooplankton. This high biomass also supports the current hypotheses explaining the NHCS high fish production. Using the method, we are able to revisit present-day and historical acoustic databases and extract high-resolution data on macrozooplankton, a key ecological compartment of the ecosystem. Since zooplankton is the link between the physically driven primary producers and the biologically driven tertiary consumers, this information is essential to achieve a mechanistic understanding of the system, from physics to top predators.  相似文献   

18.
根据在中国近海采集到的标本,描述多毛类环节动物一新种——中华竹节虫（Sabaco sinicus sp.nov.）。该物种标本在乌沙科夫等（1962）报道后一直被错误鉴定为钩齿短脊虫（Asychis gangeticus Fauvel,1932）。我们基于以下性状将该种描述为一新物种：项沟和前三个刚节具深棕色色素点;刚节4–6的腺体分布特征;肛前节无刚毛;肛漏斗喇叭状,背侧小叶呈扁盘状;具膝状的背刚毛、表面有或无螺旋刺毛带的细毛状背刚毛;腹刚毛为排成一列的鸟嘴状齿片刚毛。钩齿短脊虫和该新种的区别在于前者身体无色素点,肛板背侧小叶呈三角形。自1950s以来,该物种的分布区已经从长江口以南海区明显向北扩展。  相似文献   

19.
Zooplankton sampling at Station 18 off Concepción (36°30′S and 73°07′W), on an average frequency of 30 days (August 2002 to December 2005), allowed the assessment of seasonal and inter-annual variation in zooplankton biomass, its C and N content, and the community structure in relation to upwelling variability. Copepods contributed 79% of the total zooplankton community and were mostly represented by Paracalanus parvus, Oithona similis, Oithona nana, Calanus chilensis, and Rhincalanus nasutus. Other copepod species, euphausiids (mainly Euphausia mucronata), gelatinous zooplankton, and crustacean larvae comprised the rest of the community. Changes in the depth of the upper boundary of the oxygen minimum zone indicated the strongly seasonal upwelling pattern. The bulk of zooplankton biomass and total copepod abundance were both strongly and positively associated with a shallow (<20 m) oxygen minimum zone; these values increased in spring/summer, when upwelling prevailed. Gelatinous zooplankton showed positive abundance anomalies in the spring and winter, whereas euphausiids had no seasonal pattern and a positive anomaly in the fall. The C content and the C/N ratio of zooplankton biomass significantly increased during the spring when chlorophyll-a was high (>5 mg m−3). No major changes in zooplankton biomass and species were found from one year to the next. We concluded that upwelling is the key process modulating variability in zooplankton biomass and its community structure in this zone. The spring/summer increase in zooplankton may be largely the result of the aggregation of dominant copepods within the upwelling region; these may reproduce throughout the year, increasing their C content and C/N ratios given high diatom concentrations.  相似文献   

20.
We describe the distribution and abundance of the midwater fish community, between 200 m and 500 m, in the North Pacific. The main area of interest was the Subarctic Pacific gyres, but we include species from the Bering Sea and the Sea of Okhotsk. There were 196 species identified in each gyre, 38 of which were common to both gyres. The most abundant species belong to the family Myctophidae and the most ecologically important myctophid probably is Stenobrachius leucopsarus. This species could have a biomass of approximately 21 million tonnes (t) in the Subarctic Pacific (including the Bering Sea and Sea of Okhotsk). S. leucopsarus is a small (about 8 cm maximum length) fish that lives up to 7 years. It is prey for a variety of other fishes, birds and mammals and may migrate into the mixing layer each evening where it feeds mainly on euphausiids and copepods.The total abundance of midwater fishes appears to be large relative to total catches of other fish in the same areas. The vertical migratory behaviour of some of the residents provides a mechanism to transfer production out of the mixing layer. The movement into the surface layer by some fishes at night indicates that dynamic changes occur in the midwater community between the day and night, and the ecosystem dynamics in the surface layer are different in the day and in the night. This behaviour and the huge biomass relative to commercial species means that the dynamics of fish communities in the Subarctic Pacific are complex and need to be studied over a 24 hour period. The large biomasses may eventually attract commercial interest, thus it is important to establish international, cooperative programs now to learn more about the dynamics of these populations and the relationships with other species.  相似文献   

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