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1.
长江口基础生物资源现状及年际变化趋势分析   总被引：9，自引：1，他引：9  
探讨长江口生物资源可持续利用和对生物栖息环境的保护措施,2004年对长江口海域的基础生物资源进行4次监测。结果表明:浮游植物5月份有5门39属86种,8月份有6门48属139种,河口锋附近水域的浮游植物种类、数量较多,中肋骨条藻(Skeletonema costatum)在数量上处于绝对优势;浮游动物5月份为77种,8月份为91种,中小型浮游动物的数量在河口锋附近水域较高,主要优势种有针刺拟哲水蚤(Paracalanus aculeatus)等,大型浮游动物种类数量较高的区域位于长江冲淡水以外的海域,主要优势种为中华哲水蚤(Calanus sinicus)、五角水母(Muggiaea atlantica)、真刺唇角水蚤(Labidocera euchaeta);2004年共鉴定底栖生物146种,5月份63种,8月份108种,其中多毛类41种、甲壳类41种、软体动物32种、棘皮动物6种、鱼类21种、其他4种;长江口沿岸的崇明东滩、南汇边滩和嵊山岛共鉴定潮间带生物126种,其中5月份86种,8月份71种;鱼卵仔鱼计8目20科30属34种,鱼卵中绝大部分为凤鲚,其数量占95.7%,仔鱼以中华小公鱼出现频次最多。通过年际变化分析表明,长江口门以内区域浮游生物种类明显下降,长江冲淡水区域的生物资源基础饵料生物种类数基本保持稳定,浮游生物群落结构发生变化,硅藻比重减少,甲藻增加,长江口区域的潮下带和潮间带底栖生物种类数也存在不同程度的下降,以口门以内区域下降明显。  相似文献   

2.
Surgeonfish and parrotfish play an important role in structuring the benthic communities of coral reefs. However, despite their importance, little is known about their distribution patterns in the north sector of the Mesoamerican Reef System. This study evaluated the distribution of these fish in 34 sites in four habitats (lagoon, front, slopes and terrace) along a depth gradient (c 0.5–20 m). These herbivorous fish were assessed by visual censuses. Species dominance was evaluated for each habitat using SIMPER analysis. Habitat characteristics data were collected to determine the relationship between habitat conditions and spatial variations in herbivorous fish (using abundance and biomass as a proxy) via redundancy analysis. The herbivorous fish assemblage had a low density (fish per 100 m2) and biomass (g·100 m?2) in comparison with assemblages in similar studies. In contrast, species richness was high compared with other studies in the Caribbean. Spatial variation of the abundance, biomass and size of herbivorous fish was strongly related to coral and seagrass cover, as well as to depth and rugosity. These four variables were critical in controlling the distribution patterns of the herbivorous fish assemblages. No associations were found between fish and macroalgae or any other benthic group. The present study indicates that the species richness of surgeonfish and parrotfish was not regionally affected by the dominance of macroalgae in the habitats studied. Seagrass beds and the coral reef matrix need to be preserved for the herbivorous fish assemblages to remain healthy and capable of controlling excess macroalgae growth.  相似文献   

3.
The location and seasonal variability of the tidal mixing front in the region of Shantar Islands are studied based on an analysis of satellite data. The Shantar tidal mixing front is related to the main features of the oceanographic structure of the northwestern shelf of the Sea of Okhotsk in summer. This front separates the coastal waters mixed by tidal currents and the stratified part of the shelf. The temperature tidal mixing front forms in July after the melting ice cover and disappears in the end of October when the stratification is broken. The mean position of the front changes insignificantly and is determined by the critical value of the Simpson-Hunter parameter (logh/u 3 = 2.5); the front is located over the isobath of 50 m. The temperature tidal mixing front corresponds to the front in the distribution of chlorophyll a determined from SeaWiFS and MODIS satellite imagery. High (when compared to the stratified part of the shelf) concentrations of chlorophyll a were observed within the zone of intense tidal mixing. Satellite images in the IR range of the spectrum (Landsat-5 TM) demonstrated that the front is dynamically unstable. Mixing effects connected with frontal submesoscale baroclinic eddies have an influence on the structure of the stratified part of the shelf.  相似文献   

4.
丹麦海峡海洋锋可为局地气候变化、海峡鱼类分布以及海峡中尺度涡等方面研究提供参考,在军事领域也具有较高应用价值。然而目前国内外缺乏对于丹麦海峡锋的系统研究。本文利用WOA13数据,对丹麦海峡锋位置、强度空间分布以及相应的季节变化进行了分析。结果显示,锋轴线位置在34°W以西150 m以浅相对稳定,34°W以东锋轴线随深度和季节均有摆动。锋强度在水平和垂直方向分布不均匀,水平方向上锋轴线最大值与最小值差值一般在3倍以上。垂直方向盐度锋主要集中在100 m以浅,且强度随深度不断减小;温度锋强度最大值在表层以下且在300 m以深随深度增加锋强度减小,但有一些区域如27°W和28°W附近,受到海底地形的影响温度锋强度随着深度的变化有大幅度增加的特点。  相似文献   

5.
In the late 1990s, the southeastern Bering Sea exhibited a number of anomalous conditions, including a major die-off of short-tailed shearwaters (Puffinus tenuirostris), a trans-equatorial migrant that constitutes a major portion of the marine bird biomass in the southeastern Bering Sea. As part of a larger study of the ecological role of the inner or structural front over the southeastern Bering Sea shelf, in 1997–1999, we collected short-tailed shearwaters to determine diet composition. In spring 1997, we found that short-tailed shearwaters were consuming predominately the euphausiid Thysanoessa raschii, a diet expected on the basis of past studies. However, in subsequent years, short-tailed shearwater diets in spring contained increasingly larger proportions of fish, in particular, sandlance (Ammodytes hexapterus), as well as other species of euphausiids (T. inermis in 1999). In summer and fall collections, short-tailed shearwater diets were more varied than in spring, and included both fish (age-0 gadids, 21–35% by weight) and a wider variety of euphausiid species (T. inermis and T. spinifera). In summer and fall, crab zoea (August 1998) and copepods (August 1999) were eaten by shearwaters collected while feeding within the inner front. Diets in 1997–1999 were broader than those found in previous studies of short-tailed shearwaters over the inner shelf and Bristol Bay, which had documented diets composed almost solely of T. raschii. Our data are consistent with the hypothesis that euphausiids were less available to short-tailed shearwaters foraging over the middle and coastal domains of the southeastern Bering Sea in 1997–1999 than has previously been true. Our results are also consistent with hypothesis that the inner front can affect the availability of prey to shearwaters.  相似文献   

6.
《Oceanologica Acta》1998,21(1):87-93
A space-time approach is adopted in an attempt to understand the long-term influence of currents on the spatial pattern distribution of zooplankton, ichthyoplankton, and fish larvae communities in the Bay of Campeche, Mexico.The analysis is performed using results of a hydrodynamic numerical model, and zooplankton and ichthyoplankton abundances.Three fish larvae communities were identified in the Bay, on the basis of its physiography, i.e.: “neritic fluvio lagoon influence”, “neritic” and “oceanic” communities.A cyclonic eddy is formed in the Bay and drifts westward, facilitating transport and distribution of planktonic organisms. The eddy provides larvae transport from oceanic areas to the coastal areas and vice-versa. The seasonal distribution of fish larvae is highly influenced by the position of the eddy.The biomass pattern and fish larvae density observed in the coastal area off the Grijalva-Usumacinta delta are governed by the coastal front.  相似文献   

7.
Distribution of the sea surface temperature (SST) across the Kuroshio has been measured in the Tokara Strait by the Kagoshima Prefectural Experimental Fishery Station, using a thermometer installed on boardEmerald-Amami, a ferry that operates regularly between Kagoshima and Naha. The data from 1 October 1978 to 30 September 1981 were analyzed in this paper.A sharp temperature front is usually formed at the northern edge of the current zone of the Kuroshio, and its position is very variable and moves north and south between Satamisaki and Nakanoshima. The northward migration of the front can easily be traced, but the southward migration is obscure in many cases. Some of the southward migrations seem to be understood as arising from the alternate appearance of two different fronts, namely a weakening of the northern front and a strengthening of the southern front, which are associated with the double structure of the Kuroshio front. The temperature contrast across the Kuroshio front is very weak in August through October, and the phase of its seasonal variation lags a few months behind that of temperature itself. Transitions between the states with and without temperature contrast occur suddenly, though the transition times differ year by year. Periodical fluctuations with a period of several tens of days are often observed in the migration region of the Kuroshio front. The fluctuations sometimes look very periodical within limited time periods, but the fluctuations are very changeable in nature from year to year.The results show that continuous observation of the SST distribution across the Tokara Strait yields a good tool for monitoring fluctuations of the Kuroshio path and the occurrence of the Ohsumi Branch Current, at least in the season when a large horizontal temperature contrast exists.  相似文献   

8.
半滑舌鳎(Cynoglossus semilaevis)摄食行为感觉作用的研究   总被引：1，自引：1，他引：1  
采用实验室内鱼类行为学研究方法对半滑舌鳎的摄食行为进行研究。结果表明,嗅觉、味觉和侧线在半滑舌鳎的摄食中具有重要作用,视觉在摄食中的作用没有被发现。侧线在摄食中作用主要体现在头部侧线,躯干部侧线对侧面刺激有一定的攻击行为,但行为发生率与头部各部位受刺激时产生的行为发生率相比较低,且差异显著(P<0.05);躯干部对上部刺激仅发生警戒行为,没有观察到攻击行为。侧线和嗅觉在摄食中的作用大小比较差异显著(P<0.05),侧线的作用优于嗅觉。口咽腔对味道和硬度都有作用且对二者的敏感程度差异显著(P<0.05),对味道更为敏感。  相似文献   

9.
In this paper, I review my study on heterogeneous distribution of plankters at oceanic fronts, taking advantage of an opportunity awarded the Okada Prize 1989 of the Oceanographical Society of Japan. The main focus is on the formation and retention mechanisms of phytoplankton peak abundance at the Kuroshio Front, and the events observed at other oceanic fronts, such as the Oyashio Front, the Antarctic Polar Front and the Subtropical Convergence in the southern hemisphere are compared to those found at the Kuroshio Front.Phytoplankton standing stock was hign in the inner cold belt of the Kuroshio Front. Since the inner cold belt form from water masses entrained into the front from coastal and/or the Oyashio areas, one of the characteristics of the phytoplankton community is high contribution of microplankton fraction. Rising of isopleths of temperature and nutrient saltsetc. at the front suggested that upwelling event occurred along the front. Incubation experiments with nutrient addition showed it would flourish the entrained species but oceanic ones when upwelling occurred at the front.Zooplankton was strongly aggregated at the Kuroshio Warm-Core Ring Front, and the peak of abundance was spatially separated in each individual. I thought that biological processes, such as motility and prey-predator interaction, play an important role to control the aggregated patterns.Since the cabbeling event is expected at the Oyashio Front where the relatively cold and low-salinity Oyashio water faces the relatively warm and saline water of the perturbed area, it appears that the phytoplankton species having larger density than that of sea water sink down there. The mixture of sea waters on both sides of the front, however, may accelerate their growth by supply of some deficient element (s).I consider that raise of temperature is the most plausible factor to make phytoplankton peak abundance at the Polar Front. Experiments on board showed that raising temperature activates photosynthesis of antarctic phytoplankton. Phytoplankton standing stock and productivity were also high at the Subtropical Convergence.The primary object of this study is the biological role of oceanic fronts for fish ecology. I consider that concentrated phyto- and zooplankton communities at oceanic fronts support the energy of migrating fishes, and also support the survival of juvenile fishes just after hatching which are transported from coastal areas.  相似文献   

10.
We propose a new hypothesis, the Oscillating Control Hypothesis (OCH), which predicts that pelagic ecosystem function in the southeastern Bering Sea will alternate between primarily bottom-up control in cold regimes and primarily top-down control in warm regimes. The timing of spring primary production is determined predominately by the timing of ice retreat. Late ice retreat (late March or later) leads to an early, ice-associated bloom in cold water (e.g., 1995, 1997, 1999), whereas no ice, or early ice retreat before mid-March, leads to an open-water bloom in May or June in warm water (e.g., 1996, 1998, 2000). Zooplankton populations are not closely coupled to the spring bloom, but are sensitive to water temperature. In years when the spring bloom occurs in cold water, low temperatures limit the production of zooplankton, the survival of larval/juvenile fish, and their recruitment into the populations of species of large piscivorous fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias). When continued over decadal scales, this will lead to bottom-up limitation and a decreased biomass of piscivorous fish. Alternatively, in periods when the bloom occurs in warm water, zooplankton populations should grow rapidly, providing plentiful prey for larval and juvenile fish. Abundant zooplankton will support strong recruitment of fish and will lead to abundant predatory fish that control forage fish, including, in the case of pollock, their own juveniles. Piscivorous marine birds and pinnipeds may achieve higher production of young and survival in cold regimes, when there is less competition from large piscivorous fish for cold-water forage fish such as capelin (Mallotus villosus). Piscivorous seabirds and pinnipeds also may be expected to have high productivity in periods of transition from cold regimes to warm regimes, when young of large predatory species of fish are numerous enough to provide forage. The OCH predicts that the ability of large predatory fish populations to sustain fishing pressure will vary between warm and cold regimes.The OCH points to the importance of the timing of ice retreat and water temperatures during the spring bloom for the productivity of zooplankton, and the degree and direction of coupling between zooplankton and forage fish. Forage fish (e.g., juvenile pollock, capelin, Pacific herring [Clupea pallasii]) are key prey for adult pollock and other apex predators. In the southeastern Bering Sea, important changes in the biota since the mid-1970s include a marked increase in the biomass of large piscivorous fish and a concurrent decline in the biomass of forage fish, including age-1 walleye pollock, particularly over the southern portion of the shelf. Populations of northern fur seals (Callorhinus ursinus) and seabirds such as kittiwakes (Rissa spp.) at the Pribilof Islands have declined, most probably in response to a diminished prey base. The available evidence suggests that these changes are unlikely the result of a decrease in total annual new primary production, though the possibility of reduced post-bloom production during summer remains. An ecosystem approach to management of the Bering Sea and its fisheries is of great importance if all of the ecosystem components valued by society are to thrive. Cognizance of how climate regimes may alter relationships within this ecosystem will facilitate reaching that goal.  相似文献   

11.
The Río de la Plata Estuary presents a strong bottom salinity front located over a submerged shoal. Apparently favored by retention processes, it is a spawning ground for several coastal fishes. This estuary is very shallow and essentially wind driven and, moreover, in time scales relevant to biota, estuarine circulation is wind dominated and highly variable. Two intriguing questions are, therefore, how this system can favor retention and what the involved mechanisms are. This paper qualitatively explores mechanisms involved in the estuary where retention is favored applying numerical simulations in which neutral particles – simulating fish eggs and early larvae – are released along the bottom frontal zone and tracked for different wind conditions. Results suggest that retentive features can be a consequence of estuarine response to natural wind variability acting over bathymetric features. For winds from most directions, particles either remain trapped near their launching position or move northeastward to southwestward along the shoal. As alternation of winds that favor along-shoal motion is the dominant feature of wind variability in the region, a retentive scenario results from prevailing wind variability. Additionally, winds that tend to export particles with a poor chance of being restored to the front are neither frequent nor persistent. Results show, therefore, that physical forcing alone might generate a retentive scenario at the inner part of this estuary. The physical retention mechanism is more effective for bottom than for surface launched particles. Wind statistics indicate that the proposed mechanism has different implications for retention along the seasons. Spring is the most favorable season, followed by summer, when particles would have a larger propensity to reach the southern area of the estuary (Samborombón Bay). Fall and winter are increasingly less favorable. All these features are consistent with patterns observed in the region in organisms having different life history traits.  相似文献   

12.
We determine the characteristic features of the space structure of the Scotia-Sea Front and study the variations of its characteristics on the seasonal and interannual scales. It is shown that, in the climatic seasonal cycle, the Scotia-Sea Front is intensified in the warm period of the year when the temperature contrast between the cold waters formed as a result of thawing of ice and warmer waters transported by the South Branch of the Antarctic Circumpolar Current becomes more pronounced. The maximum seasonal variations of the intensity of this front are observed in the region of minimum intraannual displacements of the boundary of drift ice. The interannual variations of the characteristics of the front are observed in the form anomalous variations of its intensity and latitudinal displacements. A significant relationship is established between the interannual anomalies of the characteristics of the front and the areas of warm tropical waters in the Pacific Ocean caused by the events of El Niño.  相似文献   

13.
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   

14.
Although underwater visual census (UVC) is the most frequently used technique for quantifying reef fish assemblages, remote video analysis has been gaining attention as a potential alternative. In the South Atlantic Ocean, Millepora spp. (class Hydrozoa) are the only branching coral species; however, little is known about the ecological role that they play for reef fish communities. We compared these two observation methods (remote video and UVC) to estimate reef fish abundance and species richness associated with colonies of the fire‐coral Millepora alcicornis at Tamandaré Reefs, Northeast Brazil. Additionally, the two different techniques were used to compare species behaviour in association with fire‐corals in order to examine the biases associated with each technique and provide useful information for behavioural ecologists studying fish–coral associations. There were no differences in reef fish abundance or species richness sampled by remote video or UVC. However, a significant difference in the behaviour of associated fish was recorded between the two methods. In the presence of a diver carrying out a UVC, fish were observed spending more time sheltered amongst the coral branches compared with passively swimming on coral colonies with the remote video technique. Specifically, on the remote video recordings agonistic interactions between fish and passive swimming accounted for 33.3% and 22.2% of the census time, respectively. By comparison, when observed by a diver fish spent 34.8% of their time sheltering amongst the coral branches. We demonstrate that both techniques are similarly effective for recording fish abundance and species richness associated with fire‐corals. However, differences were observed in the ability of each method to detect the behaviour of coral‐associated fishes. Our findings show that behavioural ecologists studying complex fish–coral associations need to ensure that their aims are clearly defined and that they choose the most appropriate technique for their study in order to minimize methodological biases.  相似文献   

15.
The fish fauna of the Berg River Estuary was sampled from the mouth to 40 km upstream using a small-meshed seine-net before (summer 1993), during (summer 1994) and after (summer 1996) a low-oxygen, hydrogen sulphide “black tide” event that caused a mass mortality of fish in St Helena Bay. These data were compared to determine how the species composition, abundance and distribution of the fish fauna of the Berg River Estuary differed before, during and after the event as well as to ascertain which species, if any, found refuge in the estuary. The overall catch-per-unit-effort of 1637 fish.haul−1 during the event was almost double the 932 fish.haul−1 and 643 fish.haul−1 in the years before and after respectively. All the fish recorded alive in the estuary during the event were species known to have some degree of estuarine association. No representatives of the purely marine species found dead on the adjacent shoreline were recorded live in the estuary during the event. Of the 10 estuarine-associated species sampled, 5 extended their range and/or modal peaks of abundance further upstream during the event. One species, Liza richardsonii, was abundant enough to examine its size distribution in different breaches of the estuary. Large/adult fish were concentrated further upstream than small/juvenile fish, which appeared to be unable to escape tidal currents and were concentrated at the edge of the low-oxygen front. Collectively this circumstantial evidence indicates that (1) fish used the Berg Estuary as a refuge from low-oxygen conditions in the marine environment during the “black tide” event, and (2) the ability to secure refuge in the estuary was restricted to species described as “estuarine-associated” or “estuarine-dependent”.  相似文献   

16.
Whereas diel fish migration between mangrove and seagrass habitats has been recognized for decades, quantitative studies have focused mainly on diurnal patterns of fish distribution and abundance. In general, previous studies have shown that fish abundances decline with increasing distance from mangroves; however, evidence for such a pattern at night, when many fishes are actively feeding, is scarce. The present study is the first to report nocturnal fish abundances along a continuous distance gradient from mangroves across adjacent seagrass habitat (0–120 m). Here, we used nocturnal seine sampling to test the null hypothesis (based on diurnal studies and limited nocturnal work) that fish abundance would decrease with increasing distance from shoreline. We focused on species and life‐stage‐specific abundance patterns of Lutjanus griseus, Sphyraena barracuda, Archosargus rhomboidalis, and Haemulon sciurus. Results indicated that assemblage composition and structure differed significantly by season, likely influenced by temperature. However, within each season, the fish habitat use pattern at both the assemblage and species‐specific level generally failed to support our working null hypothesis. Species‐specific analyses revealed that, for most species and life‐stages examined, nocturnal abundance either did not change with distance or increased with distance from the mangrove‐seagrass ecotone. Our results suggest that analyses where taxa are grouped to report overall patterns may have the potential to overlook significant species‐ and stage‐specific variation. For fishes known to make nocturnal migrations, we recommend nocturnal sampling to determine habitat utilization patterns, especially when inferring nursery value of multiple habitats or when estimating fish production.  相似文献   

17.
基于个体模型的东海鲐鱼渔场形成机制研究   总被引：4，自引：2，他引：2  
鲐鱼Scomber japonicus资源丰富,在我国近海渔业中占有重要地位。其渔场的形成受海洋环境的制约,本文确定鲐鱼运动和物理环境之间的响应关系,建立起了基于个体的东海鲐鱼生长洄游模型。结果显示,鲐鱼集群分布与捕捞生产渔场基本吻合,鲐鱼聚集主要受台湾暖流、大陆沿岸水、黑潮影响,往往集群在一定温度范围内并在冷暖交汇区温盐梯度大偏暖水一侧。在台湾暖流和沿岸水交汇的锋面附近、台湾暖流暖水舌前端、黑潮与中国大陆沿岸水形成的潮境区域均有大量的鲐鱼聚集,并形成渔场。产卵位置的变动使偏西产卵位置的鲐鱼由于受台湾暖流影响较大,鲐鱼会呈长带状大量聚集在台湾暖流和沿岸水的锋面附近,并使在台湾暖流暖水舌前端的聚集数量增多,而偏东的产卵的鲐鱼受黑潮影响较大,聚集分布范围较大,会使黑潮形成的锋面附近聚集数量增多,而使台湾暖水舌的前端的聚集量减少。正常产卵位置在生存率方面是最佳产卵位置。研究表明鲐鱼所处空间位置不同,会影响其集群的位置,用数值模型验证了物理环境会对鲐鱼的洄游和渔场的形成产生影响。  相似文献   

18.
《Journal of Sea Research》2007,57(2-3):230-235
The live body mass, body volume, and length were determined for four groups of juvenile plaice (Pleuronectes platessa L.): (1) plaice immigrating into the Wadden Sea, (2) fish that had settled in the Wadden Sea, (3) settled fish that had been starved, and (4) laboratory-reared fish. Mass and volume values were obtained by applying Archimedes' principle to weights of fish obtained while they were immersed in waters of different densities. The allometric relationship of mass, volume, and length was determined for each group using principal component analysis. Fish in each treatment group had different mass-volume-length trajectories. Immigrating fish had the least mass and volume at fixed length, followed by settled fish, and laboratory-reared fish had the greatest mass at fixed length. Mass and volume differences between freshly captured settled fish and starved fish were slight. All treatments showed great variability in body mass and volume at fixed length, and the use of fish length alone as an index of fish size is problematic. It is suggested that centroid size is a better measurement of fish size when size is an important variate in a study.  相似文献   

19.
Two oceanographic surveys were conducted across the salinity gradient of the Río de la Plata Estuary during austral late spring and summer. Ichthyoplankton was dominated by families Sciaenidae, Clupeidae, Engraulidae, Cynoglossidae and Carangidae. The distribution of larval fish assemblages was strongly associated to the salinity structure. Oceanographic conditions were characterized by a large salt wedge (180 km long and a vertical salinity gradient up to 10 units per meter). During both sampling periods each ichthyoplankton assemblage was related to the same region of the salt wedge: the fresh water environment, the bottom salinity front, the mixohaline zone and the outer portion (surface salinity front) of the estuary. Larval assemblages were not related to the thermal structure. Reproductive activity of fishes inside the Río de la Plata Estuary is relatively common (60% of the teleosts sampled use this estuary as a nursery ground). Moreover, the high incidence of pelagic eggs spawners (75% of the species present in the ichthyoplankton) in the Río de la Plata contrasts to the vast majority of estuaries, where this reproductive strategy is poorly represented. Estuarine dynamics plays a dominant role in allowing this reproductive style in this estuary. Spring–summer wind pattern should generate a retentive environment which could minimize advective losses of eggs and larvae to the adjacent ocean.  相似文献   

20.
Catch of marine fish grew from after WW II–1989, at which point it stabilized. In 1996 it began to decline. It continues to decline now, at a time when earth's population is expected to reach 10 billion by 2050. Since the factors driving the increase are primarily the growth in income, population, technology of catching fish and ever increasing fishing effort, it is to be expected that the aggregate marine catch will continue to decline. This decline has important implications for marine ecosystems but primarily its importance relates to the human use of other global resources such as food, water and world's climate.  相似文献   

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