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1.
During intensive biannual demersal fish-sampling cruises since January 1983 over the South African west coast shelf region (Orange River to Cape Agulhas, coast to 500-m isobath), feeding studies on both species of Cape hake (Merluccius capensis and M. paradoxus) were undertaken. These studies were of two forms, random sampling of the dietary preference and constituents of the hake caught during daylight only and intensive sampling of stomachs with food throughout a 28-hour cycle. From the index of relative importance, crustaceans were shown to be most important in the majority of length classes of hake studied, with mesopelagics (fish and cephalopods) and other hake also important. Differences by hake species, by predator size, by geographical location and by season were evident. The Cape hakes, which constitute some three-quarters of the demersal fish biomass in the area, are deduced to play a major role in the marine faunal environment, both as predators and prey, and some base-line figures of the quantities of different food items consumed by the resource are presented. Hake are largely opportunistic feeders and it is concluded that they can adapt to perturbations in the availability of prey, whether they be triggered by environmental cause or by, for example, purse-seining of particularly mesopelagic fish.  相似文献   

2.
The results of multiyear observations of the seasonal and inter-annual variability of the population structure, abundance, and biomass of the arctic calanoids copepod Calanus glacialis in the White Sea are presented. The spring season represents the most crucial period for the population’s seasonal dynamics. During the spring, the maximal abundance, biomass, and contribution of C. glacialis to the total zooplankton biomass is observed. The interannual variability of the abundance is closely related to the timing of the spring warming of the upper water column and the respective shifts of the onset of reproduction and the offspring development. The development of a new generation to the overwintering copepodite stage IV is usually completed three to four weeks later in the cold years compared to the warm ones. Our multiyear observations suggest that C. glacialis could be more tolerant of Arctic warming than it is usually believed. The high abundance of the C. glacialis population in the White Sea indicates that this arctic species is able to cope with the seasonal surface warming and should continue to do so, being provided with the cold water “refuge” in the deep sea.  相似文献   

3.
A study was undertaken to investigate the spatial, diurnal and interannual dynamics of the pelagic component of Namibian hake stocks. Data were derived from trawl surveys conducted on board the R.V. Dr Fridtjof Nansen during the period 1991–1999. In addition to trawl surveys, acoustic methods were applied concurrently to quantify fish biomass off the bottom (above the headline). The pelagic component of hake was variable in both time and space – it was denser in the north and at depths of 200–500 m. Pelagic densities also correlated well with the density of adult fish (≥30 cm) at the bottom, but not with that of young hake (>18 and <30 cm) at the bottom. Pelagic densities were lowest at midday and highest at night, early morning and late afternoon densities being intermediate between the two. Incorporation of acoustic methods allows for at least partial account to be taken of diurnal variation in determining catchability. The fraction of the biomass off the bottom was not the same each year, thus affecting assessment and management advice for the stock, even if the survey results are treated only as relative indices of abundance. More field studies are needed to investigate the size range and species of hake active in vertical migration.  相似文献   

4.
A trawl catches only a portion of the fish in its path. The term catchability therefore refers to the fraction of the available fish caught. A method was developed and tested to establish catchability constants for trawl surveys of Namibian hake Merluccius spp. A catchability constant can be expressed as a simple relationship between hake area densities calculated from trawl catches and acoustic biomass estimates. Initial values were on an order of magnitude of 0.8, meaning that the catch takes 80% of the hake available to the trawl. The methodology depends on careful area selection (flat bottom, homogenous fish distribution), following the same trawl lane during subsequent hauls in an area, and thorough acoustic post-processing. A pronounced and repetitive pattern in catchability within surface daylight hours was found. Early morning and early afternoon catches were low, and the best catches of the day were made around noon, a result that may influence stock assessments based on trawl data, because morning and afternoon data would under-represent actual abundance.  相似文献   

5.
中国近海区域浮游植物生态对气候变化的响应   总被引：1，自引：0，他引：1       下载免费PDF全文
我国近海区域对气候变化高度敏感,浮游植物生态的变化关系到我国近海生态安全.采用重构的遥感数据等资料,分析并综述我国近海区域浮游植物叶绿素a浓度、初级生产力和浮游植物群落结构对气候变化背景下海水升温、风场等环境因子的响应.结果表明,东(南)中国海叶绿素a浓度略有上升(下降)的趋势,但浮游植物群落结构和生物量有明显的变化;其中,微微型浮游植物和甲藻占比增加,小型浮游植物物种成为海区优势种,暖水性种分布区北扩,而这与气候变化背景下海洋热动力环境的长期变化及其对营养盐供给的影响关系密切.分析还指出了气候变化对我国近海区域海洋生态影响研究迫切需要开展的若干工作.  相似文献   

6.
During 1997, considerable scientific differences arose about the status of the Namibian hake (Merluccius capensis and M. paradoxus) resource, and as to whether the hake Total Allowable Catch (TAC) should be substantially decreased or increased. These differences revolved primarily around whether or not abundance estimates from the swept-area trawl surveys by the Norwegian research vessel Dr Fridtjof Nansen should be considered as reliable measures of biomass in absolute (as distinct from relative) terms. The paper relates the computations underlying the Interim Management Procedure (IMP) approach that was put forward at that time as a basis to resolve this impasse. The anticipated performance, in terms of catch and risk of resource depletion, of a number of simple candidate IMPs for the Namibian hake resource is evaluated. The IMPs depend on two parameters, whose values are to be chosen by decision-makers, and adjust the TAC up or down from one year to the next according to whether trends in recent commercial catch rate and survey indices of abundance are positive or negative. Performances are evaluated across the then current wide range argued for resource abundance and status. Trade-offs in performance across the candidates considered are discussed. One of the candidates was subsequently selected by a joint meeting of scientists, industry and Ministry officials in February 1998 and served as the basis for scientific recommendations for the TAC for the hake resource for the following three years.  相似文献   

7.
In this study we examine physiological responses of Peruvian hake (Merluccius gayi peruanus) to changes in their population structure and physical environment during the period 1971–2004. In particular, we assess the relative investment of energy in growth and/or reproduction of small (20–35 cm) and large (35–50 cm) hake. We calculated the (i) condition (Fulton’s K), (ii) gonad and (iii) gut fullness indices for 42,761 female hakes sampled from commercial landings; these indices indicate fish somatic, reproductive and feeding condition, respectively. Using Generalized Additive Models we then examined potential relationships between these indices and sea surface temperature anomalies and date. Drastic energy exhaustion and a decrease in female hake fecundity were observed during El Niño events. The long-term trend showed a general increase in condition factor and a decrease in gonad index for large hake between 1971 and 2004. Small hake exhibited a different trend with an increase in reproductive activity, which was accompanied by an earlier maturation. We hypothesise that the observed low investment of energy in reproduction by large female hake might be related to the lack of large males, due to a sex-selective fishery and the impact of El Niño. We suggest that fishing diminished hake reproductive capacity, modified the sex ratio in favour of females and increased population vulnerability to environmental stress, in particular to the El Niño. The impact of multidecadal variability and predators like the squid, Dosidicus gigas, remain unresolved until longer time series become available.  相似文献   

8.
Population genetic data of marine genetic resources add up to establish their pattern of connectivity. Such pattern determines the choice of a suitable management strategy to those resources in space and time. This study addresses the pattern of connectivity among European hake grounds upon the largest sampling effort so far developed in this species. Bayesian inference made on multilocus genotypic data provides evidence that a large genetic connectivity exists among Atlantic grounds and is mediated by significant migration rates stepping up from the Celtic Sea towards its adjacent Atlantic grounds. Therefore, the spawning biomass of the northern hake population could play a crucial role at ensuring the sustainability of southern hake fish grounds. The deepest restriction to gene flow was observed at the easternmost side of the Alboran Sea what suggests that the Almeria–Oran Oceanographic Front is an effective barrier keeping apart the two major gene pools so far shown in this species. A molecular clock calibrated for cytochrome b sequences of fishes and applied to the divergence between hake pools (0.3%) suggests that the Atlantic–Mediterranean population split might date back to the Middle Pleistocene (150,000 years BP). Complementarily, Bayesian treatments of multilocus genotypes indicate that such barrier to gene flow remains active nowadays. This study shows the upmost interest of future integrative efforts to incorporate population genetic data into current assessment practices, in order to better manage European hake populations and make its exploitation sustainable.  相似文献   

9.
The factors modulating recruitment success of Cape hake Merluccius capensis in Namibian waters are still unresolved. In this study, we used generalised additive models, regression tree analysis and the conventional Ricker model to examine the effect of environmental indices and spawning stock biomass (SSB) on hake recruitment success for the period 1984–2012. Results indicated that upwelling strength explained 51% of the recruitment variability, whereas SSB had no significant influence. The effect of SSB on recruitment only became significant when combined with upwelling strength, explaining 89% of the recruitment variability. SSB influenced recruitment during periods of strong upwelling. Optimal conditions for hake recruitment were associated with moderate upwelling strength. Low and high upwelling intensities reduced recruitment success. Our results are consistent with those of other studies suggesting a significant influence of environmental conditions on recruitment at a low spawning stock level. Our study highlights the importance of assessing the combined non-linear effects of both biotic and abiotic factors on hake recruitment.  相似文献   

10.
Namibia’s most important commercial fisheries resource, the shallow-water Cape hake Merluccius capensis, is currently assessed using statistical catch-at-age analysis. Age data obtained from otoliths constitute an important data component of this model. Recent age-validation studies of M. capensis showed that growth was previously underestimated. We investigated this new fast-growth hypothesis (FGH) by using measurements and counts of translucent zones (T1 to T14, from otolith core to edge) on two survey otolith samples covering the entire range of fish lengths. We compared three hypotheses of periodicity of otolith zone formation and show that, if all zones are counted, T2 (at 9.0 mm otolith length), T5, T8, T11 and T14 are most likely to be annuli. A conversion from the slow-growth-hypothesis (SGH, currently used) age data was calculated as: FGH age group = round (0.41[SGH age group] + 0.25), and this formula should be applied to compute and test updated catch-at-age data in a future hake stock assessment. Additional adjustment for the hake stock assessment following the FGH, such as the timing of recruitment in winter and catches in summer, should be considered in future assessments.  相似文献   

11.
The Lophius vomerinus component of the monkfish resource off Namibia was assessed by means of deterministic length- and age-based models. Steady state length cohort analyses illustrated that, although the model was sensitive to the rate of natural mortality, it was relatively insensitive to changes in terminal fishing mortality. These biases may, however, not be serious provided that estimates of abundance are used to reflect relative changes in the biomass dynamics of the population. The age-structured production model, tuned to trends in General-Linear-Modelling-standardized catch-per-unit-effort data and relative abundance indices calculated from hake (Merluccius spp.) biomass surveys, together with observed commercial and survey catches-at-age, showed similar trends. Both models provided evidence that the monkfish resource was fully to overexploited, with current harvesting levels higher than those concomitant with sustainable yields.  相似文献   

12.
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13.
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   

14.
A brief history of research into, and the fishery for, Cape hake is given. From twelve monthly surveys in 1972/73, abundance of the two species (Merluccius capensis and M. paradoxus) at depth was found to vary, larger individuals of both species being found deeper than the smaller ones. Off the Cape of Good Hope, M. capensis was never found deeper than 440 m, and there was significant species overlap between 150 and 440 m. M. paradoxus was numerically dominant except at the shallowest station sampled (150 m). Little overlap of mature specimens of each species was recorded. The research data were applied to commercial trawl statistics, revealing that M. capensis accounted for only 12,9 per cent by mass and 4,8 per cent by number of the commercially caught hake in the years 1955–1974. Such low percentages justify considering the Cape hake fishery as a single-species entity for the purposes of stock assessment and management.  相似文献   

15.
The two sympatric species of Cape hake, Merluccius capensis and M. paradoxus, have been the main targets of bottom-trawl fisheries off Namibia for several decades. The feeding ecology of these hakes has been studied mainly using stomach content analyses and thus there remain some gaps in our knowledge about food assimilated over the longer term. In this study, we used fatty acid (FA) profiles to characterise the dietary relationships of M. capensis and M. paradoxus. Muscle samples from hake (n=110) and their known prey (n=68) were collected during trawl surveys off Namibia during 2011. Significant differences between the neutral FA profiles of the hake populations were detected in December 2011 but not in January 2011, an indication of temporal variations in diet and resource partitioning. Comparisons of the neutral FAs in hake and the total FAs of potential prey showed no clear trophic connections, with the exception of flying squid Todarodes sagittatus, which had FA profiles very similar to those of M. paradoxus in December 2011. Our results highlight the complex and temporally shifting relationships that exist between hake and the large pool of prey available to them, and between the two hake species that overlap in their feeding habits and distribution within the highly productive Benguela Current region.  相似文献   

16.
It is generally accepted that a climate shift occurred about 1977 that affected the dynamics of North Pacific marine ecosystems. Agreement on the possibility of further climate shifts in 1989 and the late 1990s is yet to be achieved. However, there have been changes in the dynamics of key commercial fishes that indicate changes in their environment occurred in the early 1990s, and possibly around 1998. One method of measuring climate change is to observe the dynamics of species that could be affected.Several studies have described decadal-scale changes in North Pacific climate–ocean conditions. Generally, these studies focus on a single index. Using principal components analysis, we use a composite index based on three aspects of climate ocean conditions: the Aleutian Low Pressure Index, the Pacific Atmospheric Circulation Index and the Pacific Interdecadal Oscillation Index. We link this composite index (Atmospheric Forcing Index) to decadal-scale changes in British Columbia salmon and other fish populations. Around 1989 there was a change from intense Aleutian Lows (above average south-westerly and westerly circulation patterns and warming of coastal sea surface temperatures) to average Aleutian Lows (less frequent south-westerly and westerly circulation and slightly cooler coastal sea surface temperatures in winter). These climate–ocean changes were associated with changes in the abundance and ocean survival of salmon (Oncorhynchus spp.), distribution and spawning behaviour of hake (Merluccius productus) and sardines (Sardinops sagax) and in recruitment patterns of several groundfish species.  相似文献   

17.
During the strong warm El Niño (EN) that occurred in 1997/98, Independence Bay (14°S, Peru) showed a ca. 10 °C increase in surface temperatures, higher oxygen concentrations, and clearer water due to decreased phytoplankton concentrations. Under these quasi-tropical conditions, many benthic species suffered (e.g. macroalgae, portunid crabs, and polychaetes) while others benefited (e.g. scallop, sea stars, and sea urchins). The most obvious change was the strong recruitment success and subsequent proliferation of the scallop Argopecten purpuratus, whose biomass increased fiftyfold. To understand these changes, steady-state models of the bay ecosystem trophic structure were constructed and compared for a normal upwelling year (1996) and during an EN (1998), and longer-term dynamics (1996–2003) were explored based on time series of catch and biomass using Ecopath with Ecosim (EwE) software. Model inputs were based on surveys and landings data collected by the Instituto del Mar del Perú (IMARPE). Results indicate that while ecosystem size (total throughput) is reduced by 18% during EN, mainly as a result of decreased total primary production, benthic biomass remains largely unchanged despite considerable shifts in the dominant benthic taxa (e.g. scallops replace polychaetes as secondary consumers). Under normal upwelling conditions, predation by snails and crabs utilize the production of their prey almost completely, resulting in more efficient energy flow to higher trophic levels than occurs during EN. However during EN, the proliferation of the scallop A. purpuratus combined with decreased phytoplankton increased the proportion of directly utilized primary production, while exports and flows to detritus are reduced. The simulations suggest that the main cause for the scallop outburst and for the reduction in crab and macroalgae biomass was a direct temperature effect, whereas other changes are partially explained by trophic interactions. The simulations suggest that bottom-up effects largely control the system.  相似文献   

18.
Zooplankton biomass and distribution in the KwaZulu-Natal Bight were investigated in relation to environmental parameters during summer (January–February 2010) and winter (July–August 2010). Mean zooplankton biomass was significantly higher in winter (17.1 mg dry weight [DW] m–3) than in summer (9.5 mg DW m?3). In summer, total biomass was evenly distributed within the central bight, low off the Thukela River mouth and peaked near Durban. In winter, highest biomass was found offshore between Richards Bay and Cape St Lucia. Zooplankton biomass in each size class was significantly, negatively related to sea surface temperature and integrated nitrate, but positively related to surface chlorophyll a and dissolved oxygen. Zooplankton biomass was significantly related to bottom depth, with greatest total biomass located inshore (<50 m). Distribution across the shelf varied with zooplankton size. Seasonal differences in copepod size composition suggest that a smaller, younger community occupied the cool, chlorophyll-rich waters offshore from the St Lucia upwelling cell in winter, and a larger, older community occurred within the relatively warm and chlorophyll-poor central bight in summer. Nutrient enrichment from quasi-permanent upwelling off Durban and Richards Bay appears to have a greater influence on zooplankton biomass and distribution in the bight than the strongly seasonal nutrient input from the Thukela River.  相似文献   

19.
Seasonal variations in the picoplankton community were investigated from June 2002 to March 2004 within the photic zone of Sagami Bay, Japan. The study area was mostly dominated by coastal waters during the warm period (mixed layer water temperature ≥ 18°C). During the cold period (mixed layer water temperature ≤ 18°C), the water mass was characterized by low temperature and high saline waters indicative of the North Pacific Subtropical Mode Water (NPSTMW). Occasionally, a third type of water mass characterized by high temperature and low saline properties was observed, which could be evidence of the intrusion of warm Kuroshio waters. Synechococcus was the dominant picophytoplankton (5−28 × 1011 cells m−2) followed by Prochlorococcus (1−5 × 1011 cells m−2) and picoeukaryotes during the warm period. Heterotrophic bacteria dominated the picoplankton community throughout the year, especially in the warm period. During the Kuroshio Current advection, cyanobacterial abundance was high whereas that of picoeukaryotes and heterotrophic bacteria was low. During the cold period, homogeneously distributed, lower picophytoplankton cell densities were observed. The dominance of Synechococcus in the warm period reflects the importance of high temperature, low salinity and high Photosynthetically Active Radiation (PAR) on its distribution. Cyanobacterial and heterotrophic bacterial abundance showed a positive correlation with temperature. Prochlorococcus and picoeukaryotes showed a positive correlation with nutrients. Picoeukaryotes were the major contributors to the picophytoplankton carbon biomass. The annual picophytoplankton contribution to the photosynthetic biomass was 32 ± 4%. These observations suggest that the environmental conditions, combined with the seasonal variability in the source of the water mass, determines the community structure of picoplankton, which contributes substantially to the phytoplankton biomass and can play a very important role in the food web dynamics of Sagami Bay.  相似文献   

20.
戴燕玉  林茂 《海洋学报》2007,29(3):90-97
在对南黄海和东海毛颚类生态特征的研究数量分布特征后的续篇.重点探讨调查区毛颚类群落性质和群落结构的特征及与海区环境因子的相关性.结果表明,本区共记录毛颚类28种,种类季节变化不明显,周年共有种达64.3%;但群落优势种则有季节差异与区域差异.毛颚类群落多样性指数(H')和均匀度(J')年均值分别为1.68和0.64.H'的平面分布表现出外部海区大于近岸海区的分布格局.在群落中存在4个生态类群,其中近海暖温带类群和广盐暖水类群可分别指示黄海水、浙江沿岸流和东海陆架混合水在南黄海和东海这两个海区的相互推移和消长过程.此外,从不同侧面的计算分析也表明,影响群落特征值(H')的主要因子是温度和盐度.群落性质是以近岸暖温带低盐种和暖水广布种为主,近岸暖水种和大洋暖水种占一定比例的暖温带-亚热带群落结构的特点.  相似文献   

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