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1.
Spatial and size/age distributions of seven teleost species are described from a series of five annual trawling surveys made over the continental shelf and upper slope (20–500 m) of the South African south coast between Cape Agulhas and Port Alfred. The catch was quantified and scaled up to an estimate of an annual biomass index for each of the principal species. The fish studied were the two species of Cape hake Merluccius capensis and M. paradoxus, kingklip Genypterus capensis, Agulhas sole Austroglossus pectoralis, Cape horse mackerel Trachurus trachurus capensis, monk Lophius vomerinus and panga Pterogymnus laniarius. Each showed varying patterns of biomass fluctuation annually, although the surveys constituted too short a data series to reveal significant trends. Nevertheless, declines in abundance of panga and kingklip supported data collected from other sources over a longer period. Interpretation of the data is tempered by a realization that species which are patchily distributed or less vulnerable to the sampling gear may have been inadequately sampled. The information can be used for more-effective management of the multi-species demersal trawl fishery of the region.  相似文献   

2.
A good relationship between the variability in measurements of Cape hake Merluccius capensis biomass during winter and summer swept-area research surveys and sea surface temperature off southern Namibia for the years 1983–1990 is presented. The results call into question the hypothesis that hake density is related to biomass and imply that abnormally warm summers could support higher catches, regardless of the state of the stock. A hypothesis that anomalous warm conditions could induce hake to concentrate closer to the sea bed, making them more susceptible to bottom trawling, is presented.  相似文献   

3.
To describe demersal communities off the west coast of South Africa, collections of all species of fish, cephalopods and crustaceans made during four cruises in the area during January and July have been analysed. Hierarchical classification methods were employed to determine species and sample associations. The results showed that the species could be split into two main groups, the first containing Merluccius paradoxus (the dominant species both in abundance and frequency of occurrence) and other species normally found in deep (> 380 m) water. The second group is represented by species that live on the continental shelf, M. capensis being the most abundant. The boundary separating the deep-sea community from that of the shelf follows approximately the 385-m isobath. Seasonal variations in the number of main associations as well as in their spatial location are documented.  相似文献   

4.
The trophic role of the hyperiid amphipod Themisto gaudichaudii in the southern Patagonian shelf food web was assessed from the analysis of stomach contents of the local fish assemblage. A total of 461 trawl samples were collected during seven seasonal cruises. A total of 17 out of 38 fish species were found to ingest T. gaudichaudii. This amphipod was a main prey item in five of these species, showing high values of alimentary index: Seriolella porosa (99.9%), Macruronus magellanicus (68.8%), Micromesistius australis (59.1%), Patagonotothen ramsayi (48.6%), and Merluccius hubbsi (10.9%). The contribution of T. gaudichaudii, in weight, to their summer diet was 60%, on average. This contribution was minimal in winter and maximal in summer. Fisheries studies have indicated that these five species, mainly M. magellanicus, account for almost 85% of the fish biomass in the area. Although the remaining 15% did not feed heavily on T. gaudichaudii, they are known to prey on the main hyperiid predators. Our study shows that T. gaudichaudii contributes greatly, both directly and indirectly, to supporting the fish community. We thus proposed that T. gaudichaudii plays a key role as a “wasp-waist” species in the sub-Antarctic region, similar to that of krill in Antarctic waters, channeling the energy flow and enabling a short and efficient food chain.  相似文献   

5.
Fish assemblages from two beaches, one in the inner and the other in the outer Sepetiba Bay (latitude: 22°54′–23°04′S; longitude: 43°34′–44°10′W), Southeast Brazil, were sampled by beach seine net, simultaneously, on both seasonal and diel scales, between August 1998 and June 1999. Sites were selected to encompass different environmental conditions which reflect the two bay zones, thus providing a comprehensive assessment of the factors influencing surf zone fish assemblages, and their spatial, seasonal and diel variations. A total of 55 fish species was recorded, mostly young-of-the-year. Anchoa tricolor, Micropogonias furnieri, Gerres aprion, Diapterus rhombeus, Harengula clupeola, Atherinella brasiliensis and Mugil liza were numerically dominant and contributed to 95.2% of the total fish catches. Strong differences in fish assemblages were observed between the two areas, with higher number of species in the outer bay. Increases in fish numbers occurred in winter, while the highest biomass occurred in winter and summer. Transparency, followed by salinity, was responsible for most of the spatial variability and played an important role in structuring fish assemblages. Overall, diel patterns did not reveal any significant trends; however, if we consider each season separately, an increase in fish numbers during the day with peak at sunset was observed in winter, and a higher biomass occurred at night in winter and summer. Species preferences for various combinations of environmental variables are responsible for shifts in the structure and overall abundance of assemblages and dictated some patterns. The sciaenid M. furnieri, the second most abundant species, occurred only in the inner zone, being more abundant in winter. The species of Engraulidae were more abundant in the outer zone in winter/spring during the day. The gerreids G. aprion and D. rhombeus occurred mainly in summer. Overall, temporal fluctuations act more at a specific level than at a structural one, and may be linked to some particular stages of the fish life cycle, but do not significantly influence the spatial organization.  相似文献   

6.
Samples were collected with a plankton net in the four seasonal cruises during 2006-2007 to study the seasonal variability of the zooplankton community in the southwest part of Huanghai Sea Cold Water Mass (HSCWM, Yellow Sea Cold Water Mass). The spatial and temporal variations of zooplankton species composition, biomass, abundance and biodiversity were examined. A total of 122 zooplankton species and 30 pelagic larvae were identified in the four cruises. Calanus sinicus and Aidanosagitta crassa were the most dominant species, and Themisto gaudichaudi and Euphausia pacifica were widely distributed in the HSCWM area. The spatial patterns of non-gelatinous zooplankton (removing the high water content groups) were similar to those of the total zooplankton biomass in autumn, but different significantly in the other three seasons. The seasonal means of zooplankton biomass in spring and summer were much higher than that in autumn and winter. The total zooplankton abundance averaged 283.5 ind./m~3 in spring (highest), 192.5 ind./m~3 in summer, 165.5 ind./m~3 in autumn and 65.9 ind./m~3 in winter (lowest), and the non-gelatinous groups contributed the most total abundance. Correlation analysis suggests that the non-gelatinous zooplankton biomass and abundance had a significant positive correlation in the whole year, but the relationship was insignificant between the total zooplankton biomass and abundance in spring and summer. The diversity index H of zooplankton community averaged 1.88 in this study, which was somewhat higher than historical results. Relatively low diversity in summer was related to the high dominance of Calanus sinicus, probably due to the strongest effect of the HSCWM in this season.  相似文献   

7.
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   

8.
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   

9.
The aim of this research was to study spatial and temporal variation in epiphyte cover and leaf biomass of Posidonia oceanica in Eastern Tunisia. Sampling was conducted at four stations on the Mahdia coast during October and December 2010, and April and August 2011, which correspond respectively to autumn, winter, spring and summer in this area. Posidonia oceanica shoots were collected at two depths (5 and 10 m). Cover of macroinvertebrates and macroalgae was estimated on adult leaves. The results showed that leaf and epiphyte biomasses vary significantly according to sampling date, with the highest values recorded in August. We found a high diversity of epiphytic assemblages on the leaves of P. oceanica with clear qualitative and quantitative dominance of Rhodophyceae compared to other groups, followed by Phaeophyceae. Most epiphyte species on the leaves of P. oceanica in Eastern Tunisia are the same as those in other parts of the Mediterranean Sea. No bathymetrical variation in the epiphytic community was found in our study area, which can be explained by the high levels of water clarity in Mahdia.  相似文献   

10.
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11.
In this study demersal survey data for the period 1990–1999 are used to investigate the average distribution of the Cape hake Merluccius capensis and M. paradoxus off Namibia in terms of density and mean length. Further, biomass estimates are compared on the basis of depth and density stratification. The main distribution of M. capensis was north of 27°S and that of M. paradoxus south of 24°S. M. paradoxus was deeper than M. capensis. For both species, average length increased with depth. M. paradoxus expanded its range to the north through the 1990s as its population size (off South Africa and Namibia) increased. In Namibian waters, small M. paradoxus were found only south of 25°S. Mean length of M. capensis increased north of 21°S, largely as a result of decreased numbers of small fish in shallower water. Abundance estimates stratified by depth were no different from those post-stratified on similar densities.  相似文献   

12.
Information on hake stomach contents collected during research cruises off the west coast of South Africa between 1988 and 1990 is analysed. Estimates of the annual consumption and daily ration of the Cape hakes Merluccius capensis and M. paradoxus are obtained. Coefficients of variation (CVs) for these estimates are estimated by a bootstrap variance-estimation procedure. Cape hakes are opportunistic feeders. Juveniles feed mainly on crustaceans and the diet becomes increasingly piscivorous with age. Anchovy were the dominant dietary item of juvenile M. capensis, although this may be a reflection of the fact that anchovy were readily available at the time of the surveys. The large number of anchovy found in the diet during this study highlights the dangers of extrapolating consumption estimates for opportunistic feeders to other time periods. Pooling of data across geographic and seasonal strata introduces substantial bias in estimates of consumption and daily ration for only the pelagic prey species. Estimates of annual consumption are highly sensitive to the cruise selected to provide the estimates of numbers-at-length, because these latter estimates vary substantially between cruises. Furthermore, estimates of annual consumption and daily ration by prey species are very imprecise due to the effects of small sample sizes, the opportunistic nature of and natural variability associated with feeding. This implies that it is unlikely that model-estimation procedures which utilize these data will be able to provide particularly precise predictions. Therefore, before any larger-scale stomach collection exercise is undertaken, it is advisable to perform simulation studies to assess the sampling intensity required to achieve the desired levels of accuracy and precision for predictions from multispecies model-estimation procedures which make use of such data.  相似文献   

13.
We compared fish community structure in Guaratiba Mangrove, SE Brazil, among three seasons of the hydrological cycle [(i) spring, increasing temperature and rainfall, decreasing salinity; (ii) summer/early fall, high temperature and rainfall, intermediate salinity; (iii) late fall/winter, low temperature and rainfall, high salinity] and between the years 2002–2003 and 2008–2009. The aim was to test the hypothesis that the fish community structure changes seasonally, associated with seasonal changes in environmental conditions, and that changes occurred between the two yearly periods because of increased anthropogenic activities. The sampling protocol for the two surveys was identical. A total of 63 species was recorded, with 38 species occurring in 2002–2003, and 53 species in 2008–2009. The main changes in the mangrove's fish assemblage between 2002–2003 and 2008–2009 were (i) an increase in species richness, fish density and total biomass; and (ii) an increase in the zoobenthivorous species of gerreids Eucinostomus argenteus, Diapterus rhombeus and Ulaema lefroyi, and a decrease in the planktivorous clupeid Harengula clupeola. Other abundant species, such as the opportunistic atherinopsid. Atherinella brasiliensis, the detritivorous mullet Mugil liza and the zoobenthivorous pufferfish Sphoeroides testudineus did not differ in abundance between the two yearly periods. Ten species occurred only in 2002–2003, and 23 species only in 2008–2009, indicating significant changes in community structure over the 6‐year period. Seasonal changes in community structure were more conspicuous in 2008–2009, when species abundance and richness were greatest. The highest fish abundance was recorded in spring and in summer/early fall, and the lowest in late fall/winter. The increased abundance and richness over time may be at least partially attributable to protection policies because of the effective implementation of a biological reserve in the area.  相似文献   

14.
Diel patterns in survey trawl catches for the Cape hakes Merluccius capensis and M. paradoxus off Namibia were studied in order to examine the effect of diel bias on catchability, and its implication for survey abundance estimation and the consistency of the survey time-series. Catch rates (numbers per haul) by species and length from summer demersal biomass surveys conducted during the period 2002–2013 were used, together with a computation of the corresponding light-level data from which the solar zenith angles were obtained. Generalised additive models were fitted to assess the relationship between the catches and a number of explanatory variables. Significant covariates were zenith angle, depth and geographical position. The final models explained 78% and 59% of the variability in catch rates of M. capensis and M. paradoxus, respectively. For M. capensis, the response to zenith angle increased sharply for values above 100°, which represents the time between sunset and sunrise. For M. paradoxus there was a moderate increase in the response to zenith angle during the night. In cases where some fishing took place at night in shallow water, the survey results for M. capensis were more greatly affected than was the case for M. paradoxus, which is related to the different depth preference of the two species. Fishing in depths shallower than 400?m outside daylight hours should therefore be avoided in order to reduce bias and ensure consistency in abundance estimates from surveys.  相似文献   

15.
Fish assemblages that included known predators of benthic crustaceans were compared between kelp and barren habitats in northeastern New Zealand using baited underwater video census methods. The benthic-feeding fish were observed in winter, spring/summer and autumn, as well as during night-time in spring/summer. Overall, the fish assemblages varied between barren and kelp habitat, being most marked in winter. Individual benthic-feeding species, such as grey mao mao (Scorpis lineolatus) and leatherjacket (Meuschenia scaber) were associated with barren habitat, while the spotted wrasse (Notolabrus celidotus) was more strongly associated with kelp habitat. The results suggest that changes in habitats on coastal reefs affect populations of species that are benthic predators, which may in turn influence the distribution and abundance of their prey species, such as juvenile spiny lobsters.  相似文献   

16.
Prevalence of infection with the myxozoan parasite Kudoa thyrsites is not well documented in sardine Sardinops sagax, the Cape hakes Merluccius capensis and M. paradoxus, and kingklip Genypterus capensis. Quantitative polymerase chain reaction assays were used to determine the prevalence of K. thyrsites in wild-caught S. sagax (n?=?296), Merluccius spp. (n?=?162) and G. capensis (n?=?70), harvested in 2015. Sardinops sagax were harvested from the east, south and west coasts of South Africa, whereas M. capensis, M. paradoxus and G. capensis were taken from the west coast only. Relationships between K. thyrsites infection prevalence and fish capture locality, season, sex and size were determined with Chi-square tests. Prevalence was highest in S. sagax (91%), followed by M. capensis and M. paradoxus (87%) and G. capensis (40%). Prevalence was independent of sex of the host for all fish species. In the case of S. sagax only, the prevalence of infection was independent of locality of capture but depended on season, with prevalence lowest during spring and highest during autumn. No significant relationship (p?>?0.05) was found between infection prevalence and host size for any of the fish species.  相似文献   

17.
Harvest control measures for Namibia and Angola are defined using reference points for spawning biomass, which requires knowledge of spawning potential and dynamics of fishery stocks. The reproductive biology of two species of horse mackerel, Trachurus capensis and Trachurus trecae, in these waters was investigated for spawning distribution in time and space. Samples were collected during two scientific cruises conducted in February and July/August 2007 off the coast of Namibia and Angola. A total of 396 fish ovaries was examined to determine the spawning dynamics of the two species. The ovaries were histologically analysed and their oocyte distributions mapped. The length (TL)–weight relationships of the two species differed significantly. The observed development of oocyte recruitment and absence of a hiatus in the oocyte distribution indicated that these species are indeterminate spawners. Analyses of the data from the NatMIRC database (2001–2004) indicated annual changes in gonadosomatic index (GSI) for T. capensis at different latitudes, but these changes were small. GSI also fluctuated between years and areas, but there were no clear spatial or temporal trends, with averages ≤4.0% at all stations. The GSI of both species was not indicative of the histologically determined development of gonad maturity. Oocyte maturation for T. capensis indicated that smaller and younger fish spawn first. Batch fecundity (F) for T. trecae was best described by F = 4 986 TL (cm) – 99 645 (r 2 = 0.83). This study provided the first analyses of oocyte development in time and space for the two species in the region.  相似文献   

18.
长江口近海春季鱼类群落结构的多样性研究   总被引：3，自引：0，他引：3       下载免费PDF全文
根据2007年5月、2008年5月和6月在长江口近海春季的底拖网调查资料,对该海域鱼类的种类组成、资源量变化、群落结构多样性及优势种类长度谱进行了分析.调查共捕获鱼类187种,带鱼在三次调查中占据绝对优势,其渔获量均占总渔获量的50%以上,其它优势种类,除小黄鱼、刺鲳、银鲳外,均为小型非经济种类.另外,冷温性的鳄齿鱼也...  相似文献   

19.
The feasibility of a directed trap-fishery for panga Pterogymnus laniarius was investigated with special emphasis on minimising the bycatch. A total of 1 302 fish, representing 19 species, was captured in 59 trap deployments at three locations along the Eastern Cape coast of South Africa between September 2002 and July 2004. Panga comprised over 55% of the total catch, with an average of 12 panga per trap haul. Depth and substrate type had a significant effect on the species composition of the catch. Panga comprised 77% of the catch in depths >50m, whereas other commercially important species contributed 11% to the total catch in water >50m. No kingklip Genypterus capensis were captured in the traps and shallow-water Cape hake Merluccius capensis accounted for only 0.2% of the total catch composition. The findings suggest that trap-fishing provides a new fishing opportunity in South Africa with a high catch per unit effort for the target species and a low level of bycatch.  相似文献   

20.
根据2009年8月至2010年6月4个季度月(夏、秋、冬、春季),对位于北黄海獐子岛附近海域所设的13个站进行的大型底栖动物生态调查所获得的资料,对该海域大型底栖动物的种数、密度和生物量的组成及季节变化进行分析研究,采用Shannon-Wiener指数(H’)、物种丰富度指数(D)和物种均匀度指数(J)分析该该海域大型底栖动物的物种多样性;并研究了该海域的次级生产力和P/B值的空间分布和季节变化。结果表明,北黄海獐子岛附近海域大型底栖动物全年总种数、年平均栖息密度和年平均生物量分别为211种、699.415个/m2和98.927 g/m2。各季度的种数(S)、平均密度D (个/m2)和平均生物量B (g/m2)的季节变化分别为: S春季(121)＞S秋季(118)＞S冬季(89)＞S夏季(87),D春季(794.58)＞D秋季(766.92)＞D夏季(674.62)＞D冬季(561.54),B春季(180.271)＞B夏季(107.121)＞B秋季(70.824)＞B冬季(37.493)。全年物种多样性指数H’值、物种丰富度指数D值和物种均匀度指数J值分别为2.976、4.135和0.707,该海域大型底栖动物的夏季、秋季和春节平均密度季节变化不明显,但冬季明显较少。而平均生物量和种数的季节变化比较明显,春季较高,冬季较低。该海域的平均次级生产力为15.335g(AFDW)/(m2.a),相对较高。P/B值的平均值为1.239。  相似文献   

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