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1.
Abstract. We discuss the critical problem of estimating daily egg production in the Daily Egg Production Method (DEPM), a widely used technique for the stock assessment of multiple spawning pelagic fishes. Data collected in 1999 for the stock of European anchovy ( Engraulis encrasicolus ) in the Southern Adriatic Sea are used to study the effects of different calibration techniques. We analyze the performances of the mortality model from which daily egg production is estimated. All these calibrations are based on the Bootstrap method. Nonlinear fitting (vs. linear regression on log-transformed data) is shown to guarantee more reliable parameter estimation. We show that specifying the function assigning age to staged eggs is crucial.  相似文献   

2.
Three important developments in resource assessment and management during the period of the Benguela Ecology Programme are discussed. The first is the recognition of the limited information content for assessment purposes of the available data for many of the Benguela's major pelagic and demersal resources. This has meant that management measures for these resources could attempt no more than to maintain the status quo, rather than to achieve "optimal" utilization. Secondly, a post mortem on the commercial extinction of the kingklip suggests a need to consider Bayesian methods as a means to incorporate additional information into stock assessments. Finally, changes to the management approaches for the hake and anchovy fisheries are argued to have been particular successes, notwithstanding the recent heavily reduced Total Allowable Catch (TAC) for the latter resource. "Management procedures" have been developed and simulation-tested for these two resources. The implications of this approach for the future basis, whereby TACs are set for South Africa's fisheries, are discussed.  相似文献   

3.
Understanding long-term variability of pelagic fish populations is important in developing forecasting strategies for fisheries management and planning. However, many current fisheries models have only short-term datasets available, whereas those of suitable duration often lack reliability. As resources are placed under increasing pressure, all available information should be used to assist management. Two simple rule-based deterministic modelling approaches are described, which use semi-quantitative and qualitative rules to relate recruitment success of South African anchovy Engraulis capensis to physical and biological indices. The first model relates recruitment success to indices of wind and sea surface temperature by way of a rule-based decision support system. In the second model, significant environmental and biological factors were identified and related to anchovy recruitment by way of an expert system approach. These two approaches are evaluated and compared. It is suggested that these types of models, when satisfactorily validated, have great potential in supporting the future management of the South African anchovy fishery in the dynamic environment of the Benguela Current.  相似文献   

4.
We analyzed oceanographic, hydrologic and atmospheric conditions in the northern Adriatic (section between the Po River delta and Rovinj) from 1990 to 2004 in which extreme phytoplankton blooms appeared. The largest blooms occurred in February, March, July and October, and were higher in surface layers and in the western, more eutrophic part of the region. They often appeared when the Istrian Coastal Countercurrent (ICCC; strong southward current indicating a “closure” of the northern Adriatic circulation system) was pronounced. They seem to be related to the intensities of surface fluxes and Po River discharge rates. The blooms can float over the northern Adriatic for a long period (up to 30 days; typical for March) and can appear simultaneously in separate circulation systems with a different species distribution. Finally, we present a hypothesis based on comparisons of geostrophic currents and fish stock that specific February northern Adriatic oceanographic conditions play the key role in the Adriatic anchovy stock.  相似文献   

5.
The epipelagic and mesopelagic nekton communities of the northern California Current have been sampled somewhat continuously over the last four decades with bottom and pelagic trawls, small midwater trawls, and purse seines. We review the zoogeography and community and environmental associations of the dominant pelagic micronekton and nekton species in this region with a view to understanding their role in this dynamic marine ecosystem. As is typical of many upwelling eastern boundary current regions, the pelagic biomass is dominated by a few species that fluctuate dramatically through time. The abundance trends of pelagic nekton caught in this region demonstrated large-scale ecosystem changes about the time of the regime shifts of 1976/77 and 1989 and possibly another beginning in 1999. The rapidity of the changes in composition indicates that the response was due to a change in migration or distribution patterns as opposed to recruitment patterns. The 1989 regime shift led to a dramatic increase in sardine and a decrease in anchovy populations. The most pronounced interannual signals were attributed to strong El Niño/Southern Oscillation (ENSO) conditions in 1983 and 1998 that altered the latitudinal ranges and proximity to the coast of many pelagic species. Variations in abundance and cross-shelf distribution patterns were noted for both pelegic nekton and micronektonic from surveys off California, Oregon, and Washington.  相似文献   

6.
Abstract. Investigations into the occurrence of oceanic tintinnines in the northern Adriatic Sea were carried out during 62 cruises between November 1979 and September 1995. A contribution to the knowledge of the ecological conditions behind the occurrence of oceanic tintinnines, particularly a consideration of the species Amphorides luuckmanni (Tintinnina) in the northern Adriatic Sea, is presented in this paper. Oceanic tintinnines are usually very rare organisms and their occurrence in the northern Adriatic is connected with the inflow of oligotrophic water masses from the southern Adriatic in autumn. After exceptionally high freshwater discharge from the Po River caused an intensive diatom bloom (October 1993), in early November, with an intrusion of water of high salinity from the south, the mass occurrence of the rare species A. luuckmunni in northern Adriatic open waters was noted, with a maximum of 890 indiv., I-1. This numerical abundance is the highest ever registered for oceanic tintinnines in the Adriatic Sea. Therefore it may be assumed that oceanic tintinnines occasionally play an important role in the food web in the northern Adriatic pelagic community.  相似文献   

7.
Climate variability and pelagic fisheries in northern Chile   总被引：5，自引：0，他引：5  
A time series analysis of long-term climate variability in northern Chile (18°21′–24°00′S) shows anomalies associated with the El Niño events and the longer warm period observed since 1976, followed by a cooling trend since mid 1980s. The succession of pelagic fisheries, anchovy (Engraulis ringens) and sardine (Sardinops sagax), occurring in this fishing zone was analyzed taking into account the landings, the CPUE abundance index, the fishing effort, and the environmental variables. The anchovy production model is a negative linear function of fishing effort and turbulence. For sardine, the production model is a negative linear function of fishing effort and a quadratic function of the sea surface temperature.An analysis of the relationship between recruitment, adult biomass and the environment shows that the annual recruitment of anchovy increases with turbulence intensity until wind speed reaches a value of 5.46 m s−1, decreasing for higher values. For sardine, the recruitment increases with turbulence intensity until 5.63 m s−1, stabilizing thereafter.It is deduced that the climatic variations associated to the El Niño events affect the abundance of coastal pelagic fishes, without forgetting the most likely effects upon its distribution and the fishing effort. However, it is the long-term variability that mainly affects the fishing activity.  相似文献   

8.
Anchovy and sardine in the Mediterranean are known to share the same habitat and, consequently, to interact with one another. These two sympatric pelagic species are planktivorous and consume a wide range of planktonic prey items during all their developmental stages, potentially overlapping their ecological niches, although the feeding interactions between these species have been poorly investigated. Here we compare the dietary habits of the juvenile phases of anchovy and sardine during different seasons in the northwestern Mediterranean Sea, through analysis of their stomach contents and of their feeding-related anatomical characteristics. In this study we show that juveniles of anchovy and sardine do not compete for food, and we describe significant dietary differences between anchovy and sardine due to their different alimentary tract morphology.  相似文献   

9.
Northern Adriatic Rocky Reef Fishes at Low Winter Temperatures   总被引：1，自引：0，他引：1  
Kurt  Kotrschal 《Marine Ecology》1983,4(3):275-286
Abstract. The behavioural ecology of certain northern Adriatic fishes was observed from January to April at a water temperature of 7–11 oC. The fishes studied here do not migrate to deeper waters. Most of the benthic fishes show a similar distribution over the substrate in winter and in summer; in winter, however, they are very inactive and hide in the substrate. Suprabenthic fishes aggregate in littoral caves where they form dense, inactive, mixed aggregations. Certain hemipelagic and pelagic fishes join to form very dense aggregations at specific sites in the open waters of bays. Littoral predators display high feeding activity at a time when their prey is inactive. I present a hypothesis concerning possible implications of seasonally cyclic environmental temperatures for a sympatric community of non-migrating fishes.  相似文献   

10.
Results are presented following a long-term study of the diet of the Cape gannet Morus capensis at Bird Island, Algoa Bay, examining 4 178 regurgitations representing 36 351 prey items collected during 12 breeding cycles from April 1979 to March 1991. In all, 32 teleost and two cephalopod species were recorded. The pelagic shoaling pilchard Sardinops ocellatus, anchovy Engraulis capensis and saury Scomberesox saurus scombroides were identified as the main prey species. The diet composition showed marked interannual fluctuations during the sampling period, which appear to be related to changes in the abundance of the main prey species. The relative abundance of pilchard, anchovy and saury was also found to change intra-annually, the first two species occurring more frequently in the diet during the breeding season and saury dominating the diet in the non-breeding season. These changes are thought to originate from temporal variation in the oceanographic regime within the foraging range of the gannets.  相似文献   

11.
Abstract. Phytoplankton biomass estimators – total cell number (CN), total cell volume (CV), chlorophyll a (Chl a ). phytoplankton organic carbon (PC), and their response to environmental variables, with a special emphasis on the vertical variability in the structure of the water column – were examined in the Mali Ston Bay (Southern Adriatic). Correlations among biomass estimators as well as with physical-chemical parameters varied according to hydrodynamic characteristics of the water column. The amount of Chl a per cell ranged from 0.28.06–10-6μg during stratification to 0.7–2.0.10-6μg during the mixing period. Most frequent values for PC/Chl a ratios and for Chl a /CV in mixed natural phytoplankton populations were 20–25 and 1–2, respectively.  相似文献   

12.
Stomach content analyses and measurements of gillraker morphology were used to assess the diet and feeding ecology of the East Coast redeye round herring Etrumeus wongratanai and provide data for comparisons with other small pelagic fishes off South Africa. Samples were collected by jigging from a kayak off Scottburgh, KwaZulu-Natal (KZN), over the period July 2013–May 2014. In total, 66 stomachs (containing 4 407 prey items) and 66 gill arches were examined. Stomach content analyses indicated that East Coast redeye round herring, at the time of sampling, fed on large (1 500–2 500?µm) particles, with ～2 500?µm particles making the greatest contribution to dietary carbon. The species feeds on larger particles than do anchovy Engraulis encrasicolus and sardine Sardinops sagax, but on particles of a similar size to those consumed by West Coast redeye round herring Etrumeus whiteheadi. There were significant differences in mean gillraker gaps between East Coast redeye round herring and both sardine and anchovy, but not between East Coast and West Coast redeye round herring. The differences in gillraker gaps between East Coast redeye round herring, sardine and anchovy are indicative of resource partitioning through differential feeding, based on zooplankton size. The data suggest that there is no resource partitioning between East and West coast redeye round herring, indicating that competition is likely to occur between them. However, such competition is minimised by limited spatial overlap between these two species.  相似文献   

13.
The Humboldt Current System (HCS) is dominated by two pelagic species; Peruvian anchovy or anchoveta (Engraulis ringens) and sardine (Sardinops sagax). Using data from 43 acoustic surveys conducted from 1983 through 2005 by the Peruvian Marine Institute (IMARPE), we examined the distribution of these two species relative to water masses. We tested the hypothesis that anchovy was found more frequently in upwelled cold coastal water (CCW) and mixed waters (MCW) than in other water types and that sardine was more associated with more offshore oceanic surface subtropical water (SSW). Surface temperature, salinity, latitude, season and distance to the coast data were used to define water masses. Results using generalized additive models (GAM), modelling sardine and anchovy presence–absence as a function of year, water body, bottom depth and latitude, showed that anchovy were primarily found in CCW and MCS, while sardine were more ubiquitous relative to water masses with some predilection for SSW. These results were supported by various indexes of anchovy and sardine distribution versus water mass as well as temporal and location variables.  相似文献   

14.
Marine oxygen-deficient environments with high sedimentation rates and high primary productivity can provide relevant information regarding variations of ocean–climatic conditions in the past. In the Humboldt current ecosystem, which now hosts huge populations of pelagic fishes (mainly anchovy and sardine), fish scale abundance in the sedimentary record may be useful indicators of environmental change. Here we assess such a proxy record in a 42 cm-long sedimentary core collected from 80 m in Mejillones Bay (23°S, northern Chile). We also analyse fish remains in surface sediment sampled along a bathymetric transect (from 10 to 110 m water depth) in the same bay. In the core-top record, the fluctuations of sardine and anchovy scale deposition rates (SDR) agreed with those of industrial catches for these two species in northern Chile, tending to validate the SDR as a proxy of local fish biomass when bottom anoxic conditions prevail. However, apparent SDR for records prior to 1820 have probably been influenced by dissolution processes linked to the oxygenation of the bottom environment of Mejillones Bay, as suggested by other proxy records. After 1820, the fluctuations in the relative abundance of sardine and anchovy scales point to alternating warm and cold conditions during about 30 years and then a progressively cooler period. Since ca. 1870, marked fluctuations of SDR of both species are observed, probably as a consequence of the onset of a different oceanographic regime characterized by intensified upwelling, stronger subsurface oxygen deficiency, higher primary productivity, and enhanced “ENSO-like” interdecadal variability. While anchovy SDR fluctuated in periods of 25–40 years, only two peak periods of sardine SDR occurred (late 19th century and late 20th century), suggesting that sardine abundance depends on other ocean–climatic factors.  相似文献   

15.
A particle-size spectrum model is used to estimate standing stocks of some pelagic fish from measurements of phytoplankton chlorophyll in two exploited regions in southern Africa (southern Benguela and off South West Africa/Namibia) and in two unexploited regions (the Agulhas Bank and off the eastern Cape Province). The model is based on the assumption that equal biomasses occur in logarithmically equal size classes in the pelagic marine environment. Phytoplankton, with an equivalent spherical diameter ranging from 1 to 128 μm, occupy 21 size classes on the logarithmic scale. Two different size ranges are assumed for some commercial, pelagic fish species, equivalent to exploitation with two different purse-seine mesh sizes. A mesh of 12,7 mm would catch 8 size classes of pilchard, horse mackerel and anchovy whereas a mesh of 32 mm would catch only 3,3 size classes of pilchard and horse mackerel. From the model, the potential biomass of these commercial pelagic fish is estimated, after allowing for the presence of other commercial and non-commercial fish and other taxa in the exploited size range. Total pelagic fish production is estimated by assuming constant turnover rates of 1·y?1 and 1,5·y?1 when exploited with 32 and 12,7 mm mesh nets respectively. Consideration of the maximum and mean reported catches in the exploited areas indicates that only some 25 per cent of pelagic fish production is exploitable by man. On this basis, the unexploited Agulhas Bank region may yield some 400 000 metric tons (wet) of pelagic fish of the species considered, and the East Coast region some 90 000 tons. Exploitation in these regions cannot be recommended, because the Agulhas Bank is an important spawning ground for many pelagic species, and the fish in both regions probably act as a reserve buffer for the heavily exploited pelagic resource of the Western Cape.  相似文献   

16.
Many top predators in the Benguela ecosystem feed on prey species targeted by commercial fisheries. Their roles as indicators of the state of exploited prey resources, as competitors with commercial fisheries for resources, and as susceptible to impact from commercial fishing on those resources are briefly considered. Trends in the occurrence of anchovy Engraulis capensis and pilchard Sardinops ocellatus in the diet of Cape gannets Morus capensis off South Africa's west coast are related significantly to survey estimates of the abundance of these fish species, and they provide useful confirmation of those estimates. In the 1980s, anchovy decreased in the diet of Cape gannets, but pilchard increased. In both the northern and southern portions of the Benguela system, groundfish were thought to eat most (66–73%) of the total quantity of cephalopods and vertebrates consumed by predators and man in the 1980s. South African fur seals Arctocephalus pusillus pusillus, predatory pelagic fish and man removed roughly equal amounts, with squids, seabirds and cetaceans having a smaller impact. In the 1980s, man and seals removed about two million tons live mass more than in 1930. Indices of the rate of natural mortality of anchovy and pilchard attributable to Cape gannets are not related to biomass of the prey species. That for anchovy was high in 1989 when a poor anchovy year-class was formed. Decreased abundance of anchovy led to poor breeding by Cape cormorants Phalacrocorax capensis in 1989 and 1990. A model linking the Cape cormorant population with anchovy is used to explore the impact of possible exploitation strategies for anchovy on Cape cormorants.  相似文献   

17.
Abstract. In the water-column of the Gulf of Trieste (Northern Adriatic Sea) flocculent, amorphous aggregates known as marine snow reach maximum density and dimensions up to 20cm in diameter during summer months. Sampling of these fragile macro-aggregates was carried out by SCUBA-divers.
A large detrital component was found to be embedded in an organic matrix together with a large array of intact phytoplankton species. Heterotrophic bacterial density ranged from 2–65 × 108 cells-g-1 (marine snow dry wt) and is therefore of the same order of magnitude as in the surface layer of the sediments of the Gulf. In terms of biomass, bacteria reached only 30–95% of heterotrophic microflagellates.
A mean abundance of marine snow of 5g (marine snow dry wt) m-3 was obtained for the water-column above the pyenolinc. At this boundary layer, however, a mean abundance of about 10g (marine snow dry wt) m-3 led to the formation of a distinct, 10-50cm thick layer. Based on these data and sedimentation rates of marine snow to the pyenoclinc (38 g [marine snow dry wt] m-2 d-1) the potential ecological significance of these macro-aggregates for the pelagic system of the Gulf of Trieste is discussed.  相似文献   

18.
《Marine Chemistry》2001,75(4):1149
Spatial distribution and seasonal variability of surfactant activity (SA) of seawater were studied in the northern Adriatic Sea and compared to the temperature and salinity patterns in the 2-year period from February 1998 to January 2000, based on monthly surveys. Surface-active substances (SAS) were determined with alternating current voltammetry (in-phase mode) using o-nitrophenol as an electrochemical probe. A general characteristic of the SA seasonal variability for the northern Adriatic is the sinusoidal change of SA within the period of 1 year; similar behaviour was observed for the seasonal variations of temperature. Maximum SA values appeared during the summer period, while minimum SA values were measured in winter, during the period of well-mixed water layers and lower phytoplankton production. In May and October, the highest SA values were obtained in the upper seawater layer, which was ascribed to the influence of high inflow of nutrient-rich Po River freshwater. Riverine inputs indirectly favour autochthonous production processes, which result in increased concentration of organic matter (OM), particularly surface-active substances.  相似文献   

19.
In this paper we looked for the most suitable method for benthic ecological quality status (EcoQS) assessment of the North-western Adriatic Sea. Taking into account the historical and present information, we inter-compared our expectations in terms of EcoQS, with the Bentix, AMBI and M-AMBI classification metrics results, and investigated the sensitivity of these indices to the spatial, seasonal and inter-annual variability of benthic communities. The sampling sites were located at increasing distances with respect to the Po River mouth, along an (inverse) gradient of eutrophication. Moreover, a 2-year monthly monitoring plan (October 2004–September 2006) was carried out at the CEBEB site, located in a central position with respect to the investigated area. The results highlighted that the three indices differed in the sensitivity to spatial and temporal variability of the benthic communities. The strong seasonal and inter-annual changes in the benthic communities had a significant effect on the Bentix, whereas no relevant modifications were observed on the EcoQS assignation obtained using the M-AMBI and AMBI methods. The environmental quality ratios (EQR) values and EcoQS classification obtained by the Bentix and AMBI did not match expectations, as they were not inversely correlated with the site distances with respect to the Po mouth, whereas the M-AMBI clearly discriminated the most and the least eutrophicated sites. It thus appears that the M-AMBI could be considered to be the most valuable method for the ecological-status assessment of the North-western Adriatic Sea benthic ecosystem.  相似文献   

20.
From 2002 to 2010, the jumbo squid (Dosidicus gigas) has been regularly encountered in large numbers throughout the California Current System (CCS). This species, usually found in subtropical waters, could affect coastal pelagic ecosystems and fisheries as both predator and prey. Neither the abundance of jumbo squid nor the optimal ocean conditions in which they flourish are well known. To understand better the potential impacts of this species on both commercial fisheries and on food-web structure we collected nearly 900 specimens from waters of the CCS, covering over 20° of latitude, over a range of depths and seasons. We used demographic information (size, sex, and maturity state) and analyzed stomach contents using morphological and molecular methods to best understand the foraging ecology of this species in different habitats of the CCS. Squid were found to consume a broad array of prey. Prey in offshore waters generally reflected the forage base reported in previous studies (mainly mesopelagic fishes and squids), whereas in more coastal waters (shelf, shelf break and slope habitats) squid foraged on a much broader mix that included substantial numbers of coastal pelagic fishes (Pacific herring and northern anchovy, as well as osmerids and salmonids in northern waters) and groundfish (Pacific hake, several species of rockfish and flatfish). We propose a seasonal movement pattern, based on size and maturity distributions along with qualitative patterns of presence or absence, and discuss the relevance of both the movement and distribution of jumbo squid over space and time. We find that jumbo squid are a generalist predator, which feeds primarily on small, pelagic or mesopelagic micronekton but also on larger fishes when they are available. We also conclude that interactions with and potential impacts on ecosystems likely vary over space and time, in response to both seasonal movement patterns and highly variable year-to-year abundance of the squid themselves.  相似文献   

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