Decadal-scale variations of trophic levels at high trophic levels in the Yellow Sea and the Bohai Sea ecosystem

A total of 2759 stomachs collected from a bottom trawl survey carried out by R/V “Bei Dou” in the Yellow Sea between 32°00 and 36°30N in autumn 2000 and spring 2001 were examined. The trophic levels (TL) of eight dominant fish species were calculated based on stomach contents, and trophic levels of 17 dominant species in the Yellow Sea and the Bohai Sea reported in later 1950s and mid-1980s were estimated so as to be comparable. The results indicated that the mean trophic level at high trophic levels declined from 4.06 in 1959–1960 to 3.41 in 1998–1999, or 0.16–0.19·decade^− 1 (mean 0.17·decade^− 1) in the Bohai Sea, and from 3.61 in 1985–1986 to 3.40 in 2000–2001, or 0.14·decade^− 1 in the Yellow Sea; all higher than global trend. The dominant species composition in the Yellow Sea and the Bohai Sea changed, with the percentage of planktivorous species increases and piscivorous or omnivorous species decreases, and this was one of the main reasons for the decline in mean trophic level at high tropic levels. Another main reason was intraspecific changes in TL. Similarly, many factors caused decline of trophic levels in the dominant fish species in the Yellow Sea and the Bohai Sea. Firstly, TL of the same prey got lower, and anchovy (Engraulis japonicus) as prey was most representative. Secondly, TLs of diet composition getting lower resulted in not only decline of trophic levels but also changed feeding habits of some species, such as spotted velvetfish (Erisphex pottii) and Trichiurus muticus in the Yellow Sea. Thirdly, species size getting smaller also resulted in not only decline of trophic levels but also changed feeding habits of some species, such as Bambay duck (Harpodon nehereus) and largehead hairtail (Trichiurus haumela). Furthermore, fishing pressure and climate change may be interfering to cause fishing down the food web in the China coastal ocean.     

Environmental changes and the responses of the ecosystems of the Yellow Sea during 1976–2000

The changes in the environmental features of the Yellow Sea during the last 25 years of the 20th century were studied using a set of seasonally monitored data along a transect (at 36°N) maintained by the State Oceanic Administration of China. The data included the ocean temperature (T), salinity (S) and biogenic elements, such as dissolved oxygen (DO), phosphorus (PO₄-P), silicon (SiO₃-Si) and dissolved inorganic nitrogen (DIN).The seasonal (summer and winter) values and the annual mean of these elements showed significant changes during the monitored period. Time series of T, S, DIN and N:P ratios exhibited positive trends, while those of DO, P and Si exhibited negative trends. During this period, the annual mean of T and DIN in the Yellow Sea increased by 1.7 °C and 2.95 μmol L⁻¹, respectively, while those of DO, P and Si decreased by 59.1, 0.1 and 3.93 μmol L⁻¹, respectively. In the 1980s, particularly in between 1985 and 1989, concentrations of P and Si dropped to near the ecological threshold for growth of diatoms. The N:P ratio increased from 4 in 1984 to over 16 in 2000. The climate trend coefficients, R_xt, for these time series are all above 0.43 with significance levels of 95%, except for salinity. The increases in T were consistent with the recent climate warming in northern China and the adjacent seas, i.e. the Bohai Sea and the East China Sea. The reduction of DO was probably attributable to the increase in T and decrease in primary production in these regions. The positive trend of DIN was mainly attributable to precipitation and partly to Changjiang River discharge. The negative concentration trends of P and Si were due to the decreases in their concentrations in seawater that flowed to the Yellow Sea from the Bohai Sea. As a result, N:P ratios greatly increased in the seawater of the Yellow Sea.Moreover, some important responses of the Yellow Sea ecosystems to the changes in physical variables and chemical biogenic elements were obviously displayed. These responses include strengthening nutrient limitation, decreasing chlorophyll a, primary production and phytoplankton abundance, succession of dominant phytoplankton species from diatoms to non-diatoms, changes in fish community structure and species diversity.     

Enrichment, concentration and retention processes in relation to anchovy (Engraulis ringens) eggs and larvae distributions in the northern Humboldt upwelling ecosystem

A Lagrangian model is used to simulate and quantify in the northern Humboldt upwelling ecosystem the processes of enrichment, concentration and retention, identified by Bakun [Bakun, A., 1996. Patterns in the ocean. Ocean processes and marine population dynamics. University of California Sea Grant, California, USA, in cooperation with Centro de Investigaciones Biologicas de Noroeste, La Paz, Baja California Sur, Mexico, 323 pp.] as being important for the survival and recruitment of early life stages of pelagic fish. The method relies on tracking the positions of particles within water velocity fields generated by a three-dimensional hydrodynamic model. Simple criteria for considering particles as participating to enrichment, concentration or retention are used to derive indices of the three processes. We analyse the spatial distribution of and seasonal variability in these indices. The results are discussed in relation to anchovy (Engraulis ringens) eggs and larvae distributions off Peru, and to a comparable study conducted in the southern Benguela upwelling ecosystem.     

Trophic flows,ecosystem structure and fishing impacts in the South Catalan Sea,Northwestern Mediterranean

An exploited ecosystem from the continental shelf and upper slope of the Northwestern Mediterranean Sea was described by means of an Ecopath mass-balance model with the aim of characterising its functioning and structure and describing the ecosystem impacts of fishing. This application included some complexities added to the general modelling methodology due to the high biodiversity of the Mediterranean Sea and the multispecific nature of the fishery, and to the difficulties of working with fishing data which are usually irregularly or imprecisely collected. The model comprised 40 functional groups including primary producers, the main species of benthic, demersal and pelagic invertebrates, fishes and non-fish vertebrates and three detritus groups. In addition, trawling, purse seine, longline and troll bait fishing fleets were included.Results showed that the functional groups were organized into four trophic levels with the highest levels corresponding to anglerfish, dolphins, large pelagic fishes and adult hake. The system was dominated by the pelagic fraction, where sardine and anchovy prevailed in terms of fish biomasses and catches. Detritus and detritivorous groups also played key roles in the ecosystem and important coupled pelagic-demersal interactions were described. Considering Odum's theory of ecosystem development, the ecosystem was placed on an intermediate-low developmental stage due, at least partially, to the impact of fishing activity. This highlighted the high intensity of fishing in the ecosystem, in accordance with the general assessment of western Mediterranean marine resources, and fishing fleets were ranked as top predators of the system. The low trophic level of the catch was in line with the long history of exploitation in the area. However, the steady decline of pelagic landings between 1994 and 2003, coupled with a decrease of the pelagic biomass within the system, underlined the low resistance of the system in front of perturbations. This decline was reproduced under Ecosim dynamic simulations combining different scenarios of moderate increase of fishing effort and an environmental forcing affecting the availability of preys to small and medium-sized pelagic fishes under wasp-waist flow control.     

Impact of tidal front on the distribution of bacterioplankton in the southern Yellow Sea, China

Three surveys were carried out in anchovy spawning periods in southern Yellow Sea in May and June 2001, and June 2002. Chlorophyll a (Chl-a) concentration, bacterioplankton abundance, biomass and their variations along the zone of tidal fronts were investigated. The results showed that (1) high Synechococcus abundance distributed more often in frontal area and middle-surface layer of a stratified zone; and (2) the maximal abundance of bacteria occurred in stratified and mixed zone.     

Distribution of drifting seaweeds in eastern East China Sea

In offshore waters with relatively low primary production, drifting seaweeds composed of Sargassum species form an identical ecosystem such as an oasis in desert. Commercially important pelagic fishes such as jack mackerel (Trachurus japonicus) and yellow tail (Seriola quinqueradiata) spawn in East China Sea pass their juvenile period accompanying drifting seaweeds. Therefore drifting seaweeds are very important not only in offshore ecosystem but also fishery resources. However the distribution of drifting seaweeds in East China Sea has scarcely known. Then we conducted two research cruises of R/V Hakuho–Maru in May 2002 and in March 2004. During the cruises, drifting seaweeds were visually observed from the bridge and sampled with a towing net. The observation revealed that the drifting seaweeds were distributed along the front between the Kuroshio Current and coastal waters and mainly composed of one seaweed species, Sargassum horneri (Turner) C. Agardh from spring to early summer. There are no reports on geographical distribution of this species in the coasts south of southern Kyushu Island in Japan. Kuroshio Current flows northeastward there. Buoys with GPS attached to drifting seaweeds released off Zhejiang Province, China, in March 2005 to track their transport. Their positions monitored by ORBCOM satellite showed that they were transported to the area in East China Sea, where the drifting seaweeds were observed during the cruises, in 2 months. These facts suggest that S. horneri detached from Chinese coast in March or months earlier than March could be transported to fringe area of continental shelf and waters influenced by Kuroshio Current from March to May. Therefore the Sargassum forests, especially S. horneri, along the Chinese coast play a very important role in the ecosystem of the East China Sea as a source of drifting seaweeds.     

Weaving marine food webs from end to end under global change

Marine food web dynamics are determined by interactions within and between species and between species and their environment. Global change directly affects abiotic conditions and living organisms, impinging on all trophic levels in food webs. Different groups of marine researchers traditionally study different aspects of these changes. However, over medium to long time scales perturbations affecting food webs need to be considered across the full range from nutrients to top predators. Studies of end-to-end marine food webs not only span organism sizes and trophic levels, but should also help align multidisciplinary research to common goals and perspectives. Topics are described that bridge disciplinary gaps and are needed to develop new understanding of the reciprocal impacts of global change on marine food webs and ocean biogeochemistry. These include (1) the effects of nutrients on biomass and production, (2) the effects of varying element ratios on food web structure and food quality, (3) bulk flows of energy and material in food webs and their efficiencies of transfer, (4) the ecological effects of species richness and the roles of microbial organisms, (5) the role of feeding behaviour in food web dynamics and trophic controls, (6) the spatial dynamics of communities and links between different food webs, (7) the combined effects of body size and behaviour in determining dynamics of food webs, and (8) the extent to which the ability of marine organisms (and communities) to adapt will influence food web dynamics. An overriding issue that influences all topics concerns the time and space scales of ecosystem variability. Threads link different nodes of information among various topics, emphasizing the importance of tackling food web studies with a variety of modelling approaches and through a combination of field and experimental studies with a strong comparative approach.     

Environmental causes of the fluctuations of red shrimp (Aristeus antennatus) landings in the Catalan Sea

The fluctuations of catches of the deep-sea shrimp Aristeus antennatus (Risso, 1816) were analysed in the 6 ports of Catalonia where production is concentrated, based on monthly landings from 1988 to 2004. The 6 selected ports produced 300 t of red shrimp in 2004, or 80% of the total production in Catalonia. The series for each port showed clear interannual variability, with peaks of production in the early 1990s and more recently from 2001 to 2003. The time periods of the monthly data series, estimated by frequency analysis based on Fourier transform, varied around 7–8 years in the four central ports and 12–13 years in the two northern ports. Additionally, the different curves were not in phase: even in nearby ports, the maximum production is observed in different years. Since the North Atlantic Oscillation (NAO) index is an excellent proxy for long-term series of environmental variables, we aimed to explore relationships between the fluctuation of the NAO index and A. antennatus landings in the Catalan Sea. The correlation between the mean annual NAO index and the annual catches in each port was positive and significant with some time lags (from 1 to 3 years). The existence of clear patterns linking the NAO with marine ecological processes has been demonstrated in many studies, but the underlying ecological mechanisms are far from being well understood. The variations in environmental parameters linked to the NAO may act on biological organisms at different levels (individual, population) through physiology (metabolic and reproductive processes) or through trophic relationships, including ecological cascade effects. We propose that NAO-induced environmental variability may enhance food supply to A. antennatus and hence strengthen the reproductive potential of particular year classes, which result in increased catches 1 to 3 years later, although other possible effects of environmental variability on the population dynamics of this species are worth investigating.     

The role of pelagic fish as forage for the demersal fish community in the southern Bay of Biscay

The small pelagic fish represents the link between nekton and demersal communities, and they are an important food source for some demersal fish species. In the autumns between 1993 and 2002, 74,869 stomach contents of 25 demersal fish species were analysed during the scientific surveys organized by the Instituto Español de Oceanografía. These species represent the demersal fish community of the southern Bay of Biscay. An important part of their diet (39% by volume) was composed of pelagic fish species (e.g. Engraulis encrasicolus, Gadiculus argenteus, Micromesistius poutassou, Trachurus trachurus, Sardina pilchardus, Scomber scombrus). Among all these prey species, the relevance of M. poutassou and G. argenteus stood out because of their high abundance both in the diets and during the scientific surveys. The relevance of pelagic fish as prey increased with predator size, reaching more than 60% of the diet by volume in the length range 25–29 cm. However, fish predators larger than 50 cm depended less on pelagic fish (33%), since they were also able to feed on other sources such as megafaunal invertebrates, mainly cephalopods. The pelagic fish resource was primarily exploited by 12 demersal fish species, with Merluccius merluccius and Zeus faber being the main ones feeding on small pelagics, which was related to the vertical movement of predators and prey through the water column. Survey abundance indices were used as indicators of prey abundance in the ecosystem, both for biomass and number. There was evidence for density-dependant feeding by predators on E. encrasicolus, G. argenteus and T. trachurus, while the main discrepancies between abundance in the stomachs and in the surveys were due to differential availability of prey length classes in the environment. Small prey species and individuals were exploited by most demersal fish while large prey species were less accessible to predators.     

Effects of the 1988/89 climatic regime shift on the structure and function of the southwestern Japan/East Sea ecosystem

In this study we compared the structure of the ecosystem and the role of major species before and after 1988/89 climatic regime shift (CRS) in the southwestern Japan/East Sea. The 1988/89 CRS event changed the biomass and production of fisheries resources in the southwestern Japan/East Sea. Total biomass of all species groups in the ecosystem increased by 59% after the CRS. These results indicate that there were substantial changes in the function of major species in the southwestern Japan/East Sea ecosystem. The relative contribution of walleye pollock, at trophic level (TL) III, to the total flow of energy decreased drastically from 33.0% in the pre-CRS period to 4.3% in the post-CRS period, while that of common squid, at the same TL, doubled from 34.2% to 72.2% during the periods.     

Summer distribution of seabirds in the North-East Water polynya, Greenland

The distribution at sea of seabirds was studied in the North-East Water (NEW) polynya, Greenland, during transect counts in the summers of 1991, 1992 and 1993 on board the ice-breaking RVs Polarstern and Polar Sea. Data collected within the polynya ‘box’ (78–82°N; 5–18°W) concern observations of 8000 birds counted during 1350 half-hour counts. Distribution is presented as density (N/km²) and calculated daily food intake. Five bird species were selected for discussion, representing more than 95% of the total numbers encountered: Fulmar (Fulmarus glacialis), Ivory Gull (Pagophila eburnea), Kittiwake (Rissa tridactyla), Glaucous Gull (Larus hyperboreus) and Ross's Gull (Rhodostethia rosea). For these species, densities are comparable in the NE Greenland polynya and in other European Arctic seas. The main difference is the absence in NEW of the species playing the main role in Arctic seas: Brünnich's Guillemot (Uria lomvia) and Little Auk (Alle alle). In the absence of fish-eating birds and of birds consuming zooplankton in the water column, the NEW polynya ecosystem is thus dominated by surface feeders and, closer to the coast, by benthic feeders like eiders, Somateria mollissima and S. spectabilis, and walrus, Odobenus rosmarus. The density and daily food intake for all seabirds are one order of magnitude lower in the polynya than in the Arctic seas. The distribution and abundance of seabirds in the NEW polynya seems to reflect a very low density of pelagic fish and Zooplankton in the water column, while Zooplankton must be present at ‘normal’ concentrations in the upper layer.     

Succession of the ecosystems of the Aral Sea during its transition from oligohaline to polyhaline water body

During 22 field trips from 1990 to 2002 (mainly the western basin of the Large Aral) data on salinity, phytoplankton, zooplankton, zoobenthos and fish fauna have been collected. In 2002, the salinity of the western basin reached 75 ppt, while that in the eastern basin, 150 ppt. In 1999–2002, 159 species of planktonic algae have been recorded. This is approximately twice as low as recorded before. The diversity of Cyanophyta, Pyrrhophyta and Chlorophyta in particular has dropped in the past few years. As before, currently Bacillariophyta is the most diverse plankton. However, the composition of dominants has changed. Once previously dominant species, Actinocyclus ehrenbergii, vanished from the plankton of the Aral Sea and was replaced by such diatoms as Amphora coffeaformis, A. coffeaformis var. acutiuscula and Synedra tabulata var. parva.Since 1970s, a gradual decrease in the diversity of zooplankton has been taking place. Since 1997, the formerly dominant Calanipeda aquaedulcis vanished, which apparently was the reason for the emergence of Moina salina and Artemia parthenogenetica. Since 2000, artemia has been dominant in the plankton of the Aral Sea, constituting 99% of the zooplankton biomass.In the 1970–1980s, a rapid decrease in the biodiversity of the zoobenthos was observed. In the 1990s, most aboriginal and introduced species became extinct. Currently, the bivalve mollusk Syndosmya segmentum, the ostracod Cyprideis torosa and larvae of the dipteran Chironomus salinarius can still be recorded in the western basin. In the eastern basin no benthos is observed.By 1998, in the Large Aral, only five fish species survived: baltic herring Clupea harengus membras, flounder Platichthys flesus luscus, atherine Atherina boyeri caspia and bullheads Neogobius fluviatilis and Potamoschistus caucasicus. Since 2002, only flounder and atherina have been recorded in the western basin of the Large Aral. No fish have been recorded in the eastern part of the Aral Sea in 2002.With increasing salinity and transition of the Aral Sea from an oligohaline to a polyhaline water body, its biota is becoming drastically poorer. Almost all local species became extinct in the Aral; however, some still survive (including some endemics) in some lakes around the Aral Sea. In the near future, artemia will be the only animal in the Aral Sea.     

Distribution of phytoplankton in the southern Black Sea in summer 1996, spring and autumn 1998

The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June–July 1996 and March–April and September 1998. A total of 150 species were identified, 50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×10³ cells l⁻¹ in spring to 110×10³ cells l⁻¹ in autumn and biomass from 250 μg l⁻¹ in summer to 1370 μg l⁻¹ in spring. Based on the extensive sampling grid from June–July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.     

The coastal edge of the Northeast Water polynya in spring 1993

Multidisciplinary, marine ecological observations were conducted at the shallow water edge of the Northeast Water in June, 1993. Although variable in size and shape, a small polynya was constantly present at Eskimonaes, at the fast-ice edge of Ingolfsfjord. A shallow stratified layer developed at the water sufface at negative water and air temperatures—an effect of sea ice melting in cold water early in the season. Nutrients were recorded in considerable quantities, although by mid July NO₃ had become depleted. The chlorophyll and phytoplankton maxima at 8–12 m depth had peak values of 2 mg chl a m⁻³, typical for Arctic algal blooms. The phytoplankton included over 90 species and was dominated by the Fragillariopsis group. Zooplankton was poor in biomass and density, but over 23 taxa were found, with the copepods Oithona similis and Pseudocalanus acuspes being numerically dominant. Sedimentation was approximately 0.2 g dry weight m⁻² d⁻¹ and suspended matter concentrations ranged from 4 to 19 mg l⁻¹. The benthos was represented by hard bottom forms only, with a surprisingly dense cover of macrophytes. Juvenile sea urchins (Strongylocentrotus droebachiensis), brittle stars (Ophiocten sericeum) and amphipods were dominant. Higher trophic levels were represented by benthic feeders, such as eiders and walruses. The area observed was more similar to high Arctic fjord ecosystems than to the offshore central Northeast Water polynya.     

Carbon flows in Baltic Sea food webs — a re-evaluation using a mass balance approach

The brackish Baltic Sea has been seen as particularly suitable for studies of food webs. Compared to fully marine ecosystems, it has low species diversity, which means fewer trophic linkages to analyse. The Baltic Sea is also one of the best-studied areas of the world, suggesting that most data requirements for food web models should be fulfilled. Nevertheless, the influence of physical and biological factors on trophic interactions and biogeochemical patterns varies spatially in the Baltic Sea, adding considerable complexity to food web studies. Food web structure and processes can be described and compared quantitatively between areas by estimating the flow of matter or energy through the organisms. Most such models have been based on carbon, though studies of complementary flows of other elements limiting production, such as nitrogen and phosphorus would be desirable. However, since ratios between carbon and other elements are used in calculating these flows, it is crucial, as a first step, to quantify the flows of carbon as accurately as possible.In this study, we used the EcopathII software (ver 3.1) to analyse models of carbon flow through the food webs in the three main areas of the Baltic Sea; the Baltic proper, Bothnian Sea and Bothnian Bay. A previously published study on carbon flow in the Baltic Sea [Elmgren, R. 1984. Trophic dynamics in the enclosed, brackish Baltic Sea. Rapp. P.-V. Reun. — Cons. Int. Explor. Mer. (183) 152–169.] was complemented with the data on respiration and flow to detritus [Wulff, F., Ulanowicz, R. 1989. A comparative anatomy of the Baltic Sea and Chesapeeake Bay ecosystems. In: F. Wulff, J.G. Field, K.H. Mann (Eds.), Flow Analysis of Marine Ecosystems: Theory and Practice. New York: Springer-Verlag.] in order to present complete mass balance models of carbon. The purpose of re-evaluating previous models with new analytic tools was to check how well their carbon flows balance, and to provide a basis for improved mass balance models using more recent data, including nutrients other than carbon.The resulting mass balance networks for the Baltic proper, Bothnian Sea and the Bothnian Bay were shown to deviate from steady state. There was an organic carbon surplus of 45, 25 and 18 g C m⁻² year⁻¹ in the pelagic zones of the Baltic proper, Bothnian Sea and Bothnian Bay, respectively. The Ecopath network analysis confirmed that the overall carbon flow was highest in the Baltic proper, somewhat lower in the Bothnian Sea and much lower in the Bothnian Bay. The only clear differences in food web structure between the basins was that the average trophic level was lower for demersal fish in the Bothnian Sea and higher for macrofauna in the Bothnian Bay, compared to the other basins. The analysis showed weakness in our current understanding in Baltic Sea food webs and highlighted areas where improvements could be made with more recent data.     

Changes in the shelf macrobenthic community over large temporal and spatial scales in the Bohai Sea, China

Over the past 20 years, the Bohai Sea has been subjected to a considerable human impact through over-fishing and pollution. Together with the influence of the Yellow River cut-off, the ecosystem experienced a dramatic change. In order to integrate available information to detect any change in macrobenthic community structure and diversity over space and time, data collected during the 1980s and the 1990s from 3 regions of the Bohai Sea (Laizhou Bay, 16 stations, 37–38°N, 119–120.5°E; central Bohai Sea, 25 stations, 38–39°N, 119–121°E; eastern Bohai Bay, 12 stations, 38–39°N, 118.5–119°E) were reanalyzed in a comparative way by means of a variety of statistical techniques. A considerable change in community structure between the 1980s and the 1990s and over the geographical regions at both the species and family level were revealed. After 10 years, there was a considerable increase in abundance of small polychaetes, bivalves and crustaceans but decreased number of echinoderms. Once abundant in Laizhou Bay in the 1980s, a large echinoderm Echinocardium cordatum and a small mussel Musculista senhousia almost disappeared from the surveying area in the 1990s. Coupled with the increased abundance was the increased species richness in general whereas evenness was getting lower in central Bohai Sea and Bohai Bay but increased in Laizhou Bay. K-dominance plot showed the same trend as evenness J′. After 10 years, the macrobenthic diversity in the Bohai Sea as a whole was slightly reduced and a diversity ranking of central Bohai Sea > Laizhou Bay > eastern Bohai Bay over space was also suggested. Sediment granulometry and organic content were the two major agents behind the observed changes.     

Topological constraints on the dynamics of wasp-waist ecosystems

Small pelagic fish species like anchovy or sardines are of high ecological and economical importance. As marine food webs are fished down, these small pelagics tend to be more exploited and overfished. It is not yet very well known what the possible effects of their collapse can be, therefore there is an urgent need to outline a theoretical framework for understanding their dynamics. These fish occupy very special position in food webs, ensuring energy transfer between species lower and higher levels, while forming narrow “wasp-waists” poor in number of species (but very abundant). Our purpose was to quantify the interaction structure of model food webs of equal complexity but different levels of “wasp-waistedness”. We analysed the topological properties of the webs by characterising every direct and indirect interactions between individual species, as well as by assessing the relative positional importance of each species in each web. We found that (1) the shorter the interaction pathways considered, the weaker the predictive power of node degree for positional importance, (2) the importance of species varies more in wasp-waist food webs, (3) if longer indirect chain effects are considered, indirect effects can well be stronger than direct ones, (4) interactions between coexisting wasp-waist species are stronger than the average, and (5) the “self-regulatory” looping effects are also stronger for wasp-waist species. Based on the topological properties of the networks, our results describe constraints acting on the dynamical behaviour of wasp-waist ecosystems. We give explanations, from this viewpoint, for regime shifts in which one WW species replaces another, and for the unpredictable dynamics of these fish stocks. From a marine conservation viewpoint, we illustrate that as the abundance of wasp-waist species decreases, the architecture of energy flows becomes highly vulnerable and unreliable. We provide an approach for quantifying these structural changes.     

Behavioral modeling of Pagrus major in the East Seto Inland Sea

Evaluation of the behaviors of fish that belong to higher levels of the food web is important from the viewpoint of resource management and other environmental issues. Especially for adult fish that can swim against the surrounding currents, it is quite important to consider migration effects when modeling fish behavior. In the present study, a model of fish behavior is developed that considers the migration effect by incorporating a preference for various environmental factors. The species to be modeled, Pagrus major, was chosen because its high value as a food makes it representative of an exploited fish species. In the developed model, the direction of the fish movement is determined by the strength of preference for environmental factors of water temperature, salinity, dissolved oxygen, and prey density. The model is coupled with a hydrodynamic model and a lower-trophic ecosystem model, which predict the physical environment and water quality of the target area. Numerical simulations are carried out to reproduce the spatial distribution and seasonal variations of the ecosystem in the East Seto Inland Sea. Proper parameters for fish behavior were obtained through the processes of model tuning. As a result of the simulations, we clarified that the environmental conditions have a sizeable influence on the migration and distribution of Pagrus major. The developed model is also able to reproduce the fish biomass variation in time and space, which will provide more detailed information for resource management of the fish.     

Trace element levels in some commercially valuable fish species from coastal waters of Mediterranean Sea, Egypt

Concentrations of 5 trace elements (Cd, Pb, Cu, Cr, Zn) were determined in muscle of fishes collected from two coastal areas of the Egyptian coast of the Mediterranean Sea west of Alexandria (El-Mex Bay and Eastern Harbour). Sardinella aurita, Alepes djedaba, Siganus luridus, Siganus rivulatus, Sphyraena chrysotoenia, and Scomberomorus commerson were collected from El-Mex Bay. While Boops boops, Lithognathus mormyrus, Sparisoma cretense, Serranus cabrilla and Synodus saurus were collected from the E.H. In all fishes zinc was highest (up to 57 mg/kg) followed by Cr, Cu, Pb and Cd. The concentrations of several metals in fishes were significantly different among the species in each area. For all trace element examined, the concentrations decreased significantly with body weight of some fishes. In contrast, a positive correlation with body weight was found especially for Zn and Cu concentrations in 5 fish species and for Pb in 4 fish species. The concentrations of Cd, Pb and Zn were higher in fishes from El-Mex Bay (3.76, 6.49 and 57.21 mg/kg) than those from Eastern Harbour, whereas the opposite trend was observed for Cu and Cr, revealing the direct sources of trace metal pollution present in El-Mex coastal area. Levels of Cr surpassed the Maximum Permissible Concentration in most fish tissues, followed by Pb and Cd in some species. Cu and Zn presented concentrations below the maximum permissible levels in fish tissues.     

Mesoscale high-frequency variability in the Alboran Sea and its influence on fish larvae distributions

This work analyses a multidisciplinary data set including hydrological and meteorological data, satellite images, and fish larvae abundance from a high-frequency experiment conducted along a north–south transect across the Western Alboran Sea anticyclonic gyre. Four consecutive occupations of the transect, crossing the frontal area, evidenced the high-frequency variability of hydrological structures and its influence on the latitudinal and vertical distribution of fish larvae in a period of a few days. The influence of dynamical processes on fish larvae depends on the location of the spawning as well as on the larval fish position in the water column. Wind induced upwelling and/or the southward drift of the Atlantic current transport larvae of neritic species, such as Sardina pilchardus and Engraulis encrasicolus to open sea areas. At the same time, these events bring about alterations in the latitudinal and vertical distributions of mesopelagic fish larvae. The species with a relatively surface distribution, as Ceratoscopelus maderensis, were transported and accumulated to the south of the Atlantic Jet (AJ), while those with a deeper distribution in the water column, as Maurolicus muelleri or Benthosema glaciale, would be upwelled and concentrated to the north. This study shows that it is difficult to extract conclusions about the mean or preferential distributions of fish larvae when one single/synoptic survey is considered and that time evolution of hydrological structures has to be considered in order to conclude.