The role of notothenioid fish in the food web of the Ross Sea shelf waters: a review

The Ross Sea, a large, high-latitude (72–78°S) embayment of the Antarctic continental shelf, averages 500 m deep, with troughs to 1,200 m and the shelf break at 700 m. It is covered by pack ice for 9 months of the year. The fish fauna of about 80 species includes primarily 4 families and 53 species of the endemic perciform suborder Notothenioidei. This review focuses on the diet and role in the food web of notothenioids and top-level bird and mammal predators, and also includes new information on the diets of artedidraconids and bathydraconids. Although principally a benthic group, notothenioids have diversified to form an adaptive radiation that includes pelagic and semipelagic species. In the southern Ross Sea, notothenioids dominate the fish fauna at levels of abundance and biomass >90% and are, therefore, inordinately important in the food web. Antarctic krill (Euphausia superba) and mesopelagic fishes are virtually absent from the shelf waters of the Ross Sea. Of the four notothenioid families, nototheniids show the most ecological and dietary diversification, with pelagic, cryopelagic, epibenthic and benthic species. Neutrally buoyant Pleuragramma antarcticum constitutes >90% of both the abundance and biomass of the midwater fish fauna. Most benthic nototheniids are opportunistic and feed on seasonally or locally abundant zooplanktonic prey. Artedidraconids are benthic sit-and-wait predators. Larger bathydraconids are benthic predators on fish while smaller species feed mainly on benthic crustaceans. Channichthyids are less dependent on the bottom for food than other notothenioids. Some species combine benthic and pelagic life styles; others are predominantly pelagic and all consume euphausiids and/or fish. South polar skuas, Antarctic petrels, Adélie and emperor penguins, Weddell seals and minke and killer whales are the higher vertebrate components of the food web, and all prey on notothenioids to some extent. Based on the frequency of occurrence of prey items in the stomachs of fish, bird and mammal predators, P. antarcticum and ice krill E. crystallorophias are the key species in the food web of the Ross Sea. P. antarcticum is a component of the diet of at least 11 species of nototheniid, bathydraconid and channichthyid fish and, at frequencies of occurrence from 71 to 100%, is especially important for Dissostichus mawsoni, Gvozdarus svetovidovi and some channichthyids. At least 16 species of notothenioids serve as prey for bird and mammal predators, but P. antarcticum is the most important and is a major component of the diet of south polar skua, Adélie and emperor penguins and Weddell seals, at frequencies of occurrence from 26 to 100%. E. crystallorophias is consumed by some nototheniid and channichthyid fish and can be of importance in the diet of emperor and Adélie penguins, although in the latter case, this is dependent on location and time of year.Unlike the linear phytoplanktonE. superbaconsumers of the E. superba food chain hypothesized for much of the Southern Ocean, the food web of the Ross Sea shelf is non-linear, with complex prey-predator interactions. Notothenioid fish play a key role: as predators, they occupy most of the trophic niches available in the ecosystem, relying on benthic, zooplanktonic and nektonic organisms; as prey, they are important food resources for each other and for most top predators living and foraging on the shelf. They also constitute the major link between lower (invertebrates) and higher (birds and mammals) levels of the food web. This is especially true for P. antarcticum. Along with E. crystallorophias, its ecological role in the Ross Sea is equivalent to that of myctophids and E. superba elsewhere in the Southern Ocean.     

Multivariate analysis of surface water characteristics in the summer regime of the western Irish Sea

A part of the western Irish Sea is strongly stratified during the summer months, with the boundary of stratification marked by a front with horizontal temperature gradients. Continuous on-line analysis has been used to record the surface water variability of nine characteristics along a cruise path in this area.Multivariate analysis (principal component) indicates the presence of four distinct water types separated by thermal gradients. Two of these water types show land mass influence and two illustrated the effects of the mixed and stratified regimes. The stratified surface waters had a marked degree of homogeneity with respect to the measured characteristics, while the mixed surface waters exhibited greater variability. Marked micro-variations in chlorophyll a concentrations were observed in the latter water type.No consistent relationships were observed among the physical, chemical, and biological variables across the front because of the gradual change in the gross properties of the mixed surface water during its northward passage through the Irish Sea.     

Gametogenesis and reproductive strategies in some species of the Antarctic fish genus Trematomus (Nototheniidae) from Terra Nova Bay, Ross Sea

Species of the genus Trematomus are the most common nototheniids in the High-Antarctic Zone, and several live sympatrically on the Ross Sea continental shelf. Histological analysis of gonadal development of T. bernacchii, T. hansoni and T. newnesi was carried out on samples collected during the austral summer in the coastal waters of Terra Nova Bay, western Ross Sea. Gonads of both females and males of the three species were described based on macroscopic and histologic scales. Histologically, both males and females of T. bernacchii were in postspawning or in early maturing condition, thus indicating that they recently completed spawning. Conversely, most specimens of T. hansoni were in spawning condition, as evidenced by the histological appearance of gonads and by the high value of gonado-somatic index (GSI) of females between 20 and 36%. In the same period, females of T. newnesi had maturing ovaries, whereas males were at an early developmental stage of maturity. In all species, the testicular structure closely resembled the unrestricted spermatogonial type, although spermatogonia were more concentrated in the blind end of tubules. On the basis of the pattern of oocyte maturity observed within the ovarian lamellae, all Trematomus had a group synchronous ovary, as commonly reported in Antarctic fish. Furthermore, the presence of oocytes showing the commencement of yolk deposition in spent females confirms that vitellogenesis is a slow process, extending over at least 1 year. The reproductive strategies of Trematomus are discussed in terms of timing of the spawning season of each species as a function of the seasonal production cycle and of the interspecific relationships. The spawning activities of the three species investigated appeared to be mismatched with respect to time as well as location within the range of distribution of each.     

The combined effects of temperature and food supply on Calanus sinicus in the southern Yellow Sea in summer

The effects of temperature and food availability on the lifehistory strategy of the planktonic copepod Calanus sinicus inthe southern Yellow Sea in summer were studied in this paper.The fifth copepodite stage (CV) dominates the population inthe central part of the southern Yellow Sea, where the YellowSea Cold Water Mass (YSCWM) occurs below the thermocline. Incubationexperiments were conducted on CV C. sinicus caught from theYSCWM to examine the effects of temperature and food availability.Temperature at the surface (27°C) is lethal to CVs regardlessof food availability. At the temperature in the middle of thethermocline (18°C), survival time of the specimens dependson food availability, being     

Biology of Salpa thompsoni in waters adjacent to the Ross Sea, Southern Ocean, during austral summer 2008

Depth-stratified vertical sampling was carried out during the New Zealand International Polar Year cruise to the Ross Sea on board the RV Tangaroa in February–March 2008. The distribution (horizontal and vertical), density and population biology of Salpa thompsoni were investigated. Salps were found at two of the four major sampling locations, e.g. near the continental slope of the Ross Sea and in the vicinity of seamounts to the north of the Ross Sea. Both abundance and biomass of S. thompsoni were highest near the seamounts in the Antarctic Circumpolar Current reaching ~2,500 ind 1,000 m⁻³ and 8.2 g dry wt 1,000 m⁻³ in the water column sampled. The data showed that S. thompsoni populations were able to utilize horizontal and vertical discontinuities in water column structure, in particular the warm Circumpolar Deep Water (CDW), to persist in the high Antarctic. Although salps appeared to continue migrating to the surface colder layers to feed, both aggregate chain and young embryo release seem to be restricted to the CDW. This study for the first time has provided evidence that low Antarctic salp species may successfully reproduce in the hostile high Antarctic realm.     

Some biological characteristics of early larvae Dacodraco hunteri (Notothenioidei: Channichthyidae) in the western Ross Sea

Ichtyoplankton surveys were carried out in the western Ross Sea by the R/V Italica in the austral summer 1996 and 1997–1998 to study species composition and spatial distribution of larval stages of fish. One of the most abundant icefish caught was Dacodraco hunteri, a poorly known channichthyid inhabiting the high-Antarctic Zone. Based on 382 yolk-sac larvae and 13 preflexion larvae, the study was focused to estimate spatial distribution and abundance, as well as diet and growth rate. The pigmentation pattern and some morphometric measurements were also recorded for comparative purposes. The specimens were caught in relatively restricted areas located in Terra Nova Bay and north of the Ross Ice Shelf on the Challenger and Joides Basins. The standardized abundance of early larvae ranged between 0.03 and 1.72 individuals 10⁻³ m³ in 1996 and 0.16–4.53 individuals 10⁻³ m³ in 1997–1998, respectively. Fitting a linear model to the mean length increase in larvae collected in subsequent catch dates, the growth rate was estimated to be approximately 0.11 mm/day. Based on back calculation of growth rate and presumed hatch size of 11 mm, larval hatching probably took place in mid-December. Diet of preflexion larvae consisted exclusively of larvae of the pelagic nototheniid Pleuragramma antarcticum, a key species of the high-Antarctic pelagic food web. Hence, D. hunteri probably plays a more important role than previously thought in the pelagic community of the Ross Sea.     

Organic matter composition in coastal sediments at Terra Nova Bay (Ross Sea) during summer 1995

We illustrate the spatial and vertical distribution of sediment phytopigments and organic matter biochemical composition at Terra Nova Bay (Ross Sea) during summer 1995. Coastal sediments displayed high phytopigments concentrations associated with huge amounts of labile organic matter largely dominated by proteins. This result was opposite to previous observations in the same area. Such comparison suggested that whilst organic matter quantity in the sediments depended upon the vertical input from the water column, temporal changes in its biochemical composition were related to benthic processes. As considerably high concentrations of biopolymeric organic carbon were found even at 6-cm depth and according to the “loss type” functioning of the coastal waters of the Ross Sea, we stress the summer time occurrence in coastal sediments of an important organic matter burial. Accepted: 24 October 1999     

Weight–length relationships for selected fish species of the western Mediterranean Sea

Weight–length relationships are presented for 24 species of the western Mediterranean Sea (Spain). These species were selected from samplings made during November 1995. The captures were made by beam trawl in Posidonia oceanica meadows up to depths of 20–22 m.     

The Sargasso Sea Eel Expedition 1979

The background and aims of the Sargasso Sea Expedition for exploring the reproduction of the eel (Anguilla spp.) in the North Atlantic are described. The ships used for the investigations in the Sargasso Sea and during transects across the North Atlantic were F. R. V. Anton Dohrn and R. V. Friedrich Heincke, provided by the Federal Republic of Germany. Their technical and scientific equipment and their staffs are described. The courses of the cruises are mapped and the geographical positions of the investigations as well as the different instruments employed are tabulated as a source of basic data for more specialized papers published in this issue or elsewhere. Reference is also given to relevant papers which have been published elsewhere. Important but unpublished preliminary results are mentioned. Suggestions for future research needs are made.     

Meiofauna as food source for small-sized demersal fish in the southern North Sea

Meiofauna play an essential role in the diet of small and juvenile fish. However, it is less well documented which meiofaunal prey groups in the sediment are eaten by fish. Trophic relationships between five demersal fish species (solenette, goby, scaldfish, dab <20 cm and plaice <20 cm) and meiofaunal prey were investigated by means of comparing sediment samples and fish stomach contents collected seasonally between January 2009 and January 2010 in the German Bight. In all seasons, meiofauna in the sediment was numerically dominated by nematodes, whereas harpacticoids dominated in terms of occurrence and biomass. Between autumn and spring, the harpacticoid community was characterized by Pseudobradya minor and Halectinosoma canaliculatum, and in summer by Longipedia coronata. Meiofaunal prey dominated the diets of solenette and gobies in all seasons, occurred only seasonally in the diet of scaldfish and dab, and was completely absent in the diet of plaice. For all fish species (excluding plaice) and in each season, harpacticoids were the most important meiofauna prey group in terms of occurrence, abundance and biomass. High values of Ivlev’s index of selectivity for Pseudobradya spp. in winter and Longipedia spp. in summer provided evidence that predation on harpacticoids was species-selective, even though both harpacticoids co-occurred in high densities in the sediments. Most surficial feeding strategies of the studied fish species and emergent behaviours of Pseudobradya spp. and Longipedia spp. might have caused this prey selection. With increasing fish sizes, harpacticoid prey densities decreased in the fish stomachs, indicating a diet change towards larger benthic prey during the ontogeny of all fish species investigated.     

Sea warming and fish distribution: the case of the small pelagic fish, Sardinella aurita, in the western Mediterranean

This study analyses the temporal and spatial changes in abundance and distribution of the warm water species round sardinella (Sardinella aurita) in the western Mediterranean over the last decades in relation to sea water temperature. In the western Mediterranean basin (1950–2003), a significant positive relationship was found between round sardinella landings and temperature anomalies. Along a latitudinal gradient off the Mediterranean Iberian coast (1989–2004), a gradual increase in species abundance was observed from south to north, with a certain time lag going northwards, associated with the increase in sea water temperature. The abundance of round sardinella in the two warmest and southernmost areas was positively and significantly correlated with sea surface temperature registered during the start of gonad maturation the previous year. In addition, the positive relationship established between water temperature and abundance of round sardinella in the coldest and northernmost study area demonstrates that there is a temperature limit for the distribution of this species in the western Mediterranean. In addition, this study analyses round sardinella larvae distribution and abundance in the summers of 2003 and 2004, and conducts a comparison with the situation 20 years ago (summer 1983). Results show a marked increase in larval abundance during the last decades and the present appearance of larvae in the northernmost study areas, where they did not occur 20 years ago. This indicates the successful reproduction of round sardinella in the northern part of the Mediterranean, where the species has expanded, confirming its establishment in the area.     

The chromosomal complement of the artedidraconid fish Histiodraco velifer (Perciformes: Notothenioidei) from Terra Nova Bay, Ross Sea

The karyotype of Histiodraco velifer from the Antartic Ocean was analyzed using various banding methods and in situ hybridization with a telomeric probe. A male and a female had a diploid set of 46 chromosomes (6 submetacentric + 40 acrocentric, FN = 52); the nucleolar organizer was CMA3-positive and was located on the short arm of a medium-sized submetacentric pair. All chromosomes stained uniformly with DAPI, whereas C-banding revealed heterochromatic blocks that were mostly located centromerically and telomerically and were resistant to ALUI digestion. The substantial identity of the karyotype of H. velifer with that of the other artedidraconids investigated so far suggests that chromosome changes must have played a less than significant role in the speciation among the lineages of this fish family endemic to Antarctica.     

Proximate composition and energy density of nototheniid and myctophid fish in McMurdo Sound and the Ross Sea,Antarctica

Nototheniid and myctophid fish are primary prey for marine piscivores, yet little is known about their nutritional value. In this study, we characterized the proximate composition [PC: water, fat (neutral lipids), crude protein (CP) and ash] and energy density (ED; kJ g⁻¹) of fifteen fish species from McMurdo Sound and the Ross Sea, Antarctica. We assayed the entire fish for all species except for the large Antarctic toothfish, Dissostichus mawsoni (muscle tissue only). On a wet mass basis (WM), fish were variable in composition: moisture content ranged from 64.9 to 87.3% WM, fat from 0.5 to 17.4% WM, CP from 7.7 to 16.7% WM, ash from 11.2 to 21.0% FFDM (fat-free dry mass), and ED from 2.9 to 10.3 kJ g⁻¹. Myctophids and pelagic nototheniids such as Pleuragramma antarcticum and D. mawsoni were high in fat content (7–17% WM), while a bathylagid and benthic nototheniids including most Trematomus spp. and Lepidonotothen squamifrons were low in fat (0.5–4% WM). The epibenthic Trematomus species (T. eulepidotus and T. lepidorhinus) were intermediate. Energy density tracked fat content, with highest values in myctophids and pelagic nototheniids. The variation in nutrient and energy density confirms that prey composition must be taken into account when modelling energy and nutrient fluxes within the Antarctic ecosystem. Further analyses of prey collected over a number of different locations and seasons are needed in order to determine how the nutritional value of certain species might affect annual or decadal variation in reproductive success or population size of top predators.     

Fishes of the eastern Ross Sea,Antarctica

Antarctic fishes were sampled with 41 midwater and 6 benthic trawls during the 1999–2000 austral summer in the eastern Ross Sea. The oceanic pelagic assemblage (0–1,000 m) contained Electrona antarctica, Gymnoscopelus opisthopterus, Bathylagus antarcticus, Cyclothone kobayashii and Notolepis coatsi. These were replaced over the shelf by notothenioids, primarily Pleuragramma antarcticum. Pelagic biomass was low and concentrated below 500 m. The demersal assemblage was characteristic of East Antarctica and included seven species each of Artedidraconidae, Bathydraconidae and Channichthyidae, ten species of Nototheniidae, and three species each of Rajidae and Zoarcidae. Common species were Trematomus eulepidotus (36.5%), T. scotti (32.0%), Prionodraco evansii (4.9%), T. loennbergii (4.7%) and Chaenodraco wilsoni (4.3%). Diversity indices were highest for tows from 450 to 517 m (H=1.90–2.35). Benthic biomass ranged from 0.7 to 3.5 t km^–2. It was generally higher in tows from 450 to 517 m (0.9–2.0 t km^–2) although the highest biomass occurred at an inner-shelf station (238 m) due to large catches of T. eulepidotus, T. scotti and P. evansii.     

Spermatogenesis in some Antarctic teleosts from the Ross Sea: histological organisation of the testis and localisation of bFGF

Testis structure and spermatogenetic activity were studied in two Antarctic teleostean species, Chionodraco hamatus and Trematomus bernacchii, captured during the austral summer in the Ross Sea. The specimens of C. hamatus showed full reproductive activity but, the spermatogenetic cycle being over, only spermatogonia and Sertoli cells were present in the seminiferous tubules whereas the lumina were full of sperm. By contrast, the specimens of T. bernacchii were in the stage of spermatogenetical recrudescence, having not yet entered the reproductive period. In this species, the seminiferous tubules were devoid of lumen and full of spermatogonial cysts, showing some mitoses. Many tubules contained cysts of meiotic spermatocytes I and, in one case only, small cysts of spermatocytes II. The final stages of spermatogenesis were lacking, presumably occurring later, in autumn/winter. The immunocytochemical tests aimed at identifying bFGF and FGFR1 revealed a positive reaction both in Sertoli cells and spermatogonia in the C. hamatus specimens, indicating that this species was ready to start a new spermatogenetic cycle. The weak reaction in the specimens of T. bernacchii suggests that, in this species, the stage of cell division was over and that of meiosis and differentiation was starting. These data indicate that Antarctic fish have an opportunistic spermatogenetic cycle. Accepted: 24 October 1999     

Picoplankton BIOMASS in the Ross Sea (Antarctica)

Summary Spatial distribution of picoplankton in the Ross Sea was studied. The authors discuss the biomasses of various picoplanktonic-sized fractions and of bacterial cells between 0.2 and 2.0 m capable of growing on Marine Agar 2216 (Difco). Picoplankton having a cellular diameter cf between 1.0 and 2.0 m (PP1) generally predominate, accounting for 73% of the whole picoplankton biomass. However, smaller cells (PP2) can represent 28% of the picoplankton biomass at depths corresponding to 1% of surface light. These results are in good agreement with those found in the coastal regions of McMurdo Sound (Fuhrman and Azam 1980) and in other areas of the Antarctic seas where total bacterioplankton was studied (Hanson et al. 1983b; El-Sayed 1987; Lancelot et al. 1989). Biomasses of total picoplankton (TPP) are not correlated with any of the environmental parameters studied. The PP1 is correlated with O₂ and silicates and PP2 is correlated with O₂, phosphates temperature and nitrates. Aerobic heterotrophic biomasses are correlated with O₂ and salinity.     

The flora of some relict oak woods in Wester Ross

Abstract

Species lists of vascular plants were collected for ten woodlands in Wester Ross. The localities were compared on the basis of these lists and found to be a series from acid, heavily grazed woods containing 41–50 species; to a lightly grazed, base-rich wood in which 124 species were recorded. Tree regeneration and the present status of the woods is discussed.     

Meiofauna distribution and mesoscale variability in two sites of the Ross Sea (Antarctica) with contrasting food supply

Meiofauna abundance, biomass and community structure were investigated in two comparable deep sites of the Ross Sea (Antarctica) characterized by different trophic and sediment characteristics. Site B (567 m depth, dominated by muddy sediments) and site C (439 m depth, characterized by the presence of calcareous debris and coarse sand) were located at increasing distance from the polynyas and were subject to different inputs of organic material to the seabed. Total meiofauna abundance ranged from 192.0 to 1191.2 ind. 10 cm⁻², and total biomass varied between 9.5 and 50.3 μgC 10 cm⁻². Meiofauna densities from the Ross Sea are, on average, 2–7 times lower than those reported from other similar deep polar regions and displayed significant differences between the sites. Nematodes dominated the samples at both sites, but their relative significance changed between the sites (80% at site B and 56% at site C), followed by copepods (1.6% and 35% at sites B and C, respectively). Meiofauna composition at site B appeared similar to that reported for deep-sea antarctic or temperate sediments, whereas the composition at site C was similar to that of coastal areas. On a macroscale, the different inputs of utilizable organic material at the two sites were reflected in meiofaunal distribution patterns, indicating that meiofaunal communities from the Ross Sea are dependent on particulate organic matter fluxes from the photic layer and are coupled to pelagic phenomena. Very low microscale variations (i.e. between replicates) in meiofauna density contrasted with large mesoscale variability, which was related to the concentration of the main food indicators (phytopigments, proteins, carbohydrates and lipids). Accepted: 18 February 1999     

Late Quaternary foraminiferal assemblages from western Ross Sea (Antarctica) in relation to the main glacial and marine lithofacies

Investigations on foraminifers from Upper Pleistocene–Holocene sediments were carried out on twelve cores from the western Ross Sea continental margin (Drygalski, Joides, North Victoria Land Basins) as part of a “Progetto Nazionale di Ricerche in Antartide” (P.N.R.A.) multidisciplinary project. Data on the foraminiferal frequency, species diversity, tests abundance and their state of preservation were presented as a synthesis of 404 core samples to establish their relationships with the main glacial and marine lithofacies of this area. A total of 126 benthic species, pertaining to 73 genera have been identified; just few taxa, such as Cibicides spp., Globocassidulina spp., Trifarina angulosa and Miliammina spp. being the most ubiquitous and in some cases the dominant species of these paleoenvironments. Two variants of Neogloboquadrina pachyderma, including thin and thick-shelled forms have been recovered. We propose to use these results to provide the degree of glacial control during the Last Glacial Maximum and the following Holocene retreat of the ice sheets. High test fragmentation, low diversity and density tests reflect higher glacial influence of the ice sheet in the Drygalski Basin, whereas the decreasing percentage of fragmentation and a relative increase of density and diversity in Drygalski, Joides and North Victoria Land Basins indicate the paleoenvironmental passage from the ice sheet to the ice shelf condition. The ice shelf retreat is well evidenced in the Joides Basin by a succession of levels barren of foraminifers alternating with high-density levels, rich in T. angulosa, followed by a total disappearance of the calcareous foraminifers. Open-marine settings indicative of lower glacial influence and increased corrosiveness of the water masses is testified by the Miliammina foraminiferal assemblage during the Holocene in Drygalski and Joides Basins cores. On the contrary, rich and abundant benthic and planktonic assemblages characterize the Holocene paleoenvironment of the North Victoria Land area, indicating that the water masses were less corrosive with respect to the other areas. In addition to the glacial reworking of the tests, and the dissolution due to the corrosive water mass conditions, the volcaniclastic sediments recovered in the North Victoria Land Basin cores also affected the condition of test preservation. In volcaniclastic sediments, older than about 20 ka BP, the foraminifers concentration tends to zero and, when present, their tests are highly damaged or completely broken.