Aggregation of avian predators and zooplankton prey in Otago shelf waters, New Zealand

In Otago shelf waters surface swarms of krill (Nyctiphanes australis),hyperiid amphipods (Parathemisto spp.) and galatheid crab larvae(Munida gregaria) provide an abundant summer food source forplanktivores. We tested the hypothesis that aggregation of avianplanktivores depends upon the spatial distribution of theirprey. Gulls (Larus scopulinus, L.bulleri, L.dominicanus), sootyshearwaters (Puffinus griseus) and white-fronted terns (Sternastriata) showed significantly aggregated distributions. Thedistribution of birds sitting on the sea surface was correlatedwith the abundance pattern of krill but was not correlated withthe distribution of smaller hyperiid amphipods. The distributionof flying red-billed gulls (L.scopulinus), black-billed gulls(L.bulleri) and sooty shearwaters was correlated with the krilldistribution but black-backed gulls (L.dominicanus) were not.Stomach contents of black-billed gulls were dominated by krill,in contrast to sooty shearwaters, which ate a higher proportionof Minida, and black-backed gulls, which contained Munida andfish. No amphipods were found in bird stomachs. There was nosignificant correlation between bird distributions and the hydrographicregime, water depth or distance offshore. The distribution ofprey rather than hydrographic regime was a more important determinantof bird distributions at this spatial scale (2.6–12 km)and location.     

Swimming in formation in krill (Euphausiacea), a hypothesis: dynamics of the flow field, properties of antennular sensor systems and a sensory-motor link

The act of swimming in formation by species such as Euphausiasuperba, Antarctic krill, is assumed to be regulated by a sensitivityto the characteristic and spatially elaborate flow field producedby this species of shrimp. We used a related species, Meganyctiphanes,North Atlantic krill, to visualize the flow field produced bytethered shrimps in an aquarium. In this situation, the propulsionjet flow some centimeters behind the shrimp is surrounded bya vortex ring of recoiling water motion from which, if the vortexis also produced by unrestrained swimming shrimp, a followingshrimp hypothetically can draw forces of lift and propulsionto decrease energy expense in long-distance migration. Two antennularsensitivities to water vibration in frequency ranges 5–40and 40–150 Hz were calibrated, and the activity of connectedinterneurons was traced into the abdominal pleopod-carryingsegments. Water oscillation of 3–10 Hz frequency, appliedto the antennules, was shown to entrain a closely synchronouspleopod beat in the stimulated specimens.     

An Overview of the Ecology of Antarctic Seals

Four species of seals occupy the pack-ice region of the oceanssurrounding the Antarctic Continent. These seals include thecrabeater (Lobodon cardnophagus), leopard (Hydrurga leptonyx),weddell (Leplonychotes weddellii), and ross (Ommatophoca rossii),and are true seals with special adaptations for living in thepack-ice region. Two other seal species, the southern elephantseal (Mirounga leonina) and the fur seal (Arctocephalus gazella)(the only eared seal of this region) generally occur furtherto the north and use land rather than ice during the periodof birth of young. This paper reviews the status of these species,and examines the generalecology of the four species that inhabitthe pack-ice zone. In general, the four species that occupythe pack-ice zone have specialized in habitats and habits sothat little overlap in dietsor habitat use exist among thesespecies. The exception is the interaction between the leopardand the crabeater which occupy the same regions and eat krill(Euphausia superba), particularly during the winter. The impactof the potential harvest of krill by man on these species isdiscussed. Further, the impact that recovery of the large baleenwhales that feedin this region during the summer is discussedwith regard to the changes that might occur as competition forkrill by the large vertebrate species increases.     

Behavioral and Physiological Characteristics of the Antarctic Krill, Euphausia superba

The antarctic krill, Euphausia superba, is considered a successin the intensely seasonal environment of the Southern Oceanbecause of its abundance and central role as an important fooditem for many of the larger carnivores in the ecosystem. Thebehavioral and physiological characteristics that foster thissuccess are: (1) the ability to find concentrations of foodin several types of habitat and efficiently exploit whateverfood is available; (2) the close correspondence of the lifecycle with seasonal cycles of food availability; and (3) a combinationof physiological mechanisms that enable krill to survive thelong winter period of low food availability. We evaluated therelative importance of the following four major winter-overmechanisms that have been proposed for adult krill west of theAntarctic Peninsula. The three-fold reduction in metabolic rateis the most important winter-over mechanism for these adults,although lipid utilization and shrinkage also help satisfy energyrequirements in the winter. Alternate food sources did not appearto contribute significantly as a winter energy source. However,the extent, predictability and complexity of the ice cover ina region during winter may have a great influence on the relativeimportance of these winter-over mechanisms for different populations.Ice cover in the waters west of the Antarctic Peninsula is unpredictableand smooth surfaced when it occurs, providing the krill withlittle refuge from predation. In multi-year pack ice of theWeddell Sea, however, ice cover is predictable and extensive,and there is a complex undersurface that provides hiding places.In this multi-year ice, adult krill have been observed underthe ice feeding, whereas west of the Antarctic Peninsula mostadult krill are in the water column in the winter and are notfeeding. The balance between acquiring energy and avoiding predationmay be different in these two regions in the winter becauseof differences in predictability and complexity of the ice cover.     

Time-series measurement of grazing rates of zooplankton and bivalves

The current state of the art in automated measurement of grazingrates of zooplankton and bivalves is evaluated. Limitationsto the development of automated methods are discussed with referenceto theoretical considerations. Different approaches to time-seriesmeasurements are examined, and some inadequately investigatedtime scales for measurement of grazing rates are outlined. Levelsof automation of existing experimental systems are described,and a recently developed grazing system based on in vivo fluorescenceadvanced. The new system is suited to use with zooplankton orbivalves, but depends on precise calibration of in vivo fluorescence.It facilitates rapid measurement of functional response to foodconcentration, estimation of time-series rates at constant foodconcentrations, and measurement of rates while simulating patchyfood distribution. Results derived from experiments with krill(Nyctiphanes australis) and cockles (Chione stutchburyi) usingthis grazing system are presented, and some future directionsfor instrument development suggested.     

Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994

Measurements of adult Antarctic krill (Euphausia superba) gutcontents, evacuation and egestion rates, as well as digestiveefficiency, were carried out during February-March 1994 in thevicin ity of South Georgia to estimate in situ daily ration.These were combined with acoustically derived biomass data tocalculate the grazing impact of Antarctic krill and its contributionto the carbon flux in the region. Individual levels of gut pigmentconcentrations and evacuation rates ranged from 27 to 1831 ngchlorophyll a-eq. ind.^–1 and from 0.133 to 0.424 h^–1,respectively. Losses of pigment fluor escence during digestionwere very high, ranging from 58 to 98% of the total pigmentdigested. Daily carbon consumption estimated using the gut fluorescencemethod varied from 0.234 to 0.931 mg C ind.^–1 day^–1(or 0.4–1.7% of body carbon), compared to {small tilde}2.73mg C ind.^–1 day^–1 (or {small tilde}5% of body carbon)using the faecal pellet production data. The 3-fold higher dailyration estimated using egestion rate data may be explained bypredation on micro-and mesozooplankton. Maximum krill grazingimpact ranged from 0.4 to 1.9% of the total phytoplankton stockor from 10 to 59% of the total daily primary production. However,grazing impact on the microphytoplankton (>20 µm) wassubstantially higher, at times exceeding 100% of the daily microphytoplanktonproduction. It is suggested that to meet its energetic demands,kriil must consume a substantial proportion of heterotrophiccarbon. ³Present address Zoology Department, University of Fort Hare,P/Bag X1314, Alice, 5700, South Africa     

Zooplankton in the Ligurian Sea: Part II. Exploration of their physical and biological forcing functions during summer 2000

A survey of the biological and physical oceanography of theLigurian Sea was conducted in the late summer of 2000. Forty-onestations were sampled for nutrients, oxygen, fluorescence andhydrographic information. Acoustic backscatter measurementswere used to estimate abundance of small (<5 mm) zooplanktonbiovolume versus depth and the distribution of northern krill,Meganyctiphanes norvegica. Net-tow and underwater video datawere collected to identify the zooplankton present. These datawere used to analyze the Ligurian Sea ecosystem for physicaland biological linkages that control zooplankton abundance anddistribution. Results are compared with those from a similarstudy conducted in 1999. Hydrographic sampling showed a domeof dense water in the southwestern middle of the basin. Thehighest chlorophyll a (Chl a) concentrations were measured inthis area, while small zooplankton biovolume was evenly distributedthroughout the survey. Integrated values of Chl a and smallzooplankton biovolume in 2000 were greater than in 1999. Meganyctiphanesnorvegica, siphonophores and salps were the dominant componentsof the macrozooplankton population in the upper 200 m. In thesampled depth strata, siphonophore abundance did not changeduring the day, while M. norvegica were only caught at night.Acoustic backscatter data show that higher densities of M. norvegicaoccurred in deeper water and in the western and southwesternareas of the Ligurian Sea.     

A field study on the physiology of digestion in the Antarctic krill, Euphausia superba, with special regard to chitinolytic enzymes

Endo- and exochitinase activities were determined in the stomachand midgut gland of the Antarctic krill, Euphausia superba.along a transect west of the Antarctic Peninsula. Activitieswere compared with the digestive enzymes protease, cellulase(1,4-ß-D-glucanase) and laminarinase (1,3-ß-D-glucanase)The chlorophyll and protein contents in the surface water ofthe corresponding stations were determined. Enzyme activitieswere characterized by high individual and spatial variations.Chitinolytic activity in the stomach correlated well with alldigestive enzymes investigated. In the midgut gland, a correlationwith cellulase and laminannase was evident. The amount of chlorophylla and phytoplankton protein in the surface water was not correlatedwith enzyme activity. Specific enzyme activity was higher inthe stomach than in the midgut gland. showing individual ratiosfor each enzyme. Elevated endochitinase activity in the stomachsuggests that chitinous food is digested to oligomers in thestomach, while the subsequent degradation to amino sugars occurspredominantly in the midgut gland.     

Changes in muscle tissue of shrinking Antarctic krill

We examined the mechanism by which Antarctic krill, Euphausia superba, shrink, and suggest that cellular changes occurring during shrinkage may provide a means for identifying krill that have undergone shrinkage. We compared the muscle tissue of juvenile, adult and shrunken adult krill to identify changes in cell number associated with maturity and shrinkage. Comparison of the absolute number and density of nuclei in abdominal segments of juvenile, adult and shrunken adult krill revealed differences related to maturity and shrinkage. Shrunken adult krill had nearly twice as many nuclei per unit area than adult krill that had not shrunk. This suggests that krill shrink by a reduction in cell volume, rather than cell loss. This simply detected variation in muscle cell nucleus density may be useful in distinguishing shrunken adult krill from juveniles, and contribute to our knowledge of age structure in natural populations.     

Deterioration of Antarctic Krill Muscle during Freeze Storage

The effects of freezing on the heat-induced gelation, Ca²⁺-ATPase activity and myofibril structure of Antarctic krill muscle were investigated. Muscle from freshly caught krill was immediately stored at-20°C in the presence (to prevent freezing) (glycerol krill) and absence (frozen krill) of glycerol. Several protease inhibitors, monosodium glutamate and Ca²⁺ were individually added to glycerol krill to inhibit endogenous proteolysis. The examinations described above were carried out after about 3-month storage at-20°C. In glycerol krill (unfrozen state), viscoelastic parameters of the heat-induced gels and Ca²⁺-ATPase activities of all the krill samples were similar to those of “surimi” (raw fish meat paste) of Alaska pollack which gave gel of good quality, although the micro structure (Z-lines) of myofibrils was different among the glycerol krill samples. In frozen krill, however, the parameters of the gel were different from those of “surimi”, the ATPase activity was completely lost and disruption of the myofibril structure occurred. Refreezing (-20°C) of glycerol krill after removal of glycerol resulted in a marked decrease in the gelation ability. These results suggest that freezing of krill muscle causes deterioration of the gelation ability.     

Cycles of <Emphasis Type="Italic">Euphausia superba</Emphasis> recruitment evident in the diet of Pygoscelid penguins and net trawls in the South Shetland Islands,Antarctica

Size and sex of Antarctic krill taken from chinstrap and gentoo penguin diet were compared to those from scientific net surveys in the South Shetland Islands from 1998 to 2006 in order to evaluate penguin diet as a sampling mechanism and to look at trends in krill populations. Both penguin diet and net samples revealed a 4–5 year cycle in krill recruitment with one or two strong cohorts sustaining the population during each cycle. Penguin diet samples contained adult krill of similar lengths to those caught in nets; however, penguins rarely took juvenile krill. Penguin diet samples contained proportionately more females when the krill population was dominated by large adults at the end of the cycles; net samples showed greater proportions of males in these years. These patterns are comparable to those reported elsewhere in the region and are likely driven by the availability of different sizes and sexes of krill in relation to the colony.     

Antarctic krill (Euphausia superba Dana) eat salps

 Feeding behaviour of Antarctic krill (Euphausia superba) on salps was observed in shipboard experiments during the 1994/1995 Kaiyo Maru Antarctic Ocean research cruise. The feeding rate was more than 0.5 salp/krill per day. When offered ethanol extracts of four prey types, salps, phytoplankton, krill and polychaetes, krill preferred the salp extracts. This evidence implies that the substances extracted from salps were most attractive to krill. These results might indicate a tight ecological relationship between krill and salps. Received: 24 May 1995/Accepted: 8 October 1995     

Related antipodes: a comparative study on digestive endopeptidases from Northern krill and Antarctic krill (Euphausiacea)

The Antarctic krill, Euphausia superba, and the Northern krill, Meganyctiphanes norvegica, are closely related species but occupy significantly different trophic and climatic environments. E. superba holds a key position as a phytoplankton grazer in the Southern Ocean. The omnivorous M. norvegica is an important member of plankton communities in the Northeast Atlantic. Both species expressed high proteolytic activities which were dominated by serine proteinases. In the stomachs of Antarctic krill, activities of total proteinase, trypsin, and chymotrypsin were significantly higher than in Northern krill. In the midgut glands, however, total proteinase and trypsin activities were similar in both species, but chymotrypsin activity was significantly higher in Antarctic krill. Moreover, Antarctic krill expressed four trypsin isoforms while only one isoform appeared in Northern krill. Chymotrypsin was present in either species as one single isoform. Antarctic krill adapted to the low and patchy distribution of food by elevated enzyme activities and the expression of trypsin isoforms with slightly different catalytic properties. Presumably, these enzymes facilitate in concerted action the efficient utilization of proteins from phytoplankton, the major food. Northern krill, in contrast, seems not to be equipped to face food limitation. It expresses a “simple” or “basic” set of digestive enzymes for utilizing abundant and easily digestible prey.     

Spatial and temporal variability in reproductive timing of Antarctic krill (Euphausia superba Dana)

Spawning dates of Antarctic krill, Euphausia superba Dana, were calculated from larval stage compositions, and corrected using data on maturity stage composition of the adult krill. Both original and literature data obtained from the Antarctic Peninsula-Bellingshausen Sea area and around the Antarctic continent were used. A time series (1975/76–1986/87) for several subareas of the Antarctic Peninsula-Bellingshausen Sea area indicates considerable variation in the krill spawning start, maxima and completion. In particular years (1975/76, 1980/81), krill spawning in the western Atlantic sector began relatively early, was intensive, and completed early. Some years (1977/78, 1981/82) were characterised by long and non-synchronised krill spawning. Compiled data sets for the Atlantic sector (1980/81), the entire Antarctic (1983/84) and the east Indian-west Pacific Antarctic waters (1981–85) reveal some spatial patterns in krill reproductive timing. In relation to spawning timing variation, the habitats of the krill population fall into five categories: (1) areas with an early beginning (late Novemberearly December) and a variable, but normally long, duration (3–3.5 months) of krill spawning; this is generally the southern boundary of the Antarctic Circumpolar Current, (2) areas with an early beginning, but a short duration of krill spawning (Gerlache Strait), (3) areas with a highly variable (within 1–1.5 months) beginning and a relatively long duration (ca. 3 months) of krill spawning (Bransfield Strait, Palmer Archipelago), (4) areas with a late beginning (late December–January) and a long duration of krill spawning (Bellingshausen Sea, D'Urville Sea, and Balleny Islands area), and (5) areas with a delayed beginning, but a very short duration (ca. 1.5 months) of krill spawning (Ross Sea slope, probably the Coastal Current area off the Lasarev Sea shelf and in the south-eastern Weddell Sea. These patterns can be partly explained by peculiarities of the ice regime in particular areas and by routes of krill movement within water circulation systems.     

Interannual variation in condition of the mackerel icefish

Mackerel icefish Champsocephalus gunnari are widespread on the South Georgia (54° S, 36° W) shelf. Analysis of condition indicated a strong interannual variation. High condition indices, indicative of good feeding conditions, were present when krill were abundant in the region. Years when krill were scarce and condition index was consequently low, were consistent with years when indices from land-based krill predators also indicated that krill were scarce.     

Using carapace measurements to determine the sex of Antarctic krill, Euphausia superba

Krill (Euphausia superba) carapace measurements (length and width; mm) collected from plankton tows in the South Shetland Islands (SSI), Antarctica are used to test the generality of a common discriminant function developed to reconstruct krill length frequencies in Antarctic fur seal diets for the area surrounding South Georgia (SG). Total length and sex ratio of krill in the SSI were overestimated by 5.6 and 154%, respectively, when the SG allometric equations were applied to 3 years (2003–2005) of data. These errors arise and increase as a result of krill population dynamics, specifically recruitment that contributes large proportions of immature krill, misclassified as males by the SG discriminant function. We develop sex-specific regression models based on separate discriminant functions that provide significantly better discriminatory power. However, our analysis indicates that reconstructions of krill sex ratio and length composition in the ocean environment are less reliable in years when the ratio of immature to mature krill is high. For the SSI area, five out of 14 years (35.7%) surveyed (1992–2005) had proportions of immature to mature adult krill ≥ 0.50.     

Relative changes in krill abundance inferred from Antarctic fur seal

Antarctic krill Euphausia superba is a predominant species in the Southern Ocean, it is very sensitive to climate change, and it supports large stocks of fishes, seabirds, seals and whales in Antarctic marine ecosystems. Modern krill stocks have been estimated directly by net hauls and acoustic surveys; the historical krill density especially the long-term one in the Southern Ocean, however, is unknown. Here we inferred the relative krill population changes along the West Antarctic Peninsula (WAP) over the 20th century from the trophic level change of Antarctic fur seal Arctocephalus gazella using stable carbon (δ(13)C) and nitrogen (δ(15)N) isotopes of archival seal hairs. Since Antarctic fur seals feed preferentially on krill, the variation of δ(15)N in seal hair indicates a change in the proportion of krill in the seal's diets and thus the krill availability in local seawater. For the past century, enriching fur seal δ(15)N values indicated decreasing krill availability. This is agreement with direct observation for the past ～30 years and suggests that the recently documented decline in krill populations began in the early parts of the 20th century. This novel method makes it possible to infer past krill population changes from ancient tissues of krill predators.     

Long-term changes in Krill biomass and distribution in the Barents Sea: are the changes mainly related to capelin stock size and temperature conditions?

Krill plays a significant role in the Barents Sea ecosystem, providing energy transport between different trophic levels. The current paper presents the results of a long-term study (1980–2009) based on pelagic trawl catches from August to September. Our investigations show that the krill species were distributed widely in the Barents Sea and that the largest krill concentrations were restricted to the west-central and eastern parts of the Barents Sea. The current paper presents the relative biomass indices, and the estimates must be interpreted as minimum biomass. The mean annual krill biomass was estimated to be 22 million tonnes in wet weight, with the highest values being as much as 48 million tonnes. Capelin is the largest pelagic stock, and in some years, their biomass can amount to 4–7 million tonnes, which can impose high predation pressure on krill. When their biomass is high, capelin may consume close to 26 million tonnes annually. The predation from pelagic (herring and blue whiting) and bottom (cod and haddock) fish species was much lower, being 9 and 1 million tonnes, respectively. A negative relationship between krill biomass and capelin stock size above 74°N was observed during the study period. However, during the last decade, the krill biomass has increased despite heavy predation from capelin in some years. A positive significant linear relationship between the mean annual Kola temperature and the krill biomass seems to indicate that the recent warming conditions have favourable impacts on the krill populations in the Barents Sea.     

Analysis of the flow field of the krill,Euphausia pacifica

Velocity measurements were performed for the flow field generated by tethered krill Euphausia pacifica. The particle image velocimetry (PIV) technique was used to measure the velocity field in vertical planes aligned with the krill body axis. The krill generates a narrow jet-like flow behind and below the pleopods (roughly 25° below horizontal). The volume of fluid moving at greater than 10% of the maximum velocity near the pleopods is roughly 18 times larger than the volume of the krill. Thus, the hydrodynamic disturbance occupies a significantly larger region than the animal body. Other krill, sensing the flow disturbance, may take advantage of the flow induced by a neighbor to locate a mate or to draft for efficient propulsion.