Laboratory and field estimates of the rate of faecal pellet production by Antarctic krill,Euphausia superba

The time course of faecal pellet production (egestion) was monitored in January 1985 for a population of Antarctic krill, Euphausia superba Dana, maintained in flowing seawater aquaria at Palmer Station, Antarctica. Following transfer to filtered seawater, krill produced faecal strings for roughly 40 min, after which time faecal egestion virtually ceased. Similar results were obtained for freshly-trawled krill at sea in February and March 1985. There were wide daily variations in total faecal egestion rate; mean rates varied from 0.54 to 1.66 mg dry wt h^-1 and individual rates from 0.25 to 2.35 mg h^-1 (all data corrected to a standard krill of 600 mg fresh weight). Despite these wide fluctuations in total faecal egestion, the loss of organic matter showed no significant daily variation, with a mean value of 0.13 mgh^-1. The relationship between faecal egestion rate and faecal organic content suggested that feeding rate was governed by food quality; when inorganic load was high, feeding rate increased to ensure sufficient energy intake. The data suggest that superfluous feeding does not occur in krill and that values of gut-clearance time calculated from time intervals greater than about 40 min will not be representative of previous feeding history. the rates of faecal egestion observed in this study indicate that the flux of faecal pellets from krill is substantial. They imply an energy intake in E. superba of 17 to 28% body weight per day, much higher than estimated previously for this species by summing known energy losses, but similar to estimates for other euphausiids.     

Effect of temperature on the molting,growth and maturation of the antarctic krill Euphausia superba (Crustacea: Euphausiacea) under laboratory conditions

The effect of temperature on molting, growth, and maturation rates was studied on laboratory-maintained Euphausia superba. The length of intermolt periods (IMP's) was inversely proportional to temperature (20.10 d, SD=1.60, at 0.12°C; 16.87 d, SD=1.68, at 0.97°C; and 12.48 d, SD=0.90, at 4.48°C), and directly proportional to krill size at 0.12°C and 0.97°C. For individually maintained krill the maximum growth rate at 4.48°C (0.068 mm d^-1) was nearly twice that at 0.68°C (0.037 mm d^-1). There was no observable temperature effect on maturation rates. The maturation changes of juveniles at all temperatures indicated that more than two years are probably required to reach maturity. Mature males and females regressed to immature forms, suggesting that E. superba may reproduce in successive years. These results and previously reported field and laboratory data for E. superba and other euphausiid species suggest a 4+ year life span for this species.This work was supported by NSF grant DPP 76-23437     

Change in the gross biochemistry and mineral content accompanying the moult cycle in the Antarctic krill Euphausia superba

Concentrations of water, ash, protein, chitin, lipid, calcium, magnesium, sodium, potassium, strontium and copper were measured in individuals from a laboratory population of Antarctic krill, Euphausia superba Dana, over the course of a moult cycle. Significant changes in all variables were encountered. Total ash, lipid, calcium, magnesium and strontium all increased in concentration following moulting. Water, protein and copper concentrations all decreased following ecdysis and increased again towards the end of the moult cycle. The major ions sodium and potassium fluctuated around mean levels. Cast moults of E. superba were shown to be a drain on the ionic load of the krill, and the losses inherent in exuviation could account for much of the variation observed during the moult cycle.     

Comparative lipid dynamics of euphausiids from the Antarctic and Northeast Pacific Oceans

To better understand the feeding and reproductive ecology of euphausiids (krill) in different ocean environments, lipid classes and individual lipid components of four different species of euphausiids from Northeast Pacific (temperate species) and Southern Ocean (Antarctic species) were analyzed in animals from multiple life stages and seasons. The dominant krill species in the Northeast Pacific Euphausia pacifica and Thysanoessa spinifera, were compared to the two major Antarctic species, Euphausia superba and E. crystallorophias. Analysis comprised total lipid and lipid classes together with individual fatty acid and sterol composition in adults, juveniles, and larvae. Antarctic krill had much higher lipid content than their temperate relatives (10–50 and 5–20% of dry mass for Antarctic and temperate species, respectively) with significant seasonal variations observed. Phospholipids were the dominant lipid class in both temperate krill species, while neutral storage lipids (wax esters and triacylglycerols for E. crystallorophias and E. superba, respectively) were the major lipid class in Antarctic krill and accounted for up to 40% of the total lipid content. Important fatty acids, specifically 16:0, 18:1ω9, 20:5ω3, and 22:6ω3, were detected in all four krill species, with minor differences between species and seasons. Detailed lipid profiles suggest that krill alter their lipid composition with life stage and season. In particular, larval Antarctic krill appear to utilize alternate food resources (i.e., sea-ice associated organisms) during austral winter in contrast to juveniles and adults (i.e., seston and copepods). Lipid dynamics in krill among krill in both systems appear closely linked to their life cycle and environmental conditions including food availability, and can provide a more complete comparative ecology of euphausiids in these environmentally distinct systems.     

Omnivorous feeding behavior of the Antarctic krill Euphausia superba

Feeding experiments were conducted at Palmer Station from December 1985 to February 1986 to examine the potential role of copepod prey as an alternative food source for Euphausia superba. Copepod concentration, copepod size, phytoplankton concentration, the duration of krill starvation and the volume of experimental vessels were altered to determine effects on ingestion and clearance rates. Krill allowed to feed on phytoplankton and copepods in 50-litre tubs showed greatly increased feeding rates relative to animals feeding in the much smaller volumes of water traditionally used for krill-feeding studies. Clearance rates on copepods remained constant over the range of concentrations offered, but clearance rates on phytoplankton increased linearly with phytoplankton concentration. Feeding rates increased when larger copepods were offered and when krill were starved for two weeks prior to experiments. Clearance rates of krill feeding on copepods were higher than, but not correlated with, their clearance rates on phytoplankton in the same vessel. E. superba may have a distinct mechanism for capturing copepods, perhaps through mechanoreception. Although our observed clearance rate of 1055 ml krill^-1 h^-1 indicates that krill can feed very efficiently on copepod prey, such feeding would meet less than 10% of their minimum metabolic requirements at the typical copepod concentrations reported for Antarctic waters. However, substantial energy could be gained if krill fed on the patches of high copepod concentrations occasionally reported during the austral summer, or if krill and copepods were concentrated beneath the sea ice during the winter or spring months. Our results, indicating efficient feeding on zooplankton and higher clearance rates on phytoplankton than previously believed, represent a step towards balancing the energy budget of E. superba in Antarctic waters.     

Reproductive trade-off in male Antarctic krill,Euphausia superba

Freshly caught male and female Euphausia superba from the same swarm exhibited different rates of mortality subsequent to capture. Mortality was significantly higher for reproductive males (100%, n=68) than for females (3%, n=186) within the first 3 d of capture. Total lipid and triacylglycerol levels in male, female and juvenile Euphausia superba were analysed and compared. All reproductive male krill analysed from this swarm had low lipid levels (1 to 3% dry weight) with negligible triacylglycerol stores (0 to 2% of total lipid). Somatic lipid stores in female and juvenile krill ranged from 8 to 30% of which up to 40% was triacylglycerol. The levels of algal sterols in the digestive gland of males, females and juveniles indicate that all krill had been feeding recently. An analysis of the sex ratio of krill catches derived from data collected over seven summers from the Prydz Bay region showed a decrease in the proportion of males with increasing size. There was a sharp decline in numbers of male krill once they attained a length of 51 to 55 mm. Low lipid levels in redroductive male krill may be due to reproductive costs. The resulting low storage-lipid levels are accompanied by high mortality in male krill.     

Using a high-powered strobe light to increase the catch of Antarctic krill

Adult euphausiids are difficult to capture with the nets typically used by oceanographers due in part to avoidance of the net. A cruise of the Southern Ocean GLOBEC program (NBP0103) in April/May 2001 permitted an evaluation of the efficacy of a high-powered strobe light (150 W) mounted on a 1-m² multiple opening/closing net and environmental sampling system (MOCNESS) in reducing net avoidance. Three horizontal tows were made in Laubeuf Fjord, Marguerite Bay (western Antarctic Peninsula, 67.89°S; 68.30 W) on 28–29 May in an area that had a high abundance of adult krill (mostly Euphausia superba, Dana and a few E. crystallorophias, Holt and Tattersall). Each tow consisted of a series of paired down and up casts through a set depth interval (e.g. 50–90 m), with each successive net (335 µm mesh) sampling both a down and up cast. The strobe light was either on or off while each net was open, and when on, the light flashed at 4-s intervals. During a tow, four of the eight nets sampled with the strobe flashing and four sampled with the strobe off, in a random sequence. Total zooplankton displacement volume was significantly higher (P<0.05), on average by a factor of ~1.5, when the strobe light was on. The increased biovolume was due to the enhanced catch (factor of ~2) of adult krill (15–60 mm length). There was no enhanced catch of smaller krill (5–15 mm length: a mixture of E. superba, E. crystallorophias, and Thysanoessa macrura, GO Sars). In addition, the average length of the large krill fraction was not changed substantially with the strobe light on. These results suggest that krill avoidance of nets can be overcome by intense strobe lighting.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00227-003-1228-z.Communicated by J.P. Grassle, New Brunswick     

Growth of Antarctic krill Euphausia superba at South Georgia

 Antarctic krill Euphausia superba has a central role in the ecosystem of the Southern Ocean and knowledge of its growth rate is central to determining the factors influencing population dynamics. The length of Antarctic krill in the diet of Antarctic fur seals Arctocephalus gazella at South Georgia revealed a consistent increase in size between ca. 42 and ca. 54 mm over the period October–March, indicating growth rates much higher than predicted by existing models. Geographical variation in growth rate may result in 2-year-old krill at South Georgia attaining the same size as 3-year-old krill in the Antarctic Peninsula region. The effect of geographical variation in growth rate on the population structure of krill has important implications for comparing the fate of individual cohorts over large scales and in the interpretation of krill life-cycles. Received: 20 May 2000 / Accepted: 11 August 2000     

Flexible foraging strategies of gentoo penguins <Emphasis Type="Italic">Pygoscelis papua</Emphasis> over 5 years in the South Shetland Islands,Antarctica

Gentoo penguins Pygoscelis papua show considerable plasticity in their diet, diving, and foraging behaviors among colonies; we expected that they might exhibit similar variability over time, at a single site, since flexible foraging habits would provide a buffer against changes in prey availability. We examined interannual changes in the foraging strategies and diet of gentoo penguins in the South Shetland Islands, Antarctica, over 5 years with variable prey abundance. Antarctic krill Euphausia superba was the primary diet item, and fish the secondary, though the importance of these items varied among years. Diving behavior also varied over time: different dive depth distributions were observed among years. Nonetheless, chick-rearing success remained relatively constant, indicating that gentoo penguins were able to maintain chick provisioning by altering their foraging strategy among years. Variable abundance of krill in the region did not have observable impacts on the diet, foraging behaviors or chick-rearing success of gentoo penguins. We suggest that foraging plasticity may be one reason that gentoo penguin populations have remained stable in the region, while their congeners (P. antarctica and P. adeliae) with less flexible foraging strategies have declined.     

Effects of simulated light regimes on maturity and body composition of Antarctic krill, <Emphasis Type="Italic">Euphausia superba</Emphasis>

The effect of different light regimes on the development of sexual maturity and body composition (carbon, nitrogen, lipid and protein) of Antarctic krill, Euphausia superba, was studied over 12 weeks under laboratory conditions. Krill were exposed to light-cycle regimes of variable intensity to simulate Southern Ocean summer, autumn and winter conditions, respectively using: (1) continuous light (LL; 200 lux max), (2) 12-h light and 12-h darkness (LD 12:12; 50 lux max), and (3) continuous darkness (DD). The sexual maturity of female and male krill exposed to LL and LD 12:12 showed an accelerated succession of external maturity stages during the experimental period, while krill exposed to continuous darkness showed no changes in external maturity during the course of the study. Changes in the maturity development of krill between the different light regimes are reflected in changes in body composition. Krill exposed to LL and LD 12:12 showed an increase in lipid utilization, indicating that the development of external maturation may be fuelled preferentially by lipid reserves. In contrast, values of total lipid content of krill held under continuous darkness indicated an unchanged lipid catabolism during the course of the study. Thus, the maturity development of krill was affected either directly or indirectly by the different simulated light conditions. Based on these results, and observations on the effects of simulated light regimes on feeding and metabolic rates of krill available from a previous study, we suggest that the Antarctic light regime is an essential cue governing the seasonal cycle of krill physiology and maturity, and highlight the importance of this environmental factor in the life history of krill.     

Lipid metabolism of the Antarctic krill Euphausia superba and its ecological implications

 Various developmental stages (early larvae to adults) of Euphausia superba have been collected in different seasons in the Weddell Sea, the Lazarev Sea and off the Antarctic Peninsula to investigate the role of lipids and fatty acids in the life cycle of the Antarctic krill. The total-lipid data for E. superba exhibited seasonal variations, with low lipid levels in late winter/early spring and the highest levels in autumn. Seasonal changes were most pronounced in the immature and adult specimens, increasing from about 10% lipid of dry mass to more than 40%. The fatty-acid compositions of the younger stages were dominated by 20:5(n-3), 22:6(n-3) and 16:0. These are typical phospholipid fatty acids, which are major biomembrane constituents. The phospholipid composition was similar in the older stages. With increasing storage of triacylglycerols in the lipid-rich immature and adult stages, the fatty acids 14:0, 16:0 and 18:1(n-9) prevailed, comprising about 70% of total triacylglycerol fatty acids. The trophic-marker fatty acids 16:1(n-7) and 18:4(n-3), indicating phytoplankton ingestion, were less abundant. They reflected, however, the dependence of the larvae on phytoplankton as well as the seasonal changes in algal composition. The generally close linear relationships between fatty acids and lipid suggest that the fatty-acid compositions of the collected specimens were largely independent of the respective developmental stage, season and region. The linear fit indicates that triacylglycerol accumulation started at a level of about 5% of total lipid. Considering the various overwintering scenarios under discussion, the life cycle and reproductive strategies of krill are discussed in the context of the lipid metabolism and fatty-acid composition of E. superba. Lipid production is effective enough to accumulate large energy reserves for the dark season, but E. superba does not exhibit the sophisticated biosynthetic pathways known from other Antarctic euphausiids and copepods. Although important, lipid utilisation appears to be just one of several strategies of E. superba to thrive under the extreme Antarctic conditions, and this pronounced versatility may explain the success of this species in the Southern Ocean. Received: 16 June 2000 / Accepted: 18 December 2000     

Predicting changes in the Antarctic krill, Euphausia superba, population at South Georgia

Variability in the Southern Ocean is frequently reflected in changes in the abundance of Antarctic krill Euphausia superba and subsequent effects on dependent predators. However, the nature and consequences of changes in krill population dynamics that accompany fluctuations in its abundance are essentially unknown. A conceptual model, developed from quantitative measures of krill length in the diet of predators at South Georgia from 1991 to 1997, allowed predictions to be made about the abundance and population structure of krill in 1998 and the consequences for predators. Consistent with model predictions, in 1998 there was a serial change in krill population structure, low krill biomass and low reproductive performance of predators. The change in the modal size of krill, from 56 mm in December to 42 mm in March, was apparently a result of the transport of krill into the region. This is the first occasion when the future status and structure of the krill population at South Georgia has been successfully predicted. By representing local krill population dynamics, which may also reflect large-scale physical and biological processes, predators have a potential key role as indicators of environmental variation in the Southern Ocean at a range of spatial scales. Received: 6 March 1999 / Accepted: 3 September 1999     

Metabolic activity and elemental composition of krill and other zooplankton from Prydz Bay,Antarctica, during early summer (November–December)

Oxygen uptake and ammonia excretion rates, and body carbon and nitrogen contents were measured in krill (Euphausia superba) and eight other zooplankton species collected during November–December 1982 in the Prydz Bay, Antarctica. From these data, metabolic O:N ratios (by atoms), body C:N ratios (by weight) and daily metabolic losses of body carbon and nitrogen were calculated as a basis from which to evaluate seasonal differences in metabolism and nutritional condition. Comparison of the present data with mid-summer (January) data revealed that early-summer E. superba were characterized by higher metabolic O:N ratios (58.7 to 103, compared with 15.9 to 17.5 for mid-summer individuals). Higher O:N ratios of early-summer E. superba resulted largely from reduced ammonia excretion rates and, to a lesser degree, from slightly increased oxygen uptake rates. Body C:N ratios of E. superba were low in early-summer (3.8 to 4.2) compared with mid-summer krill (4.1 to 4.7) due to lowered body-carbon contents in the former (42.6 to 43.6% compared with 43.2 to 47.5% dry weight of midsummer individuals); gravid females formed an exception, since no seasonal differences in body elemental composition were detected for these. No significant changes in water content (75.3 to 81.4% wet wt) and nitrogen content (9.9 to 11.1% dry wt) in E. superba were evident between the two seasons. Seasonal differences in metabolic rates and elemental composition were less pronounced in a salp (Salpa thompsoni), but a higher metabolic O:N ratio occurred in early-summer individuals. Interspecific comparison of the seven remaining zooplankton species studied with twelve species from mid-summer zooplankton investigated in an earlier study indicated that higher metabolic O:N ratios in early-summer are characteristic not only of herbivore/filter-feeders, but also of some carnivores/omnivores. No relationship between metabolic O:N ratios and body C:N ratios was apparent either intraspecifically or interspecifically, within or between early-summer and mid-summer seasons.     

Antarctic krill aggregation characteristics from acoustic observations in the Southwest Atlantic Ocean

The distributional features and physical characteristics of 4830 krill (Euphausia superba Dana) aggregations detected acoustically in the Southwest Atlantic between 26 January and 21 February 1981 are described. Results are compared with aggregations detected in the Indian Ocean. Aggregations in the Atlantic were larger, closer to the surface and to each other than in the Indian Ocean. Similar patterns in the distribution of aggregation spacing along survey transects were found in the two areas, although the pattern of spacings in the Atlantic indicates differences in the scale of aggregation. Serial interdependence of aggregation variables was minimal in the Atlantic, with aggregation thickness, length and spacing showing weak inter-relationships. Weak functional association, between water depth and aggregation thickness was evident. Investigation of variability in aggregation structure in relation to prevailing environmental conditions gave equivocal results and no clear association between any aggregation variable and prevailing hydrography was observed. The implications of these results for future studies on krill aggregation are discussed in relation to a conceptual framework which was developed from the present results and aimed at linking krill aggregation characteristics to environmental features.     

Ultrastructure of the integument of a pelagic Crustacean: moult cycle related studies on the Antarctic krill,Euphausia superba

The ultrastructure of euphausiid integument was examined in relation to the moult cycle and supplemented by investigations of chitinase activity in the integument and content of N-acetyl--D-glucosamine in the hemolymph. The Antarctic krill, Euphausia superba was collected in 1983 in Admiralty Bay, King George Island, Antarctica. Some specimens of the Northern krill, Meganyctiphanes norvegica, from the Danish Kattegat served for comparison. As a major aim of the study, the moult staging system developed for living tissue could be verified by ultrastructural findings. Under experimental high production conditions of the Antarctic summer, no period of rest or intermoult between post- and premoult was observed in subadult E. superba. Neither was a resting phase seen at the cellular level, the epidermis remained active. The epidermal gland cells did not show any cyclical changes, and the organelles of protein synthesis were generally well developed in all moult stages. In order to follow the physiological course of events, structural and biochemical methods were combined and showed as a result that the last moult stage before ecdysis is characterized by massive cuticular resorption. The epicuticle remained ultrastructurally unchanged before and after ecdysis, even though its permeability should alter at ecdysis. The existence of muscle insertions which connect the old and the new cuticle across the exuvial space suggests an answer to the question why E. superba is hardly impaired in swimming almost up to the time of ecdysis.Dedicated to Professor Dr. H. Leonhardt on the occasion of his 70th birthday     

Ecological implications of fecal pellets produced by the Antarctic krill Euphausia superba in the Antarctic Ocean

Organic analyses and electron microscopic observations on fecal pellets produced by the Antarctic krill Euphausia superba Dana showed that krill fed on choanoflagellates, the abundant heterotrophic flagellate in the Antarctic Ocean. Two new pathways of organic materials in the Antarctic ecosystem are proposed: (1) a new food chain including non-living particulate and dissolved organics, and bacteria-choanoflagellate-krill-vertebrate, which coexists with the traditional diatom-krill-vertebrate food chain; (2) non-phytoplanktonic organic materials in surface waters are transferred into choanoflagellates and are transported to deep water as fecal pellets which are still useful as nutrition for other organisms there.     

Krill evolution and the Antarctic ocean currents: evidence of vicariant speciation as inferred by molecular data

The phylogenetic relationships of the Antarctic krill Euphausia superba, the key species in the Antarctic food web, and other Antarctic and sub-Antarctic cuphausiids have been investigated using the 16S ribosomal mitochondrial gene. The phylogenetic reconstructions indicated that the Antarctic species form a monophyletic clade separated by the non-Antarctic species. The results revealed a large genetic divergence between the Antarctic (E. superba and E. crystallorophias) and sub-Antarctic species (E. vallentini). The time of separation between these species, estimated from the molecular data, is around 20 million years ago, which is comparable with the geological time of the formation of a circum-Antarctic water circulation and the Antarctic Polar Frontal Zone. The euphausiid molecular phylogeny therefore represents evidence for vicariant speciation.     

Contribution of heterotrophic material to diet and energy budget of Antarctic krill, Euphausia superba

A novel approach was used to estimate the heterotrophic carbon component in the diet of the Antarctic krill Euphausia superba. Over 200 specimens from seven samples collected in the Lazarev Sea (January 1993 and 1995), at the Antarctic Polar Front (January 1993), and around South Georgia (February/March 1994) were dissected, and the total carbon content of their stomachs was estimated with a CHN-analyser. Gut-pigment contents were also measured by the gut-fluorescence technique in specimens collected at the same time, and the equivalent amount of their gut carbon was then subtracted from the total organic carbon content of guts from the same samples. The remaining carbon was assumed to originate entirely from heterotrophic food sources. This heterotrophic component accounted for a substantial proportion of the total food consumed by Antarctic krill, ranging from 17.4 to 98.9% of the mass of the gut contents (mean = 78.8% ± 21.2 SD). The results make an important contribution to the elucidation of the energy budget of krill and its daily carbon ration. With a few exceptions, previous estimates were largely calculated from a solely autotrophic carbon source, and were unable to account for the metabolic requirements of E. superba. Krill plays an important role in Antarctic food webs, as it often constitutes ≃50% of the total biomass of the zooplankton, and produces fast-sinking, dense faecal pellets which are important in the vertical transport of organic carbon from the euphotic layer to the deep ocean. High consumption rates of smaller heterotrophic organisms by krill suggest that this large microphage may be more important than previously believed in re-packaging micro- and mesozooplankton into a longer-lasting and more easily sequestered carbon pool. Received: 26 October 1998 / Accepted: 14 October 1999     

Validation and quantification of extractable age pigments for determining the age of Antarctic krill (<Emphasis Type="Italic">Euphausia superba</Emphasis>)

Antarctic krill, Euphausia superba, hatched from eggs and maintained for four years, were sampled periodically for age-pigment analysis. Extractable pigments from the eye and eyestalk ganglia were quantified using fluorescence intensity and standardised against protein. Three peak fluorescence intensities were detected at wavelengths of excitation 280 nm, emission 625 nm (pigment 1); excitation 355 nm, emission 510 nm (pigment 2); and excitation 463 nm, emission 620 nm (pigment 3). There was a positive correlation between the quantity of pigments 1 and 3 and the age of Antarctic krill. A model was developed to predict age from pigment 3 and to compare it with other age proxies (carapace length and eyeball diameter). The quantity of pigment 3 was the best predictor of age. The pigment method can discriminate between similar sized krill aged 12 and 36 months. Age pigments provide an improved tool for age estimation in Antarctic krill, particularly if used in conjunction with other demographic information.     

Linking predator and prey behaviour: contrasts between Antarctic fur seals and macaroni penguins at South Georgia

Antarctic fur seals Arctocephalus gazella and macaroni penguins Eudyptes chrysolophus are the two main land-based krill Euphausia superba consumers in the northern Scotia Sea. Using a combination of concurrent at-sea (predator observations, net hauls and multi-frequency acoustics), and land-based (animal tracking and diet analysis) techniques, we examined variability in the foraging ecology of these sympatric top predators during the austral summer and autumn of 2004. Krill availability derived from acoustic surveys was low during summer, increasing in autumn. During the breeding season, krill occurred in 80% of fur seal diet samples, with fish remains in 37% of samples. Penguin diets contained the highest proportion of fish in over 20 years of routine monitoring (46% by mass; particularly the myctophid Electrona antarctica), with krill (33%) and amphipods (Themisto gaudichaudii; 21%) also occurring. When constrained by the need to return and feed their offspring both predator species foraged to the northwest of South Georgia, consistent with an area of high macrozooplankton biomass, but fur seals were apparently more successful at exploiting krill. When unconstrained by chick-rearing (during March) penguins foraged close to the Shag Rocks shelf-break, probably exploiting the high daytime biomass of fish in this area. Penguins and seals are able to respond differently to periods of reduced krill abundance (in terms of variability in diet and foraging behaviour), without detriment to the breeding success of either species. This highlights the importance of myctophid fish as an alternative trophic pathway for land-based predators in the Scotia Sea ecosystem.