sign in sign up
<<
Home , Antarctic petrel, Blue petrel, Prion (bird), Southern fulmar

DENSITIES OF ANTARCTIC AT SEA AND THE PRESENCE OF THE EUPHA USIA SUPERBA

BRYAN S. OBST Departmentof Biology,University of California,Los Angeles, California 90024 USA

ABSTRACT.--Theantarctic krill Euphausiasuperba forms abundant,well-organized schools in the watersoff the AntarcticPeninsula. Mean avian densityis 2.6 timesgreater in waters where krill schoolsare present than in waters without krill schools. density is a good predictorof the presenceof krill. Seabirddensity did not correlatewith krill density or krill schooldepth. Disoriented krill routinely were observedswimming near the surface above submergedschools, providing potential prey for surface-feedingbirds. Responsesof seabird speciesto the distribution of krill schoolsvaried. The small to me- dium-sizeprocellariiform were the best indicatorsof krill schools;large procellari- iforms and coastalspecies were poor indicators.Pygoscelis penguins occurredat high den- sitiesonly in the presenceof krill schools.These responses are consistentwith the constraints imposedby the metabolicrequirements and reproductivestrategies of eachof thesegroups. Krill schoolswere detectednear the seasurface throughout the day. Correlationsbetween seabirddensity and the presenceof krill during daylight hourssuggest that diurnal foraging is important to the seabirdsof this region. Received19 December1983, accepted4 December 1984.

RELATIVELY little is known about the factors depend on directly or indirectly for food influencing the distribution of seabirdsin the (Haury et al. 1978). These observationssuggest marine habitat. The past decade has produced that relatively small-scalephenomena, such as a number of studiesattempting to correlatepat- local concentrationsof prey, may be of major terns of avian abundance and distribution with importance in determining the patterns of sea- physical featuresof the oceansuch as currents distribution within the broad limits set by and convergences,water masses,and temper- featuresof the physical . ature-salinity fronts, features presumed to in- It is well known that seabirdssometimes ag- fluence the distribution of marine prey. Many gregateover concentrationsof prey (seeBrown such studies have demonstrated broad-scale 1980 for a review). But because the number of correlationsbetween these oceanographicfea- potential prey speciesis often quite large and tures and bird distributions or species assem- the movementsof prey within the open ocean blages (e.g. Jehl 1973, 1974; Shuntoy 1974; are complexand difficult to monitor, systematic Brown et al. 1975;Pocklington 1979; Griffiths studies of the influence of prey distribution et al. 1982; Schneider 1982; Gould 1983). upon the density and distribution of seabirds Recently,however, Ainley and Boekelheide in pelagicenvironments are lacking. (1983) concluded that the major, classical The present study documentsthe influence oceanographicboundaries in the south Pacific of a principal prey species,the krill Euphausia do not act as effective distributional barriers for superba,on the densitiesof seabirdsin waters seabirdsinhabiting the region. Furthermore, off the .The overwhelming where associationsbetween seabirdsand large- dominance of E. superbain this ecosystemand scale oceanographicfeatures have been tested its habit of forming large, well-defined schools statistically, explained variances of seabird (Hamner et al. 1983) facilitated the location of numbers have been low (Abrams and Griffiths this abundant prey organismby SONAR, pro- 1981). Hunt and Schneider (in press) suggest viding a uniquely simple systemin which dis- that "fine-scale" patchinessin bird distribution tributions of seabirdsand their prey could be caneffectively mask differences at larger scales, monitored simultaneously. The relationships e.g. differencesbetween water masses.Patchi- between krill distribution, school size, and ness at scales of meters to hundreds of meters schooldepth and the densitiesof the seabirds is evident for planktonic organismsthat sea- comprisingthis communityare examined.

540 The Auk 102: 540-549. July 1985 July1985] AntarcticSeabirds and Krill 541

SEA

V., o

Fig. 1. Map of the northern Antarctic Peninsulaand surroundingwaters indicating cruisetracks followed during the study.

METHODS elevated and those for penguins somewhat de- Seabird densitieswere determined by performing pressed,relative to the true levels. strip transectsfrom the R.V. 'Hero' during 3 cruises Euphausiid distribution was monitored via a con- between 12 January and 12 February 1983. I per- tinuously reading SONAR echosounderthat pro- formed 226 10-min transects(48 from 12-17 January, duceddiscrete, easily recognizable tracings as the ship 83 from 25-30 January,and 125 from 2-12 February) passedover krill schools.The identity of the organ- in the waters to the northwest of the Antarctic Pen- ismsproducing these tracings was verified by netting insula,including the BransfieldStrait, Gerlache Strait, within the schools;over 99% of the tracings were and Drake Passage(Fig. 1). One 10-min transectwas attributable to E. superba.Several problems are asso- carried out during each 30-min period that the ship ciatedwith the useof hydroacousticsto monitorprey. was in transit and visibility permitted. Only birds First, becausethe SONAR systemused was vertically seenwithin a quadratdefined by a line out from the oriented, only schoolsdirectly under the ship were ship's bow and a second,perpendicular line off the detected. Second, because the transducer was mount- beam out to 300 m were counted. The area of each ed in the ship's hull, the upper 3 m of the water transectwas calculatedusing ship'svelocity, time of column (the ship's mean draft) were not monitored. travel, and the width of the transectstrip, and bird Finally, vertical SONAR does not provide an abso- numberswere convertedto units of density. lute measure of a school's size because it bisects it Many antarctic seabirds,including most procellar- only in a singleplane. However, the frequencywith iiforms,, and larids,are inveterateship follow- which distinct krill schools appear on the echo- ers. Becauseseabird numbers typically were low, ship sounder, as well as their duration, provides an index followers usually could be recognized and moni- of the general abundance of krill in the waters cov- tored throughout the 10-min census;such individu- ered during a transect.Each transectwas assigneda als were not recorded. However, the presence of a ranking (0-10) corresponding to this apparent prey ship may attract such speciesinto the transectlimits abundance.A value for krill depth correspondingto even if they do not follow, thereby increasingtheir the shallowest school detected also was assigned to densityartificially. On the other hand, penguinscan each transect. be quite difficult to detectduring a shipboardcensus, Finally, SCUBA and blue-water diving techniques particularly in rough seas,because they float low in (Hamner 1975) were used to observe shape and be- the water and may dive upon approach.Thus, den- havioral characteristics of several selected, shallow sity estimatesfor volant seabirdsmay be somewhat (10-30 m) krill schoolslocated by SONAR. 542 BRYANS. OBST [Auk,Vol. 102

TABLE1. Comparisonof avian .tensityand biomassfor waterswith and without Euphausiasuperba schools.

Transects without krill Transects with Factorial schools krill schools increase pa Mean density (birds/km 2) 8.8 22.8 2.6 <0.001 Mean biomass(kg/km 2) 23.2 45.4 2.0 <0.001 n 56 167 Significanceof the difference between means.

RESULTS oceanicus),were found in transects with krill Krill schoolswere presentin 167 (75%)of the significantly more often than expected by 226 transects.Mean avian density was 2.6 times chancealone (Chi-squarepaired test of associ- greaterin waterswhere krill schoolswere pres- ation, P < 0.05 for each). Both regularly feed ent, and mean avian biomass was double that on krill during the breeding season. One found in waters without krill schools(Table I). species,the South Polar (Catharactamac- The differencebetween these meanswas sig- cormicki),showed a significantnegative associ- nificant in each case (t-test, P < 0.001). ation with krill schools(P < 0.05). These skuas When seabirddensity (all speciescombined, regularly include krill in their summer diet, rounded to the nearestbird/km 2) was plotted but are restrictedprimarily to coastalwaters againstthe probability that krill schoolswere during this season. The Black~browedAlba- present(i.e. the fractionof transectswith a giv- tross (Diomedeamelanophris) tended toward a en bird density in which krill schoolswere negative associationwith krill schools,but this present), a regular, increasingtrend was ob- tendency was not statisticallysignificant (P = vious (Fig. 2). Of the transectswhere avian 0.07). densitywas 1-10 birds/km2, 68% also had krill; Densities of individual specieswithin the of thosetransects with an avian density of 31- seabird community varied in relation to the 40 birds/km 2, 92% had krill schools. Where ob- distribution of krill schools. Three species, served seabird density was greater than 40 SouthernFulmar, Cape (Daption capense), birds/km2, krill schoolswere always present. and Wilson'sStorm-Petrel, showed clear posi- This suggeststhat seabirdsare effectivelycon- tive correlations between their densities and centratingtheir activitiesin responseto E. su- the probabilitythat krill were present(Fig. 3). perba,and that seabird density is a good pre- Thesespecies are all small to medium-sizepro- dictor of krill's presence. cellariiforms,pelagic foragers,and take much Krill abundance did not correlate with sea- krill during the breeding season(Table 2). No bird density (Spearman'srank correlation,P > clear correlation was detected for three other 0.05). The presenceof large or numerouskrill surface-feedingspecies, Black-browed Alba- schoolsin a transect did not necessarilypro- tross,Southern Giant-Petrel (Macronectes gigan- duce high densitiesof seabirds.Thus, while teus),and South Polar Skua (Fig. 4). Although seabird aggregationsare associatedwith krill thesespecies all take krill, it is a lessimportant schools,waters supporting the highest krill componentof the diet than for the CapePetrel densitiesdo not necessarilysupport the great- and Wilson's Storm-Petrel. estavian densities.Similarly, no correlationbe- Becausethe three commonpenguin species tween schooldepth and bird density was de- of the Antarctic Peninsula [Ad•lie Penguin tected(Spearman's rank correlation,P > 0.05). (PygosceIisadeIiae), Chinstrap Penguin (P. ant- Shallow schoolsdid not consistentlysupport arctica),and Gentoo Penguin (P. papua)]are higher concentrationsof birdsthan did deeper often impossibleto distinguishat sea,all pen- schools,despite the predominanceof surface- guin datawere pooled for analysis.While their feeding birds in most transects. foraging rangesand specificdiets vary (Volk- Of the ten avian speciesrecorded frequently man et al. 1980),this pooling may be justified enough to permit statistical analysis, two ecologicallybecause they sometimestravel and species,the Southern (Fulmarusglaci- foragetogether in mixed flocks(pets. obs.),and aloides) and Wilson's Storm-Petrel (Oceanites all have diets comprisedof 80-99% krill. At low July1985] AntarcticSeabirds and Krill 543

0.9

0.8

WILSON'S STORM-PETREL

ALL BIRD SPECIES

0 • io ii-20 2•-30 3i-40 ai-50 5•-60 6•-70 7•-80 >80 BIRD DENSITY (BIRDS/KH 2) [JJ Fig. 2. The probability that Euphausiasuperba schoolsare presentin a given transectas a function i of seabirddensity. Data for all bird speciesare pooled; ß SOUTHERN FULMAR values are rounded to the nearest whole bird/kin 2.

penguin densities,no clear correlationwas seen 0.9 between density and the probability that krill 0.8 schoolswere present. However, at high pen- guin densities(> 30 penguins/ km2) krill schools 0.7 ß were alwayspresent (Fig. 5). This relationship held whetherall penguinsobserved (foraging, 0 1-5 S-10 11-15 iS-20 21-25 26-30 >30 porpoising,and loafing birds) were considered BIRD DENSITY (BIRDS/KM2) or just thoseseen making foragingdives. Fig. 3. The probability that Euphausiasuperba The following specieswere observed too schoolsare presentas a function of avian density for infrequently to permit a meaningful plot of three small to medium-sizeprocellariiforms. density vs. the probability of krill presence: Wandering Albatross (DiDmedea exulans), Grey-headedAlbatross (D. chrysostoma),Light- concentratingtheir foraging activitiesin waters mantiedAlbatross (Phoebetria palpebrata), Snow where krill schoolsoccur; (2) seabirdsare for- Petrel (Pagodromanivea), Dove ( aging on honkrill prey whose distribution is desolata), (Halobaenacaerulea), Black- correlated with the distribution of krill; and (3) bellled Storm-Petrel(Fregetta tropica), Blue-eyed seabirdsand krill are respondingsimilarly but Cormorant (Phalacrocoraxatriceps), Great Skua independentlyto somefeature of the environ- (Catharacta skua lonnbergi), Southern Black- ment.While theseneed not be mutuallyexclu- backed Gull (Larusdominicanus), and Antarctic sive alternatives, each will be discussed in turn. Tern (Sterna vittata). The first hypothesisis supportedby the fact that most of the seabirdspecies in the study DISCUSSION region(75%) regularly include E. superbain their diets, sometimes to the near exclusion of other SEABIRD DENSITY AND THE prey (Table2). Only 3 of the 20 speciesrecord- PRESENCE OF KRILL ed during the studytake negligible amountsof krill. Thesespecies, the Blue-eyedCormorant, The clear and substantial increase in mean Wandering Albatross,and Great Skua, seldom avian density associatedwith the presenceof were recordedduring transects,because they krill schoolslends support to the idea that local are either uncommonin the region (albatross) phenomena,such as concentrationsof prey, or restricted to coasts (cormorant and skua). strongly influence patterns of avian distribu- Thus,the majorityof birdscontributing to den- tion observedat sea. Three hypothesescould sity and biomassvalues were krill feeders.Sim- account for the observed correlations: (1) sea- ilarly, Ainley et al. (1984)found euphausiidsto birdsare locatingand preyingon krill, thereby be numerically the most important prey for the 544 BRYAN$. OBST [Auk, Vol. 102

T^BLE2. The importance of Euphausiasuperba in the diets of seabirdsof the Antarctic Peninsula region.

Breeding % krill in diet % diet samples Sourceof Breedingrange • stageb (by wt)c with krill c diet dataa Specieseating mostlykrill Ad•lie Penguin T C 99.6 100 1 Chinstrap Penguin T C 99.6 100 1 Gentoo Penguin SS,PA, MP C 85 100 (?) 1 Cape Petrel SS, PA C 85 98 2,3 SS, MP C,I 80 -- 2 Dove Prion NB -- 57 87 4 Blue Petrel NB -- 75 -- 4 Wilson's Storm-Petrel T I, C 85 95 5 Speciesregularly taking krill in lesseramounts Black-browed Albatross NB -- 41 70-92 6 Grey-headedAlbatross NB -- 17 53-69 6 Light-mantled Albatross NB -- 36 -- 7 Southern Giant-Petrel SS, PA C 1-21 3-65 8 Speciesregularly taking krill, importanceunknown Southern Fulmar SS, NP I, C -- 50 9 South Polar Skua SS, PA, MP I, C -- -- 10 Southern Black-backed Gull T C -- -- 5 Antarctic Tern T I, C -- 67 11 Speciestaking little or no krill Blue-eyedCormorant T C n 5 12 WanderingAlbatross NB -- < 10 -- 13 Great Skua T I, C n -- 14 aData from Watsonet al. (1971).SS = SouthShetlands, PA = PalmerArchipelago, NP = northern Antarctic Peninsula, MP = middle Antarctic Peninsula, T = throughout study region, NB = not breeding in study region. bBreeding stage of AntarcticPeninsula seabirds during the studyperiod. C = chickrearing, I = incubation. cData derived from samplesregurgitated by chicksor adultsreturning to feed chicks,except in the cases of SouthernFulmar and AntarcticTern where data refer to stomachcontents of birds collectedat sea.Dietary information for the penguins, cormorant,storm-petrel, and charadriiform speciescome from the Antarctic Peninsula;values for the remaining speciesare from the ScotiaSea (SouthGeorgia or South Orkneys).n = negligible quantities. a 1 = Volkman et al. (1980), 2 = Croxall and Prince (1981), 3 = Beck (1969), 4 = Prince (1980a), 5 = Obst (unpubl. data), 6 = Prince (1980b), 7 = Thomas (1982), 8 = Hunter (1983), 9 = Bierman and Voous (1950), 10 = P. Pietz (unpubl. dissertation),11 = D. Parmelee (unpubl. data), 12 = Schlatter and Moreno (1976), 13 = Croxall and Prince (1980), 14 = Trivelpiece and Volkman (1982).

majority of seabird species in the . surface.Thus, it seemslikely that the density However, they estimated that on the basis of of seabirds is correlated with the presence of weight, and are much more impor- krill schools. tant than studies from western would However, the hypothesis that seabirds ag- indicate (Croxall and Prince 1980, 1981). The gregate over waters with krill schoolsto ex- probableexplanation for this differenceis the ploit larger, nonkrill prey cannotbe dismissed. far greater abundanceof krill in the ScotiaSea- Many seabirds'prey species,such as and AntarcticPeninsula region comparedwith oth- , are known to feed on krill (Mauchline er Antarctic regions (Mart 1962). 1980). Many of these (e.g. cephalopodsand Many instancesof birdsfeeding on krill were myctophid fishes) are probably most available witnessed during transects.The most impres- during the darkest hours, and these interac- sive event was a mixed-speciesflock of over tions would be unlikely to influence the diur- 700 birds, including Wilson's Storm-, nal distributional patterns recorded in this Cape Petrels, Southern , Southern study. Some krill-eating fishesdo occurin sur- Giant-Petrels, and Black-bellied Storm-Petrels, face waters during the day, and piscivorous feeding on a small schoolof krill visible at the birds prey on them. For example,in March 1984 July1985] AntarcticSeabirds and Krill 545

0.9 BLACK-BROWEO ALBATROSS 0.9 ALL PENGUINS

0.8 0.8 0.7

•Z O.S •rr 0.5

SOUTHERN

0 i-iO ii-20 2i-SO S•-40 >40

BIVING PENGUINS

SOUTH POLAR SKUA nO:0.7

0.8

0.5

0 i-5 8-i0 •i-i5 •8-20 2i-25 2B-SO >SO 0 i 2 S 4 >4 PENGUIN DENSITY (PENGUINS/KM2) BIRD DENSITY (BIRDS/KN 2) Fig. 5. The probability that Euphausiasuperba Fig. 4. The probability that Euphausiasuperba schoolsare present as a function of Pygoscelispen- schoolsare present as a function of avian density in guin density. Data for all penguins observedand for two large procellariiformsand the SouthPolar Skua. feeding penguins only are plotted separately.

I observedlarge numbersof South Polar Skuas and Southern Black-backedGulls catching sil- become increasingly clear that these crusta- very fish, probably Pleuragrammaantarcticum, ceans are far from the passive, planktonic or- amid a large surfacekrill school.Although the ganismsthey once were regardedto be. Krill birdswere associatedwith the krill school,they "swarms" are in fact highly organized schools ignored the abundant euphausiidsand clearly that orient and travel in set directions, often selected the fish as prey. Pleuragrammaeats againstthe flow of prevailing currents(Kanda mostly krill (DeWitt and Hopkins 1977),and it et al. 1982). Their behavior includes a set of is reasonable that its local distribution would complex,antipredatory patterns (Hamnor et al. be correlatedwith prey availability. 1983). Finally, the correlationbetween seabirdden- sity and the presenceof krill may result from SEABIRD DENSITY AND KRILL an independent tendency of both to occur in AVAILABILITY waters of a particular physical/chemicaltype and not from any trophic interactions.No water No correlation was found between seabird temperature or salinity data were gathered density and the relative abundanceor depth of during the study, but the dietary information krill schools. In view of the limitations of the discussedabove argues strongly against this hull-mounted, vertically oriented SONAR, it is hypothesis.Krill schoolsand bird aggregations difficult to know whether this lack of correla- were broadly distributed throughout the re- tion accuratelyreflects nature or is merely a gion covered, rather than being restricted to methodologicalartifact. The lack of correlation watersof particular depth, distancefrom shore, between seabird density and krill abundance etc. While the factors governing the distribu- may simply stem from the fact that seabirdsare tion of E. superbaare poorly understood,it has not omniscient.A bird aggregationmay begin 546 BRYANS. OBST [Auk,VoL 102

when a few birds discover a krill school and available patches of krill. An individual eu- grow via social facilitation with a dynamic of phausiid represents a substantial package of its own, regardlessof whether a larger school energy relative to the overall daily energy bud- is present nearby. get in these moderate-sizespecies (see below). The lack of correlation between seabird Although these speciesare opportunisticfeed- numbers and krill depth also may be a result ers and will take a wide variety of food when of the inability to detect schoolsin the upper available, during the antarctic summer they 3 m of ocean, the region of particular impor- feed heavily upon E. superba. tance to volant seabirds. However, this limita- LargeProcellariiformes.--The two large pro- tion cannot accountfor the strong correlations cellariiform speciescommon in the study area, between the densitiesof some surface-feeding the Black-browed Albatross and Southern seabirdsand submerged krill schools.Charac- Giant-Petrel, were not good indicators of krill. teristically, krill schoolsobserved while SCU- Like the smaller procellariiforms, these species BA diving were centered between 10 and 30 m are highly pelagic and, owing to their larger depth. However, divers regularly encountered size, have even greater fasting abilities. They individual euphausiidsswimming apart from may remain at sea for 1-3 weeks during the the well-organized school between its upper incubation phase of the reproductive cycle; edge and the ocean surface. These individuals adultsof both speciesforage for 1-4 dayswhile often could be seen from the ship when it was rearing chicks (Tickell and Pinder 1975, Hun- directly over a school.A small fraction of these ter 1983). individuals were whitish and opaque,probably These larger speciesmust have much greater due to disease or damage. The majority ap- absolutedaily energy requirements, such that peared normal but were disoriented with re- an individual euphausiidrepresents a relative- spectto the swimming direction of the school. ly minor input of energy. Allometric equations These damaged and disoriented individuals predicting daily energy expenditures of flee- must provide an easy food source for surface- living birds (nonpasserinesat 0øC,Kendeigh et feeding and shallow-diving seabirds, even al. 1977; Walsberg 1983) suggest that the en- when the main school is submerged. Thus, the ergy requirementsof a 4-kg albatrosswould be presence of these individuals may account for 10-15 times greater than that of a 40-g storm- the increased density of surface-feeding birds petrel. Using the more conservative equation associatedwith submerged krill schools. Iso- of Kendeigh et al. to predict daily energy re- lated euphausiids were rarely encountered quirements, and assuming an energy content during SCUBA dives away from the proximity for krill of 4.35 kJ/g wet weight (Clarke 1980) of schools. and an assimilation efficiency of 80% for both species,it would take 24 min for a storm-petrel INTERSPECIFIC DIFFERENCES to use the energy assimilablein 1 g of krill (2- 4 adult euphausiids)but only 2.4 min for an While the density of seabirdsgenerally cor- albatrossto use the same quantity. Thus, krill relates with the presence of krill, individual probably provides an attractive food sourceto speciesvary in their responses.The factorscon- large procellariiforms only when it is highly tributing to these differences are doubtlessly concentrated,as on those infrequent occasions many and complex, but the specificresponses when krill schools surface. are consistentwith aspectsof the foraging and While speculative,this predictionagrees with reproductive ecology of the various seabird the observed behavior of procellariiforms. Al- species. batrossesand giant-petrels are adapted for sus- Small-to-mediumProcellariiformes.--The best tained, soaring flight (Pennycuick 1982), and avian indicatorsof the presenceof krill are the they appear to interrupt their progressto feed Southern Fulmar, Cape Petrel, and Wilson's infrequently. On several occasions,however, Storm-Petrel. These are all highly pelagic for- groups of giant-petrels or Black-browedAlba- agers.Foraging trips may last from 2 to 4 days trosses were observed sitting on the water in the storm-petrel(Beck and Brown 1972;pers. scooping up beakfuls of krill from surface obs.)and up to severaldays in the other species schools.Smaller petrels, on the other hand, fly (Pinder 1966). This mobility permits the for- low over the water, frequently picking or dip- aging bird to searchout and exploit waterswith ping at the surface,and they were observedto July1985] AntarcticSeabirds andKrill 547 feed on isolated euphausiidssurfacing above TABLE3. Potential foraging ranges of four seabird krill schools as well as on surface schools them- speciesof the Antarctic Peninsularegion during selves.The bestforaging strategy for the Black- the chick-rearingphase of reproduction. browed Albatross and Southern Giant-Petrel Mean may be to stay on the wing and cover broad Tray- foraging- Poten- areasin searchof concentratedprey, ratherthan eling trip tial to chasedisoriented individual euphausiidsas- speed duration range sociated with submerged schools. The alba- Species (kin/h) ' (h)b (kin)c trossesand giant-petrelsappear to depend more Ad•lie Penguin 11.2 11.7 65.3 heavily upon nonkrill prey than do their Southern Giant-Petrel 39.2 23.9 469 Wilson's Storm-Petrel 24.5 60.8 744 smaller relatives (Table 2). Ainley et al. (1984) South Polar Skua 48.4 •2.0 48.4 discusssome advantagesof feeding on squid and fish relative to krill for antarctic seabirds. aAd•lie Penguin value is for birds porpoising to and from rookeries(Obst and Hamner in prep.), val- Coastal species.--Several species recorded ues for the two procellariiformsare ground speeds during transectsare primarily coastalin their publishedin Pennycuick1982, and value for the skua distribution during the breeding season.These is from Young 1963. species undergo frequent incubation ex- bAll values are for adults feeding young chicksat changesand consequentlyare limited in their Palmer Station in the Palmer Archipelagoarea. Data for penguin and procellariiformsare from unpub- potential foraging ranges. The potential for- lished field studiesin 1983-1984 by B. Obst, G. Bar- aging ranges of the four commonestbreeding tholomew, and K. Nagy. The skua estimateis from speciesat PalmerStation, located in the Palmer P. Pietz (pers. comm.). cTraveling velocity x trip duration/2. This is the Archipelago,are presentedin Table 3. Of the distance that would be achieved if 100% of the time coastalspecies, only the South Polar Skua was awayfrom the nestwere spent traveling and the path observedfrequently enough to permit analysis taken to and from the nest were a straight line. of the relationship between krill distribution and skua density. This speciesshowed a nega- tive associationwith krill, and its density was than 25 birds/km 2were within 25 km of a large independentof the probability that krill were colony.Foraging behavior was observedin 33% present. South Polar Skuas do include krill in of these transects. their diets;however, the krill probably are tak- It is tempting to speculatethat the relation- en opportunisticallyas schools move into waters ship between high penguin density and the near the bird coloniesor are pirated from other presenceof krill may be the result of a long- species.Other coastalspecies, which take little term processin which Pygosceliscolonies locat- krill, may conform to this opportunistic pat- ed near waters with dependablekrill popula- tern. tions have flourished, while those away from Penguins.--Highconcentrations of penguins such waters have not. Although the factors invariably were associatedwith krill schools, controlling patternsof krill distribution are not but the probabilityof schoolsbeing presentdid understoodat present,regions of dependably not increase steadily with penguin density high krill populationsdo exist.It would indeed throughout the range of densities observed. be surprising if the geography of penguin Foraging excursionsin the Pygoscelisspecies breeding has not been influencedby this. typically last no longer than 12-24 h, depend- ing upon the species(Volkman et al. 1983;pers. NOCTURNAL FORAGING AND VERTICAL obs.).This, coupledwith their inability to fly, MIGRATION effectively restricts them to waters near their rookeries.The potential foraging range of the Imber (1973, 1976) argued that several sub- Ad•lie Penguin at Palmer Station is a fraction antarcticpetrel speciesfeed primarily at night, of the potential rangesof the volant giant-pe- taking advantageof the nocturnal vertical mi- trel and storm-petrel,and is in fact more com- grations of bioluminescentprey toward the parableto the restrictedrange of the South Po- oceansurface. Similarly, Lishman and Croxall lar Skua (Table 3). Unlike the procellariiform (1983) suggestedthat Chinstrap Penguinsfeed species,high penguin densitiesgenerally were chiefly at night in shallow krill schools.Be- encountered near major penguin colonies;78% causeall the transectsduring this study were of the transectswith penguin densitiesgreater performedduring daylight, nocturnalforager 548 BRYANS. OBST [Auk,Vol. 102

GeorgeBartholomew, John Cooper,Zoe Eppley, Eliz- abeth Flint, William Hamner, Judith Hand, Thomas Howell, Patrick Mock, David Nettleship, and an anonymous referee for their helpful comments on earlier drafts of this paper. This work was supported by the National ScienceFoundation Division of Polar Programsgrant 80-23851.

LITERATURE CITED ABRAMS,R. W., & A.M. GRIFFITHS.1981. Ecological structureof the pelagic seabird community in 0700 0900 llO0 i300 1500 1700 1900 2100 the BenguelaCurrent region. Marine EcoLProg. HOUR OF DAY Ser. 5: 269-277. Fig. 6. The percentageof transectsin which Eu- AINLEY, D. G., &: R. J. BOEKELHEIDE.1983. An eco- phausiasuperba schools were recordedwithin 3 m of logical comparison of oceanic seabird commu- the seasurface during each hour of the day. nities of the south Pacific Ocean. Pp. 2-23 in Tropical seabird biology (R. W. Shreiber, Ed.). Studies Avian Biol. No. 8. densities were less likely to show correlations --, E. F. O'CONNOR, &; R. J. BOEKELHEIDE.1983. with krill distributions. The marine ecologyof the birds in the RossSea, Euphausiasuperba is consideredto be a verti- Antarctica.Ornithol. Monogr. 32. cally migrating speciesthroughout its range BECK,J. R. 1969. Food,moult and age of first breed- (Mauchline 1980, Kalinowski and Witek 1980). ing in the Cape Pigeon, Daptioncapensis Lin- However, B. H. Robisonet al. (in prep.) found naeus. Brit. Antarctic Surv. Bull. 21: 33-44. no evidence of a significant vertical migration --, & D. W. BROWN.1972. The biology of Wil- in krill near the Antarctic Peninsula. I found son's Storm-Petrel Oceanitesoceanicus (Kuhl), at krill schools throughout the day in the shal- Signy Island, South Orkney Islands. Brit. Ant- arctic Surv. Sci. Rept. 69: 1-54. lowest waters;roughly 42% of all transectshad BIERMAN, W. H., & K. H. VOOUS. 1950. Birds ob- schools within 3 m of the surface. Shallow krill servedand collectedduring the expe- schools appeared to become somewhat less ditions of the Wilhelm Barendszin the Antarctic, common toward midday but were detected at 1946-1947 and 1947-1948. Ardea, extra no.: 1- all hours (Fig. 6). Surface schools were ob- 123. served during daylight on several occasions BROWN, R. G. B. 1980. Seabirds as marine . during the study. The periods of darknessvar- Pp. !-39 in Behavior of marine animals, vol. 4 (J. ied from 3 h in early January to about 5 h in Burger, B. Olla, and H. E. Winn, Eds.).New York, mid-February. The shortness of nights, the Plenum Press. presenceof krill at or near the surfacethrough- --, F. COOKE,P. K. KINNEAS, &: E. L. MILLS. 1975. Summer seabird distributions in the Drake Pas- out the day, and the interactionsof birds and sage, the Chilean fjords and off southern South krill witnessed during daily transectssuggest America. Ibis 117: 339-366. that nocturnal foraging may be less pervasive CLARKE,A. 1980. The biochemicalcomposition of in seabirdsin this region than in communities krill Euphausiasuperba from SouthGeorgia. J. Exp. at lower latitudes.The routine presenceof ver- Marine Biol. Ecol. 43: 221-236. tically migrating cephalopodsand midwater CROXALL,J.P., &:P. A. PRINCE.1980. Food,feeding fishes in the diets of many antarctic seabirds ecology, and ecologicalsegregation of seabirds (Croxall and Prince 1980, Ainley et al. 1984) is at South Georgia. Biol. J. Linnean Soc. London evidencethat nocturnal foraging occurs,and E. 14.' 103-131. superbamay be taken at night as well. --, & --. 1981. A preliminary assessment of the impact of seabirdson marine resourcesat South Georgia. Colloq. Ecosyst•mesSubantarc- ACKNOWLEDGMENTS tiques 51: 501-509. DEWITT,H. H., &: t. L. HOPKINS. 1977. Aspectsof I am sincerelygrateful to William Hamner for sup- the diet of the antarcticsilver fish, Pleuragramma plying ideas,advice, and encouragementthroughout antarcticum.Pp. 557-567 in Adaptations within the courseof this research.Captain Pieter Lenie and antarcticecosystems (G. A. Llano, Ed.). Proc. 3rd the crew of the R.V. 'Hero' provided invaluable lo- SCARSymp. on AntarcticBiol. Washington, D.C., gisticand technicalsupport. Assistance during SCU- Smithsonian Inst. BA dives was generouslyprovided by Peggy Ham- GOURD, P. J. 1983. Seabirds between Alaska and ner, William Hamner, and James Stretch. I thank Hawaii. Condor 85: 286-291. July1985] AntarcticSeabirds and Krill 549

GRIFFITHS, A.M., W. R. SIEGFRIED, & R. W. ABRAMS. PENNYCUICK,C. J. 1982. The flight of petrels and 1982. Ecologicalstructure of a pelagic seabird albatrosses(), observed in South community in the .Polar Biol. 1: Georgiaand its vicinity. Phil. Trans. Royal Soc. 39-46. London B 300: 75-106. HAMNER, W. H. 1975. Underwater observations of PINDER,g. 1966. The Cape Pigeon,Daption capensis blue-water plankton: logistics,techniques, and Linnaeus,at Signy Island, South Orkney Islands. safety procedures for divers at sea. Limnol. Brit. Antarctic Surv. Bull. 8: 19-47. Oceanogr.20: 1045-1051. POCKLINGTON,R. 1979. An oceanographicinterpre- ß P. HAMNER, S. STRAND, & R. GILMER. 1983. tation of seabird distributions in the Indian Behaviorof antarctickrill, Euphausiasuperba: che- Ocean. Marine Biol. 51: 9-21. tooreception,feeding, schooling, and molting. PRINCE,P. A. 1980a. The food and feeding ecology Science 220: 433-435. of Blue Petrel (Halobaenacaerulea) and Dove Prion HAURY, L. g., J. A. McGowEN, & P. H. WIEBE. 1978. (Pachyptiladesolata). J. Zool. 190: 59-76. Patterns and processesin the time-space scales 1980b. The food and feeding ecology of of plankton distributions. Pp. 277-327 in Spatial Grey-headedAlbatross Diomedea chrysostoma and pattern in plankton communities (J. H. Steele, Black-browed Albatross D. melanophris.Ibis 122: Ed.). New York, Plenum Press. 476-488. HUNT, G. L., JR., & D. SCHNEIDER.In press. Scale SCHLATTER,R. P., & C. g. MORENO. 1976. Habitos dependent processesin the physicaland biolog- alimentarios del cormoran antartico Phalacroco- ical environment of marine birds. In Seabird tax atricepsbransfieldensis (Murphy) en Isla Green, feeding ecology:the role of seabirdsin marine Antattica. Set. Cient. Inst. Antart. Chileno 4(1): ecosystems(J.P. Croxall, Ed.). Cambridge, En- 69-88. glandßCambridge Univ. Press. SCHNEIDERßD. 1982. Fronts and seabird aggrega- HUNTER,S. 1983. The food and feeding ecology of tionsin the southeasternBering Sea. Marine Ecol. the giant petrelsMacronectes halli and M. gigan- Prog. Set. 10: 101-103. teusat South Georgia.Emu 82: 92-100. SHUNTOV,V. P. 1974. Seabirdsand the biological IMBER,M. J. 1973. The food of Grey-faced Petrels structureof the ocean.Washington, D.C., U.S. [Pterodromamacroptera gouldi (Hutton)]ß with spe- Dept. Interiorß transl. TT 74-55032. cial reference to diurnal vertical migration of THOMAS,G. 1982. The food and feeding ecologyof their prey. J. Anita. Ecol. 42: 645-662. the Light-mantled Sooty Albatross at South --. 1976. Comparison of prey of the black Pro- Georgia.Emu 82: 92-100. cellariapetrels of New Zealand. New Zealand J. TICKELL,W. L. N., & g. PINDER. 1975. Breeding bi- Marine Freshwater Res. 10: 119-130. ology of the Black-browed Albatross Diomedea JEHL,J.g. 1973. The distribution of marine birds in melanophrisand Grey-headedAlbatross D. chry- Chilean waters in winter. Auk 90: 114-135. sostomaat Bird Island, South Georgia.Ibis 117: 1974. The distribution and ecology of ma- 433-450. rine birds over the continental shelf of Argen- TRIVELPIECE,W., & N.J. VOLKMAN. 1982. Feeding tina in winter. Trans. San Diego Soc. Nat. Hist. strategiesof syrupattic South Polar (Catharacta 17: 217-234. maccormicki)and Brown skuas (C. lonnbergi).Ibis KALINOWSKI,J., & Z. WITEK. 1980. Diurnal vertical 124: 50-54. distributionof krill aggregationsin the western VOLKMAN,N.J., P. PRESLER,& W. TRIVELPIECE.1980. Antarctic. Polish Polar Res. 1: 127-147. Diets of pygoscelidpenguins at King GeorgeIs- KANDA, K., K. TAKAGI, •: Y. SEKI. 1982. Movement land, Antarctica. Condor 82: 373-378. of the larger swarms of the antarctic krill, Eu- --, W. TRIVELPIECE,& S. TRIVELPIECE.1983. Py- phausiasuperba, populations off Enderby Land goscelidpenguins in the AntarcticPeninsula re- during the 1976-1977season. J. Tokyo Univ. Fish. gion: their role as predators of euphausiids. 68: 25. Abstr., 101ststated meeting of the Amer. Orni- KENDEIGH, S. C., V. g. DOL'NIK, & V. M. GAVRILOV. thol. Union: 123. 1977. Avian energetics.Pp. 127-204 in Graniv- WALSBERG,G.E. 1983. Avian ecologicalenergetics. orousbirds in ecosystems(J. Pinowski and S.C. Pp. 161-220 in Avian biology, vol. 7 (D. S. Far- Kendeigh, Eds.). Cambridge, England, Cam- net, J. R. King, and K. C. Parkes,Eds.). New York, bridge Univ. Press. Academic Press. LISHMAN,G. S., & J.P. CROXALL.1983. Diving depths WATSON,G. E., J. P. ANGLE,P. C. HARPERßM. A. BRIDGE, of the ChinstrapPenguin Pygoscelis antarctica. Brit. g.P. SCHLATTER,W. L. N. TICKELL,J. C. BOYD, & Antarctic Surv. Bull. 61: 21-25. M. M. BOYD. 1971. Birds of the Antarctic and MARR,J. W.S. 1962. The natural history and geog- .Antarctic Map Folio Set. 14: 1-18. raphy of the antarctic krill (Euphausiasuperba YOUNG,E. C. 1963. Feeding habits of the South Po- Dana). DiscoveryRepts. 32: 33-464. lar Skua. Ibis 105: 301-318. MAUCHLINE,J. 1980. The biology of mysidsand eu- phausiids.Advances in Marine Biol. 18: 527-542.