Deep-Sea Research I ] (]]]]) ]]]–]]]

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Deep-Sea Research I

0967-06

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volker.s

PleasAnta

journal homepage: www.elsevier.com/locate/dsri

Distribution and abundance of Antarctic krill (Euphausia superba) along theAntarctic Peninsula

Volker Siegel a,n, Christian S. Reiss b, Kimberly S. Dietrich b, Matilda Haraldsson c, Gerhard Rohardt d

a Thunen Institut fur Seefischerei, Palmaille 9, 22767 Hamburg, Germanyb Antarctic Ecosystem Research Division, Southwest Fisheries Science Center, NOAA, La Jolla, CA 92037, USAc Department of Marine Ecology—Kristineberg, Kristineberg 566 450 34, Fiskebackskil, Swedend Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany

a r t i c l e i n f o

Article history:

Received 13 August 2012

Received in revised form

12 February 2013

Accepted 16 February 2013

Keywords:

Southern Ocean

Antarctic krill

Large-scale distribution

Geographical zonation

Demography

Recruitment

37/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.dsr.2013.02.005

esponding author. Tel.: þ49 40 38905221; fa

ail addresses: volker.siegel@ti.bund.de,

iegel@ish.bfa-fisch.de (V. Siegel).

e cite this article as: Siegel, V.,rctic Peninsula. Deep-Sea Res. I (201

a b s t r a c t

Net-based data on the abundance, distribution, and demographic patterns of Antarctic krill are quanti-

fied from a contemporaneous two ship survey of the Antarctic Peninsula during austral summer 2011.

Two survey areas were sampled focussed on Marguerite Bay in the south, and the tip of the Antarctic

Peninsula in the north. Data from 177 stations showed that the highest concentrations of krill were

found in the southern sampling area. Differences between areas were associated with a few large

catches of one year old krill found in anomalously warm and productive waters in Marguerite Bay, and

small krill catches in the less-productive, offshore waters in the north. Estimated krill density across the

survey area was 3.4 krill m�2, and was low compared to the long-term average of 45 krill m�2 for the

Elephant Island area. Overall recruitment between the two survey regions was similar, but per capita

recruitment was about 60% lower than historical mean recruitment levels measured at Elephant Island

since the late 1970s. Demographic patterns showed small krill concentrated near the coast, and large

krill concentrated offshore on the shelf and slope all along the survey area. The offshore distribution of

adult krill was delineated by the warm (�1 1C), low salinity (33.8) water at 30 m, suggesting that most

krill were present shoreward of the southern boundary of Antarctic Circumpolar Current Front.

Distributions of larvae indicated that three hotspot areas were important for the production of krill:

slope areas outside Marguerite Bay and north of the South Shetland Islands, and near the coast around

Antarctic Sound. Successful spawning, as inferred from larval abundance, was roughly coincident with

the shelf break and not with inshore waters. Given the rapid changes in climate along the Antarctic

Peninsula and the lower per capita recruitment observed in recent years, studies comparing and

contrasting production, growth, and recruitment across the Peninsula will be critical to better

understand how climate change will impact krill populations and their dependent predators in the

Scotia Sea.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The impacts of climate change on the Antarctic Peninsulaecosystem are among the most dramatic on the planet (King,1994; Vaughan et al., 2003), and are likely to continue to impactall trophic levels of the ecosystem (Clarke et al., 2007; Ducklowet al., 2007). Since the mid-1970s, changes attributed toclimate variability are declines in sea-ice duration and extent(Stammerjohn et al., 2008), changes in nutrient and elementalsources (Prezelin et al., 2004; Hewes et al., 2009), concomitantchanges in phytoplankton productivity and community structure

ll rights reserved.

x: þ49 40 38905263.

et al., Distribution and3), http://dx.doi.org/10.101

(Montes-Hugo et al., 2008; Moline et al., 2004), populations ofhigher predators (Trivelpiece et al., 2011; Lynch et al., 2012), andalso declines of up to 80% in Antarctic krill (Euphausia superba)density (Atkinson et al., 2004). Given the importance of Antarctickrill to higher trophic levels, its role in mediating carbon flux,and its large temporal and spatial variability in abundance,understanding the broad-scale distribution of production, demo-graphic stages, and recruitment are critical to evaluating whetherchanges in the environment have further impacted krill popula-tions along the Antarctic Peninsula.

The few large-scale studies that have been conducted onAntarctic krill have documented the standing biomass and thebroad-scale spatial patterns and associations with oceanographicfeatures. Perhaps the best known study of the distribution andgeneral biology of krill was the Discovery surveys (Marr, 1962)that described the general pattern of euphausiid distribution,

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

75˚W 70˚W 65˚W 60˚W 55˚W

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5000 m

4000 m

3000 m

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Fig. 1. Net sampling and CTD station locations during the German RV ‘‘Polarstern’’

and US AMLR cruises west of the Antarctic Peninsula in January 2011; color

shading indicates depth contours, EI: Elephant Island, SSI: South Shetland Islands,

BrS: Bransfield Strait, GS: Gerlache Strait, AnI: Anvers Island, AdI: Adelaide Island,

MB: Marguerite Bay.

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]2

including the high abundances of Antarctic krill in the AntarcticPeninsula area, and the spatial structure of demographic compo-nents. However, the patterns established were the results of amulti-year effort, potentially confounding synoptic patterns ofkrill demography (Siegel and Loeb, 1995).

More recent synoptic and broad-scale studies in the Atlanticsector of the Southern Ocean have sought to better understandkrill distribution and biology, but also were designed to provideestimates of abundance or biomass to inform management. Thefirst two of these were during the international BIOMASS program(Biological Investigations On Antarctic Systems and Stocks;El-Sayed, 1994) which undertook two multi-ship surveys, FIBEX(1981) and SIBEX (1985). FIBEX covered the western Atlanticsector and produced the first large-scale population estimate ofAntarctic krill (Trathan and Everson, 1994). SIBEX concentratedon seasonal aspects of krill biology in the Antarctic Peninsula area,especially in Bransfield Strait and the waters around the SouthShetland Islands (SSI). Data collected during SIBEX confirmed thatBransfield Strait is an area with predictably high krill biomass,as first suggested by the Discovery results. SIBEX also identifiedBransfield Strait as a nursery ground, while the offshore regionnorth of the SSI was identified as a spawning area (Siegel, 1988).A third international broad-scale survey in the Atlantic wasconducted by CCAMLR member nations in 2000, specifically todetermine the biomass of krill for input to management models(Hewitt et al., 2004).

Regional German studies west of the Antarctic Peninsula havebeen conducted since 1978, extending from Elephant Island in thenorth to Adelaide Island in the south. Since 1990, data have beencollected annually by the U.S. Antarctic Marine Living Resources(AMLR) Program around Elephant Island, the SSI, and BransfieldStrait. The results of these long-term studies show that the meanabundance of krill around Elephant Island is 30 krill m�2, represent-ing a biomass of approximately 10 g m�2 (Siegel, 2005), and providea baseline from which to compare estimates of abundance.

Since the early 1990s, regional studies have been conductedfarther south and west by the Palmer Long Term EcologicalResearch (LTER) Program in the Western Antarctic Peninsula(WAP) region from just north of Anvers Island south to Marguer-ite Bay (Ducklow et al., 2007; Quetin and Ross, 2003; Ross et al.,2008). These studies have documented much about the dynamicsof krill in this region, which is ‘‘upstream’’ from the Peninsulaarea, and have shown a general correlation of recruitment alongthe Peninsula (Siegel et al., 2003), but have not documented thespatial patterns of spawning thoroughly (Ashijan et al., 2008; butsee Ross and Quetin, 2000). Additionally, the LTER studies havenot quantitatively examined larval production consistently, pre-senting a major challenge in understanding whether changes inproduction or survival are occurring in this region.

Together, the regional studies in the Antarctic Peninsula areahave linked the fluctuations in sea ice extent and duration tovariability in krill recruitment and populations dynamics (Siegeland Loeb, 1995; Loeb et al., 1997; Quetin and Ross, 2003). Theyhave also linked these temporal patterns to major modes ofclimate variability (Ross et al., 2008; Loeb et al., 2009), and haveassociated the decline in krill over the last 35 years to overalldeclines in sea ice associated with decadal and longer changes inatmospheric forcing (Atkinson et al., 2004).

Despite the large number of distributional studies, there arestill important gaps in our understanding of the spatial structureof krill demographic patterns and their broad-scale relationshipto the environment. For example, some data from surveys duringthe 1980s suggest that the more southerly areas, especially theBellingshausen Sea, could be important areas supplying larvaeand young krill to downstream areas (Siegel, 1988). Modelingstudies have shown that areas of the eastern Bellingshausen Sea

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

exhibited environmental properties conducive to the develop-ment of eggs and larvae (Hofmann and Husrevoglu, 2003).However, other studies have suggested that some areas are self-recruiting and that some stocks are quasi-stationary (Wiebe et al.,2011), whereas others have argued that there is considerableconnection among areas (Brierley et al., 1998; Ross et. al., 2003).Despite the general correspondence between patterns of abun-dance and recruitment areas along the Antarctic Peninsula (Siegelet al., 2003), the oceanographic and environmental conditions aredistinct in the south compared to the north. This suggests that,given climate change, there could be future deviations if someareas remain colder and conducive to krill spawning or recruit-ment while other areas remain warm and become less productive.

Here we describe the peninsula-wide distribution and abun-dance of the larvae and post-larval stages of Antarctic krill duringa largely synoptic two ship survey conducted during three weeksin January 2011. We have three specific goals: (1) examinerelative abundance and recruitment patterns between northernand southern areas of the Antarctic Peninsula ecosystem and comparethese to historical data; (2) relate patterns of larval and post-larval krill abundance to oceanographic, hydrographic, and envir-onmental conditions during the survey; and (3) examine thespatial patterns of occurrence in relation to the potential sourcesand sinks of krill that could be important in the future dependingon climate change impacts on the Antarctic Peninsula system.

2. Material and methods

2.1. At-sea data collection

The survey was conducted in the area west of the AntarcticPeninsula during austral summer 2010/11, the main spawningseason for Antarctic krill. We sampled 177 stations along24 transects from 9 to 31 January, 2011. An additional sevenhydrographic stations were sampled at the tip of the AntarcticPeninsula. The area extended between 541W (east of ElephantIsland) and 751W (west of Adelaide Island/Marguerite Bay in theBellingshausen Sea), and from 601 S to 711 S. In general, transectswere located perpendicular to the coast. Ninety-six stations werelocated in the traditionally-sampled U.S. AMLR survey grid, and81 stations were located along 10 Polarstern transects in thesouthern area of the western Antarctic Peninsula area (Fig. 1).

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 3

Although the southern transects covered the area of the LTER grid,stations and transect lines were not identical in their geographicalposition and they extended further into the oceanic region thanthe LTER survey grid. Throughout the paper we refer to thesouthern area as that sampled by the Polarstern cruise, and thenorthern area as that area sampled by the U.S. AMLR Program.

2.2. Physical oceanography

Aboard the Polarstern, a Sea-Bird Electronics, Inc. (SBE Inc.)model 9/11plus Conductivity, Temperature, and Depth Instru-ment (CTD) attached to a carousel water sampler (model SBE-32)with 24 12 L bottles was used to sample the water column. TheCTD was equipped with double sensors for temperature andconductivity. These sensors were calibrated before the cruiseand salinity samples were taken to calibrate the conductivitysensors of the CTD. The salinity samples were taken in homo-geneous layers from shallow, mid, and full depths. All CTD castsextended to the sea floor or within 5 m when possible.

During the AMLR survey, a SBE9/11 CTD was used to deter-mine profiles of water column properties. A single set of sensorswas used on the CTD, and no salinity samples were collected dueto a broken salinometer. The CTD was calibrated prior to and afterthe cruise by SBE, Inc. and no significant differences in calibra-tions were found, indicating little if any drift in the instrumentsduring the survey. The CTD carousel (SBE-32) was equipped with11 10 L bottles and was routinely lowered to 750 m depth or towithin 10 m of the bottom at the shallower stations.

CTD data from both ships were combined in Ocean Data View(Schlitzer, 2008) to visualize water mass properties and subsur-face spatial patterns of temperature, salinity, and density at 30,70, and 200 m. CTD data were also used to derive the dynamicheight at the surface relative to 500 dBar. Two-dimensional sur-face plots were generated using Ocean Data View software(Schlitzer, 2008). Other data visualization was produced in MATLAB(The Mathworks, Natick MA, USA).

Because only the US AMLR survey collected chl-a data duringtheir surveys, we used satellite based data to represent pro-ductivity during the survey period. Satellite data for surfacechlorophyll-a concentration (chl-a) and Sea Surface Temperature(SST) for January 2011 were retrieved from the NASA Oceancolorwebsite (oceandata.sci.gsfc.nasa.gov/MODISA/Mapped/). These datawere mapped to provide broad-scale interpretation of surfaceproperties in relation to the distribution of krill during the survey.Satellite based chl-a concentrations were highly correlated withsurface chl-a measured at 93 stations in the AMLR area (r¼7,po0.001), and so we consider the satellite data to be representa-tive of the patterns of chlorophyll across the survey area duringthis study.

Additionally, surface drifter data for the western AntarcticPeninsula area were extracted from the NOAA Global drifterrepository (http://www.aoml.noaa.gov/envids/gld/dir/) for allmonths and years between 1979 and 2011. Data were griddedonto a 0.25 by 0.25 degree map, and averaged over time. Surfacecurrent vectors were plotted across the west Antarctic Peninsulato provide a context to interpret hydrographic, environmental,and biological data in that region.

2.3. Krill sampling

For the Polarstern survey, the RMT 1þ8 (Baker et al., 1973) wasused to catch juvenile and adult krill with the RMT8 (mesh size4.5 mm) and krill larvae with the RMT1 (mesh size 330 mm).The RMT1þ8 was equipped with a real-time time-depth-recorder(TDR) and sampled the upper 200 m surface layer or to within10 m of the bottom in shallow areas. A double oblique net tow

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

was carried out routinely at all stations. Calibrated flowmetersmounted on the net frame were used to estimate net speedduring the haul as well as the total distance traveled. The net typi-cally sampled about 23,000 m3 using equations from Pommeranzet al. (1982) at a tow speed of 2.570.5 knots.

The U.S. AMLR survey sampled krill using a 1.8 m Isaacs-KiddMidwater Trawl (IKMT 2.54 m2 effective mouth opening) fittedwith a 505 mm mesh plankton net. Real-time tow depths werederived from a depth recorder mounted on the trawl bridle. Alltows were fished in a double oblique manner to 170 m or to ca.10 m above the bottom in shallower waters. Flow volumes weremeasured using a calibrated General Oceanics flow meter (model2030R) mounted on the frame in front of the net. Tow speedswere approximately two knots with flow volumes averagingapproximately 3850 m3. All krill catch data were adjusted tofiltered water volume before estimating densities.

2.4. Krill counts and measurement

All samples from both surveys were processed at sea. Imme-diately after the tow, postlarval krill were removed and countedprior to other sample processing. All krill from samples of o100individuals were analyzed. For larger samples, at least 100individuals were measured (total length TL, mm) and sexed(juvenile, male, female) following Makarov and Denys (1981).For samples collected by the U.S. AMLR Program, demographicanalyses of krill postlarvae were made using fresh or freshlyfrozen specimens. On the Polarstern, krill samples were usuallyanalyzed after one to two days of preservation in formalin. ThePolarstern samples were measured using the Discovery method forE. superba, i.e., total length from the anterior margin of the eye tothe tip of the telson (Everson, 2000). The US AMLR Programmeasured krill from the tip of the rostrum to the posterior tip ofthe uropods (Standard 1 as described by Mauchline (1980)),which required a correction of the AMLR data of þ1 mm for krilllarger 30 mm (Siegel, 1982). All length data presented in thefollowing paper refer to the Discovery method.

2.5. Krill larvae

Krill larvae were counted and staged from the RMT1 net on thePolarstern and from the IKMT used by the US AMLR program.Although mesh sizes were different between the two nets, the C1larval stage, at approximately 1.5 mm, was still effectively cap-tured by the IKMT. Additionally, the smaller size of the RMT1 netshould not have resulted in significant avoidance; therefore, wedid not expect that patterns of abundance would be biased by themesh differences for these stages. For the Polarstern data, one totwo fractions of 1/2–1/16 of the preserved amount were usuallyexamined to estimate the number and stages of euphausiid larvae(Kirkwook, 1982; Baker et al., 1990). For the US AMLR samples,the plankton samples were split until a manageable fraction wasobtained, and total numbers were extrapolated from the splits.Calyptopis and furcilia stages were identified using the descrip-tion of Kirkwook (1982) and Baker et al. (1990); numbers wereconverted to abundance (N m�2) based on the volume filteredand the maximum depth of the tow.

2.6. Statistical analysis

Krill biomass (g m�2) values were calculated by adjusting alllength frequency distributions to the total krill count at eachstation and multiplying counts per length class with the length–weight relationship W¼2.05�10�6

� TL3.325 (wet weight ingrams) (Siegel, 1992a). This relationship included measurementsof juvenile and adult length classes with gravid and spent krill

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

Table 1Krill numerical (N m�2) and biomass (g m�2) densities west of the Antarctic

Peninsula in January 2011 using the TRAWLCI software described by de la Mare

(1994a).

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]4

representing the adult stages, thus reflecting the stock composi-tion in summer 2011. Mean abundance and biomass estimateswere calculated for the southern and northern areas both sepa-rately and together using the TRAWLCI software (de la Mare,1994a). This method uses Aitchison’s Delta-distribution toaccount for the high number of hauls with zero-value observa-tions and a log-linear model to account for the patchy nature ofthe krill catches. Krill recruitment indices were calculated apply-ing the CMIX software (de la Mare, 1994b). The recruitment indexis defined as the ratio of numbers in age class 1 to the totalnumber of krill in the samples. Since krill distribution is extre-mely patchy, neither stock density estimates nor density for eachlength class of standardized length frequency distributions showa Gaussian distribution. The underlying statistics of these dis-tributions was analyzed using CMIX (de la Mare, 1994b) to createdensity-at-length (instead of length frequency) data from scien-tific surveys as a basis for recruitment estimates. This methodmodifies and extends the Macdonald and Pitcher (1979) least-squares approach of fitting the length frequency distributions bymaximum likelihood. As a result, the quantitative proportions aswell as the mean length of the distribution mixture componentscan be calculated, including a statistical test for the goodness of fitto the data.

Relative krill length frequency data from station hauls sampledin the survey area were analyzed by an agglomerative hierarchicalcluster analysis to determine potential recognizable krill sizegroupings over the survey area, and a geographical considerationof the distribution of such cluster members. A hierarchical fusionof clusters was performed with Ward’s Linkage Method and theuse of the Euclidean distance between stations as a distancemetric. Only stations with a minimum of 20 measured specimenswere used in the cluster analysis to avoid random fluctuations inclassification (Siegel, 1988). Differences among composite length-frequency distributions for resulting clusters were tested usingthe Kolmogorov–Smirnov (K–S) test. The analyses were carriedout using the STATISTICATM software. All deviations from meansare reported as 7standard error, unless otherwise noted.

Surveyarea

Number ofstations

Density(N m�2)

Std.Error

LowerCI

UpperCI

(A)

Northern 96 1.7 0.6 0.9 4.2Southern 81 6.1 2.8 2.5 21.3Total 177 3.4 1.1 1.9 7.4

(B) BiomassNorthern 96 1.3 0.5 0.7 3.6Southern 81 2.9 1.2 1.4 8.5Total 177 2.1 0.6 1.2 4.2

3. Results

3.1. Distribution and abundance

During austral summer 2011, juvenile and adult krill werecaught at 133 out of 177 stations across the survey area. In thenorthern area, krill were absent at 31 of 96 stations (32%),whereas krill were absent from only 11 out of 81 stations (14%)in the southern area. In both areas, these zero hauls were at

Fig. 2. Distribution of Antarctic krill, Euphausia superba, in January 2011: (A) numeric

distributional limit of krill postlarvae during this survey.

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

offshore stations. In the northern area, krill were also absent at afew stations within Bransfield Strait. Overall, the patterns ofcapture indicate that the survey area covered almost the entiredistribution range of the species between the continent and theoceanic region (Fig. 2).

The mean abundance of krill for the survey area was3.471.1 krill m�2, and biomass of krill was 2.17 0.6 g m�2

(Table 1). Abundance and biomass were higher in the southernarea than in the northern area. The mean abundance in thesouthern area was 6.7 krill m�2 (35.9 krill 1000 m�3) comparedto 1.8 krill m�2 (11.0 krill 1000 m�3) in the northern area(including the zero haul stations in both areas). Median abun-dances between areas exhibited a similar pattern to the means,with median abundances of 0.33 krill m�2 in the southern areaand 0.11 krill m�2 in the northern area. The largest catches of krilloccurred at three stations: one in the southern Bransfield Strait(12,700 krill), one in Gerlache Strait (21,200 krill), and one offAdelaide Island (10,600 krill) (Fig. 2). These three stations were indifferent areas of the survey grid, all on the inner shelf and alldominated by juvenile krill.

The distribution of krill biomass (g m�2) across the westernAntarctic Peninsula was geographically broader than the distribu-tion of abundance that was dominated by the three large catchesof small krill. Biomass was roughly twice as high in the southernarea compared to the northern area (Table 1). Highest biomass ofkrill occurred in Marguerite Bay, around Anvers Island in thesouth, and around Elephant Island in the north. Additionally, highbiomass of krill was present at the entrance of Antarctic Sound.Although numerical densities on the outer shelf were much lowerthan inshore along the coast, the average size and weight of krillwere much greater (see next section), resulting in higher biomass

al density (N m�2) and (B) biomass density (g m�2). The solid line indicates the

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 5

values across the outer Peninsula shelf areas compared to patternsin abundance.

3.2. Length and maturity composition

The size distribution of krill was not uniform across the surveyarea. Small juvenile krill were concentrated in near-shore areas,while adults dominated the outer shelf and oceanic regions. Theoverall composite length frequency distribution showed a bimo-dal pattern (Fig. 3). The first mode was centered at approximately27–28 mm, which is the expected size range of one-year-old krillin the Antarctic Peninsula region during summer. The secondmode consisted of larger 45–62 mm krill that are between threeand five years of age.

Cluster analysis performed on length-frequency data from 77samples with more than twenty individuals resulted in threedistinct krill groupings with spatially coherent distribution pat-terns (Fig. 4). The smallest krill were observed in cluster 1 andwere distributed in a nearly continuous band along the continent,

Fig. 3. Composite length frequency distribution of krill, Euphausia superba, west of

the Antarctic Peninsula in January 2011 from the entire set of 177 net samplings.

0

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Fig. 4. Krill size clusters: spatial distribution of stations clusters (C1–C3) with similar kr

‘‘0’’ in the Bransfield Strait represents stations with no krill; length frequency distribu

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

including Marguerite Bay in the south, and Gerlache Strait,Bransfield Strait, and south of Elephant Island in the north. Themodal size of this cluster was 28 mm and most of the krill weresmaller than 40 mm. Krill in cluster 2 were larger, with anintermediate size mode at 48 mm, and with a higher proportionof individuals around 40 mm length. Cluster 2 comprised animalscollected on the outer shelf, just north and south of the SSI andnorth of Elephant Island. Cluster 3 comprised predominately largeadult krill and was defined from animals collected along thecontinental slope and in oceanic regions in the south. This clusterwas dominated by krill of approximately 52 mm in modal length.

3.3. Recruitment

More than 64% of the krill collected during this surveywere juvenile size classes. The Peninsula-wide recruitment index,derived using CMIX, for one-year-old krill (R1all) was 0.43,indicating a relatively successful recruitment of the year classspawned in 2010. The absolute recruitment index was 9.6 recruits1000 m�3 (Table 2). In the northern area, the recruitment indexfor one-year-old krill, R1north¼0.35, was slightly less than theoverall recruitment index and substantially smaller than therecruitment index in the south (R1south¼0.54). The correspondingmean length of the first cohort from CMIX output for each areawas 27.3 and 27.4 mm, indicating that different growth rateswere not responsible for the spatial difference in the recruitmentindex. It is likely that the differences arose from the few towswith extremely large catches of one-year-old krill in the southernarea. Due to the high catch rates of juveniles in the inshore areaacross the WAP, these hauls were particularly important for theweighting of the recruitment index. A re-calculation of therecruitment index without these three highest catch ratesrevealed an index of R1all¼0.25.

There were some differences in the proportion of the differentage groups in the krill stock between the northern and southernareas (Table 3). While recruitment indices showed that age1 recruits were relatively more abundant in the southern section

20 25 30 35 40 45 50 55 60 65

Cluster 1

20 25 30 35 40 45 50 55 60 65

Cluster 2

Total Length (mm)20 25 30 35 40 45 50 55 60 65

Cluster 3

ill length frequency distributions west of the Antarctic Peninsula; area indicated by

tions for the various krill size clusters.

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]6

of the survey grid, age 2 and 4þ krill dominated the compositionin the northern section. It is also worth noting that age class 3(year class 2008) and 2 krill (year class 2009) were under-represented in samples across the entire Antarctic Peninsula.

3.4. Larval abundance and distribution

Krill larvae were found at 115 of 177 stations and wereconcentrated at the southernmost and offshore stations of thegrid (Fig. 5; Table 4). The highest densities of larvae exceeded66,000 larvae m�2. More than 94% of all larvae were in the firstcalyptopis stage, whereas furcilia stages only occurred in lownumbers at the shelf slope and as far north as Elephant Island.There was some evidence for increased numbers of calyptopisstages within Gerlache Strait and higher concentrations of furciliastages at the mouth of Antarctic Sound.

3.5. Oceanographic conditions

Sea surface chlorophyll and temperature exhibited consider-able variability across the Antarctic Peninsula region (Fig. 6). Thehighest chl-a concentration was observed in a band conforming tothe shape of Marguerite Bay in the southern area, where surfacechl-a exceeded 5 mg m�3. In the northern area, around the SSI,chl-a concentrations rarely exceeded 2 mg m�3. Highest

Table 2Recruitment indices for the krill stock west of the Antarctic Peninsula in 2011, and

distribution mixture analysis according to de la Mare (1994b); R1 one-year-old,

R2 two-year-old krill.

R1 Std.error

R2 Std.error

Abs. recruit(N; 1000 m�3)

Meanlength-at-age1þ (mm)

Northern 0.35 0.086 – 3.9 27.3

Southern 0.54 0.017 – 19.4 27.4

Total area 0.43 0.14 0.31 0.038 9.6 26.8

Table 3Proportion of krill age classes observed in January 2011 in the stock west of the

Antarctic Peninsula.

AC 1 AC 2 AC 3 ACZ4þ

AC was spawned 2010 2009 2008 2007

Northern 35.6 15.4 o0.1 49.0

Southern 54.0 8.0 3.3 34.7

Total area 42.9 17.9 5.5 33.6

75˚W 70˚W 65˚W 60˚W 55˚W

68˚S

66˚S

64˚S

62˚S

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0

500

1000

1500

2000

Fig. 5. Distribution of Antarctic krill (A) calyptopis and (B) furcilia larvae (N m�2), in

indicates the 1000 m bathymetric contour line which represents the continental slope

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

concentrations of chl-a in this area was in a band around themajor islands, with low concentrations in offshore waters and insouthernBransfield Strait.

Sea surface temperature across the region also varied. In thesouthern area, highest temperatures were associated with head-lands and smaller embayments. In the northern area, hightemperatures were present on the north side of Bransfield Straitand the shelf-slope region north of Livingston Island. Cool waterswere associated with inflow from the Weddell Sea, aroundJoinville Island and Antarctic Sound, as well as the area southeastof Elephant Island.

Near surface water temperature and salinity, derived from CTDdata, showed variability associated with the presence of theAntarctic Circumpolar Current (ACC), Weddell Sea Shelf Water(WSSW), and freshening due to ice melt (Fig. 7; c.f. Martinsonet al., 2008; Hewes et al., 2009). At 30 m, highest temperatures(42.5 deg) were found offshore northwest of Elephant Island,and on the north side of Bransfield Strait. At 200 m, warm waterbetween 1.5 and 2 1C was found at most offshore stations, as wellas within Marguerite Bay just north of Adelaide Island. Cold waterwas present in Bransfield Strait, especially on the southern side ofthe strait, indicative of advection of WSSW into Bransfield Straitaround the tip of the Antarctic Peninsula. Patterns of salinityreflected the mixing of these water sources as well. Low salinitywater (o33.8) indicative of the ACC was found offshore, but alsonearshore in Marguerite Bay and south of Anvers Island, reflectingseasonal ice melt (with salinities o33.6). High salinity waterindicative of the advection of upper Circumpolar Deep Water(CDW) onto the shelf was present on the shelf north of MargueriteBay at 200 m, off Joinville Island, the cold band of water was alsovery saline (434.3), further indicating the presence of WSSW.

Dynamic height relative to 500 m showed a strong inshore tooffshore gradient in the northern area, with a difference of about20 cm between Elephant Island and the tip of the AntarcticPeninsula (Fig. 8). In the southern area, the gradient was lower,with a difference of about 5–10 cm. The pattern of dynamicheight indicated a general northward flow along the WAP, with

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Table 4Krill larvae (CI–FI) numerical densities west of the Antarctic Peninsula in January

2011 using the TRAWLCI software described by de la Mare (1994a, 1994b).

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Northern 96 15.2 6.8 6.3 33.0

Southern 81 3885.4 2574.2 957.3 29,834.4

Total 177 910.0 500.8 302.4 3991.9

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Fig. 6. Sea surface environmental conditions during January 2011 derived from

MODIS-A data, with sampling stations (black dots) and offshore extent of krill in

net tows (heavy black line). (A) Chlorophyll-a (log10 mg m�3); and (B) Sea surface

temperature (1C).

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 7

a stronger flow around the tip of the peninsula into BransfieldStrait from the Weddell Shelf. The seaward distribution of post-larvae was weakly bounded by the higher dynamic height valuesoffshore.

3.6. Associations between krill abundance and environmental

variables

The offshore distribution of postlarval krill was defined by thelow-salinity, high-temperature water at 30 m that is associatedwith the ACC and the dynamic height of about 35 cm (Figs. 7and 8), but krill were also associated with other environmentalvariables. Log10 abundance of postlarval krill was positivelycorrelated with Log10 chl-a concentration (r¼0.39, po0.01,n¼169) when examined across the entire Antarctic Peninsula.Almost all of the highest values of chl-a and krill were associatedwith cluster 1 stations, indicating a stronger association withchl-a for smaller krill than for larger krill (Fig. 9). When krill and

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

chl-a were plotted against surface temperature and surface salinityto examine water mass association, many of the large catches ofkrill were associated with high temperature, low salinity waters(Fig. 10A). High chl-a concentration in surface waters was alsoassociated with the warm (41 1C) lowest salinity (o33.4)inshore stations (Fig. 10B). Without exception, these stationswere located in the vicinity of Marguerite Bay, and comprisedabout six stations. In contrast to the associations between chl-a

and postlarval krill, analysis of the water mass associations of krilllarvae showed the strongest affinity for ACC type waters withcharacteristic low salinity (33.8) and relatively high (1 1C) tem-perature (Fig. 10C).

The patterns of abundance of larvae and postlarvae were alsoreflective of different advective environments in the southernarea (Fig. 11). Larval krill were found offshore where the currentsare strongly northeastward. In contrast, many of the smallpostlarvae (cluster 1) were found inshore where the coastalcurrent is southwestward. Thus, it is likely that krill larvae weretransported to the north along the shelf slope, and could betransported shelfward in onshore flows associated with eddies, orwith intrusions of water within canyons.

4. Discussion

The results of this survey represent the most complete pictureof the spatial distribution of krill abundance, demography, andproduction on the north side of the Antarctic Peninsula since thelate 1980s. Two significant sources of krill larvae were presentand indicate that areas within the Bellingshausen and the Wed-dell Sea are likely important and different inputs of krill intothe Antarctic Peninsula system. The demographic pattern ofkrill distribution we observed in this study (high abundancesof small krill inshore, and lower abundances of large krill off-shore) is similar to the average patterns described in largercompilations of data (Atkinson et al., 2008; Fig. 14a) and withprevious studies (Siegel, 1988). The importance of the GerlacheStrait/Bransfield Strait and Marguerite Bay for juvenile krillwas already reported by Makarov (1979) and described bySiegel (1988) in the conceptual view of the spatial inshore–offshore succession of juvenile and adult krill. Siegel (1988) alsodiscussed the possibility of a movement of late stage larvaewith the retreating pack-ice in spring from the offshore spawningareas to the inshore juvenile nursery grounds to explain theirhigh densities in those areas. We can also infer from historicrecords and from this study that the inner shelf zone and itsfunction as a successful spawning habitat is less well supported.The distribution of the adult spawning stock in the offshore areasduring summer was already reported by Makarov (1979) andSiegel (1988), and interestingly this group showed a slightlylarger size in the southern region compared to the north(Makarov, 1979). This indicates that the slight difference in adultkrill size between these two areas in this study was not causedby the use of different gear types, but is a known and recurrentphenomenon.

The long-term mean density from net sampling surveysaround Elephant Island since 1978 is approximately 45 krill m�2

and 230 per 1000 m�3 (c.f. Siegel, 2005). Records from theBIOMASS program in the 1980s include six surveys that coveredthe southern as well as northern region. Abundance and bio-mass estimates were concordant in their inter-annual variationbetween the southern areas of those surveys and the ElephantIsland area (Siegel, 1992a), although abundance values weregenerally higher in the northern area. The results of thissurvey indicate that krill abundance was relatively low west ofthe Antarctic Peninsula in summer 2010/11 compared to the

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

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Data were interpolated in Ocean Data View. Thick black line shows offshore extent of krill collected during the survey.

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Fig. 8. Dynamic height (m) at the surface relative to 500 m derived from CTD data

collected during January 2011. Data were interpolated in Ocean Data View. Thick

black line shows offshore extent of krill collected during the survey.

Fig. 9. Antarctic krill abundance in relation to chlorophyll-a concentration across

the southern (red) and northern (yellow) sampling areas, coded by demographic

cluster (C1, C2, C3) for the Antarctic Peninsula area. Symbols with no cluster code

have samples less than 20 individuals and were excluded from the cluster

analysis. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]8

almost-continuous data record from the Elephant Island region,and that the overall krill density was in the lower third of allobserved values over the years. The density values observed for

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

2011 would be even lower if we eliminated the four highestjuvenile krill densities from the most inshore stations. However, itshould be mentioned that these hauls with very high densities

abundance of Antarctic krill (Euphausia superba) along the6/j.dsr.2013.02.005i

Fig. 10. Surface water mass (salinity vs. temperature) relationships across southern (red triangles) and northern (yellow circles) sampling areas as a function of (A) log10

chl-a concentration and (B) log10 postlarval krill abundance coded by demographic cluster (C1, C2, C3); and (C) calyptopsis (yellow circles) and furcilia (red triangles)

abundance across the northern and southern sampling areas, all as expanding symbols. (For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.)

Fig. 11. Map of post-larval krill (green circles) and calyptopsis stages (red circles) on

the west Antarctic Peninsula overlain on a satellite image of chl-a concentration during

January 2011, and the climatological surface circulation derived from drifters released

between 1979 and 2011 and averaged over 0.25�0.25 degree boxes. The 10 and

100 krill m�2 symbols are drawn for reference.

V. Siegel et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 9

were not just random hits of single krill swarms or schools.The hydroacoustic records as well as the observation of highnumbers of feeding baleen whales in those inshore areas at the

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

time of the survey indicate the presence of larger patches withnumerous krill swarms (V. Siegel unpublished data).

Recent studies (e.g., Schmidt et al., 2011) have suggested thatepibenthic behavior of krill may be an underappreciated source ofvariability in the estimate of total abundance of krill. In shallowareas, it is possible that some proportion of krill was missedbecause they were not within the water column. However, otherstudies have shown that the amount of krill captured by netsat depths greater than 200 m was between 0.05 and 1.4% ofthe upper 200 m (Siegel, 1985). Regardless, relative estimates ofabundance, sampled in a similar manner over time, should reflectthe relative changes in abundance of krill, even if certain fractionsare consistently missed because of sampling limitations.

From the proportional recruitment index (R), the 2010 yearclass was successful (R1¼0.54). However, in absolute terms, thenumber of recruits was still relatively low, since the absoluterecruitment index (RI1) was less than 10 recruits 1000 m�3.Between 1990 and 2000, the mean recruitment success was 33recruits 1000 m�3 for the northern area, although poor recruit-ment also occurred during that decade; the 1998 and 1999 yearclasses were almost complete failures (Siegel et al., 2002).

Although there were small differences in the recruitment indicesbetween the northern and southern areas, the results of the 2011survey demonstrated that the two areas are highly concordant. Thelargest differences were related to the small number of stations withextremely large catches of one-year-old krill in the south. Recentmodeling studies of the circulation in the WAP region have demon-strated the strong southwestward-flowing coastal current, and the

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influence of bathymetry on the circulation in this region (Moffatet al., 2008). It is possible that the high concentration of young krillin Marguerite Bay and in the Gerlache Strait may result from theinteraction of krill behavior and the topographically-controlledcirculation (Mackas et al., 1997), or tidal currents, as observed forinvertebrates in other locations (Mackas et al., 1997; Michaud,2005). The outer portion of the circulation on the continentalshelf west of the Antarctic Peninsula is dominated by the north-easterly flow of the ACC, while the inner flow is provided by thesouthward coastal flow. As suggested by Stein (1992), there may beone or more mesoscale eddies within this clockwise flow. Thus,because of krill behavior and the prevailing currents, one-year-oldkrill may be more affected by retention in the southern area ofthe Peninsula region compared to the northern area of BransfieldStrait–SSI–Elephant Island region, impacting the magnitude of therecruitment signal.

The data from this study are consistent with other studies thathave documented a gradient in krill size from inshore to offshore(e.g., Siegel, 1988; Lascara et al., 1999), and the consistency of thepattern along the Peninsula indicates that the pattern is not alatitudinal phenomenon. It is, however, interesting to note thatacoustic studies within Marguerite Bay during autumn and winteralso show similar patterns of extremely high abundance of smallerkrill (Lawson et al., 2008). The krill distribution in summer 2011was similar to the krill distribution patterns during the BIOMASSperiod of the 1980s, but we cannot support or refute the hypothesisdeveloped by Siegel (1988) on the summer–winter offshore–inshoremigration based on the 2011 summer data alone.

When comparing recent data from this cruise survey toprevious surveys (e.g., Witek et al., 1980; Makarov et al., 1990;Siegel, 1992b), it becomes obvious that some areas repeatedlyshow high concentrations of krill larvae. For example, duringautumn cruises in 1985 and 1986, furcilia stages were present,and hot spots of krill larvae occurred in the southwest (northernBellingshausen Sea), and smaller spots north of Livingston Island,and northwest of Elephant Island. Additionally, the continuedpresence of later stage furcilia in the Antarctic Sound area is ofconsiderable interest, as well as the fact that krill larvae are oftenpresent in fairly high abundances (262 larvae 1000 m�3; Lipsky,2006) in this region at times.

The large number of calyptopis 1 (C1) larvae in offshore watersin the southern area is likely the result of both local spawning andsubsequent development on the shelf-slope (off Marguerite Bay),and also advection of larvae from farther south within the shelf-slope current (Fig. 11). The fact that the number of C1 larvae wasmuch lower inshore near Marguerite Bay and off Elephant Islandand the SSI lends some support for the hypothesis that theBellingshausen Sea is the major source of larvae potentially feedingboth Marguerite Bay and areas to the north, in contrast to conclu-sions based on small scale regional studies (Wiebe et al., 2011).

The presence of measurable quantities of krill larvae nearAntarctic Sound suggests some production in this area. Theselarvae were later stage furcilia and could have different growthdynamics given differences in water temperature, salinity andproductivity (Siegel et al., 2004) given their locations. Very fewsamples have historically been collected from the eastern side ofthe Antarctic Peninsula, and yet the presence of these animalscould greatly impact predictions about the impact of climatechange on krill populations. If production in this region remainsfairly constant over time because of the more stable polarenvironment there, it could become proportionately more impor-tant as a source of krill to the SSI area, given the warmingoccurring on the west side of the Peninsula.

The presence of large numbers of calyptopis in the southernarea and first furcilia stages offshore along the shelf and nearAntarctic Sound also suggests that the spawning season began

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

considerably earlier than the historical January–February peak inthis region. However, the paucity of data on larval abundance andPeninsula-wide distribution complicates the evaluation of howthese less well studied areas contribute to reproductive success ofthe broader population. Siegel and Loeb (1995) have shown thatearly spawning (December–January) leads to successful recruit-ment the next year, whereas late hatching of larvae results in lessadvanced growth of larvae and insufficient energy storage tosurvive the winter (Daly, 2004; Meyer et al., 2002; Meyer andOettl, 2005). Survival of these larvae from different origins willdiffer given the variability in temperature, chl-a, and subsequentice formation timing (Quetin and Ross, 2003; Siegel et al., 2003),and it is not currently feasible to back calculate the origin ofsurvivors in subsequent years given the lack of hard parts in krill(c.f. Dorval et al., 2007).

It has been demonstrated, especially by Makarov (1982), thatcalyptopis and early furcilia larvae tend to be more abundant inthe 200–500 m depth layer during December and January beforethey concentrate in the 0–200 m layer from February onward.This would indicate that our larvae abundance values are mini-mum values. However, since earlier studies such as the FIBEX dataset have worked in the same depth stratum our data arecomparable. It is unlikely that the patterns of krill larvae wereinfluenced greatly by the gear types used in this study. Both thecalyptopis and furcilia stages are larger in size than the net meshsizes, and are efficiently caught in the larger 505 mm mesh used inthe 1.8 m IKMT, and are weak swimmers that could not avoid thesmaller 1 m2 RMT1 net. Larger post-larvae could potentially haveavoided the small IKMT relative to the larger RMT8. Support forthis hypothesis is mostly drawn from the difference in meanabundance driven by the three large, inshore catches of krill in thesouthern area, as well as the higher proportion of zero haulsduring the day in the northern area (46.5%) compared to thesouthern area (12.5%). But it is also hard to determine the severityof this because the proportion of samples collected in offshore,krill-poor waters was much greater in the northern area. Despitethese potential biases, the overall demographic patterns inabundance among areas (that have historically used the samegears) indicate that mean densities from this study were lowerthan the historical means (Siegel, 2005).

Drifter data averaged into 0.25�0.25 deg and plotted over thechl-a concentration estimated from satellite data, along with thedistribution and abundance of krill and larvae, illustrate thecomplexity inherent in this system (Fig. 11). Along the coast ofMarguerite Bay, a strong southward jet separates cold coastalwater from high chl-a, warm, and krill-rich areas on the shelf that,given the difference in current magnitude, temperature, and chl-a

content, must have a higher residence time. Offshore, calyptopisstages of krill are associated with lower chl-a, ACC water, and arealso embedded within the strong shelf break jet. Since larger,mature krill are also present offshore, it is unclear how importantthe highest chl-a areas of the shelf are to spawning females andvulnerable larval krill during summer. Similarly, the clear asso-ciation of calyptopis stages with ACC water is less a result ofactive preference for specific water masses, but rather reflects therise of larval krill from deeper depth strata to the surface as theydevelop (Makarov, 1982). This can create an overlap that ispossibly viewed as a physiologically or ecologically importantassociation. What still remains an important and open-endedquestion is the influence of deep (41000 m) westward-flowingcounter currents at depth near the shelf edge that could transportor retain eggs in areas to the southwest and the temporalvariability in larval transport onto the shelf from offshore.Hofmann and Husrevoglu (2003) have used modeling studies todocument potential spawning areas, but given the stages of krilllarvae found in this study, the eggs of these larvae would have

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had to have been produced in late November, farther south,during the spring bloom in those areas (Marrari et al., 2008).

Although there was a general correlation between the abun-dance of krill and chl-a, the relationship was weak, and was afunction of a small number of southern area near-shore stationswith high numbers of juvenile krill. We expected a strongercorrelation between abundance and chl-a concentration becauseof the distinctly different productivity regimes associated withcoastal and offshore environments. In the ACC, iron limits primaryproductivity, resulting in consistently low chl-a concentration(Smetacek et al., 2012). Along the coast, higher productivity isassociated with higher stratification, sources of iron from resus-pension, upwelling of iron rich waters, or from runoff and icemelt. There is a high correlation between monthly indices of chl-a

based on satellite data and instantaneous measures of chl-a

(r¼0.7, n¼95; US AMLR Program unpublished data) taken atfixed sites within the northern area. This suggests that the chl-a

patterns based on satellite data also reflect the general patterns ofproduction available to krill during the survey period. Thus thelack of a strong relationship with chl-a and krill across the entiresampling area is less likely due to differences in scale or timing ofthe sampling, but rather due to the oceanographic differencesbetween the two sampling areas, since primary production isdriven by different combinations of light, nutrients, and stratifi-cation (Moline et al., 2004; Prezelin et al., 2004; Mitchell et al.,1991). The southern area is essentially modified and eroded ACCand CDW water masses, with the addition of low salinity waterderived from ice melt (Hofmann et al., 1996; Martinson et al.,2008). In the northern area, the water mass composition issubstantially more complex, and is mostly a mixture of ACC,CDW, and WSSW (Gordon and Nowlin, 1978; Hofmann et al.,1996). Together, these different water masses impact the overalllevels of production as well as the seasonal timing of the blooms,and therefore the local correlations. The second reason why nostrong relationships with chl-a are present is that postlarval krillare highly efficient omnivores and can effectively feed on phyto-plankton 45 mm and at low chl-a concentrations (40.36 ug L�1;Ross et al., 1998) as well as zooplankton prey. Therefore, post-larval krill are relatively independent of the average concentra-tions of chl-a observed in surface waters during this study.Finally, surface chl-a measured by satellite may indicate thequantity present, but not the species composition of chl-a, whichmay affect krill grazing during summer (Moline et al., 2004).This is especially true in the southern area, where recent studieshave shown that near coastal phytoplankton communities aredominated by species less available to krill (Moline et al., 2004),and where phytoplankton community structure may have chan-ged over the last 25 years (Montes-Hugo et al., 2008). It isinteresting to note again that, excluding the several stations withanomalous hydrographic properties in the southern area, therewould be little difference between the pattern and ranges of krillabundance and chl-a concentration, and including subsurfacechl-a concentration that may exist in ACC waters would notchange the inferences here.

The west Antarctic Peninsula is one the most rapidly changingenvironments on the planet. Declines in sea-ice extent andduration, potential changes in phytoplankton community compo-sition, and changes in water column temperature all suggest thatkrill populations will be substantially impacted in the future.Future studies could profitably examine the relative importanceof the Weddell Sea outflow as a mechanism for the transport ofkrill larvae into the northern area of the WAP. Growth rates andproductivity could be compared to the western Antarctic Penin-sula and Bellingshausen Sea to understand whether differencesexist, and whether the warming to the south is negativelyimpacting the growth, mortality, and recruitment dynamics of

Please cite this article as: Siegel, V., et al., Distribution andAntarctic Peninsula. Deep-Sea Res. I (2013), http://dx.doi.org/10.101

krill. Of course, the magnitude of difference in productivitybetween the two regions strongly suggests that the Bellingshau-sen will remain important from a numerical perspective, but ifthe survival rates of krill from the Weddell Sea are slightly higher,they could influence population dynamics in the future givenclimate change predictions (IPCC, 2007).

The development of feedback management systems to controlthe harvest of krill may need to consider the patterns in sizestructure in this area, the role of the Weddell Sea in producingkrill in this area, and the impact of climate change that will openmore nearshore areas to fishing as the ice recedes. Despite morethan 30 years of continuous research, the complexity of the lifecycle, the large spatial habitat, and the impact of environmentalvariability, our understanding of Antarctic krill is still incomplete.Future studies using both integrated, spatially-explicit modelingand directed field studies in less well sampled areas may help totest hypotheses that could be used to examine likely impacts ofclimate change in the Antarctic Peninsular ecosystem that arelikely given changes already observed.

Acknowledgments

The authors acknowledge the support and dedication of thecaptain and crews of both the Polarstern and the Moana Wave foralways completing the work with smiles, energy and positiveoutlooks. We thank our field teams for working under sometimesdifficult conditions to ensure that the data are always of thehighest quality. The authors thank four anonymous referees fortheir comments and suggestions that have helped shape some ofthe discussion of this paper.

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