Habitat selection of overwintering Calanus finmarchicus in the NE Norwegian Sea and shelf waters off...

Habitat selection of overwintering Calanus finmarchicus inthe NE Norwegian Sea and shelf waters off Northern Norwayin 2000–02

ELISABETH HALVORSEN,1,* KURT S.TANDE,1 ARE EDVARDSEN,1 DAGSLAGSTAD2 AND OLE PETTER PEDERSEN1

1Norwegian College of Fisheries Science, University of Tromsø,9037 Tromsø, Norway2SINTEF, Fisheries and Aquaculture, 7465 Trondheim,

Norway

ABSTRACT

The abundance and distribution of overwinteringCalanus finmarchicus in the NE Norwegian Sea andshelf waters off North Norway was studied duringJanuary for 2000–02. Depth integrated distribution ofC. finmarchicus CV showed aggregations with highabundances in the Lofoten Basin and southwest ofTromsøflaket for all three years. The exact location ofthe aggregation areas and the maximum abundancesdid, however, vary between the years. The concen-trations southwest of Tromsøflaket were almosttwofold higher in 2000 at 150 000 ind. m)2 comparedto 70 000 and 90 000 ind. m)2 in 2001 and 2002,respectively. Vertical distribution of the animals wassimilar for the three years, with most of the CVs of thepopulation residing in the depth interval between 700and 1200 m. Peak abundances of 350 ind. m)3 werefound at 850–1000 m west of Tromsøflaket in 2000,whereas in 2001 the maximum abundances werelocated in the Lofoten Basin at 700–900 m, in theorder of 150 ind. m)3. In 2002, the highest concen-tration of animals was found west of Vesteralenbetween 1100 and 1200 m, with a concentration of385 ind. m)3. The vertical and horizontal distributionof C. finmarchicus CV closely followed the hydrogra-phic structures in the area, with the highest abun-dances associated with cold (<2�C), less saline (34.85–34.9) water with a density of 27.95–28. The patches ofhigh abundance were located in confined areas along

the continental shelf slope and in the Lofoten Basin,indicating that the animals may have been extractedfrom the highly flushed surface areas in late summerduring their ontogenetic descent, and trapped in me-soscale physical features in the deep-water massesthroughout the winter. We argue that deep-watermesoscale anticyclonic eddies, which are frequentlyformed along the continental slope and in the LofotenBasin, may provide favourable retention areas for theoverwintering population of C. finmarchicus. Conse-quently, the impact of ocean climate as well as moreregional and local effects on the creation and persist-ence of these mesoscale features is likely to influencethe Calanus abundance and advection paths in thefollowing spring and summer.

Key words: Calanus finmarchicus, habitat selection,hydrography, NE Norwegian Sea, overwintering

INTRODUCTION

The calanoid copepod Calanus finmarchicus is widelydistributed in the North Atlantic, and has one of itsthree distribution centres in the southern NorwegianSea gyre (Aksnes and Blindheim, 1996). In spring, theoverwintering population ascends to surface waters,where it matures and spawns. The majority of the firstgeneration enters diapause in June and July, and des-cends to depths >600 m to overwinter, mainly ascopepodite stage IV–V (Hirche, 1991, 1996; Østvedt,1955). It spends the winter months in a state ofarrested development, living on accumulated lipid re-serves and with reduced respiration rates (Hirche,1996). In the northeast Atlantic, accumulations ofoverwintering animals have been identified in theFaroe-Shetland Channel, southwest of Iceland, and inthe southern and eastern Norwegian Sea (Heath et al.,2000). The animals were generally concentrated in the600–2000 m isobath. Enhanced probabilities of survi-val because of lower temperatures, and the avoidanceof predators and parasites are considered as the mainadvantages of overwintering at depth (Krause andRadach, 1989; Kaartvedt, 1996).

*Correspondence. e-mail: [email protected]

Received 25 October 2002

Revised version accepted 13 June 2003

FISHERIES OCEANOGRAPHY Fish. Oceanogr. 12:4/5, 339–351, 2003

� 2003 Blackwell Publishing Ltd. 339

The overwintering habitat also plays an importantrole with regard to the persistence of spatial patterns ofC. finmarchicus populations in the Northeast Atlanticand the Norwegian Sea. In the southern part of theNorwegian Sea the depth layers occupied duringwinter correspond to the upper layer of the NorwegianSea Deep Water (NSDW), a polar water mass runningas a counter-flow beneath the poleward slope jet cur-rent. The animals are thus slowly transported from theNorwegian Sea via the Faroe-Shetland Channel to theAtlantic (Heath et al., 2000). As C. finmarchicusascends from its overwintering habitat to spawn inspring, it is directed by the surface circulation, whichin turn reroutes the animals towards the Norwegianand North Sea, as well as the shelf areas (Backhauset al., 1994; Bryant et al., 1998).

Bryant et al. (1998) used a particle tracking modelcoupled to a hydrodynamic model in order to study theadvection and spatial persistence patterns of C. finm-archicus in the Northeast Atlantic and the NorwegianSea. They simulated the interaction between seasonalontogenetic migration and the flow regime, and forpopulations that persisted for 5 or more years, themodel predicted a winter distribution that correspon-ded well with the observed distributions in the Faroe-Shetland Channel and the eastern and southern areasof the Norwegian Sea. Further north along the Nor-wegian coast, seasonal vertical migration interactedwith the model flow field and produced complex pat-terns around the Vøring Plateau and the LofotenBasin, resulting in retention within most of these

areas. The model also indicated that particles wererapidly washed out from the northern part of theNorwegian Sea, particles which most likely end up inthe Barents Sea. The overwintering distribution ofC. finmarchicus in this area has not previously beendescribed, but if areas of overwintering animals wereidentified, it could potentially improve our under-standing of the inter-annual variations in abundanceof this species in this northern part of the NorwegianSea as well as in the Barents Sea in summer.

The present paper describes the winter distributionof C. finmarchicus CV between the southeastern part ofthe Lofoten Basin and the southwestern border of theBarents Sea for 2000–02. Based on what is known ofthe flow fields in the region, and with the hydrog-raphical distribution for the three years, the presentpaper describes: (i) the large-scale deep water accu-mulation of overwintering animals and (ii) how theinteraction between the seasonal ontogenetic verticalmigration of the animals and the flow regime mightresult in a persistent spatial distribution pattern of theC. finmarchicus population in this northern part of theNorwegian Sea.

MATERIALS AND METHODS

Field sampling

During January 2000, 2001 and 2002, three cruiseswere carried out with R/V Jan Mayen off the coast ofNorth Norway between Bear Island and the Lofoten

Figure 1. Description of the study area,cruise track for 2002, topography andnames.

340 E. Halvorsen et al.

� 2003 Blackwell Publishing Ltd, Fish. Oceanogr., 12:4/5, 339–351.

Islands (Fig. 1, Table 1). The surveys covered both theshelf and the deep waters off-shelf in order to map thewinter distribution of the calanoid copepod C. finm-archicus, as well as the hydrographic conditions of thearea. Owing to rough weather conditions, not all 58stations were sampled in 2000 and 2001 (see Fig. 5 forexact positions and number of stations for all threeyears).

Temperature, salinity and density were measuredusing a Seabird 911 Plus CTD. Salinity measurementswere calibrated from water sampled at the deepestsampling depth of each cast.

After each CTD profile, zooplankton was sampledwith a 0.5 m2 MultiNet (Type Midi; Hydro-BiosApparatebau GmbH, Kiel-Holtenau, Germany), meshsize 180 lm, which was hauled vertically at a speed of0.5 m s)1. The MultiNet was lowered with all five netsclosed to 10 above the bottom (or to 1800 m when thebottom depth was deeper than 1800 m), after whichthe five nets were opened and closed sequentially atpre-programmed depths during the upward haul, faci-litated by a built-in depth sensor and an electronic netrelease motor. A flow meter mounted in the netopening measured the volume filtered for each net anddepth interval. On the shelf and over the shelf slope,the depth strata sampled were 10 m above the bottomto 500 m, 500–300, 300–200, 200–100 and 100–0 m.At the deep stations the depth strata sampledwere 1800–1200, 1200–700, 700–500, 500–200 and200–0 m. However, the ‘700–500 m stratum’ wasadjusted so as to cover the temperature interval from 0to +2�C at each station, as defined from the CTDprofile.

Sample analysis

The zooplankton samples were preserved in a 4%hexamine buffered formaldehyde/seawater solution,and analysed in the laboratory to determine theabundance and copepodite development stage for eachcopepod species. C. finmarchicus and C. helgolandicuswere only discriminated from each other for the femalestage VI specimens. However, the abundance of femaleC. helgolandicus was so low and sporadic that the

majority of the CI-CVs most probably were C. finm-archicus, and have been treated as such in the following.

Data analysis

The latitude/longitude components of the survey datawere transformed into a polar stereographic grid (Figs5–7). This was done in order to obtain coherence withthe bathymetry data, which were available on a polarstereographic projection. The method of interpolationapplied was a cubic spline method (spline with degreenot exceeding 3) using continuous first- and second-order derivatives (Kreyszig, 1988).

In order to test whether the abundance of CVs inthe +2–0�C depth layer differed significantly from theabundances in the other depth layers, a non-para-metric statistical test, the Wilcoxon rank-sum (Mann–Whitney) test for two independent samples, was ap-plied. The test consists of combining the two samplesinto one sample of size n1 + n2 sorting the result,assigning ranks to the sorted values (giving the averagerank to any ‘tied’ observations), and then letting T bethe sum of the ranks for the observations in the firstsample. If the two populations have the same distri-bution then the sum of the ranks of the first sampleand those in the second sample should be close to thesame value. The test returns a P value for the prob-ability that the two distributions are the same. Thus,for all deep stations (i.e. stations covering the +2–0�Ctemperature interval), the rank sum of the +2–0�Cdepth layer was compared to the rank-sum of each ofthe other depth layers.

RESULTS

Hydrography

The hydrographic structures for the three years arepresented as vertical distributions of temperature,salinity and density along four transects (Figs 2–4).Surface temperatures were 6–7�C with the highesttemperatures found as a wedge stretching westwardsfrom the shelf break, narrowing at the northernmostsections (Fig. 2). The temperature interval from

Table 1. Cruise information.

Cruise Date Stations sampledDeployments ateach station

PCB 1 31.01–13.02 2000 4–8, 10–11, 13, 16–24,28–31, 35 (19 stations)

CTD MultiNet

PCB 3 10.01–25.01 2001 1–9, 11–18, 20, 22–37(51 stations)

CTD MultiNet

PCB 5 16.01–02.02 2002 1–58 (58 stations) CTD MultiNet


Habitat selection of Calanus finmarchicus 341

Figure 2. Temperature distribution along four transects for 2000–02.D

epth

(m

)

Dep

th (

m)

Ocean data view

Dep

th (

m)

Dep

th (

m)

Figure 3. Salinity distribution along four transects for 2000–02.



+2–0�C covered a layer of about 200 m (being slightlynarrower close to the continental shelf compared tothe westernmost area), and for transects 2 and 4 thislayer was located between 600 and 800 m depth. Ontransect 3 this layer was located deeper, between 800and 1100 m.

Atlantic water masses (salinity >35 psu) dominatedthe upper 500 m along transects 2–4, but coastal waterwith salinity of 34.7 psu or less was also present in thesurface waters along the shelf slope (Fig. 3). The rh 28isopychnal denoting the NSDW (temperature <0�C,salinity <34.95 psu) was found between 750 and1250 m depth (Fig. 4).

Depth integrated distribution of C. finmarchicus CV

The depth integrated distribution of C. finmarchicusCV (Fig. 5) shows patches with high abundances inthe Lofoten Basin for all three years, and southwest ofTromsøflaket in both 2000 and 2001. In 2002, thelargest signal was found along the shelf slope outsideVesteralen. Peak concentrations at one station west ofTromsøflaket were, however, more than twofoldhigher in 2000 compared to 2001, with abundancepeaks of 150 000 and 70 000 ind. m)2 for the twoyears, respectively. Peak concentrations outside Vest-eralen in 2002 were in the order of 90 000 ind. m)2.Very low abundances (typically below 2000 ind. m)2)

of C. finmarchcius CV were found over the shelf areas.The number of stations sampled in each year varied,mainly because of severe weather conditions, and thistogether with the fact that only three years werestudied makes it difficult to assess whether there was asignificant difference in abundance for the whole areabetween years. Averaging all stations within each yeardoes not indicate that there was a significant differ-ence in abundance between years (Table 2).

Vertical distribution of C. finmarchicus CV andtemperature

Four depths (550, 750, 900 and 1100 m) were chosenin order to present the vertical distribution ofC. finmarchicus CV (Fig. 6). In 2000, small patches ofCV with concentrations of 100–125 ind. m)3 werepresent already at 550 m depth. These patches wereconfined to the southwestern corner of the sampledarea in the Lofoten Basin, and also to the north to-wards Bear Island. However, the main concentrationsof copepods were found in the depth interval between700 and 800 m in the Lofoten Basin, and between 800and 1200 m west of Tromsøflaket.

In addition to the vertical distribution of tem-perature (Fig. 2), the horizontal distribution of tem-perature is shown for the same depths as thedistribution of C. finmarchicus CV above (Figs 6–8).

Dep

th (

m)

Dep

th (

m)

Dep

th (

m)

Dep

th (

m)

Ocean data view

Figure 4. Density distribution along four transects for 2000–02.



Upward doming of cold, deep water was seen as smallareas of cold water (<2�C) in the Lofoten Basin and inthe northernmost corner towards Bear Island at550 m, the area broadening and getting colder(1–0�C) at 750 m depth. Abundances in the depthinterval covering the temperature interval of +2–0�Cwere �100 ind. m)3 in the Lofoten Basin and100–350 ind. m)3 west of Tromsøflaket. The highestabundance west of Tromsøflaket was found ataround 900 m where temperatures ranged between 0and –0.5�C.

In 2001, there were also small patches of animals atconcentrations of �100 ind. m)3 in the Lofoten Basinand southwest of Tromsøflaket at 550 m, but as for2000, the highest concentrations of C. finmarchicusCVs were found in the Lofoten Basin and west ofTromsøflaket in the depth interval between 700 and1000 m (Fig. 7). Peak abundances were between�100 and 150 ind. m)3. The upward doming of coldwater was seen in the Lofoten Basin and to thenorthwest of the survey area at 550 m, and a depres-sion of warmer water was evident at 750 and 900 mon transect 3.

In 2002, most of the CVs were located in the depthinterval between 650 and 1200 m. As for the otheryears, an upward doming of cold water was seen in theLofoten Basin at 550 m, and a depression of warmerwater was evident at 750 and 900 m depth along tran-sects 2 and 3. Peak abundances ranged from 100 to200 ind. m)3, and were found at 700–1100 mdepth. Northwest of Vesteralen a concentration of350 ind. m)3 was found at 1100–1200 m depth.

For all three years, peak concentrations above500 m and below 1200 m were generally less than25 ind. m)3.

Considering all years and including only the deepstations (83 in total), the abundance of CVs in thedepth interval covering the temperature interval of +2to 0�C was found to differ significantly from theabundances in the four other depth layers (Wilcoxonrank-sum test: P � 0, Table 3).

DISCUSSION

Winter distribution

Outside the shelf areas of North Norway and theBarents Sea in January 2000–2002, C. finmarchicusCVs were mainly distributed in water of temperaturesbelow 2�C. Considering all deep stations (i.e. stationscovering the depth interval between +2 and 0�C),between 71 and 99% (average 93%) of the CVs werefound below the 2�C isoline. Few animals were found

Table 2. Average station abundance (ind. m)2) of Calanusfinmarchicus CV for 2000–02.

Station average(ind. m)2) Std. dev.

Numberof stations

All stations2000 30 387 ±37 026 192001 28 809 ±23 336 512002 23 178 ±20 423 58

Deep stations2000 55 956 ±34 641 102001 42 983 ±15 753 332002 32 513 ±17 015 41

65

66

67

Latit

ude

NORWAY

65

66

67

Latit

ude

12 15 18 21 24 27 65

66

67

Longitude

Latit

ude

0 2 4 6 8 10 12 14

x 104

Ind. m-2

NORWAY

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

Bear Island

Tromsøflaket

Lofoten Basin

Bear Island

Tromsøflaket

Lofoten Basin

2000

2001

2002

> 150 000

Figure 5. Depth integrated abundance of Calanus finmar-

chicus CV for 2000–02.



in the Atlantic water mass, which dominated theupper 500 m of the water column, and the highestabundances were found associated with the +2–0�Cdepth interval between 700 and 1200 m depth. Thisdistribution agrees well with previous findings from theFaroe-Shetland Channel where overwinteringC. finmarchicus were found distributed between the600 and 2000 m isobath, in water masses of tempera-tures <0�C and corresponding to the upper layer of theNSDW (Heath and Jonasdottir, 1999). The abun-dances found in the present study were also ofthe same order of magnitude: a maximum of70–150 000 ind. m)2 outside the North Norwegianshelf compared to 50 000 ind. m)2 in the Faroe-Shetland Channel. South of the Scotland-Iceland

ridge in the North Atlantic the distribution ofC. finmarchicus in winter has been found to be asso-ciated with increasing proportions of cold (2–4�C)Labrador Sea Water in the Iceland Basin and theIrminger Sea at depths between 1000 and 2400 m(Heath et al., 2000). These findings all emphasize theimportance of temperature for the overwinteringhabitat of C. finmarchicus. In the present study, thehigh abundances of C. finmarchicus CV found atrelatively shallow depths (550 m), but strictly con-fined to the areas in the Lofoten Basin where coldwater (<2�C) was doming upwards, further strengthensthis notion.

However, Calanus were not evenly distributedwithin this temperature domain. Rather, high

65

66

67

Latit

ude

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

65

66

67

Latit

ude

65

66

67

Latit

ude

12 15 18 21 24 27 65

66

67

Longitude

Latit

ude

12 15 18 21 24 27Longitude

0 20 40 60 80 100 120 140 160

Ind. m -3–1 0 1 2 3 4

˚C

> 6˚C

> 350

> 350

Abundance (ind. m-3 ) Temperature (˚C)

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

550 m

750 m

900 m

1100 m

550 m

750 m

900 m

1100 m

Figure 6. Distribution (ind. m)3) of Calanus finmarchicus CV (left) and temperature (right) at 550, 750, 900 and 1100 m depthin 2000.



aggregations of animals were found in specific areas inthe Lofoten Basin and southwest of Tromsøflaket,indicating the existence of additional mechanismsconfining the main overwintering habitat to theseareas. The explanation for these aggregations probablylies within the flow fields at the appropriate depths.The upward doming of cold water at around 500 mdepth close to the continental shelf and further west inthe Lofoten Basin was evident from the vertical

transects of temperature and salinity (Figs 2–4), as wellas from the horizontal distribution of temperature(Figs 6–8). This could be the result of internal waves,or another mechanism that could produce such afeature is the presence of deep-water mesoscalecyclonic eddies. Likewise, the depression of the iso-lines of temperature seen at 750 m (Figs 6–8) couldindicate the presence of anticyclonic eddies. Meso-scale eddies (cyclonic or anticyclonic) are energetic,

65

66

67

Latit

ude

65

66

67

Latit

ude

65

66

67

Latit

ude

12 15 18 21 24 27 65

66

67

Longitude

Latit

ude

12 15 18 21 24 27Longitude

0 20 40 60 80 100 120 140 160 –1 0 1 2 3 4

Ind. m -3 ˚C

Abundance (ind. m-3 ) Temperature (˚C)

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

6˚C

550 m

750 m

900 m

1100 m

550 m

750 m

900 m

1100 m




65

66

67

Latit

ude

65

66

67

Latit

ude

65

66

67

Latit

ude

12 15 18 21 24 27 65

66

67

Longitude

Latit

ude

12 15 18 21 24 27Longitude

0 20 40 60 80 100 120 140 160 –1 0 1 2 3 4

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

NORWAY

Bear Island

Tromsøflaket

Lofoten Basin

Abundance (ind. m-3 ) Temperature (˚C)550 m

750 m

900 m

1100 m

550 m

750 m

900 m

1100 m

Ind. m -3 ˚C

6˚C

200

350


Table 3. Wilcoxon rank-sum test re-sults. Depth

interval 0–200 m 200–500 m�500–700 m(+2–0�C) 700–1200 m 1200–1800 m

Rank sum 121 192 385 344 203P 0 0 1 0.000438 0H 1 1 0 1 1

Abundance of Calanus finmarchicus CV in the +2–0�C depth interval is comparedwith abundances in the other depth intervals. The P value is the probability thatthe two distributions are the same. H ¼ 1 signifies that the result is significant.



swirling, time-dependent circulations about 100 km inwidth, which are found almost everywhere in theocean. They are often generated by unstable mean-dering of an intense current, such as the NorwegianAtlantic Current, and may be strongly influenced bybottom topography (Haugan et al., 1991; Sætre, 1999).The strong, jet-like characteristic of the NorwegianAtlantic Current and its interaction with the steepcontinental slope north of the Lofoten area are likelyto create mesoscale eddies, possibly formed from theconservation of potential vorticity as the current isdeflected northwards. The exact mechanisms at workin this particular area, however, are yet to be clarified.With respect to the Lofoten Basin, the presence of aslow topographically steered cyclonic flow around thebasin has been shown from surface drifter experiments(Poulain et al., 1996), and from hydrographic profiles(Furevik, 1998). The study of Poulain et al. (1996) alsodescribed the presence of many mesoscale eddieswithin the basin, and although these were confined tothe upper layers of the water column, it is probablethat the strong topographic influence on the flowregime in this area leads to the creation of such fea-tures also at greater depths. In the area southwest ofTromsøflaket, model simulations (SINMOD) havedemonstrated the presence of an anticyclonic eddy at adepth between 400 and 1000 m (Slagstad, unpub-lished). The presence of semi-permanent eddies couldexplain the aggregation pattern of C. finmarchicus CVsas observed in the Lofoten Basin and along the con-tinental slope in the three years studied. As the Cal-anus migrate to greater depths in late summer foroverwintering, they may be trapped in the convectivecircular flows associated with an anticyclonic eddy.The temporal scale of these mesoscale eddies mightnot cover the whole overwintering phase of C. finm-archicus (6–8 months). However, if they are createdfrequently enough in the same area throughout thewinter, it would be possible to imagine that the Cal-anus are re-aggregated within a newly formed eddyshortly after the dissipation of the first, and so on,resulting in the animals being confined to a relativelyrestricted area throughout the winter months. As wedo not have simultaneous measurements of eddystructures from the winter cruises, we cannot comparethe exact distribution of Calanus in relation to these,but rather propose this mechanism as a preliminaryexplanation. From the three years of observations, itseems that the positions of the two areas of aggregationin the Lofoten Basin were quite stable, whereas theposition of the areas of aggregation along the shelfslope further north varies between years. This mayindicate that the creation of these mesoscale features is

more dynamic along the shelf slope as opposed to inthe Lofoten Basin. More detailed studies on the flowpatterns at depth and their temporal and spatial vari-ability, as well as a longer time series of the distribu-tion of C. finmarchicus in the area, are needed in orderto fully understand these processes.

Spatial distribution throughout the life cycle

Recent studies have shown that C. finmarchicus is anoceanic species, dependent on the colder deep-waterfor overwintering and on the basin scale circulation toobtain spatial closure of its lifecycle (Backhaus et al.,1994; Bryant et al., 1998; Heath and Jonasdottir, 1999;Heath et al., 2000). The continental shelf areas, beingtoo shallow to provide an overwintering habitat, arenonetheless invaded by this species in spring andsummer (Gallego et al., 1999; Heath et al., 1999;Gaard, 2000; Gislason et al., 2000; Pedersen et al.,2000), when it constitutes an important prey item forlarval and planktivorous fish (Skjoldal and Rey, 1989;Gjøsæter, 1998; Gjøsæter et al., 2002).

The spring invasion of the North Norwegian shelfand the Barents Sea is partly the result of the northwardadvection of C. finmarchicus from the overwinteringsites in the southern part of the Norwegian Sea, but thepresent study demonstrates that there are also import-ant overwintering sites in the Lofoten Basin and alongthe continental slope off North Norway. We have triedto draw a conceptual picture of how C. finmarchcius isdistributed in this area in winter, its ascent in spring,the advection pattern during summer, and its descentto the overwintering habitat in late summer/earlyautumn (Fig. 9). The 700–1100 m depth intervalwhere the majority of the C. finmarchicus CVs werelocated is indicated by white lines along the contin-ental slope, and the main aggregation areas are given asgrey circles in the Lofoten B Basin and southwest ofTromsøflaket. The water mass below 1000 m is con-sidered to be the NSDW (<0�C, <34.95 psu).

As the CVs ascend from the overwintering depthsbetween 700 and 1200 m in spring (black arrows), themajority of the population is probably transported withthe cyclonic surface flow within the Lofoten Basin andcontinue their population development within theoceanic area (indicated as a grey circle). However, apart of the population ascending close to the contin-ental slope will inevitably be captured by theNorwegian Atlantic Current and be transportednorthwards along the continental slope (grey arrows).This current bifurcates at the entrance of the BarentsSea, one branch turning northeastwards into theBarents Sea, and one branch continuing northwards asthe East Spitsbergen Current. How much of the Cal-



anus population that is actually transported into theBarents Sea each summer is influenced by the vari-ability of the inflow of Atlantic water between BearIsland and the Norwegian coast (Helle and Penning-ton, 1999). However, this inflow has been shown to behighly variable on times scales of days and months, andthe position of the inflowing/outflowing cores also varysignificantly (Ingvaldsen et al., 2002). This implies thatthe ‘successful’ invasion of the Barents Sea each yearcannot be predicted on the basis of the average inflowduring spring, but may be dependent on the populationdensity in these overwintering aggregations as well asthe timing of ascent and the outflow/inflow regimeacross the Barents Sea Opening. A study of the inter-annual variation in abundance of C. finmarchicus at theKola section in the eastern Barents Sea did not showany correlation between abundance and the NorthAtlantic Oscillation index (Halvorsen et al., unpub-lished), an index that might be considered as a proxyfor the inflow of Atlantic water to the Barents Sea(Adlandsvik and Loeng, 1991; Hurrell, 1995; Dicksonet al., 2000; Dippner and Ottersen, 2001).

Most of the Calanus in the Norwegian AtlanticCurrent are probably transported northwards, butprevailing southeasterly winds in spring along theNorwegian Coast from Lofoten to the Barents Sea(Nordby et al., 1999) also lead to a shoreward flow inthe upper meters of the water column, transportingmany of the eggs and nauplii further onto the NorthNorwegian shelf (Halvorsen and Tande, 1999). As thespring population reaches copepodite stages IV and Vin late summer/early autumn, they start their onto-genetic vertical migration to deep waters to overwinter(white arrows). Owing to cross shelf currents, thepopulation on the North Norwegian shelf are trans-ported off the shelf as they descend, leading to a returnto the overwintering sites in the Lofoten Basin andsouthwest of Tromsøflaket (Slagstad and Tande,1996). For the Barents Sea population, there is also apotential route for the return to the Norwegian Sea viathe Bear Island Trough. The re-circulation of Atlantic

water in the Bear Island Trough (Haugan, 1999;Furevik, 2001) and the rather constant westwardbottom flow in the northern part of the trough(Ingvaldsen et al., 2002) would transport the animalsoff shelf and back into the Norwegian Sea. However,considering the high predation pressure in the BarentsSea, and the rather limited area for return transport, itis unlikely that this pathway is important for thepersistence of the Norwegian Sea population at large.Below the highly flushed surface water along thecontinental shelf and in the Lofoten Basin thedescending Calanus encounters colder, low velocitywater masses in which they reside throughout thewinter, possibly trapped within frequently occurringsemi-permanent mesoscale eddies.

The studies of Poulain et al. (1996) indicated thatthere is little exchange of surface water across theridges separating the basins of the Nordic Seas (Ice-landic, Greenland and Norwegian Sea). It is possiblethat part of the C. finmarchicus population is advectedsouthwards with the NSDW across the Vøring Plateauand into the Norwegian Sea Basin during winter, butotherwise the mixing of the Calanus populations ofthese two basins seems restricted to the part of thepopulation that is advected by the Norwegian AtlanticCurrent during spring and summer.

CONCLUSIONS

This study demonstrates the presence of importantoverwintering habitats for C. finmarchicus in theLofoten Basin and along the shelf slope off NorthNorway. The habitats seem to be confined withinhydrographic structures that aggregate the animals inrestricted areas, resulting in ‘hotspots’ with highabundances of Calanus that are found in the sameareas from year to year. The mechanism behind theseaggregations is believed to be the entrapment ofdescending animals in late summer within semi-permanent mesoscale anticyclonic eddies. These eddiesare most likely created as a result of the dynamic

Figure 9. Life cycle distribution of Cal-

anus finmarchicus in the northeasternNorwegian Sea and the western BarentsSea: a conceptual figure showing thepathways between the oceanic and shelfsystems. See text for explanations. Thecruise track for 2002 is superimposed as ablack stippled line.



nature of the northward flowing Norwegian AtlanticCurrent and the strong topographic influence of theflow regime in the area. If these mesoscale featuresenhance the survival of C. finmarchicus during winter,the impact of ocean climate as well as more regionaland local effects on the creation and persistence ofthese eddies are likely to influence the Calanusabundance in the following spring and summer. Thisimplies that the explanation for the large inter-annual variations in the abundance of C. finmarchciusin the Barents Sea is probably far too complicated tobe well represented by the inflow of Atlantic watermasses alone.

ACKNOWLEDGEMENTS

This work was funded by the Norwegian ResearchCouncil project number 133904/120, Plankton Cli-matology in the Barents Sea (PCB).

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Habitat selection of overwintering Calanus finmarchicus in the NE Norwegian Sea and shelf waters off...

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