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Variations in Minke Whale (Balaenoptera acutorostrata)Diet and Body Condition in Response to EcosystemChanges in the Barents SeaTore Haug a; Ulf Lindstrøm a; Kjell T. Nilssen aa Norwegian Institute of Fisheries and Aquaculture, NO-9291, Tromsø, Norway.

To link to this article: DOI: 10.1080/0036482021000155715URL: http://dx.doi.org/10.1080/0036482021000155715

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Variations in minke whale (Balaenoptera acutorostrata) diet and body conditionin response to ecosystem changes in the Barents Sea

Tore Haug, Ulf Lindstrøm & Kjell T. Nilssen

Haug T, Lindstrøm U, Nilssen KT. 2002. Variations in minke whale (Balaenoptera acutorostrata) dietand body condition in response to ecosystem changes in the Barents Sea. Sarsia 87:409–422.

SARSIA

Substantial changes have occurred in the Barents Sea ecosystem over the past 30 years, the mostconspicuous being related to the rise and fall of stocks of the two dominant pelagic shoaling fishspecies: capelin (Mallotus villosus) and herring (Clupea harengus). Based on data from annual studies,the effects of these ecological changes on the diet and food consumption of the minke whale(Balaenoptera acutorostrata), an important top predator in the system, were assessed for the period1992–1999. Following a collapse in the capelin stock in 1992/1993, minke whales foraging in thenorthern Barents Sea apparently switched from a capelin-dominated diet to a diet almost completelycomprised of krill (Thysanoessa sp. and Meganyctiphanes norvegica). The southern region of theBarents Sea represents important nursery areas for the Norwegian spring-spawning herring. Goodrecruitment to this stock gives strong cohorts and large numbers of young, immature herring (0–3 yearsold) which serve as the main food for minke whales feeding in the area. Recruitment failure withsubsequent weak cohorts seems, however, to reduce the availability of immature herring to such anextent that minke whales switch to other prey items such as krill, capelin and, to some extent, gadoidfish. The annual changes in prey abundance and body condition, measured as a girth and blubber index,were weakly correlated. However, the comparison of minke whales’ body condition in years of highand low abundance of immature herring showed that minke whales in the southern Barents Sea,particularly immatures and adult females, were in significantly better condition in years when theabundance of immature herring was high.

Tore Haug*, Ulf Lindstrøm & Kjell T. Nilssen, Norwegian Institute of Fisheries and Aquaculture, NO-9291 Tromsø, Norway.E-mail: tore.haug@fiskforsk.norut.no

*Corresponding author

Keywords: Barents Sea; minke whales; diet; body condition; ecological variations

INTRODUCTION

Minke whales (Balaenoptera acutorostrata) in thenortheast Atlantic have a boreo-arctic distribution withmigrations to feeding areas in the north in spring andearly summer, and southwards to breeding areas in theautumn (Jonsgard 1966). During their northward migra-tion the whales appear to be segregated by sex and size.Adult females and juveniles usually inhabit more coastalareas, whereas adult males tend to stay in more openwaters. It is well known that the Barents Sea is animportant feeding area for the northeast Atlantic stock ofminke whales (e.g. Jonsgard 1951; Haug & al. 1996).

The Barents Sea, a shallow continental shelf sea ofhigh productivity and low biological diversity (Hamre1994), serves as a nursery area for several fish stocksspawning at the western and northwestern coast ofNorway. Among these are herring (Clupea harengus)and the gadoids cod (Gadus morhua), haddock (Mela-nogrammus aeglefinus) and saithe (Pollachius virens)

(Bergstad & al. 1987; Gjøsæter 1995; Dragesund & al.1997). The adult stocks of cod and haddock reside in theBarents Sea in the feeding and wintering periods, butmove out of the Barents Sea and further south along theNorwegian coast to spawn in spring, whereas capelin(Mallotus villosus) and polar cod (Boreogadus saida)spend their entire lifecycle in the Barents Sea. The twolatter species constitute the main plankton feeders alongwith immature herring (Gjøsæter 1995), while cod,haddock and saithe feed at a higher trophic level,primarily on fish, which include these planktivores(Bergstad & al. 1987).

The Barents Sea ecosystem has undergone substan-tial changes over the past 30 years. The mostconspicuous ones relate to the collapses and subsequentpermanent or intermediate rises of stocks of theNorwegian spring-spawning herring and Barents Seacapelin (Røttingen 1990, 1992; Hamre 1994; Gjøsæter1995, 1998; Gjøsæter & al. 1998; Dragesund & al.1997; Anonymous 2000).

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After the collapse in the late 1960s, the Norwegianspring-spawning herring stock started to increaseslowly. Since 1988, when the major part of the strong1983 year class spawned for the first time, the southernBarents Sea has served as the main nursery area forimmature herring (Røttingen 1990; Hamre 1994;Gjøsæter 1995; Dragesund & al. 1997). Good recruit-ment to this stock gives strong cohorts and a largeabundance of immature (0–3 years old) herring in thearea. Upon attainment of maturity, however, the herringmigrate westwards out of the Barents Sea. Recruitmentfailure with subsequent weak cohorts or low larvalsurvival will, therefore, reduce the abundance of youngherring in the southern Barents Sea. Considerablevariation in herring recruitment has been observedduring the 1990s, with particularly strong cohorts beingproduced in 1990–1992 and in 1998 (Anonymous 2000;Lindstrøm & al. 2002).

The Barents Sea capelin stock collapsed in 1985/1986, giving a minimum size of stock biomass in 1987.However, in 1989 spawning was successful with asubsequent rapid recovery of the stock. A new dramaticdecline in stock size followed during the winter of1992/1993, with a subsequent stock biomass minimumin 1995 (Hamre 1994; Gjøsæter 1995, 1998; Gjøsæter& al. 1998; Anonymous 2000). However, reducedlarval mortality after 1995 and very good year classesin 1997–1999 allowed a rebuilding of the stock after1995 (Anonymous 2000).

Krill of the genus Thysanoessa is a key food

organism for many commercially and ecologicallyimportant Barents Sea fish species, including herringand capelin. In fact, it appears that there is a predator–prey interrelationship between capelin and krill in theBarents Sea where krill populations to a large extentseem to be controlled by predation (Dalpadado &Skjoldal 1996). There was a marked increase inzooplankton biomass during the period 1991–1994,which gave a standing stock at a rather high level andsubsequent prospects for good feeding conditions forplanktivorous predators in the Barents Sea (Anonymous2000). In 1995–1999, however, a more or lesscontinuous decrease in zooplankton biomass prevailed.

Krill, herring and capelin are known to be importantprey for northeast Atlantic minke whales Balaenopteraacutorostrata (Haug & al. 1995a, 1995b, 1996; Folkow& al. 2000; Lindstrøm & Haug 2001; Lindstrøm & al.2002). The observed stock changes of these prey itemsare, therefore, likely to have had an impact on thefeeding habits of the whales. The scientific (1992–1994) and commercial whaling operations in 1993–1999 have provided a time series of minke whale dietand body condition data which may serve as a tool toassess the effect of recent ecosystem changes on thefeeding ecology of minke whales in the Barents Sea.The purpose of this study was to address this particularquestion by comparing variations in whale data withvariations in the abundance of actual prey species. Theminke whale diet data have previously been used toassess spatial and seasonal diet variations (Haug & al.

Fig. 1. Catch positions of minke whales sampled during the Norwegian scientific (1992–1994) and traditional (1993–1999)whaling operations in the northeast Atlantic in the period 1992–1999.

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1995a, 1995b, 1996), foraging behaviour (Haug & al.1997), prey selectivity (Haug & al. 1996; Skaug & al.1997; Harbitz & Lindstrøm 2001; Lindstrøm & Haug2001), the particular impact of minke whale predationupon adolescent Norwegian spring-spawning herring(Lindstrøm & al. 2002) and the total annual consump-tion by minke whales in the northeast Atlantic in theperiod 1992–1995 (Folkow & al. 2000).

MATERIAL AND METHODS

PREY AVAILABILITY IN THE STUDY PERIOD

The given abundances of immature herring and capelinin the Barents Sea in the period 1992–1999 were basedon information given in Anonymous (2000), while thecodfish abundance (cod, haddock and saithe) in thesame period was taken from Bogstad & al. (2000).

SAMPLING REGIME OF WHALES

In 1993, the International Whaling Commissiondecided that the so-called “small areas” boundariesshould be retained for the management of northeastAtlantic minke whales (Anonymous 1993). Two ofthese areas, EB (coast of Finnmark and the southern andeastern Barents Sea) and ES (Bear Island and Spitsber-gen waters), contain the entire Barents Sea (see Fig. 1).The whale data included in this study were collectedduring scientific (1992–1994) and commercial (1993–1999) whaling in the period 1 May to 1 August. Whalestaken in scientific whaling operations were sampledrandomly along predetermined transects (see Haug &al. 1996), whereas the whales taken in commercialwhaling operations were sampled opportunistically inareas with expected high densities of whales (seeChristensen & Øien 1990).

The scientific whaling showed pronounced differ-ences between areas EB and ES with respect to dietcomposition, and also sex composition (Haug & al.1996, 1997). Therefore, the two areas were treated

separately. In total, 169 and 120 stomach contentsamples were collected in the periods 1992–1999 and1992–1997 in EB and ES, respectively (Table 1).Morphometric measurements were, however, made ofall whales taken in both areas (1504 in EB, 654 in ES)during the entire period (Table 1).

ANALYSIS OF BODY CONDITION

Immediately after death, the whales were taken onboardand hauled across the fore-deck of the boat. Total bodylength was measured in a straight line from the tip of theupper jaw to the apex of the tail fluke notch; girth wasmeasured right behind the flipper; and blubber thick-ness was measured at two sites, dorsally behind theblowhole and laterally just above the centre of theflipper. Blubber measurements were made perpendicu-lar from the skin surface to the muscle–connectivetissue interface. Length and girth measurements weremade to the nearest centimetre, while blubber measure-ments were to the nearest millimetre. The abovemorphometric measurements are identical to thosereferred to as G1 (girth) and D8/L5 (blubber) by Næss& al. (1998). In the analyses, the girth measurementswere expressed as a girth index (residuals from thegirth–length regression), while the blubber index was acombination of the two blubber measurements. Thelatter index was achieved by factorization (a linearmethod to reduce variables; Manly 1994) of the twoblubber measurements. Prior to factorization, the lengtheffect was removed from both blubber measurementsbecause these measurements tend to increase exponen-tially with the total length of the whales.

Minke whales generally become sexually mature at alength of approximately 7 m (Christensen 1981). There-fore, we defined juveniles as animals <7 m (both sexes)and adults as animals �7 m. In order to obtain enoughsamples from each reproductive class, the conditionmeasurements were analysed for one period (1 May to30 June). Adult males were excluded from the body

Table 1. Sample sizes of minke whales by area (ES and EB, see text for explanation) and year. The numbers of whales used in dietstudies and body condition studies are given separately.

Number of whales sampled

1992 1993 1994 1995 1996 1997 1998 1999

Diet studiesES 31 18 18 16 19 18 – –EB 31 5 16 16 28 22 28 23

Body condition studiesES 35 48 34 60 112 125 128 112EB 39 107 184 118 203 282 281 290

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condition analysis in the ES area due to few samples.Additionally, all adult females were checked forpregnancy.

Because different people (25–35 vessels wereinvolved in the hunt each year) made the morphometricmeasurements, the consistency of the data had to beassessed. This was achieved by randomly selecting sixindependent vessels from the same year and period, andtesting whether the slope of the girth index relationshipand blubber index as a function of date were similaramong the six vessels. This was tested by treating eachvessel as a dummy variable and date as the independentvariable. The result from this analysis showed nostatistical difference between the six vessels withrespect to these measurements (girth index:F5,80 = 0.945, p � 0.05; blubber index: F5,80 = 0.711,p � 0.05). Additionally, it was tested whether thedegree of stomach fullness (0–20, 20–40, 40–60,60–80 and �80%) affected the girth measurement.The result showed that the girth measurement was notsignificantly affected by stomach fullness(F4,271 = 0.252, p = 0.91). As a result of the latterfinding, stomach fullness was not taken into considera-tion for the girth index analysis.

To test whether the more substantial changes inresource abundance had any significant effect on bodycondition, possible changes in girth index were investi-gated for two selected resource abundance situations: (1)high and low abundance of immature herring in the EBarea, and (2) high and low abundance of capelin in theBarents Sea. To test if high and low abundance ofimmature herring had any effect on the body condition ofthe whales, the girth indices in 1993, 1994 and 1999 werepooled and compared with the pooled girth index from1995–1998. For the second analysis, i.e. to test whetherhigh and low capelin abundance had any effect on theminke whale body condition, the girth index of adultfemales in the ES area from 1992 (not presented) wascompared with the girth index from 1993–1997 pooled.

ANALYSIS OF STOMACH CONTENTS

The complete digestive tract was taken out of eachexamined whale as soon as possible (1–3 h post-mortem). Minke whale stomachs consist of a series offour chambers (Olsen & al. 1994), but Lindstrøm & al.(1997) have shown that sampling and analyses ofcontents from the first chamber (the fore-stomach) issufficient to describe the minke whale diet. Thus, onlyfore-stomach contents were used in these analyses. Theonboard and laboratory treatment of the fore-stomachcontents was as described in detail by Haug & al.(1995a, 1996).

Intact specimens of fish were identified according togross morphological characteristics (Pethon 1985),while sagittal otoliths and crustaceans were identifiedto the lowest possible taxon (Enkell 1980; Breiby 1985;Harkonen 1986). The total number of each fish specieswas calculated by adding the number of fresh speci-mens, intact skulls and half the number of “free”otoliths. For large gadoids, such as cod, haddock andsaithe, free otoliths were separated into right and leftotoliths and divided into geometrical classes (with0.2 mm intervals), whereas otoliths present in skullswere removed and measured. Random subsamples of200 undigested otoliths (or as many as possible) fromeach fish species were used to calculate the initial preymasses at the time of ingestion in each stomach.Available relationships between otolith lengths and fishlengths/weights (e.g. Harkonen 1986) were used in thecalculations.

Estimation of crustacean biomass at the time ofingestion is a problem when reconstructing the fore-stomach contents of minke whales, not only becausethey lack hard parts that are resistant to the fore-stomach microbes (Nordøy & al. 1993; Olsen & al.1994), but also due to passage and degradation rates thatmost likely differ from those of other prey types.Therefore, the in situ biomass of crustaceans was usedin the reconstructions involving this prey item.

Several feeding indices are commonly used instomach analyses of top predators (Hyslop 1980; Pierce& Boyle 1991), but in this study the relative contribu-tion of each prey species (krill, herring, capelin, cod,haddock, saithe and sandeel Ammodytes sp.) to the totaldiet was expressed as: Bi = (bi/bt) � 100, where bi is thetotal mass of prey category i, and bt is the total biomassof all prey categories.

Prior to all statistical analyses, the diet data weredivided by whale length (m) and square root trans-formed in order to reduce the importance of largestomachs.

STATISTICAL FRAMEWORK

Minke whale diets are known to vary considerably, bothin time and space (e.g. Haug & al. 1995a, 1995b). Inorder to illustrate and better understand the mechanismsbehind this temporal and spatial variation in dietcomposition, it is necessary to study simultaneouslythe effect of various environmental variables on minkewhale diets. Multivariate statistics, both indirect anddirect gradient analyses (Legendre & Legendre 1998),were therefore applied. Indirect gradient analysis wasused to illustrate the major patterns of variation in dietcomposition in relation to the environmental variables,

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while direct gradient analysis was used to extract themajor environmental variables responsible for thevariation in diet composition. The environmentalvariables in this study were a mixture of spatial (latitudeand longitude), temporal (date and year) and biotic(whale length and sex) variables, where sex was codedusing two binary dummy variables (female and male).

A principal component analysis (PCA) was used toordinate the transformed whale diet data along the firsttwo axes of variation. The site scores, i.e. the meansample scores, were calculated from each area and yearand then plotted along with the environmental vari-ables. In order to choose the optimum model todescribe the response of the diet composition tomeasured environmental variables, the eigenvaluesfrom a redundancy analysis were compared with theeigenvalues from a canonical correspondence analysis(CCA) to see if the prey species displayed a linear or

unimodal response to the environmental gradients(Legendre & Anderson 1999; Corfield 2000). Theeigenvalues derived from these analyses (not presentedhere) indicated that minke whale diets displayed apredominately unimodal response to the major envir-onmental variables, hence a CCA was used. Eachenvironmental variable was tested, by forward selec-tion and 1000 Monte Carlo permutations. Only vari-ables significant at the 0.1% level (p = 0.001) wereconsidered significant.

In order to construct 95% confidence intervals for therelative importance of each prey group, the diet datawere bootstrapped 5000 times. All confidence intervalswere corrected for possible acceleration and bias (seeEfron & Tibshirani 1993). Similar bootstrap techniqueshave proved useful in other analyses of predator diets(see Jiang & Jørgensen 1996; Lawson & Stenson 1997;Lindstrøm & al. 1998; Lindstrøm & Haug 2001).

Fig. 2. Principal component analysis (PCA) ordination plot of axes 1 and 2, with environmental variables (arrows), prey species(�) and sampling sites of areas EB (�) and ES (�). The first two axes account for 30 and 25% of the variation in the species data,respectively.

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RESULTS

VARIATION IN WHALE DIETS

The site (mean sample scores) and species scores of thePCA ordination plot by the first two axes (Fig. 2)explained, respectively, 30 and 25% of the variation indiet composition. Four defined species groups wereidentified: (1) krill, (2) capelin, (3) herring and (4) cod,haddock, saithe and sandeel. The three major preygroups (krill, capelin and herring) were well separated,implying that these prey species seldom coexist in thediet, while the fourth group was located near the originof the plot.

The whales from the ES area were strongly asso-ciated with krill, particularly in the period 1995–1997.The mean whale scores from the EB area were,however, widely scattered on the right-hand side ofthe diagram and more associated with various fishspecies. In 1993–1994, the whales were stronglyassociated with herring, while in 1995, 1997, 1998and 1999, the whales appeared to have fed on a mixtureof several prey species. The whales sampled in areas ESin 1992 (es92) and EB in 1996 (eb96) had a strongerassociation with capelin than the other whales.

Effects of latitude and longitude appear to benegatively correlated with each other and more stronglyassociated with diet composition compared with theother environmental variables. The importance of krillseems to increase with latitude and decrease with

longitude while the opposite applies for fish. Addition-ally, males tend to feed more intensively on herringcompared with females. This was also confirmed by thedietary comparison between sexes in pooled samplesfrom the EB area (Fig. 3) where males had asignificantly higher preference for herring comparedwith females. On the other hand, females appear to feedmore intensively on capelin and saithe. The CCAconfirmed the major patterns of the PCA, and fourenvironmental variables (latitude, longitude, date andsex) were retained by the forward selection procedure(Table 2). Latitude and longitude were the mostimportant predictors. Furthermore, the CCA revealedthat the major environmental variables explained only asmall fraction (15.9%) of the total variation in dietcomposition.

WHALE DIETS AND PREY AVAILABILITY

Figure 4 illustrates the relative importance of fourminke whale prey types (left) and the abundance ofthree prey types (right) in the Barents Sea ecosystem in1992–1999.

The importance of krill in the whale diets has showna monotonically increasing trend in both areas, with theexception of a decline in 1999 in the EB area (Fig. 4).Krill was significantly more important in the ES areacompared with the EB area, which is evident from theconsistently non-overlapping error bars (95% confi-dence interval). Unfortunately, due to the lack ofquantitative data on krill biomass in the sea, it wasnot possible to make predator–prey comparisons.

Minke whale predation on herring varied consider-ably in the EB area in 1992–1999, from Ca 42–73% in1992–1994 to 2% in 1996 (Fig. 4). With the exceptionof 1997, the dietary importance of herring showed ahigh correlation with the abundance of immatureherring in the sea (Spearman’s rank correlation coeffi-cient rs = 0.92).

Fig. 3. Relative abundance of seven diet items in male (–) andfemale (�) minke whales sampled in the EB area in thenortheast Atlantic in the period 1992–1999. Bars indicate 95%confidence intervals as estimated by bootstrapping of theoriginal data. herr – Herring; cap – Capelin; had – Haddock;sai – Saithe; sand – Sandeel.

Table 2. Test statistics for the canonical correspondenceanalysis (CCA) of whale diets and significant environmentalvariables (p = 0.001) retained by the forward selection proce-dure.

Predictors Fit F ratio

Latitude 0.31 19.25Longitude 0.11 10.40Date 0.07 7.08Sex 0.04 4.07

EigenvaluesConstrained 0.53Unconstrained 3.34

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Fig. 4. Relative importance (left panels) of four minke whale prey types in two areas in the northeast Atlantic (stippled lines – ES;solid lines – EB) and the abundance (right panels) of three prey types in the sea in the period 1992–1999. The abundances ofimmature herring and capelin (total stock) were based on data given in Anonymous (2000), while the codfish abundance was takenfrom Bogstad & al. (2000). Error bars indicate 95% confidence intervals as estimated by bootstrapping of the original data.

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Figure 4 shows the considerable spatial and temporalvariation in capelin importance in the whale diets. Thedietary importance of capelin in the ES area correlatedwell with the capelin abundance in the sea until 1995when the increase in capelin abundance was notobserved in the diet (rs = 0.60). In contrast, the dietaryimportance of capelin in the EB area appeareduncorrelated with the capelin abundance in the sea in1992–1996, but since 1997 the importance of capelin inthe whale diets has corresponded more with the increasein capelin abundance in the sea (rs = 0.02).

Minke whale predation on codfish (cod, haddock andsaithe) showed large variation in space and time (Fig.4). In the ES area, this prey group contributed very littleto the diet, with the exception of 1993 and 1994 (Ca34%). When comparing the dietary importance ofcodfish with their abundance in the sea, no clearrelationship appeared (ES: rs = 0.60; EB: rs = 0.62).

BODY CONDITION: GIRTH AND BLUBBER INDICES

In all ANCOVA tests performed to compare reproduc-tive classes with respect to girth index, the variablesyear, date, and pregnancy were treated as covariates,while pregnancy was excluded as a covariate when thetests were performed with respect to the blubber index.

The three reproductive classes of minke whale(immature, adult male and adult female) showed largeannual variation in girth index within and between areasin May–June (upper panels, Fig. 5). The girth indexvaried most significantly in the whales from the EBarea, while the variation in the whales from the ES areawas less pronounced (Table 3). The most conspicuousvariation was observed in adult females in the EB area,varying from Ca 2 in 1993–1994 to �0.4 in 1995,followed by adult males (from Ca �1.8 to 0) andimmatures (from Ca 0.9 to �0.8). Adult females and

Fig. 5. Annual changes in minke whale girth (upper panels) and blubber (lower panels) indices by reproductive class from subareasEB and ES in the Barents Sea. Dotted lines indicate immatures (<7 m), dashed lines are adult males and solid lines are adultfemales. Error bars indicate 95% confidence intervals. The condition indices have been corrected for the date effect.

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immatures displayed similar trends in annual bodycondition, while adult males varied differently (Fig. 5).There was a significant difference in the average girthindex between immatures, adult males and females inthe EB area (–0.09 versus �1.02 versus �0.08;ANCOVA, F2,1129 = 7.44, p < 0.01). The average adultmale had a significantly poorer condition (–1.02)compared with an average adult female (–0.08; ANCO-VA, F1,840 = 13.68, p < 0.001) and immature (–0.09;F1,496 = 8.41, p < 0.01), while there was no significantdifference in condition between the two latter repro-ductive classes (ANCOVA, F1,914 = 0.03, p = 0.87).Additionally, the average adult female from the EBarea was in significantly better condition than acounterpart in the ES area (–0.05 versus �0.40;ANCOVA, F1,1115 = 4.68, p = 0.03).

The second condition index (blubber index) dis-played a rather different condition pattern comparedwith the girth index (lower panels, Fig. 5). Theimmatures, adult males and females in the EB areademonstrated almost identical annual trends in blubber

index, with a peak in 1993 (0.20, �0.20 and 0.55)decreasing to their lowest condition level in 1995(–0.47, �0.72 and �0.32) until their condition roseagain. The average condition indices of adult femaleswere consistently higher than those of immatures andadult males, except for immatures in 1996 (Fig. 5). Thiswas also confirmed by an ANCOVA test(F2,1116 = 19.43, p < 0.01). There was, however, nosignificant difference between adult females andimmatures (ANCOVA, F1,903 = 2.00, p = 0.16). Thelatter was also confirmed in the ES area (ANCOVA,F1,522 = 0.08, p = 0.78), despite the fact that adultfemales in the ES area were on average significantlyfatter than their counterparts in the EB area (0.79 versus0.08; ANCOVA, F1,1094 = 9.05, p < 0.01). The averageblubber indices given along with the test statistics inparentheses are corrected for date but not pregnancy.

An analysis designed to test how the abundance ofimmature herring might influence the whale bodycondition showed that the girth index for both im-matures (0.49 versus �0.70; ANCOVA, F1,81 = 7.79,

Table 3. Annual variation in minke whale body condition (measured as girth and blubber indices) with respect to threereproductive classes in two areas (EB and ES) in the northeast Atlantic (1992–1999). The table shows the results from ANCOVAwith date and pregnancy as covariates, year as the main effect, and girth and fat indices as dependent variables. Pregnancy was onlyincluded as a covariate with respect to the girth index analysis.

EB ESDependent variable Source of variation d.f. F ratio d.f. F ratio

Girth indexAdult females

Year 6 5.78** 6 1.67 nsDate 1 12.56** 1 10.62**Pregnant 1 8.06** 1 12.93**Residual 625 482

Adult malesYear 6 2.85*Date 1 10.28**Residual 208

ImmaturesYear 6 2.15* 6 1.50 nsDate 1 0.28 ns 1 0.28 nsResidual 282 48

Blubber indexAdult females

Year 6 4.04** 6 4.34**Date 1 0.54 ns 1 3.36 nsResidual 626 468

Adult malesYear 6 0.61 nsDate 1 6.48*Residual 206

ImmaturesYear 6 2.29* 6 0.71 nsDate 1 0.87 ns 1 3.87 nsResiduals 278 47

**p < 0.01; *p < 0.05; ns, not significant.

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p < 0.01) and adult females (1.14 versus �0.36;ANCOVA, F1,81 = 31.04, p < 0.01) was significantlyhigher when the abundance of immature herring washigh (1993, 1994 and 1999) compared with when it waslow (1995–1998). Adult males also showed a lowergirth index in years when the abundance of immatureherring was low, but this was not significant (–0.65versus �1.28; ANCOVA, F1,213 = 3.57, p = 0.06).Results from an analysis designed to test the effect onwhale condition of capelin abundance, in which thegirth index of adult females was compared between1992 (not presented here) and 1993–1997, showed thatadult females were in similar condition in the twoperiods (–0.61 versus �0.16; ANCOVA, F1,354 = 0.96,p = 0.33). The average girth indices given along withthe test statistics in parentheses are adjusted for date andpregnancy.

DISCUSSION

During the scientific whaling operations in 1992–1994,substantial spatial and temporal heterogeneity wasobserved in minke whale feeding habits in the BarentsSea area (Haug & al. 1995b, 1996). This study confirmsthat this situation has not changed since 1994, withwhales taken in the northern area (ES) having a dietconsiderably different from those taken further south(in the EB area). The significant spatiotemporal varia-tion in minke whale feeding habits observed indicatesthat minke whale feeding habits are affected by theecosystem changes that occurred in the Barents Sea inthe period 1992–1999. This was also confirmed by theCCA, which showed that the geographical and bioticvariables explained only a small fraction of thevariation in diet composition.

The recent prominent role of krill in the northern areaseems consistent with the current status of the BarentsSea ecosystem which, since 1992, has been character-ized by relatively large amounts of zooplankton and avery low abundance of capelin (Anonymous 2000).Although krill has been shown to play an important rolein minke whale diets in the ES area in previous years(Jonsgard 1951, 1982; Nordøy & Blix 1992), it isknown that capelin, if abundant in sufficient amounts,may be the most important dietary constituent as, e.g.observed in 1992. After the collapse of the capelin stockin 1992/1993 (Hamre 1994; Gjøsæter 1995, 1998;Gjøsæter & al. 1998), capelin was apparently easilyreplaced by krill in the minke whale diet. The BarentsSea capelin stock biomass has rebuilt slowly after the1995 minimum level: in 1997 it was close to the 1993level, and in 1999 it had attained a little less than half ofthe biomass observed in 1992 (Anonymous 2000).

Apparently, the rebuilding of the stock had notproceeded far enough in 1997 to make capelin aninteresting food item for the minke whales in area ES in1997.

The mixed fish diet documented in the EB areaduring the scientific whaling period (1992–1994) wasalso maintained in 1995–1999. However, in the latterperiod with significant contributions from krill. Therelative importance of the various fish species seems tohave changed considerably since 1994. This applies inparticular to herring, which was the most importantfood constituent for the whales in this area in 1992–1994. The herring component was greatly reduced in1995 and absent in 1996. Herring occurred again on thewhale menu in 1997–1999, but never at the same levelsas in 1992–1994. Interestingly, the observed patternsseem to relate closely to the fate of the herring stock inthe area in the same period. Improvements in recruit-ment of Norwegian spring-spawning herring from 1988onwards have increased the abundance of immatureherring in the minke whale feeding areas in the southernBarents Sea (EB), and the strong 1990–1992 cohorts(Anonymous 2000) explain the dominant role ofimmature herring in the minke whale diet in 1992–1994 (see Haug & al. 1996). However, during 1995most of the herring of the 1990–1992 cohorts hadmigrated westwards out of the Barents Sea, and as the1993–1997 cohorts were all weak (Anonymous 2000),the availability of adolescent herring in the Barents Seawas low in the whole period 1995–1998. In 1999, due tothe strong 1998 cohort (Anonymous 2000), a markedincrease in herring availability occurred in the area.Summer predation by minke whales upon herring hasbeen observed in coastal areas of north Norway inprevious years (Jonsgard 1951, 1982; Lydersen & al.1991; Nordøy & Blix 1992). Lindstrøm & al. (2002)suggested that minke whales’ exploitation of herringmight increase in a non-linear relationship with theabundance of herring in the sea.

Evidently, the reduction in herring availabilitycaused minke whales to broaden their diet and to feedmore intensively on other less abundant species such ascapelin and krill. This is in accordance with optimalforaging theory (Stephens & Krebs 1986; Hughes1993), which in contrast to the competition-based view,predicts that foragers will broaden their diets when theabundance of favourable prey (capelin and herring)declines. Possibly, the habitat quality of the Barents Seaecosystem, with respect to minke whales, may havebeen reduced in 1995, when the major part of the strong1992 year class of herring migrated out of the BarentsSea. Results from the sighting surveys indicated areduced abundance of minke whales in the southern

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Barents Sea in 1995 as compared with 1989 (Schweder& al. 1997). While krill was scarce in the diets of minkewhales in the southern coastal areas of the Barents Seaand north Norway in 1992–1994, it contributedsignificantly to the EB area diet in the years when theavailability of capelin and herring was low (1995–1997). Krill has also been reported as important minkewhale food in the southern Barents Sea in previousyears, e.g. 1972–1973 (Christensen 1972, 1974). Thesewere years when the herring stock had very recentlycollapsed and the abundance of this species wasextremely low (Røttingen 1990). Even though minkewhales are rather euryphagous and flexible in theirchoice of prey, the observations made in areas ES andEB may indicate that krill represent an important foodalternative for the whales in periods when their morepreferred food, such as capelin and herring (see Haug &al. 1996; Skaug & al. 1997; Lindstrøm & Haug 2001),are less available. The suggested predator–prey inter-relationship between capelin and krill, i.e. that thereduction in capelin abundance is more or less auto-matically followed by a subsequent increase in krillabundance (Dalpadado & Skjoldal 1996), may explainthe enhanced whale predation on krill in the period1994–1998.

The reduced contribution from krill to the whale dietin 1999 seems logical, given the observed reduction inkrill biomass (Anonymous 2000) and the simultaneousincrease in availability of both capelin and herring inthis year. Given the lack of capelin in the whale diet inthe ES area in the whole period 1994–1997, thereappearance of capelin as a major food component inthe EB area after 1994 is interesting. Despite the lowavailability of capelin, particularly in 1995–1997(Anonymous 2000), this prey item contributed signifi-cantly to the EB whale diet in the whole period 1995–1999. The fact that immature herring was close toabsence in 1995–1998 is probably a contributory factor.Piatt & Methven (1992) suggested that northwesternAtlantic minke whales exhibit a sigmoidal response toincreasing capelin abundance, i.e. capelin is not preyedupon below a certain minimum density level, which isprobably influenced by the availability of other pre-ferred food items such as herring. This may explain whycapelin was eaten in significant amounts in a periodwith low capelin biomass and simultaneous lowimmature herring availability (1995–1998), whereas itwas almost absent from the whale menu in the yearswhen herring was abundant.

With the exception of the scientific catch activities in1992, all of the whales were taken in the NorwegianEconomic Zone (NEZ), i.e. in the western parts of theBarents Sea. Both capelin and immature herring have a

distribution that extends more eastwards, usually farinto the Russian Economic Zone (REZ) of the BarentsSea (Hamre 1994; Gjøsæter 1995, 1998; Lindstrøm &al. 2002). Minke whales are also known to be numerousin REZ (Øien & al. 1987; Schweder & al. 1997), andhence, sampling whales on these important feedinggrounds (e.g. along the Murman coast in Russia) wouldprobably have given a different diet composition in thisarea.

The present investigation clearly reveals the dynamicnature of minke whale feeding habits in the Barents Seaecosystem. With the rather large changes observedthroughout the study period 1992–1999, it appearsadequate to ask whether the different prey itemsinvolved were equally efficient in filling the energyrequirements of the whales, or were the diet shiftsaccompanied by variations in the general body condi-tion of the whales? Changes in body fattening, whichcould be related to food availability, were observed infin whales Balaenoptera physalus in Icelandic watersbetween 1977 and 1982 (Lockyer 1986) and in minkewhales in the Antarctic between 1990 and 1995 (Ichii &al. 1998). The present study does not seem to be equallyconclusive with respect to the significant year-to-yearvariations. However, the comparison of minke whalebody condition in years of high and low abundance ofimmature herring showed that minke whales in the EBarea, particularly immatures and adult females, were insignificantly better condition in the years when theabundance of immature herring was high. No suchtendency was, however, observed in the years of lowand high capelin abundance with respect to adultfemales in the ES area, implying that minke whalesare able to compensate sufficiently with krill. Thesefindings further reinforce the presumption of a generallower habitat quality of the southern Barents Sea in1995–1997, a period of low abundance of immatureherring and capelin, implying that the low abundance ofimmature herring probably did not single-handedlycontribute to the observed low body condition of minkewhales in 1995–1998. Thus, it appears that the general-istic foraging behaviour of minke whales may, to someextent, compensate for changes in food availabilitysimply by switching diets, and they probably do thisbetter than fin and sei whales Balaenoptera borealiswhich are more stenophagous krill lovers. Under suchcircumstances, however, the minke whales have toswitch to less favourable prey with the consequence ofpoorer condition. The extreme is the Antarctic minkewhale that has nothing to switch to when krill becomesless available (see Ichii & Kato 1991).

There was a significant seasonal increase in the girthindex with respect to adult males and females, while

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immature animals did not display any seasonal varia-tion in girth in May–June. Seasonal increases in bothgirth and blubber measurements have also beenobserved in North Atlantic fin and sei whales (Lockyer& al. 1985; Vikingsson 1990, 1995). Previous studies ofnortheast Atlantic minke whales have demonstratedsignificant seasonal variations in dorsal and lateralblubber thickness measurements, but the major part ofthe fat deposition occurred between summer andautumn, whereas no seasonal changes were observedearly in the season (between spring and summer; Næss& al. 1998). The blubber measurements, and hence theblubber index, may therefore have been made too earlyto assess the seasonal increase.

Certainly, the seasonal fattening demonstrates thegeneral tendency of baleen whales to increase their fatreserves during feeding periods, and thereby storingenergy reserves for wintering and breeding at lowerlatitudes when the feeding activity is probably reduced.Interestingly, immatures and adult females showedsimilar annual patterns in body condition and weresignificantly fatter than adult males. This is partlyconsistent with observations made on North Atlantic finand sei whales by Lockyer & al. (1986, 1987), whofound that adult females had significantly better bodycondition than immatures and adult males. It wassuggested that this could be a result of heterogeneous

physiological abilities among sexual classes to depositenergy and/or due to different arrival patterns ofreproductive classes to the feeding grounds. If the latteris true it would require that the effect of date had to becontrolled for in the body condition analysis. Fromprevious catch data it is evident that the sexes tend to besegregated when minke whales migrate to their feedinggrounds in the northeast Atlantic (Jonsgard 1951; Øien1988). The present observations of fat immatures andadult females may present an indication that they arriveearlier at the feeding grounds than the males. Unfortu-nately, details of the migration pattern of minke whales,e.g. their arrival time to and movements on the feedinggrounds, are poorly known. If feeding occurs in otherareas, e.g. on the southern breeding grounds or en routeto the Barents Sea feeding grounds (e.g. in the Nordicand Norwegian seas), the condition indices willcertainly be affected, and interpolation based only onobservations made in the EB and ES areas in theBarents Sea will be complicated.

ACKNOWLEDGEMENTS

The ecological studies of northeast Atlantic minke whaleswere funded by the Research Council of Norway (projects108146/110 and 121385/122). We thank Raul Primicero forhelpful comments on parts of the manuscript.

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Accepted 28 September 2001 – Printed 28 February2003Editorial responsibility: Tore Høisœter

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