ORIGINAL PAPER

Distribution and diet of 0-group cod (Gadus morhua) and haddock(Melanogrammus aeglefinus) in the Barents Sea in relation to foodavailability and temperature

Padmini Dalpadado Æ Bjarte Bogstad ÆElena Eriksen Æ Laura Rey

Received: 12 November 2008 / Revised: 28 May 2009 / Accepted: 1 June 2009 / Published online: 28 June 2009

� Springer-Verlag 2009

Abstract Distribution of 0-group cod (Gadus morhua)

and haddock (Melanogrammus aeglefinus) in August–

September 2005 and 2006 was mainly restricted to the

Atlantic waters of the western and central areas of the

Barents Sea. The main distribution of 0-group fish over-

lapped largely with areas of high biomass ([7 gm-2 dry

weight) of zooplankton. The copepod Calanus finmarchi-

cus and krill Thysanoessa inermis, which are dominant

zooplankton species in both Atlantic and boreal waters of

the Barents Sea, were the main prey of 0-group cod and

haddock. The main distribution, feeding areas and prey of

0-group cod and haddock overlapped, implying that com-

petition for food may occur between the two species.

However, though their diet coincided to a certain degree,

haddock seems to prefer smaller and less mobile prey, such

as Limacina and appendicularians. As 0-group fish

increased in size, there seems to be a shift in diet, from

small copepods and towards larger prey such as krill and

fish. Overall, a largely pelagic feeding behaviour of

0-group cod and haddock was evident from this study.

Keywords 0-group � Cod � Haddock � Diet �Zooplankton � Trophic interactions � Barents Sea

Introduction

Many studies in the recent years (Falk-Petersen et al. 2007

and references therein, Wassmann 2008) indicate that the

warming climate may have large impact on the arctic

ecosystem. Warming conditions may favour organisms that

thrive in warm waters but may have adverse effects on

cold-water species. Such impacts are most likely to have

consequences on the food web as a whole. An example of

such changes is indicated by Falk-Petersen et al. (2007),

where it is suggested that a warmer climate with reduced

ice cover and new phytoplankton regimes will lead to a

shift in zooplankton size spectrum, towards a smaller

zooplankton size spectrum dominated by, e.g. C. finmar-

chicus, with lower energy content. These authors indicate

that sea birds such as little auk (Alle alle) specialised,

feeding on larger, energy-rich species, e.g. Calanus gla-

cialis and C. hyperboreus for their breeding success, may

change their ecology when they have to switch to less

energy-rich prey such as C. finmarchicus. The warmer

conditions in the Barents Sea have also led to changes in

distribution of many fish species (Stiansen and Filin 2008).

For cod, it has been shown that not only their distribution

but also their spawning sites and recruitment success is

affected by climate (Sundby and Nakken 2008). Changes in

distribution of fish species could also cause changes in

species overlap and hence predator–prey interactions.

The spawning grounds of North East Arctic cod (Gadus

morhua) and North East Arctic haddock (Melanogrammus

aeglefinus) are located along the west coast of Norway.

During spring, both the Norwegian Atlantic and the Nor-

wegian Costal Currents transport the pelagic eggs into the

Barents Sea (Sonina 1969; Dingsør et al. 2007; Sundby and

Nakken 2008). The cod spawns mainly along the Norwe-

gian coast (Lofoten and Møre Coast) from February to

April (Fig. 1). Although some overlap between cod and

haddock spawning areas exists, the main spawning of

haddock occurs in deeper waters and close to the shelf’s

edge from May to June. The major spawning areas of

haddock are located off Møre, Lofoten and Tromsø (Fig. 1)

P. Dalpadado (&) � B. Bogstad � E. Eriksen � L. Rey

Institute of Marine Research, P.O. Box 1870, Nordnes,

5817 Bergen, Norway

e-mail: padmini.dalpadado@imr.no

123

Polar Biol (2009) 32:1583–1596

DOI 10.1007/s00300-009-0657-7

(Sonina 1969). The Barents Sea is used as a nursery ground

by both species, and thus their distribution and feeding

areas may overlap to a great extent as 0-groups and as

adults. However, the distribution of cod extends farther to

the north and east than that of haddock, as cod has a lower

temperature limit than haddock (Bergstad et al. 1987).

‘‘The international 0-group survey in the Barents Sea

and adjacent waters’’ has been carried out since 1965, and

annual indices of 0-group fish have been calculated

(Nakken and Raknes 1996; Dingsør 2005; ICES 2008;

Eriksen et al. 2009). This survey is now a part of a joint

Norwegian–Russian ecosystem survey (Anonymous 2006).

The diet of 0-group cod in the Barents Sea has been

investigated both during spring and summer (Sysoeva and

Degtereva 1965; Pedersen and Fossheim 2008) and in

October–December (Ponomarenko 1983). For diet of

0-group haddock, scattered information is available from

spring/summer and from October–December (Sysoeva

1972; Baranova 1972). Little information is available for

the whole Barents Sea on the diet of 0-group cod and

haddock in August–September. Thus, a project was initi-

ated at the Institute of Marine Research, Bergen, Norway,

to study trophic interactions among 0-group cod and had-

dock in August–September 2005 and 2006. In addition to

investigating feeding interactions, we have also extended

this study to examine the distribution patterns of 0-group

cod and haddock in relation to oceanographic conditions

and zooplankton abundance.

Materials and methods

0-group cod and haddock were sampled during Joint

Norwegian–Russian ecosystem survey in the Barents Sea

Fig. 1 Schematic presentation

of the main spawning areas of

North East Arctic cod and North

East Arctic haddock. Redrawn

from Sundby and Nakken

(2008), Dingsør et al. (2007)

and Gjøsæter et al. (2008)

1584 Polar Biol (2009) 32:1583–1596

123

in August–September 2005 and 2006 (Anonymous 2005,

2006). This survey is conducted just prior to the bottom

settling of the 0-group. The samples included in our anal-

ysis are solely from the Norwegian vessels, and cod and

haddock caught by pelagic and bottom trawl was used for

stomach analysis. The reason for limiting our study to data

from Norwegian vessels is that Russian vessels only sam-

pled stomachs of cod from bottom trawl hauls, and no

stomachs from pelagic trawl hauls at all. 0-group cod and

haddock were sampled using a small mesh (8 mm mesh

size) mid-water trawl (‘‘Harstadtral’’). The trawl was towed

at three depths with the headline of the trawl positioned at

0, 20 and 40 m and a towing speed of three knots. The

duration of towing carried out at each depth strata was

10 min. Additional tows were carried out in deeper layers,

when 0-group was recorded by the echosounder at depths

[40 m. The density of 0-group cod and haddock was

calculated using standard methods described by Dingsør

(2005), Anonymous (2006) and Eriksen et al. (2009). The

number of fish per nm2 at each station is estimated using

the number caught, trawl opening of 20 or 30 m (variable

from year to year), and total distance towed, corrected for

number of depth steps. In order to calculate the total

0-group abundance in the Barents Sea, the total area is

divided into 22 subareas (strata), average densities in each

stratum are calculated and corrections for the coverage area

in each stratum are made.

Ten individuals of both cod and haddock were selected

randomly from the catch at each station for diet investi-

gations. The stomachs were then removed and individually

frozen; these were later analysed in detail in the laboratory

to identify the prey to species and size/stage when possible.

The individual prey taxa or groups were put into pre-

weighed aluminium foils and then dried at 60�C in the

laboratory for about 1 day or in the case of larger organ-

isms, e.g. large krill or fish larvae, until a constant weight

was obtained.

In addition, cod stomachs were sampled from bottom

trawl investigations by the use of a Campelen 1800 shrimp

trawl. This trawl has an 80 mm stretched mesh size in the

front, and a 22 mm mesh size in the cod end. The towing

time was 30 min. A single cod was sampled from each

5 cm length group (5–9 cm, 10–14 cm and so on) at each

station, and the 0-group cod was selected based on age

readings. The stomach content investigations of cod is a

long-term study (1984 to present), providing input to the

main joint Russian/Norwegian cod stomach data base and

includes diet information of all ages/sizes of cod (Dolgov

et al. 2007). Similar to the pelagic stomach analysis, the

stomachs of fish caught by the bottom trawls were also

frozen individually, and prey was subsequently identified

to the lowest possible taxonomic level when possible. For

practical reasons, in the bottom trawl investigations where

all size groups of cod are incorporated and thus the stom-

achs may contain large fish prey, the wet weight of prey

organisms were taken. A conversion factor of 0.2 (wet

weight/dry weight ratio of 5.0—see Skjoldal et al. 2004)

was used to convert the wet weight to dry weight.

As stomach analysis is time consuming, we had to limit

the number of samples analysed. The main focus was on

the analysis of 0-group cod stomachs. When the analysis of

haddock stomachs was carried out, only the stations that

overlapped or were within the immediate vicinity of sta-

tions with cod diet data were chosen.

For each station, the mean partial fullness index (PFI)

was calculated to compare quantities of prey groups in the

stomachs of predators of various sizes (Lilly and Fleming

1981). The PFI is based on the assumption that stomach

capacity is a power function of fish length. The index was

estimated with prey grouped by either species or species

group. The PFI of prey group i in predator k is given in

Eq. 1

PFIi;k ¼Si;k

Lkð Þ3� 104 ð1Þ

where Si,k is the weight (g) of prey species i found in the

stomach of predator k, and Lk is the length (cm) of predator

k. The total fullness index (TFI) (PFI summed for all prey)

was also calculated.

On the above-mentioned cruises of 2005 and 2006,

zooplankton was regularly sampled using WP2 and

MOCNESS plankton nets with 180 lm mesh size. The

WP2 is a simple ring net with a 56 cm diameter, and

vertical hauls were taken from bottom to surface using this

net. The MOCNESS is a 1 m2 net, which is equipped with

multiple nets (up to 8) with opening and closing devises.

The WP2 was operated more often than the MOCNESS

(ratio approximately 3–1). Only the integrated biomass

from bottom to surface was used in this study. The biomass

estimates was expressed as g dry weight (dw) per m2. For

each MOCNESS and WP2 profile, the biomass was cal-

culated using the dry weight of zooplankton organisms in

each depth stratum (applies only to MOCNESS), the

sampled depth, and the volume of water filtered. Gjøsæter

et al. (2000) have shown that although the two gears

sampled different size fractions, the total biomass sampled

by the two gears, WP2 and MOCNESS were comparable

and not significantly different. Therefore, the results of the

two gears were combined in this study in order to obtain a

more comprehensive data set of zooplankton abundance. A

CTD Sonde was regularly used throughout these cruises to

obtain salinity and temperature data. The temperature and

salinity at depths of 10, 50 and 100 m were used to classify

the zooplankton stations into different water masses using

the descriptions given by Loeng (1989, 1991) and Gjøsæter

et al. (2002).

Polar Biol (2009) 32:1583–1596 1585

123

The interpolated distribution patterns of temperature,

salinity, zooplankton and 0-group fish were visualised

using ARCGIS spatial analyst. These figures show average

values within 3 9 3 km grid cells. The interpolation was

carried out using the gridding technique inverse distance

weighting. To investigate how the distribution of cod and

haddock is associated with physical (water temperature and

salinity at 10, 50 and 100 m depth) and biological (biomass

of plankton) parameters, the fish survey area was divided

into 40 9 40 nm grid cells. Number of observed fish,

temperature, salinity and plankton biomass were assigned

to each grid cell by spatial overlay method in ‘‘Manifold

System 8.0’’. Each grid cell contains information about fish

abundance and physical and biological parameters, and

constituted the study units in this statistical analysis. Also,

mean values of the physical and biological parameters were

calculated for four categories: cells with very low (0–1,000

individuals per nm2), low (1,000–10,000 individuals per

nm2), high (10,000–100,000 individuals per nm2) and very

high (more than 100,000 individuals per nm2) abundance.

Pearson’s correlation was estimated to determine if the log

transformed fish abundance is proportional to physical and/

or biological variables.

Five individual stations were selected for the compari-

son of diet of 0-group cod and haddock. It should be noted

that in general, the 0-group haddock were larger than

0-group cod. The percentages calculated under this chapter

are based on PFI values. For each of the selected stations,

an overlap index of cod and haddock diet was calculated

using Eq. 2 (Schoener 1970):

Ro ¼ 100 1� 0:5Xn

i¼1

jpxi � pyij !

ð2Þ

where Ro is the overlap index expressed as a percentage,

and pxi and pyi are the relative proportions of each food

item i obtained for species x and y. n is the number of prey

item categories. These proportions are calculated using

mean PFI and TFI for each station and species.

Results

Distribution in relation to hydrographical and

zooplankton conditions

The distribution of 0-group cod and haddock in 2005 and

2006 shows that the main concentrations are restricted to

the Atlantic waters of the Barents Sea (Figs. 2, 3) and that

the distribution of cod and haddock overlap to a great

extent. Atlantic waters are defined as waters with tempe-

rature [3�C and salinity [35% (Loeng 1991). The

largest abundance (100,000–2,000,000 individuals nm-2)

of 0-group cod and haddock was located in central areas of

the Barents Sea. For both cod and haddock the abundances

in 2005 was higher than that of 2006.

Additionally, the temperature distributions show that the

warmer Atlantic water penetrated more north and eastward

in 2006 than in 2005 (Fig. 3). The average temperatures at

50 m were higher in 2006 (5.6�C, SD = 2.7) than in 2005

(4.6�C, SD = 3.2). In addition, the areas with Arctic water,

was much less in 2006 than in 2005. Arctic waters are

defined as waters with temperature [0�C and salinities

between 34.3 and 34.8% (Loeng 1991).

The zooplankton distribution shows that the main con-

centrations ([7 gm-2 dw) in the western and central parts

of the Barents Sea overlaps with the main distribution areas

of 0-group cod and haddock (Table 1; Figs. 2, 3). The

highest biomass of 9.5 and 11.3 gm-2 dw were obtained in

Atlantic waters for 2005 and 2006, respectively (Table 1).

The second highest biomass was obtained in the Arctic

waters (8.5 gm-2 dw), where the values for 2005 and 2006

were equal. The main contribution to the biomass in

Atlantic (C. finmarchicus, Thysanoessa spp.) and Arctic

waters (C. glacialis, Themisto libellula) comes from dif-

ferent species (Orlova et al. 2005). The average zooplank-

ton biomass increased from 2005 (7.1 ± 12.5 gm-2 dw,

n = 184) to 2006 (9.2 ± 7.1 gm-2 dw, n = 193).

Table 2 contains the estimated Pearson’s correlation

between 0-group cod and haddock abundance and physical

and biological parameters. Significant correlations were

found between the cod and haddock abundance and tem-

perature and salinity at 10 and 50 m depth in 2005 and

2006 (Pearson’s r varied between 0.41 and 0.73), while

water temperature at 100 m depth was significantly corre-

lated with cod and haddock abundance only in 2005.

Salinity at 10 and 50 m depth was also correlated with fish

abundance, and the correlation coefficient r varied between

0.25 and 0.49. Significant correlations were found between

cod and haddock abundance and biomass of plankton in

2006, while no correlations were found in 2005.

Diet composition

A total of 852 and 288 stomachs of cod and haddock,

respectively, were examined from pelagic and bottom trawl

in 2005 and 2006 (Table 3). The 0-group cod examined in

this study had an average length of ca. 9 cm and a weight

of ca. 7 g. The 0-group haddock was somewhat larger than

the cod and had an average length of ca. 10 cm and a

corresponding weight of 10 g. The locations of the stations

from 2005 and 2006 are shown in Fig. 4.

The distribution of PFI for cod and haddock pooled for

2005 and 2006 shows that crustaceans dominated the diet

at most stations (Fig. 5). In 0-group cod, krill and copepods

were the main prey items. However, in haddock, in

1586 Polar Biol (2009) 32:1583–1596

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addition to krill and copepods that were found in their

stomachs, other organisms such as appendicularians and

Limacina species were also present, thus showing that

these are important prey items in some areas. The main

copepod species preyed on by both species were C. finm-

archicus, which appeared to dominate in both abundance

and biomass in Atlantic waters. Occasionally a deeper

dwelling (100–200 m), cold-water species, Metridia longa

was also preyed upon. Of the krill species, Thysanoessa

inermis dominated in all stomachs examined. In addition,

T. longicaudata and Meganyctiphanes norvegica were

important prey in some stations.

0-group cod and haddock preyed occasionally on fish. In

most cases, the fish found in stomachs of cod and haddock

were much digested and therefore registered only as Te-

leostei. However, in some 0-group stomachs, fish such as

young capelin (Mallotus villosus) and long rough dab

(Hippoglossoides platessoides) were found and in a few

haddock stomachs sculpins were identifiable.

TFI of 0-group cod and haddock presented in Fig. 6

indicates the degree of stomach fullness. The TFI values

obtained from this study were combined with subjectively

assigned filling degree to get an indication of which values

of TFI correspond to various degrees of stomach fullness.

Empty stomachs had a TFI of 0 whereas values around 0.4–

0.6 correspond to almost full stomachs. Fish with extended

stomachs, and with transparent stomach wall due to large

amount of stomach contents, had TFI values up to 1.0–1.3.

The stations with high TFI values for haddock generally

also have high TFI values for cod.

A one-way ANOVA was carried out to determine the

significance of differences among the mean TFI of fish

caught by the two trawls (pelagic and bottom trawl). The

test was carried out on pooled data from 2005 and 2006 due

to the low number of observations in some years. Both cod

and haddock specimens caught by the pelagic trawl had

higher TFI values than those caught by the bottom

trawl (cod F = 32.4, P \ 0.0001; haddock F = 13.6,

P \ 0.0001) indicating a largely pelagic feeding behaviour

by the 0-group fish. Day and night samples were separated

using inclination of the sun as criteria. The inclination was

calculated with commonly known algorithms (Olseth and

Fig. 2 Temperature distribution at 50 m overlaid with distribution of

0-group cod, haddock (log number nm-2), and zooplankton biomass

(g dry weight m-2) in 2005. The cod and haddock data were redrawn

from Anonymous (2005, 2006) because the area between 75 and 80�N

and 15 and 40�E was covered mainly by Russian vessels. Zooplank-

ton biomass dry weight data are not available for 2005 from this area

from the Russian vessels

Polar Biol (2009) 32:1583–1596 1587

123

Skartveit 1985), using position, time and date at each sta-

tion. In cod caught by the pelagic trawl, TFI values were

significantly higher at nighttime than at daytime in both

2005 and 2006 (Table 4). However, TFI values of cod

caught with the bottom trawl at daytime from both years

were not significantly different from those specimens

caught at nighttime. For haddock, the number of observa-

tions was too low to draw firm conclusions, although the

results indicate that fish have a continuous feeding pattern,

feeding at both daytime and nighttime.

Ontogenetic differences in diet

To examine ontogenetic differences in diet, fish were

divided into two distinctive size categories, larger or

smaller than 10 cm, and data from 2005 and 2006 were

pooled. The main prey items of small cod were copepods

(54%) and krill (31%). In large cod, these prey items also

dominated, with krill being the most dominant prey (45%).

Copepods accounted for a much lesser amount (25%) in the

diet of smaller cod. These results imply a shift towards

larger prey items with increasing size of 0-group cod. Such

a change was also seen in haddock, but the diet of this

species was more diverse. The dominant prey groups of

smaller haddock were Appendicularia (29%), Limacina

(18%), copepods (18%) and krill (15%). In larger haddock,

krill were most abundant (25%), followed by smaller prey

items such as Limacina (17%), Appendicularia (15%) and

copepods (14%). It should also be noted that 5% of the diet

of larger haddock consisted of Teleostei, while this taxon

was absent in the diet of smaller haddock.

Diet overlap between cod and haddock

For diet overlap studies, the following seven prey catego-

ries were selected from five stations: (1) C. finmarchicus

inclusive Calanus; (2) Calanoids including M. longa,

and Copepoda; (3) Euphausiidae including T. inermis,

Fig. 3 Temperature distribution at 50 m overlaid with distribution of

0-group cod, haddock (log number nm-2), and zooplankton biomass

(g dry weight m-2) in 2006. Cod, haddock and zooplankton data were

redrawn from Anonymous (2006) because the area 75–80�N and

15–40�E was covered mainly by Russian vessels

1588 Polar Biol (2009) 32:1583–1596

123

M. norvegica, T. longicaudata; (4) Limacina; (5) Teleostei;

(6) Other including Hyperiids and Amphipods; (7)

Unidentified including unidentified crustaceans.

The positions of the stations, gear used, number of

stomachs sampled and mean length is given in Table 5,

together with the overlap index. Figure 7 presents the prey

items observed in the diet of cod and haddock at the five

different stations. There was almost complete overlap

([90%) between cod and haddock diet at stations 547 and

561. The diet at station 547 mostly consisted of copepods,

mainly C. finmarchicus, for both cod and haddock, while

krill, mainly T. inermis, dominated at station 561. Con-

sidering station 547, it should be noted that a single large

haddock examined (12.7 cm) had preyed on a fish

(PFI = 1.6, not included in the figure) again implying a

shift in diet, as the fish grew larger. At this station, the

average TFI would have been more than doubled if this fish

had been included. The diet of cod and haddock at station

572 was dominated by krill, but Limacina spp. (average

length 3 mm) also contributed significantly (26%) to the

haddock diet. The diet overlap at this station was moderate

(63%).

At station 510, copepods and krill dominated the diet of

cod, while most of the haddock diet could not be identified.

At station 550, copepods dominated the diet of cod, while

the identifiable part of diet of haddock consisted about

equally of copepods and krill. The diet overlap was fairly

low (\40%) at stations 510 and 550.

Data from all five stations were pooled to examine the

distribution of stages and size of copepods and krill that

were consumed by cod and haddock, respectively (Fig. 8).

These results must be interpreted with caution as the state

of digestion may play a role, depending on the time after

ingestion when one can identify and measure prey organ-

isms. However, looking at the krill length distribution, we

see a clear indication that cod preyed on larger krill

(TL [ 20 mm) mostly, whereas haddock seem to prefer

the smaller ones (TL \ 20 mm). The stage distribution of

Table 1 Zooplankton biomass in different water masses

Year Type Dry weight (g/m2)

N Mean SD

2005 AW 16 8.53 5.70

CW 6 2.33 1.35

CW-NAW 12 3.98 1.85

NAW 99 9.52 6.15

PFW 48 7.67 4.77

MW 1 1.42

2006 AW 21 8.51 4.33

CW 4 1.61 1.64

CW-NAW 19 7.32 3.66

NAW 110 11.31 7.89

PFW 38 5.12 4.95

Zooplankton biomass is based on combined WP2 and MOCNESS

data. This table contains four stations less than the total number of

stations as these stations dropped out when coupling to hydrographic

parameters

AW Arctic water, CW coastal water, NAW Atlantic water, PFW polar

front water, MW melt water

Table 2 Average temperature, salinity, zooplankton biomass, and fish numbers (trawl catch per nm) in 40 9 40 nm grid cells for four categories

of fish numbers for cod and haddock

Year/category Fish numbers nm-2 Cod Haddock

T10 T50 T100 S10 S50 Plankton gm-2 T10 T50 T100 S10 S50 Plankton gm-2

2005

1 0–1,000 5.3 2.0 2.0 34.2 34.7 6.7 4.6 2.0 2.0 34.3 34.8 6.7

2 1,000–10,000 6.0 4.1 3.7 34.4 34.9 5.9 5.8 2.6 2.4 34.4 34.9 6.1

3 10,000–100,000 8.7 5.9 5.1 34.7 35.0 6.2 7.9 5.4 4.6 34.7 35.0 6.4

4 [100,000 8.4 6.0 5.2 34.9 35.0 8.2 8.8 6.2 5.5 34.8 35.0 7.4

Pearson r 0.51 0.56 0.49 0.42 0.41 0.14 0.70 0.73 0.64 0.45 0.44 0.00

2006

1 0–1,000 6.4 4.3 1.8 34.6 34.8 6.8 5.5 3.2 2.5 34.6 34.8 5.7

2 1,000–10,000 7.9 6.4 1.9 34.8 35.0 6.8 7.8 6.4 5.5 34.8 35.0 6.4

3 10,000–100,000 7.9 6.7 5.6 34.8 35.1 6.8 8.5 7.1 6.0 34.8 35.0 8.2

4 [100,000 8.4 6.1 4.0 34.9 35.0 13.7 8.3 6.1 5.0 34.9 35.0 11.2

Pearson r 0.41 0.45 0.06 0.25 0.45 0.23 0.59 0.60 0.10 0.33 0.49 0.34

Correlation analysis was done on total data sets for each species and year. The variables used in the correlation analysis were: log abundance of

cod and haddock, temperature at 10, 50,100 m, salinity at 10 and 50 m, and zooplankton biomass (gm-2 dry weight). Bold values indicate

significance at 5% level

Polar Biol (2009) 32:1583–1596 1589

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copepods preyed on, indicates that in general, CIV–CVI

stages were dominant, however, in some stations CI–CIII

were also abundant and with no clear difference between

cod and haddock on the selection of copepodite stages.

Discussion

Feeding behaviour

Though the main prey consumed did not differ much, some

variations in the diet was observed between cod and

Table 3 Summary of number of stomachs examined, mean length, mean weight, and total fullness index (TFI, dry weight) of cod and haddock

Trawl type No. of stations No. of fish Length (cm) Weight (g) TFI

Average SD

Cod

2005 Bottom 115 159 9.2 6.65 0.137 0.298

Pelagic 42 418 8.9 5.59 0.243 0.257

2006 Bottom 42 96 9.9 8.11 0.185 0.433

Pelagic 16 179 9.4 7.16 0.377 0.294

Haddock

2005 Bottom 1 4 8.4 5.30 0.144 0.098

Pelagic 12 89 10.5 10.31 0.159 0.201

2006 Bottom 3 33 11.4 13.06 0.106 0.072

Pelagic 9 162 10.1 10.78 0.359 0.308

Fig. 4 Location of stations in 2005 and 2006 where cod and haddock

stomach samples were investigated. Stations where diet overlap

between cod and haddock (referred in Table 5) was examined, are

marked in red

Fig. 5 Distribution of partial fullness index (PFI) in cod and haddock

from 2005 and 2006 cruises. For cod and haddock only stations with

more than five and three fish, respectively, were included in this figure

1590 Polar Biol (2009) 32:1583–1596

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haddock. In general, it is known that haddock fed on

smaller prey items than cod. The mouth gape of 0-group

haddock is smaller than in 0-group cod, and this may be the

reason for feeding on smaller prey (Robb and Hislop 1980).

In the northern North Sea, Robb (1981) observed that

0-group haddock fed mostly on appendicularians. In this

study, we observed that haddock taken at the same station

fed on other prey such as Limacina spp. and appendicula-

rians in addition to copepods and krill. Limacina spp. and

appendicularians are smaller and less mobile organisms

than krill, for example.

The diet of 0-group cod found in our study has many

similarities with the results found by Ponomarenko (1983)

based on a long time series of diet data from 0-group cod

sampled by bottom trawl in October–December in the

Barents Sea. She found, however, that the main prey

groups were euphausiids, fish and gammarids, while

copepods were of little importance. Early in the summer,

copepods are the most important prey items (Sysoeva and

Degtereva 1965; Pedersen and Fossheim 2008), so August–

September seems to be a transition stage from feeding

mainly on copepods to feeding mainly on other, larger,

prey items.

Sysoeva (1972) found that the diet of haddock larvae in

June–July was dominated by krill, copepodites and ap-

pendicularians, which correspond well to our results. The

diet of 0-group haddock found in our studies was different

from the diet of 0-group haddock sampled by bottom trawl

in October–December in the Barents Sea, which was

dominated by Polychaeta and amphipods (Baranova 1972).

Diurnal variations in diet indicate that the 0-group

haddock seem to have a continuous feeding pattern

including both daytime and night. In contrast, the cod seem

to mainly limit their feeding to night time. Our observa-

tions on diurnal feeding pattern of 0-group cod and had-

dock seem to fit well with observations from the North Sea

Fig. 6 Distribution of total fullness index (TFI) in cod and haddock

from 2005 and 2006 cruises. For cod and haddock only stations with

more than five and three fish, respectively, were included in this figure

Table 4 TFI distribution at day and night, in 2005 and 2006, for pelagic and bottom trawl investigations

Cod Haddock

Trawl type No. of fish Mean SD F value Pr [ F No. of fish Mean SD F value Pr [ F

2005

Day Bottom 106 0.105 0.18 4 0.144 0.10

Night Bottom 53 0.198 0.44 3.48 0.063 ND ND ND NA

Day Pelagic 289 0.209 0.24 74 0.165 0.21

Night Pelagic 129 0.321 0.28 17.70 <0.0001 15 0.128 0.12 0.41 0.5254

2006

Day Bottom 46 0.192 0.58 1 0.139

Night Bottom 50 0.179 0.22 0.020 0.887 32 0.105 0.07 NA

Day Pelagic 84 0.313 0.26 126 0.335 0.27

Night Pelagic 95 0.434 0.31 7.800 0.005 36 0.442 0.41 3.49 0.064

Day and Night are differentiated based on calculations of inclination of the sun. Calculations are based on commonly known algorithms using

position, time, date at each station. Day is regarded as sun inclination[-0.8, and night as sun inclination\-0.8. Summary of ANOVA carried

out to test differences in day and night by trawl type is also given. Bold values indicate that the values are significantly different at 1% level

ND no data, NA not applied due to few data

Polar Biol (2009) 32:1583–1596 1591

123

(Robb and Hislop 1980; Robb 1981) and Icelandic waters

(Palsson 1973).

Even though krill is a major part of the diet of 0-group

cod and haddock, there seem to be size segregation, with

haddock preferring smaller size krill than cod. The prey

selectivity of these fish is based on a limited number of

stations and should be investigated more. Results from this

study also indicate that there is a shift in diet towards krill

and fish as 0-group cod and haddock grow larger. Onto-

genetic differences in diet are common for many fish

species and were also observed for 0-group haddock and

cod from the North Sea (Robb and Hislop 1980; Robb

1981, and from Icelandic waters (Palsson 1973).

Observations indicated above from the Barents Sea and

other areas indicate that although the main prey types of

cod and haddock to a large degree overlap, there are dif-

ferences in feeding behaviour in relation to: (1) type of

food preyed on, e.g. cod feeding on active pelagic prey

whereas haddock feed on slow moving prey, (2) size of

prey, e.g. cod feeding on larger prey than haddock and (3)

when the main feeding occur, e.g. cod feed mainly at night

whereas haddock seem to feed both during day and night.

These differences likely reflect a strategy adopted by the

two species to reduce intra-specific competition, which

would be beneficial especially at times when food is

limiting.

Distribution and abundance in relation to temperature

and food conditions

Distribution maps based on combined Norwegian and

Russian investigations from 2005 and 2006 and additional

years show that the abundances of both 0-group cod and

haddock were extremely low in the eastern parts of the

Barents Sea (Anonymous 2006). High abundances of

0-group cod and haddock are largely restricted to Atlantic

waters in the central and western Barents Sea, and overlaps

with high biomass of zooplankton in the Barents Sea.

In the Atlantic waters, copepods, e.g. C. finmarchicus,

contribute largely to the zooplankton biomass (Orlova et al.

2005; Tande 1991). At euphausiid-rich locations, such as

areas around the Svalbard bank (Einarsson 1945;

Dalpadado and Skjoldal 1996; Zhukova et al. 2009),

0-group fish seem to feed largely on krill. Zhukova et al.

(2009) report that large concentrations of krill (exceeding

1,000 individuals/m3) dominated by T. inermis, are located

south and south east of Svalbard. The large amount of krill

found in 0-group stomachs investigated at stations 424

(75�540N, 30�280E), 510 (73�520N, 30�280E) and 561

(73�330N, 21�410E), which are located adjacent to the

Svalbard bank, may reflect the presence of high concen-

trations of krill in these areas. Astthorsson and Palsson

(1987), studying euphausiid predation by cod, reported that

T. inermis was the most abundant krill species found in

stomachs of 0-group cod ([7 cm) in sub-Arctic waters off

Iceland.

As the distribution, feeding areas and prey of 0-group

cod and haddock overlap, some degree of competition for

food likely exists between the two species. Dingsør et al.

(2007) reports that high densities of 0-group cod may lead

to a reduction in the survival of 0-group haddock, due to

competition for food among the two species. Results from

this study show that the main prey of 0-group cod and

haddock to some extent overlap and competition for food

may occur. Relative to the plankton availability, the fish

seem to select the most abundant prey organisms such as

copepod C. finmarchicus and krill T. inermis. Floeter and

Temming (2003) examined whether North Sea cod feeding

on other fish is based on preference or availability and

concluded that prey abundance is the main determinant of

the diet composition.

Denser concentrations of cod and haddock over larger

areas and, correspondingly, higher abundance indices of

both species were observed in 2005 when compared with

2006. The abundance of 0-group haddock was higher than

that of 0-group cod in both years. This is unusual; normally

the abundance of 0-group cod is much higher than that of

0-group haddock. It should also be noted that the 2005 and

2006 year classes of haddock were very abundant at ages

1–2 (ICES 2008). The low biomass of zooplankton

observed in the southern Barents Sea in 2005 may be a

result of high predation by abundant 0-group fish such as

cod and haddock. In years with high abundance of one

species, e.g. 0-group cod may lead to inter-specific

Table 5 Diet similarity at five selected stations

Station Latitude Longitude Trawl No. of fish sampled Mean length (cm) Similarity index

Cod Haddock Cod Haddock

510 73�520 30�280 Bottom 12 16 10.6 11.5 32.98

547 73�170 20�190 Pelagic 15 14 8.9 9.9 93.54

550 72�430 20�360 Pelagic 14 18 9.2 10.9 35.93

561 74�330 21�400 Pelagic 13 8 9.7 11.3 92.99

572 74�090 34�380 Pelagic 23 22 9.7 10.1 63.24

1592 Polar Biol (2009) 32:1583–1596

123

0 GROUP COD 0 GROUP HADDOCK

37%

15%

33%

1% 14%

0%

Calanoid copepods

C.finmarchicus

Euphausiids

M.longa

Unidentified

Other

2006ST:510 Bottom trawl

88%

9%

Calanoid copepods

M.longa

C.finmarchicus

Other

2005ST:547 Pelagic trawl Cod

L=8.9cm, W=6.1g,N=15

CodL=10.6cm, W=9.3g,N=12

24%

3%

Calanoids copepods

C.finmarchicus

Euphausiids

Unidentified

2005ST:550 Pelagic trawl

CodL=9.2cm, W=6.6g, N=14

18%

12%

69%

Calanoid copepods

Euphausiids

Hyperiids

Unidentified

2006ST:510 Bottom trawl

HaddockL=11.5cm, W=12.6g, N=16

79%

9%

6% 3%

Calanoid copepods

C.finmarchicus

Euphausiids

Crustaceans

Hyperiids

Unidentified

Other

2005ST:547 Pelagic trawl

HaddockL=9.9cm, W=8.9g, N=14

28%

43%Calanoid copepods

C. finmarchicus

Euphausiids

Limacina spp.

2005ST:550 Pelagic trawl Haddock

L=10.9cm, W=11.8g, N=18

72% Other

7%

8%

85%

Calanoid copepods

Euphausiids

T.inermis

Other

2005ST:561 Pelagic trawl

46%

21%

15%

14%

4%Calanoid copepods

Euphausiids

T. inermis

M. norvegica

Teleosts

Hyperiids

Unidentified

Other

2006ST:572 Pelagic trawl

CodL=9.7cm,W=8.0g, N=23

100%

Euphausiids

Other

2005ST:561 Pelagic trawl

HaddockL=11.3cm, W=11.5g, N=8

3%

36%

17%

26%

5%

7%5%

Calanoid copepods

Euphausiids

T.inermis

T.longicaudata

M.norvegica

Limacina spp.

Teleosts

Unidentified

Other

2006ST:572 Pelagic trawl

HaddockL=10.0cm, W=9.6g,N=22

22%

Unidentified

Other

CodL=9.7cm, W=7.9g, N=13

Fig. 7 Comparison between cod and haddock of distribution of PFI at selected stations from 2005 and 2006 cruises

Polar Biol (2009) 32:1583–1596 1593

123

competition for food resources, resulting in unfavourable

conditions for the recruitment success of other fish species.

Role of 0-group in the Barents Sea ecosystem

0-group cod and haddock may have an important role in the

Barents Sea ecosystem both as a predator and as a prey.

Based on the 0-group survey indices corrected for length-

dependent catch efficiency (Anonymous 2007; Eriksen

et al. 2009), the biomass of 0-group cod was estimated to

360,000 tonnes in 2005 and 125,000 tonnes in 2006, while

the corresponding figures for haddock were 779,000 and

257,000 tonnes, respectively. The value for haddock in

2005 is the highest in the time series, which goes back to

1980, while the highest value for cod was 1,286,000 tonnes

in 1995. These biomasses are lower than the biomass of

pelagic fishes in the Barents Sea (Stiansen and Filin 2008).

The consumption/biomass ratio is, however, higher for

small fish such as 0-group than for pelagic fishes. The

0-group fish, especially cod, are distributed widely through

the Barents Sea. Thus, 0-group cod and haddock (and

0-group fish of other species as well) are a key energy

transporter in the Barents Sea ecosystem in some years.

Impact of climate warming on prey distribution

and abundance

The Barents Sea is experiencing warming climate and the

sea temperatures in the entire Barents Sea in 2007 were

amongst the warmest ever observed (Stiansen and Filin

2008). The continued significant reduction in the extent of

the summer sea ice cover is a dramatic illustration of the

pronounced impact increased global temperatures has on

the Arctic regions. The warming conditions may influence

the ecosystem in both a positive and negative ways. If the

climatic warming continues, one could expect a more north

and eastward distribution than today for 0-group cod and

haddock as well as their main prey C. finmarchicus and

T. inermis. However, the extension of the northward dis-

tribution may be limited for the 0-group due to settlement

to the bottom in August–September. Krill studies carried

out by Zhukova et al. (2009) indicate that the abundance of

Atlantic species, e.g. T. inermis increased due to increased

seawater temperatures in the Barents Sea. The C. finmar-

chicus population is also known to increase in abundance

and extend their distribution in warm years (Orlova et al.

2005; Kwasniewski et al. 2003). Similar observations are

0

5

10

15

20

25

34323028262422201816141210

34323028262422201816141210

Nu

mb

er

870

0

50

100

150

200

250

300

C I -III

C VI -IV

0

1

2

3

4

5

6

7

Nu

mb

er

0

10

20

30

40

50

60

Total length (mm)

Total length (mm)

Copepods in cod stomachs

Copepods in haddock stomachs

CopepoditeKrill in cod stomachs

Krill in haddock stomachs

ST 510 ST 547 ST 550 ST 561 ST 572

ST 510 ST 547 ST 550 ST 561 ST 572

(a)

(b)

(c)

(d)

Fig. 8 Length distribution of krill and stage distribution of copepods

from cod and haddock stomachs at selected stations. In krill, total

length (TL)—from tip of the rostrum to end of telson was measured.

Pooled data from 2005 and 2006 cruises are presented. Copepod (CI–

CIII vs. CIV–CVI) were determined based on prosome length and

other characteristics

1594 Polar Biol (2009) 32:1583–1596

123

made from other regions as well. A regime shift in zoo-

plankton community in the North Sea (Beaugrand et al.

2003) is believed to have a negative impact on higher

trophic levels, thus possibly leading to recruitment changes

in some key fish species.

Haddock that seems to have a more preference to war-

mer waters than cod (Sonina 1969; Bergstad et al. 1987;

results from this study) may extend their distribution even

further. Another possible consequence of warming climate

is the reduced ice cover, which in turn influence the bio-

logical production in the northern Barents Sea. Reduced ice

cover may lead to reduction of the very large primary

production close to the ice edge, which takes place in a

short period each year. This is again quite important to

zooplankton and fish production in the northern regions,

having impact on the entire ecosystem.

Acknowledgments This project was part of the research pro-

gramme ‘‘Ecosystem Barents Sea’’ at the Institute of Marine

Research, Bergen, Norway. We thank Harry Williams, Haakon Hop,

Fran Mowbray and one anonymous referee for constructive comments

on a previous version of the manuscript.

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