ICES Journal of Marine Science, 62: 671e682 (2005)
doi:10.1016/j.icesjms.2004.12.018
Trends in cpue and related changes in spatial distribution
of demersal fish species in the Kattegat and Skagerrak,
eastern North Sea, between 1981 and 2003
Michele Casini, Massimiliano Cardinale, Joakim Hjelm,
and Francesca Vitale
Casini, M., Cardinale, M., Hjelm, J., and Vitale, F. 2005. Trends in cpue and related
changes in spatial distribution of demersal fish species in the Kattegat and Skagerrak,
eastern North Sea, between 1981 and 2003. e ICES Journal of Marine Science, 62:
671e682.
We explored the trends in ln-transformed catch per unit effort, defined as average weight
(kg) per 1 h trawling, and the spatial distribution of 32 demersal fish species in the Kattegat
and Skagerrak using International Bottom Trawl Survey data collected between 1981 and
2003. As in other areas, the biomass of roundfish species such as cod, pollack, hake, and
ling drastically decreased during this period most likely owing to fishing pressure.
However, other commercially important fish species, e.g. haddock, whiting, and some
flatfish, showed a constant or increasing trend during the same period. Non-commercial
species showed no or an increasing trend in ln-cpue, by as much as 40 times in hagfish.
Furthermore, analyses of the spatial distribution of 14 selected fish species by means of
distribution maps of ln-cpue suggested that fish stocks contracted and expanded in response
to decrease and increase of the stock biomass, respectively, with some flatfish species (i.e.
plaice and flounder) and hagfish representing the exceptions to this general pattern.
Ó 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: aggregation, catch per unit effort, dispersion, flatfish, non-target fish, roundfish.
Received 30 January 2004; accepted 20 December 2004.
M. Casini, M. Cardinale, J. Hjelm, and F. Vitale: National Board of Fisheries, Institute of
Marine Research, Box 4, 453 21 Lysekil, Sweden. Correspondence to M. Casini: tel: C46
0523 18728; fax: C46 0523 13977; e-mail: michele.casini@fiskeriverket.se.
Introduction
The last few decades of overexploitation have led to
a worldwide decrease, and even collapse, of several fish
stocks (e.g. see Myers and Worm, 2003). Fishing activities,
with removal of target fish species and habitat perturbation,
may also lead to changes on all trophic levels (Jennings and
Kaiser, 1998). In fact, the global collapse of predatory fish
in the northern hemisphere has produced a change in the
entire ecosystem, and the species at lower trophic levels
(zooplanktivorous pelagic fish and invertebrates) may have
steadily increased due to predation release (Pauly et al.,
1998; Myers and Worm, 2003).
The effects of changes in target fish abundance on their
spatial distribution have been discussed extensively (e.g.
Winters and Wheeler, 1985; Crecco and Overholtz, 1990;
Gordoa and Hightower, 1991; Marshall and Frank, 1994;
Swain and Sinclair, 1994; Rose and Kulka, 1999, and
references therein). Variations in spatial fish distributions,
when not adequately accounted for, can lead to overestimation of the stock biomass and underestimation of the
1054-3139/$30.00
fishing mortality (Crecco and Overholtz, 1990; Rose and
Kulka, 1999). This phenomenon may lead, in extreme
cases, to wrong management decisions and ultimately to the
collapse of commercial stocks (Rose and Kulka, 1999).
Although the causes for the spatial distribution of a fish
population are still poorly understood, density-dependent
processes are often suggested as a possible explanation
(Myers and Stokes, 1989; MacCall, 1990). Alternatively,
spatial distributions may also be affected by the degree of
stock exploitation (Winters and Wheeler, 1985) and
environmental factors (Corten and van de Kamp, 1996;
Heessen, 1996; Rose and Kulka, 1999). Information about
the factors affecting spatial distribution of non-commercial
species is scarce in the literature (but see Heessen and
Daan, 1996).
The eastern North Sea including the Kattegat and
Skagerrak (ICES Division IIIa) is a transitory area between
the central North Sea and the Baltic Sea that has not
received as much attention as other regions of the North
Sea (see Daan et al., 1996). However, both the inshore and
offshore areas of the eastern Skagerrak have undergone
Ó 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
672
M. Casini et al.
high levels of exploitation. The considerable reduction in
abundance of large individuals (O30 cm) of several
commercial fish species during the last 30 years was likely
an effect of high fishing pressure (Svedäng, 2003).
Nevertheless, only cod has been extensively investigated
in the area (Svedäng, 2003; Svedäng and Bardon, 2003).
Factors affecting fish abundance and distribution often
operate at regional scales (Heessen and Daan, 1996; Rose
and Kulka, 1999). Hence, in this paper we investigate
the changes in biomass and spatial distribution of both
commercial and non-commercial demersal fish species in
the open areas of the Kattegat and Skagerrak during the last
23 years.
Material and methods
Trends in biomass
We analysed the trends in cpue of 32 demersal fish species
collected in the Kattegat and Skagerrak (Figure 1) during
the winter International Bottom Trawl Survey (IBTS)
between 1981 and 2003. The IBTS has been carried out
in the area since 1979 in January/February, and from 1991
also in September, each year by the Swedish National
Board of Fisheries onboard the RV ‘‘Argos’’. The main aim
of the survey is to provide annual estimates of recruitment
for several commercial species, but it is also a reliable
source of stock abundance data that agree with stock
assessment estimations (Heessen, 1996). We used winter
data because of the longer time-series. Moreover, in winter
Figure 1. Depth profile of the Kattegat and Skagerrak.
most individuals recruited the previous spawning season
have already reached a size caught by the survey gear
(Heessen and Daan, 1996). Catch rates (cpue) were
calculated in weight (kg) per 1 h of trawling, including
for each species only those hauls whose depths are
considered suitable in the literature. Cpue data were lntransformed [(ln (cpue C 1)] to satisfy the assumption of
normality and homogenous variance. According to the
IBTS protocol (see for details ICES, 1992), the sampling
area was stratified by ICES rectangle of 0.5( latitude and
1( longitude. Haul duration was 30 min at 4 knots except in
adverse weather conditions. Between 23 and 49 trawl hauls
were performed each year, with a larger number of hauls in
the past decade (Table 1). The trawl employed was
a standard GOV (Grande Ouverture Verticale) with
a 16 mm mesh size in the codend. In this paper the terms
cpue and biomass are used as synonyms.
Spatial distribution
We analysed the change in spatial distribution of 14 fish
species. The species were chosen to represent three different
groups, including five commercial roundfish (cod, pollack,
saithe, haddock, and whiting), five flatfish (plaice, flounder,
common sole, lemon sole, and long rough dab), and four
non-target fish species (poor cod, four-bearded rockling,
lumpsucker, and hagfish). The spatial distribution of each
Table 1. Summary information for Swedish IBTS trawl hauls in the
Kattegat and Skagerrak between 1981 and 2003.
Year
Period
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
03e19
09e18
Jane16
06e21
04e21
03e19
02e19
01e18
06e23
05e21
12e28
03e20
08e25
Jane17
Jane16
Jane15
Jane13
Jane12
Jane11
Jane10
Jane08
Jane07
Jane12
31
31
30
29
27
26
25
24
22
21
27
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
No. of
hauls
Bottom
depth (m)
32
23
35
35
32
42
49
39
43
45
38
43
45
48
48
48
46
45
46
46
45
45
46
22e183
30e232
20e210
21e135
21e150
21e248
22e300
22e265
18e234
18e235
19e265
18e258
18e258
18e259
19e228
18e257
19e250
19e256
18e256
19e253
18e233
19e237
19e257
Trends in cpue and related changes in spatial distribution of demersal fish species
species was qualitatively examined by means of the Natural
Neighbor interpolation method. This method does not
extrapolate values beyond the range of data and it is
particularly suitable for data sets containing dense data in
some areas and sparse data in other areas (see Sambridge
et al., 1995 for details). For the sake of consistency, the ratio
(maximum range divided by the minimum range) and angle
(preferred orientation of the major axis) of anisotropy were
fixed at 1 and 0, respectively, for all maps. This means that
we did not assume any preferred orientation in the data. Two
distribution maps were created for each species, corresponding to the highest and lowest level of ln-cpue observed in the
time-series. We used ln-cpue values in order to be consistent
with the estimated temporal trends and to render maps more
comprehensive. The period 1981e1985 was avoided for this
purpose because of the low number of hauls (Table 1).
Natural Neighbor interpolation was performed using Surfer
8 (2002) computer software.
673
Results
Trends in biomass
The 32 demersal fish species displayed large fluctuations in
ln-cpue in the Kattegat and Skagerrak during the period
1981e2003. Commercial roundfish species, such as cod,
saithe, hake, and ling, showed a generally decreasing trend
in ln-cpue, sometimes reaching very low values during the
latest years, as in the case of pollack (Figure 2a). However,
only for cod and pollack were the negative trends
significant (Table 2). On the other hand, whiting exhibited
a remarkably stable ln-cpue, whereas haddock and saithe
showed large fluctuations without a clear trend (Figure 2a).
Only the ln-cpue of Norway pout increased significantly
during the study period (Table 2).
In contrast, the ln-cpue of flatfish species displayed
a constant or increasing general trend (Figure 2b). A
significant positive trend was observed in plaice, dab, long
Table 2. The 32 fish species analysed in the study. Coefficients of determination and significance values (linear correlation) of ln-cupe
temporal trends are also shown. n.s.: not significant, p O 0.05.
Group
Scientific name
r2
p
Cod
Haddock
Whiting
Hake
Ling
Pollack
Saithe
Norway pout
ÿ0.57
0.18
0.15
ÿ0.11
ÿ0.08
ÿ0.58
0.06
0.24
!0.001
n.s.
n.s.
n.s.
n.s.
!0.001
n.s.
!0.05
Common name
Commercial roundfish
1
2
3
4
5
6
7
8
Flatfish
9 Arnoglossus laterna
10 Glyptocephalus cynoglossus
11 Hippoglossoides platessoides
12 Limanda limanda
13 Microstomus kitt
14 Platichthys flesus
15 Pleuronectes platessa
16 Scophthalmus maximus
17 Scophthalmus rhombus
18 Solea solea
Scaldfish
Witch
Long rough dab
Dab
Lemon sole
Flounder
Plaice
Turbot
Brill
Common sole
0.52
0.26
0.56
0.66
0.08
ÿ0.01
0.34
ÿ0.02
0.41
0.18
!0.001
!0.05
!0.001
!0.001
n.s.
n.s.
!0.01
n.s.
!0.001
!0.05
Non-target species
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Wolffish
Dragonet
Spotted dragonet
Lumpsucker
Grey gurnard
Angler
Snake blenny
Vahl’s eelpout
Bullrout
Hagfish
Starry ray
Four-bearded rockling
Greater weever
Poor cod
ÿ0.07
0.25
0.41
ÿ0.05
0.29
0.07
0.44
0.26
0.63
0.58
ÿ0.29
0.02
ÿ0.01
ÿ0.46
n.s.
!0.05
!0.01
n.s.
!0.01
n.s.
!0.001
!0.05
!0.001
!0.001
!0.01
n.s.
n.s.
!0.001
Gadus morhua
Melanogrammus aeglefinus
Merlangius merlangus
Merluccius merluccius
Molva molva
Pollachius pollachius
Pollachius virens
Trisopterus esmarkii
Anarhichas lupus
Callionymus lyra
Callionymus maculatus
Cyclopterus lumpus
Eutrigla gurnardus
Lophius piscatorius
Lumpenus lampretaeformis
Lycodes vahlii
Myoxocephalus scorpius
Myxine glutinosa
Raja radiata
Rhinonemus cimbrius
Trachinus draco
Trisopterus minutus
674
M. Casini et al.
a
Commercial roundfish
Ln CPUE (Kg)
6.0
Cod
Norway pout
5.0
4.5
Haddock
Whiting
4.0
3.0
3.0
2.0
1.5
1.0
0.0
1980
1985
1990
1995
1.6
Ln CPUE (Kg)
6.0
2000
Pollack
Saithe
1.2
0.0
1980
1985
1990
1995
1.0
2000
Hake
Ling
0.8
0.6
0.8
0.4
0.4
0.0
1980
0.2
1985
1990
1995
2000
0.0
1980
1985
1990
1995
2000
1990
1995
2000
1990
1995
2000
1990
1995
2000
Year
Year
b
Flatfish
Ln CPUE (Kg)
3.0
3.0
2.0
1.0
1.0
3.0
1985
1990
1995
2000
0.0
1980
1.0
Dab
Witch
1985
Lemon sole
0.8
2.0
0.6
0.4
1.0
0.2
0.0
1980
1985
1990
1995
2000
0.8
Ln CPUE (Kg)
Long rough dab
Flounder
2.0
0.0
1980
Ln CPUE (Kg)
4.0
Plaice
Common sole
0.0
1980
1985
0.10
Turbot
Brill
0.6
0.08
Scaldfish
0.06
0.4
0.04
0.2
0.0
1980
0.02
1985
1990
Year
1995
2000
0.00
1980
1985
Year
Figure 2. Trends in ln-cpue of 32 fish species collected during the first quarter IBTS performed in the Kattegat and Skagerrak between
1981 and 2003. Lines represent 2-year moving averages. Vertical bars represent upper 95% confidential intervals.
Trends in cpue and related changes in spatial distribution of demersal fish species
675
c
Non-target species
Ln CPUE (Kg)
1.0
Poor cod
Bullrout
0.8
0.3
0.4
0.2
0.2
0.1
1985
1990
Ln CPUE (Kg)
2.0
1995
2000
Greater weever
Starry ray
1.5
Wolffish
Angler
0.4
0.6
0.0
1980
0.0
1980
1.2
1985
1990
1995
2000
1995
2000
1995
2000
Four-bearded rockling
Vahl`s eelpout
0.8
1.0
0.4
0.5
0.0
1980
1985
1990
4.0
Ln CPUE (Kg)
0.5
1995
2000
Lumpsucker
Grey gurnard
3.0
0.0
1980
0.6
1985
1990
Dragonet
Spotted dragonet
0.4
2.0
0.2
1.0
0.0
1980
1985
1990
1995
2000
0.0
1980
1985
1990
Year
Ln CPUE (Kg)
0.5
0.4
Snake blenny
Hagfish
0.3
0.2
0.1
0.0
1980
1985
1990
1995
2000
Year
Figure 2. (continued)
rough dab, common sole, scaldfish, witch, and brill (Table
2). Conversely, flounder ln-cpue was fairly stable, and those
of turbot and lemon sole fluctuated without a clear trend.
Among non-target fish species, the positive trend in the
ln-cpue of hagfish, snake blenny, and spotted dragonet is
noteworthy (Figure 2c). Grey gurnard, dragonet, bullrout,
and Vahl’s eelpout also significantly increased (Table 2).
Poor cod and starry ray were the only two non-target
species whose ln-cpue significantly decreased during
the study period (Table 2). Some species (wolffish, angler,
greater weever) showed large interannual fluctuations in
ln-cpue without any trend, not surprising considering that
they were caught in very small quantities.
Spatial distribution
Although there were slight differences among years in the
spatial location of IBTS hauls, the eastern Kattegat was
evenly covered every year. No hauls were made in the
western part of the central Kattegat. Moreover, the western
and central regions of the Skagerrak had a spatially
irregular coverage (Figure 3aec). This uneven spatial
676
M. Casini et al.
coverage needs to be kept in mind when interpreting
species distributions. In particular, the interpolation between the southwestern Kattegat and the western Skagerrak, where there are no hauls, is a mere artefact produced
by the software used and should not be considered reliable.
Therefore, in order to avoid misinterpretations of the maps,
the locations of the hauls are also indicated on Figure 3.
Analysis of the fish distribution showed that commercial
roundfish tended to be aggregated at a low level of biomass
and dispersed over a wider geographic range at high
biomasses (Figure 3a). This pattern was particularly evident
for pollack and saithe that, at low ln-cpue, were aggregated
in restricted areas of the central Skagerrak. Haddock were
mostly in the Skagerrak and southern Kattegat at low
Figure 3. Spatial distribution of 14 fish species at the highest (left map) and lowest (right map) values of ln-cpue observed during the IBTS
survey in the Kattegat and Skagerrak between 1981 and 2003. Circles represent haul positions. Scale bars represent ln-cpue. The Natural
Neighbor interpolation method was used to create the distribution maps (see text for details).
Trends in cpue and related changes in spatial distribution of demersal fish species
677
Figure 3. (continued)
ln-cpue and over the whole studied area at high levels of lncpue. Cod seemed to be spread over the whole Kattegat and
to have a more patchy distribution at high and low levels of
biomass, respectively (Figure 3a). For whiting the contrast
between low and high ln-cpue was small, which might
explain the lack of any clear difference in spatial
distribution.
Flatfish, on the other hand, did not show clear
distribution patterns. Plaice and flounder did not show
any differences in the degree of aggregation/dispersion at
different levels of biomass (Figure 3b). The distribution of
common sole was little changed in the years of lowest and
highest ln-cpue, with two major aggregations, one in the
central Kattegat and one in the southern Skagerrak.
However, at high ln-cpue, the dense concentrations were
more extensive. Lemon sole was aggregated in some areas
of the Kattegat at low values of ln-cpue and distributed over
the whole Kattegat and in the Skagerrak at high ln-cpue,
whereas long rough dab was distributed over a somewhat
broader area at high levels of biomass.
Spatial distribution of the non-target fish species resembled that of the commercial roundfish species. Poor cod
were present in the Skagerrak at low ln-cpue and
additionally in the Kattegat at high ln-cpue (Figure 3c).
Four-bearded rockling and lumpsucker expanded over the
whole study area at high biomasses. Hagfish, on the other
hand, seemed to have a constant geographic range, being
concentrated in small regions of the eastern Skagerrak at
both high and low levels of biomass.
Discussion
Trends in biomass
Overall, the results based on fish ln-cpue for the Kattegat
and Skagerrak are similar to those in other areas of the
North Atlantic, i.e. a steady decrease of several commercially important roundfish (cod, pollack, hake, ling). Since
the non-target fish species did not show any decrease in lncpue (except starry ray and poor cod), the decrease in
roundfish is probably attributable to fishing pressure, even
though we cannot rule out other possibilities, such as
environmental change. However, two roundfish species,
haddock and saithe, showed large fluctuations in ln-cpue
without any clear trend. Similarly, whiting showed no trend
in biomass during the period analysed. The results agree
with stock assessment values for cod, haddock, and saithe
evaluated for the whole North Sea, whereas the stable
whiting biomass in the Kattegat and Skagerrak is different
from stock assessment estimates (ICES, 2003). It must be
kept in mind that our analysis covered only a limited area of
678
M. Casini et al.
Figure 3. (continued)
the North Sea, so comparisons could be misleading and,
although the ln-cpue was relatively constant in the area, the
average size of whiting has decreased (JH, unpublished
data). The discrepancy between stock assessment values
and our ln-cpue estimates suggests that in order to better
understand the dynamics of fish stocks, biomass estimation
should be performed at regional and local scales and
implemented using as many data sets as possible.
Flatfish species displayed no or positive trends in lncpue. This agrees with the findings of Heessen and Daan
(1996) in different areas of the North Sea, and could be due
to the different ecology and life history traits of flatfish
compared with roundfish. For example, dab and long rough
dab grow rapidly and mature at relatively low size
compared with most commercial demersal fish (Heessen
and Daan, 1996; Jennings and Kaiser, 1998). Therefore,
Trends in cpue and related changes in spatial distribution of demersal fish species
679
Figure 3. (continued)
balancing the higher mortality of the older and bigger
individuals, those species are less vulnerable to intensive
exploitation than roundfish (Jennings and Kaiser, 1998). On
the other hand, there are indications of increased invertebrate productivity in coastal waters of the North Sea
over the last two decades. This could have increased the
food supply for juvenile flatfish and, thus, positively
affected flatfish populations (Rijnsdorp and van Leewen,
1996). However, the IBTS is carried out in open waters
where, even in a small system such as the Kattegat and
Skagerrak, the increase in invertebrates as an effect of
eutrophication could be minor compared with that in
coastal zones.
Non-commercial species, which are neither a target nor
a common bycatch of commercial fisheries (e.g. those with
a vermiform body shape), showed also no or a positive
trend in ln-cpue during the period analysed. An extreme
example is the hagfish, whose ln-cpue increased almost 40
times during the last decade. This species is a scavenger
(Britton and Morton, 1994), feeding mostly upon dead or
dying carcasses of other organisms on the sea bed.
Therefore, one possible explanation of its expansion could
be the increased amount of discarding from commercial
vessels (Jennings and Kaiser, 1998; Martini, 1998). This
mechanism could also in part explain the increased biomass
of the flatfish acknowledged to benefit from organisms
damaged by fishing activity (Millner and Whiting, 1996;
Rijnsdorp and van Leewen, 1996; Jennings and Kaiser,
1998). The decrease in abundance of several top-predatory
fish species (e.g. roundfish, see above) could represent an
alternative explanation for the increase in abundance of
several flatfish and non-target species, through predation
release (Pauly et al., 1998; Myers and Worm, 2003).
However, predatoreprey coupling is difficult to demonstrate especially in open high-diversity systems and in
exploited areas (Jennings and Kaiser, 1998), and the
cascading impact on other parts of the ecosystem should
be evaluated in order to provide evidence for this
hypothesis.
The IBTS has been carried out in the Kattegat and
Skagerrak in a standard manner (unvaried sampling design,
same fishing procedure and gear used) since 1979.
Therefore, in our study the factors affecting fish catchability
and cpue data in commercial fisheries (such as different
fishing power, non-random trawling, diurnal and seasonal
variation in fish distribution; Garrod, 1964; Gulland, 1964)
can be ruled out. However, other possible sources of
change in the availability of fish to the survey, such as
680
M. Casini et al.
Figure 3. (continued)
age-dependent behaviour (Swain et al., 1994), cannot be
excluded. Furthermore, the IBTS covers only the open sea
areas, thus missing the inshore part of fish distributions.
Spatial distribution
Our spatial analysis suggests a general trend for all
commercial roundfish species: a tendency to be aggregated
and dispersed at low and high levels of biomass, respectively. Although we tried to be as consistent as possible
in the analysis, the Natural Neighbor interpolation method
used is merely a qualitative method and provides a visual
representation of a phenomenon that needs to be confirmed
using quantitative statistical tools. Moreover, the spatially
uneven coverage of the survey and the interpolation between
the southwestern Kattegat and the western Skagerrak needs
Trends in cpue and related changes in spatial distribution of demersal fish species
681
Figure 3. (continued)
to be kept in mind in the interpretation of the maps. The fact
that fish populations seem to aggregate at low levels of
biomass and disperse at high levels has previously been
demonstrated for pelagic shoaling fish (reviewed by Winters
and Wheeler, 1985) and for target demersal fish stocks such
as haddock (Crecco and Overholtz, 1990; Marshall and
Frank, 1994) and cod (Swain and Sinclair, 1994; Rose and
Kulka, 1999). In our study we have shown a similar
aggregation/dispersion phenomenon in non-target fish
species. The only exception was that of hagfish, which were
always restricted to a small area of the Skagerrak in spite of
a huge interannual change in ln-cpue. This finding is not
surprising considering that hagfish are highly site-specific,
requiring specific substrata and high salinity (Martini, 1998).
Although the contraction and expansion behaviour of marine
fish stocks is considered to occur in response to densitydependent habitat selection mechanisms (MacCall, 1990),
the factors originally triggering these processes are still
poorly understood. However, food requirement and predation avoidance possibly drive the observed pattern (MacCall, 1990). Another explanation could be related to fish
spawning behaviour. In fact, as the data were collected in
February, i.e. at the beginning of the spawning season for
several species, the observed changes in spatial distribution
might be driven by density-dependent behaviour related to
spawning or by shifts in spawning time. An alternative
reason could be associated with environmental change
(Gordoa and Hightower, 1991; Rose and Kulka, 1999).
Flatfish, on the other hand, seem not to follow the same
general pattern. Long rough dab, common sole, and lemon
sole showed a higher and lower level of aggregation at low
and high ln-cpue, respectively. For plaice and flounder,
conversely, this pattern was not evident. Swain and Morin
(1996) found that the range of distribution of American
plaice was not related to changes in abundance in the Gulf
of St Lawrence. This possibly shows a generally different
behaviour of flatfish compared with roundfish. An alternative hypothesis is that plaice and flounder have never
reached, during our time-series, biomass levels high enough
for density-dependent effects to be triggered.
The implications of spatial redistribution of fish stocks in
response to density-dependent factors are huge. Variations
in fish spatial distribution can lead to overestimating the
stock size and underestimating fishing mortality (Crecco
and Overholtz, 1990; Rose and Kulka, 1999), with resulting
risks for overexploitation and stock collapse. In fact, Rose
and Kulka (1999) showed that the crash of the northern cod
stock off Newfoundland was the result of misinterpretation
of fishery cpue influenced by hyper-aggregation. Furthermore, they stressed that cpue from the fishery should not be
used as an index of demersal fish abundance without prior
knowledge of the spatial characteristics of the population
and of the fishing vessels (see also Harley et al., 2001;
Salthaug and Aanes, 2003). Therefore, the change in spatial
distribution of fish stocks in the Kattegat and Skagerrak
area presented in this paper is a phenomenon particularly
important not only from an ecological point of view but
also from a management perspective.
Acknowledgements
We thank the staff at the Institute of Marine Research in
Lysekil and the crew of the RV ‘‘Argos’’ who contributed
to data collection, and to Julia Blanchard, Henk Heessen,
and Verena Trenkel for helpful comments and suggestions
on an early draft of the manuscript.
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