ICES Journal of Marine Science, 57: 324–329. 2000
doi:10.1006/jmsc.1999.0527, available online at http://www.idealibrary.com on
Cod (Gadus morhua L.) cannibalism in the Central Baltic:
interannual variability and influence of recruit abundance and
distribution
D. Uzars, and M. Plikshs
Uzars, D., and Plikshs, M., 2000. Cod (Gadus morhua L.) cannibalism in the Central
Baltic: interannual variability and influence of recruit abundance and distribution. –
ICES Journal of Marine Science, 57: 324–329.
Baltic cod feed mainly on clupeoids and invertebrates. Other fish, including cod, form
only a small part of their diet. To examine the frequency of cannibalism in the Central
Baltic, stomach data were analysed for 1963–1990. During this period, cod in cod
stomachs in the Gotland Basin constituted 2.8% by weight of the total food content,
while the frequency of occurrence was 1.1%. Cannibalism was recorded mainly in cod
>35 cm, and the size range of cod found in stomachs was 5–15 cm. Cannibalism was
observed in restricted areas only and varied strongly between years. The intensity of
predation on cod was significantly different between the periods 1969–1975, 1976–
1979, and 1980–1990. Until 1980, the population in the eastern part mainly inhabited
near-bottom layers. Higher abundance of recruits and their wide distribution raised
the level of cannibalism. Development of stagnation during the 1980s resulted in the
reduction of recruitment and stock size. The distribution of the adult population
changed from near-bottom to mid-water during the first half of the year. The low
abundance of small cod and the differences in distribution between small and large fish
appear to have been the major factors determining very low predation on cod during
1980–90. Overall, the influence of cod cannibalism on recruitment success is negligible
in the Central Baltic. Stock dynamics in the area are determined by the combined
effects of variable oceanographic conditions and the fishery.
2000 International Council for the Exploration of the Sea
Key words: Baltic Sea, cannibalism, cod, habitat overlap, year-class strength.
D. Uzars and M. Plikshs: Latvian Fisheries Research Institute, Daugavgrivas St 8,
LV-1007 Riga, Latvia. Tel: +371 2 458731; fax: +371 7 616946; e-mail: [email protected]
Introduction
The diet of cod (Gadus morhua L.) consists of prey from
pelagic, hyperbenthic, and benthic habitats. This wide
range of food components suggests an adaptive flexibility to feeding conditions. Predation on conspecifics
constitutes a minor source of food for cod but has been
shown from many areas, e.g., the North Sea (Daan,
1973; Kikkert, 1993), the Barents Sea, Icelandic waters
and eastern Newfoundland (Bogstad et al., 1994).
Studies on food composition and feeding of cod in the
Baltic Sea have been conducted since the 1940s. Predation on cod in the western Baltic was studied by Arntz
(1978), Bagge (1979), Schulz (1988), and Weber (1991).
In the Southern Baltic, cannibalism has been reported by
Chrzan (1962), Strzyzewska (1962), and Zalachowski
(1977, 1986). The highest amount of cod in cod stomachs was observed in 1978 and 1979 (Zalachowski,
1986).
1054–3139/00/010324+06 $30.00/0
Investigations of the food composition of cod in the
Central Baltic were carried out irregularly during the
1940s and 1950s (Naumov et al., 1953; Spasski et al.
1956, 1958). Predation on fish was low, and the occurrence of cod in stomachs was mentioned only briefly.
Systematic investigations on cod feeding started in 1963
and continued until the 1990s.
We summarize the data on cannibalism of cod in the
Central Baltic based on the samples collected regularly
during 1963–1990 in an attempt to evaluate variations in
natural mortality of the juveniles. The intensity of cannibalism is analysed in relation to the annual variability
in juvenile (=prey) and old (=predator) cod abundance
and their distribution by area and depth.
Material and methods
The study area is the central part of the Baltic Sea (ICES
Subdivisions 25, 26, and 28, Fig. 1). Cod were caught
2000 International Council for the Exploration of the Sea
Cod (Gadus morhua L.) cannibalism in the Central Baltic
325
59°
N
58°
57°
600
28
25 26
400
200
56°
100
50
55°
25
10
1
54°
15°
16°
17°
18°
19°
20°
21°
22°
23°
24° E
Cod stomach sampling stations
Locations of more frequent occurrence of cannibalism
Figure 1. Study area in the Baltic with identification of ICES Subdivisions. Cod stomach sampling stations (small dots) and
stations where cannibalism was frequently observed (large dots) are shown in relation to the average spatial distribution of age-1
cod (N h 1, grey scale) during research surveys in March–April, 1977–1981.
Table 1. Number of cod stomachs (N) with food, empty, and containing remains of cod as prey by
ICES Subdivision and size class, 1963–1990.
28
26
ICES Subdivision
Size class (cm)
20–35
>35
20–30
>35
25
>35
Total
N with food
N empty
N containing cod
Total N
% empty
10 741
2863
21
13 604
21
12 448
5517
116
17 965
31
7056
2947
13
10 003
30
14 272
13 502
150
27 774
49
1751
2230
65
3981
56
46 268
27 059
365
73 327
37
during random depth-stratified bottom-trawl surveys.
The surveys in Subdivisions 26 and 28 were carried out
mainly during the 1st, 2nd, and 4th quarters of 1963–
1990, while stomach samples in Subdivision 25 were
collected in March–June, 1963–1976. All fish in the trawl
were measured and counted. The biological analysis
included 100 randomly sampled specimens from each
haul, of which about 50 stomachs containing food were
analysed. The numbers of empty stomachs were also
recorded. Sampling data are summarized in Table 1.
Stomach contents of individual cod were analysed for
type of prey as well as weight and total length of
each prey.
Cannibalism was characterized by the percentage
contribution of cod to the total stomach content
weight and the frequency of occurrence of cod in
non-empty stomachs. The survey catches from
which cod were sampled for diet analysis were also
used to determine relative densities of juvenile and
adult cod. These were calculated as the average catch in
number per hour fishing (c.p.u.e.) for cod <20 cm and
>40 cm.
An annual index of cod recruitment (age 1) was
calculated from research survey demersal trawling in the
Central Baltic during 1975–1990. Data on young cod
distribution during 1963–1976 in Subdivision 25 was not
D. Uzars and M. Plikshs
Results
Empty cod stomachs (Table 1) were more frequent
(30–49%) in samples from the Gotland and Gdańsk
spawning grounds (Subdivision 26) than in the coastal
areas of Subdivision 28, where mainly immature and
mature prespawning cod are found (21–31%). More
detailed information on food composition has been
given by Uzars (1994).
Cases of cannibalism in the Central Baltic during
1963–1990 were recorded in 300 stomachs (Table 1).
Conspecifics constituted 2.8% of the stomach content
weight. The mean frequency of occurrence of cod in
stomachs was 0.93% and 1.05% in Subdivisions 28 and
26, respectively. Although the frequency of occurrence
was very low, some patterns are obvious. Cannibalism
was observed mainly in large cod (>35 cm) and rare in
the smaller size range (<35 cm). Both large and small
cod preyed mainly on 0 and 1 group cod (5–15 cm). The
prey lengths were similar in both areas.
Differences in predation on cod were observed
between areas (Fig. 1) and depth strata. In Subdivision
28, cannibalism was more intense in coastal waters at
depths of 40–80 m. In Subdivision 26, cod in stomachs
(a)
30
25
%
20
15
10
% Weight of other fish
% Weight of cod
5
19
6
19 3
6
19 5
6
19 7
6
19 9
19 71
7
19 3
7
19 5
7
19 7
7
19 9
8
19 1
8
19 3
8
19 5
8
19 7
89
0
% Occurrence of cod
Year
(b)
%
available. Although the surveys were carried out with
the same trawl, the area coverage varied. A General
Linear Modelling approach was applied on area means
and log-transformed c.p.u.e. data, weighted by depth
strata to obtain the index (age 1):
Age-1=Y+A+S+D+A*D+S*A+, where Y is year
effect, A is subarea effect (Southern Gotland, Central
Gotland, Northern Gotland, and Gdańsk Deep), S is
season effect (January–February, March–April, and
November–December), D is depth strata effect (21–
40 m, 41–60 m, 61–100 m, 101–120 m, and 121–140 m),
A*D and S*A are interaction terms, and is the error
term. Descriptive statistics for the model were: df=371;
R2 =0.53; F=8.71; p<0.0001.
Cod cannibalism was analysed for three periods
separately, which differ either in major oceanographic
characteristics or in recruitment level (HELCOM, 1990;
ICES, 1996):
1969–1975. Inflow of high-saline water was intense
and stagnation below the halocline lasted only a short
time. Recruitment and biomass of cod were relatively
stable.
1976–1979. Oxygen and salinity conditions in the
near-bottom water layers were favourable for cod
reproduction. Abundance of cod increased sharply
owing to successful recruitment.
1980–1990. This period is characterized by deepwater
stagnation. The biomass of cod reached a maximum
in the early 1980s and decreased sharply from 1986
onwards.
10
9
8
7
6
5
4
3
2
1
0
% Weight of other fish
% Weight of cod
19
6
19 3
6
19 5
6
19 7
6
19 9
19 71
7
19 3
7
19 5
7
19 7
7
19 9
8
19 1
8
19 3
8
19 5
8
19 7
89
326
% Occurrence of cod
Year
Figure 2. Annual variability in the frequency of occurrence of
cod as prey in cod stomachs and in the percentage contribution
of cod and other fish (excluding clupeoids) by weight to the
total diet of cod in the central Baltic, 1963–1990. A. Size class
>35 cm. B. Size class 20–25 cm.
was found mainly offshore at depths>80m. The frequency of occurrence varied by season. In both areas
it was generally higher in late autumn and winter
(November–January) and in spring (March–April).
Cannibalism in large and small cod varied between years
(Fig. 2). A higher frequency of occurrence (2–4%)
among large cod was found during 1963–1964, 1973,
and 1977–1978. Values did not exceed 2% in 1968–1970,
1975–1976, 1979, and 1981, and were <0.5% during
other years. The amount of other fish in adult cod diet
increased in the early 1960s and during the second half
of the 1980s.
Small cod (20–35 cm) were almost never cannibals,
although in 1963 and 1976–1977 a slight increase can be
observed (Fig. 2). High percentage contributions
by weight in 1984 and 1987 were related to the consumption of relatively large specimens. Predation on fish
other than Clupeidae and cod was higher during the
1980s.
Trends in cannibalism and in the abundance of cod
<20 cm and >40 cm at the stomach sampling locations
during 1969–1990 are shown in Figure 3. Up to 1979, an
increase in cannibalism was observed in years when 0and 1-group cod were abundant. The highest numbers of
cod observed in stomachs were recorded in 1973, 1976,
and 1977 (23, 25, and 54, respectively).
Abundance of adult cod appears to have been
relatively stable during the 1970s (Bagge, 1994), but
600
3.0
500
2.5
% of cod as the prey
Number h
–1
Cod (Gadus morhua L.) cannibalism in the Central Baltic
400
300
200
100
327
2.0
1.5
1.0
0.5
19
6
19 9
7
19 0
7
19 1
7
19 2
7
19 3
7
19 4
7
19 5
7
19 6
7
19 7
7
19 8
7
19 9
8
19 0
8
19 1
8
19 2
8
19 3
8
19 4
8
19 5
8
19 6
8
19 7
8
19 8
8
19 9
90
0
0
Year
Number of cod as prey
Number of cod < 20 cm
Number of adult cod > 40 cm
Figure 3. Trends in survey catches of juvenile and adult cod (N
per hour) and observed number of cod as prey in cod stomachs
in the Central Baltic, 1969–1990.
considerable changes in abundance and distribution
were observed in the 1980s. In 1980–1985, the cod stock
was the highest ever observed. Although the strongest
year classes were formed in this period, the number of
small cod recorded in cod stomachs was low (4–16 per
year). Since 1986, the abundance of young cod has
decreased in the Central Baltic and spawning stock
biomass has declined below 200 000 t in the early 1990s
(ICES, 1996). The number of cod larger than 40 cm in
the samples was extremely low and predation on cod
was very rare or not even observed.
As another measure of the frequency of cannibalism,
we estimated the proportion of trawl catches in which
cannibalism was observed and related this frequency
to the catches of small cod (age-1) and large cod. In
this analysis, data from all years within each of the
three study periods were merged. During 1969–1975,
cannibalism was observed in 15% of the sampled trawl
catches, during 1976–1979 in 30%, and during 1980–
1990 in 10%. The corresponding mean numbers of
adult cod in these hauls were 83, 90, and 179 and
the numbers of age-1 cod 51, 151, and 40, respectively.
These figures suggest that a higher ratio of cannibalism is associated with high abundance of juveniles.
Regression analysis of frequency of occurrence of cannibalism against age-1 cod abundance indices from
research surveys (Fig. 4) indicates a significant positive
trend.
Cannibalism was mainly recorded in a few locations
that correspond to established centres of juvenile cod
distribution (Fig. 1), where c.p.u.e. of 100–200 h 1 has
been recorded. In addition, in certain localities of Subdivision 25, relatively high rates of cannibalism were
observed during 1963–1976. In March–June the mean
frequency of occurrence was 1.8%. Juvenile cod was
5
10
15
Age 1 indices
20
25
Figure 4. Relationship between frequency of occurrence of cod
as prey in cod stomachs and the index of abundance of age-1
cod based on research vessel surveys in the Central Baltic,
1975–1990 (R2 =0.43).
consumed more intensively in 1967–1968, 1970, and
1973–1975, years of abundant year classes (Netzel,
1974).
Discussion
The feeding success of cod can be characterized by the
proportion of empty stomachs (not due to regurgitation)
and stomachs containing food. The occurrence of empty
stomachs increased from small to large fish. Highest
fractions were observed on the spawning grounds during
the first half of the year, which may be related to
gonadal maturation and spawning activity. We quantified cod cannibalism in the Central Baltic as frequency
of occurrence of cod with reference only to stomachs
containing food. In view of the high frequency of empty
stomachs, it might have been more appropriate to
consider the total number stomachs, including empty
ones.
Abundant year classes in the Central Baltic, indicating
favourable conditions, were observed in 1964, 1967,
1969, 1972, 1976, and 1977 (Netzel, 1974; Lablaika
et al., 1975; Plikshs et al., 1993). Owing to frequent
influxes of high-saline and oxygen-rich Kattegat waters
up to the mid-1970s, the Central Baltic deeps were
regularly replenished (Matthäus and Franck, 1992). In
the 1960s and early 1970s, young and adult cod were
distributed in the near bottom water layers, but
separated in time and space. After settlement, young
fish inhabited shallow bottom waters in late autumn.
Adult cod were distributed mainly on the feeding
grounds along the coast and on the banks near the
Gotland Deep. In the winter, adult cod migrate to their
spawning grounds in deeper water of the Gotland and
Gdańsk Basins, while young fish move offshore to
warmer waters, particularly during cold winters
(Lablaika and Uzars, 1983). The diet composition of
328
D. Uzars and M. Plikshs
cod during the 1970s indirectly indicates the benthic
behaviour of cod. Although young cod were abundant
in 1964, 1967, 1969, and 1972, cannibalism was relatively
limited compared to 1976–1978. The spatial distribution
of demersal juveniles and migratory behaviour of the
mature cod are likely to have influenced the intensity of
the cannibalism.
The peak in cannibalism occurred in 1977–1978. After
the appearance of extremely abundant year classes in the
late 1970s, the population expanded. Cod distribution
during that period was influenced mainly by population
density and food availability. Competition for food
increased in the areas where traditionally the main
concentrations were observed. Cod from the Central
Baltic migrated to the Gulf of Riga, and northwards
to the Gulf of Finland and Bothnian Sea (Ojaveer
et al., 1981; Aro, 1989). The incidence of cannibalism
increased on the feeding grounds at depths above 80 m
during November–January and was higher in Subdivision 26 compared with 28. The abundance of young
cod increased at larger depths during the first half of the
year (Lablaika et al., 1983), when a large part of the cod
concentrated on the spawning grounds of the Central
Baltic. Owing to habitat overlap of adult and juvenile
cod, cannibalism was higher in waters deeper then 100 m
during March–April.
The long stagnation period in the 1980s caused the
extreme changes in the Gotland Basin and significant
changes were also observed in cod abundance and
ecology. Although the biomass of adult cod was higher
in the early 1980s, the trend in offshore catches and
stock distribution had a southward displacement
(Lablaika et al., 1991). In addition, the vertical distribution of mature cod in the Gotland Deep shifted to a
pelagic life style. Although relatively strong year classes
were formed in 1980 and 1982, the frequency of occurrence of young cod in cod stomachs was very low. The
changes in distribution in relation to environment, and
the associated reduction in spatial overlap between
juveniles and adults, are probably the principal reasons
for the low rates of cannibalism. From the late 1980s the
abundance of both juveniles and adults was reduced.
Adult cod adhered to their pelagic life style, while young
cod were distributed in the coastal regions of the Central
Baltic. Cannibalism was very low because of low spatial
overlap.
Cod cannibalism in the Baltic appears to be controlled
mainly by the abundance of juveniles and by the overlap
between distribution patterns of juveniles and adults.
The hypothesis that cannibalism increases in years of
high spawning stock biomass is not supported by the
available data. However, the influence of cannibalism on
recruitment success in the Central Baltic is negligible and
cod stock dynamics in the area are mainly determined
by the combined effects of variable oceanographic
conditions and intensive fishing.
Acknowledgements
We thank Dr Sture Hansson at the University of
Stockholm, Sweden, for helpful comments on an earlier
draft of the manuscript, and Dr Michael J. Fogarty and
an anonymous reviewer for valuable suggestions for
improvement.
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