Vol. 25, No. 1
March, 1994

JOURNAL OF THE
WORLD AQUACULTURE SOCIETY

Egg Quality in Wild and Broodstock Cod Gadus morhua L.
ELINK J 0 R S V I K
University of Trondheim. Brattma Research Station, Section of Aquaculture,
N-7055 Dragvoll, Norway

Abstract
Problems concerning egg viability or egg quality have received increasing attention, in relation
to cultivation as well as to the assessment of reproduction of wild fishes. One important aspect of
this paper is to discuss the use of cell morphological characteristics as indicators of egg quality in
cod. Early cellular malformation rates in egg batches show a positive correlation with their later
egg and larval survival, and can be used as a rough prediction of normal development and mortality
rates.
The natural mortality rate of fish eggs is not well understood, and observed egg viability of captive
cod is are discussed in relation to egg viability in the wild. Results from recent work indicated that
approximately 1Oo/o of naturally spawned cod eggs from both wild fish (planktonic samples) and
from a broodstock had abnormal cleavagesat the 2-128 cell stage. Eggs with observed early abnormal
cell-cleavage showed very poor survival and hatching rates. For broodstock cod, egg viability was
highest during the peak of the spawning season.
The North Atlantic cod may provide a good model species for studies of egg viability variations
in wild and captive individuals, as its reproductive biology is well studied. It is now documented
that the egg production in cod is dependent on fish age, nutritional status, batch number, and stress
factors.

relatively well known (Sars 1879; Hjort
1926; Rollefsen 1954; Ellertsen et al. 1989;

Sundby et al. 1989; Ellertsen and Solemdal
1990).
The studies on cod mentioned in this paper have concentrated on cellular malformation in early developmental stages as a
possible egg quality characteristic, on the
effect of maternal age and condition on the
spawning result, on viability of eggs obtained from successive egg batches during a
spawning season, as well as on an assessment of the egg quality of newly spawned
eggs in the sea. The paper is a discussion of
how these factors seem important in determining the egg quality of cod.

Very little is known about the causes of
mortality in marine fish eggs, both for eggs
spawned in the wild, and for eggs from captive spawners (Blaxter 1989). Variations in
egg viability are often considered as unimportant in the wild, and poor egg quality is
therefore regarded as a laboratory or a
hatchery phenomenon. However, results
from various field studies on early stages of
fish eggs indicate that genetic defects and
other biological factors may contribute
largely to natural egg mortality (Longwell
and Hughes 1981; Yannopoulos and Yannopoulos 198 1;Longwell et al. 1984; Kjsrsvik et al. 1984, 1990).
For cod Gadus rnorhua L. both quantitative and qualitative aspects concerning
offspring viability have been investigated,
in relation to the assessment of reproduction of wild fishes as well as to cultivation.
The cod is an important species in North
Atlantic fisheries and is also regarded as a
promising species for aquaculture and sea
ranching. The recruitment dynamics of this
species have been studied for many years,
and spawning period, overall egg and larval

mortality rates as well as larval ecology are

Cell Morphology as an Indicator of
Egg Quality
The symmetry of the earliest cells is considered as a possible direct indicator of viability in fish eggs (Kjsrsvik et al. 1990).
Pelagic fish eggs-like cod eggs-are transparent, and the cell cleavage pattern is therefore easy to observe. Malformations in the
early, undifferentiated cells will also influence the further development of the embryo

Q Copyright by the World Aquaculture Society 1994

22


EGG QUALITY IN COD

FIGURE1.

23

Typical cellular malformations (arrows) observed in early blastulae of cod Gadus morhua L.

more strongly than defects occurring in single cells later in the development. An example of typical observed malformations is
shown in Fig. 1.
If the occurrence of cellular malformations in early cleavage stages of fish eggs is
correlated to later egg and larval survival,
it follows that such morphological characteristics can be used to predict the viability
potential of an egg sample. For cod eggs, the
observed relationship between malformation rate and hatching rate in different egg
groups is shown in Fig. 2. Westernhagen et
al. (1 988) found a highly significant correlation between observed rate of normal fish

embryos (several species) in natural Baltic
Sea eggs and their viable hatch. A similar
relationship existed for eggs spawned by the
cod broodstock at the University of Tromsra
(Fig. 2; Kjersvik, unpublished).
Incubation of eggs from plankton, which
were separated into groups characterized as

“normal” and “abnormal” blastulae,
showed that the normal groups had a significantly higher hatching rate (Fig. 3;
Kjrarsvik, unpublished). Larvae hatching
from the “abnormal” groups also seemed
to be less pigmented and less active than
larvae hatching from “normal” embryos.
Further quantitative analyses need to be
done, but defects visible in the early blastula
stage will affect the subsequent survival potential, even though many embryos survive
through hatching. It should also be recommended to make such observations at
the 4-128 cell stage, as irregularities become
more difficult to observe as the cells become
smaller and more numerous (Westernhagen
et al. 1988; personal observations).

Viability of Cod Eggs in the Sea
Several recent studies now conclude that
the proportion of developmental abnormalities in pelagic fish eggs in an area may


24


KJBRSVIK

”t
10

o !
0

I

10

20

30

40

50

70

60

80

90100

Hatching (%)
FIGURE 2. Relationship between rate of normal embryos and hatching rate of cod eggs. .-planktonic

samples
in the Baltic, data from Westernhagen et al. 1988; A-naturally spawned eggs from a broodstock, Kj0rsvik.
unpublished.

be used as a monitoring tool for the state of
the marine environment. Studies reporting
on such phenomena have had various approaches, but have mostly been concerned
with monitoring pollutant effects on reproduction in certain areas.

-

In a study from the polluted New York
Bight, Longwell and Hughes (1 98 1) found
high malformation rates and mitotic aberrations in mackerel eggs. Dethlefsen et al.
(1985, 1987) made similar observations on
pelagic fish eggs in the southern North Sea.

3

-at

0

U

n

80 --

60


--

#

2
ZJ 4 0 - -

I
I

0
Q)

Y

0 4
0

i

10

20

30

40

50


60

70

80

90

100

Hatching rate (46)
FIGURE 3. Hatching rate of cod egg groups characterized as “normal” or “abnormal’’. The eggs were selected
by observed morphological aberrations at the early blastula stage lfrom Kjmsvik and Stene. in preparation).
W-eggs with visible cellular malformations; 0-eggs with normal appearance.


EGG QUALITY IN COD

Investigations in the Baltic showed that it
was possible to establish a positive correlation between pollutant load in the environment and incidence of abnormalities in
developing pelagic eggs (Westernhagen et al.
1981, 1988; Graumann 1986), and interspecific differences in malformation rates
were also found in these areas. Up to 80%
of the sampled dab eggs were abnormal in
the most polluted areas close to the Rhine
estuary (Cameron et al. 1989), which is close
to the maximal values of 87-94% for mackerel eggs found in the New York Bight
(Longwell and Hughes 198 1). Mean values
of the abnormal rate of cod eggs from the

Baltic and the North Sea are reported between 18 and 36%, the rates varying somewhat between years (Graumann 1986;
Westernhagen et al. 1988; Cameron et al.
1989).
There are no systematic records of abnormal development in situ from early investigations on planktonic fish eggs (Strodtmann 1906; Jacobsen and Johansen 1908;
Rollefsen 1930), when the burden of manmade pollution was insignificant in the open
sea. Rollefsen (1929) mentioned fish eggs
with abnormal looking embryos from his
field studies, but these were dying eggs, most
probably caused by tough handling during
the sampling procedure.
On a spawning ground in the unpolluted
Balsfjord (northern Norway), Kjarsvik et al.
(1 984) found that 6-6O% of mitoses in cod
eggs from plankton were abnormal, and that
the incidence of abnormal embryos was up
to 20%. The temperature in Balsfjord is usually below 3 C in the upper layer during the
spawning season (Eilertsen et al. 198 l), and
may fall below 0 C. Such an environment
may be considered as very cold for cod egg
development (Westernhagen 1970), although normal development has been reported for temperatures down to - 1.5 C for
eggs originating from cod adapted to cold
environments (Makhotin et al. 1984).
A baseline for the natural occurrence of
developmental defects in pelagic fish eggs
should therefore be sought in areas without

25

severe hydrographic conditions or pollution. The Lofoten area in northern Norway
is the most important spawning ground for

the Arcto-Norwegian cod. There are no major sources of pollution in the area, and it
could provide such a baseline for cod eggs.
The spawning and overall egg and larval
mortality is documented over several years.
The area is more oceanic than spawning
grounds in many Norwegian fjords, with respect to environmental factors like salinity
and temperature (Sundby and Bratland
1987).
Field studies in 1990 in Lofoten showed
that approximately 10% (range 8-1 4%) of
the early blastula stages of the cod eggs underwent abnormal cleavages (Kjrarsvik, unpublished). The field work was carried out
during the first days of April, which is at
peak spawning (Pedersen 1984; Sundby and
Bratland 1987), and more than 90% of the
collected eggs were spawned less than 24 h
before sampling (early blastula stage). Temperature in the upper layer (0-50 m) ranged
between 3-6 C, which is within the acceptable range for normal egg development
(Westernhagen 1970). The weather was calm
during the sampling period. Egg viability
from captive cod seems to be highest during
the peak spawning period (see next section),
and the sampling in Lofoten was made during the peak spawning in this area. It is
therefore possible that the egg malformation rate found in Lofoten is a baseline level
for natural cod egg mortality in the sea.
However, the results are based on one sampling period, and the studies should continue during several spawning seasons before
any conclusions on egg viability and mortality causes are drawn.
Quality of Eggs from Broodstock Cod
In fish farming, optimum egg and larval
viability is desirable for successful rearing
of fish fry. Cod, like many other marine

fishes, is a batch spawner. Knowledge about
factors like nutrition, fish age, and “batch
number” is therefore important for handling and selection of broodstock. Investi-


26

KJQRSVIK

4

1

09

13

17

I_____________ --------

21

I

r

30 06
----- I I


10

13

17 20
Apd

24

27

II

05

08
&y

12

I

-_-_--_____

FIGURE4. Hatching rate of naturally spawned eggs from a cod broodstock spawning basin. Egg viability was
highest during the period ofpeak spawning, with more variable results in the latter harfof the spawning season.
Eggs were collected at the early blastula stage twice a week, and each point is a mean of three replicates (Kjmvik.
unpublished). The horizontal axis gives collection dates.

gations on whether the rate of developmental defects can be different between wild and

broodstock cod, and whether it is all or just
a few fish that contribute to the occurrence
of abnormalities, could also elucidate more
on the natural mortality of fish eggs in general.
The spawning of a broodstock was followed through one spawning season at the
University of Tromsra to investigate whether egg viability changed during the season
and how the rate of developmental defects
compared with observations made in nature. The spawning fish were kept in a
spawning basin (3-5 C) and fertilized eggs
were collected every morning. They were
caught from the wild two years before egg
sampling took place and were not first-time
spawners (T. Pedersen, University of Tromso, personal communication). Egg viability
was highest during the period of peak
spawning, with more variable results in the
latter half of the spawning season (Fig. 4;
Kjrarsvik, unpublished). The incidence of
morphological aberrations in the early blas-

tula stage was lowest during peak spawning,
with a malformation rate similar to the rate
found at sea in the Lofoten area, about 10%.
Another approach to observe the spawning result of captive cod during a spawning
season is applied by P. Solemdal and 0. S.
Kjesbu at the Institute of Marine Research
in Bergen, where the effects of fish age, nutritional status and batch number on
spawned eggs are studied. This is done by
using reared cod with a common and known
history from the egg stage, and separating
each spawning female.

A normal cod will spawn 15-20 batches
in 50-60 d (Fig. 5 ; Kjesbu 1989), and the
egg size will decrease as the spawning progresses, with an egg dry weight reduction of
20-30% from the first to the last batch. Older fish produce more viable eggs; however,
the spawning result of the younger first-time
spawners seems more variable (0.S. Kjesbu, personal communication).
A stressed fish, on the other hand, will
behave differently with irregular spawning
intervals, and the egg batches have low fer-


27

EGG QUALITY IN COD
I

1.5

I-

i'*:

...*....

.*.*

.*

13-


*...

d 121.1 ,

Batch number

,

,

,

,

,

,

,

,

II

,

,

,


I

,

,

,

I

I

Batch number

FIGURE5 . Observed spawning result of one regularly spawning cod female (Kjesbu 1989)

tilization rates with increased occurrence of
abnormal embryos (Kjesbu 1989). Up to '/3
of the fish will show such signs of stress in
spawning experiments like these (Kjesbu,
personal communication).
In an experiment where reared cod were
given excess food, average condition factor
and fecundity were 1.5 and 2.5 times that
of wild cod respectively (Kjesbu 1988). Initial egg size seemed to be dependent on fish
size and independent of fecundity (Kjesbu,
1988), but later studies showed that egg size
will decrease more rapidly during spawning
if the fish is starved (Kjesbu et al. 199 1). In
broodstock fish held on different feeding rations for 6-9 mo prior to spawning, the degree of atresia (resorption of vitellogenic oocytes) was substantially affected. 20-80% of

the potential fecundity was realized, dependent on the nutritional status of the female
spawner (Kjesbu et al. 1991). This is also
known for other species such as rainbow
trout (Scott 1962). Eggs from starved cod
had a lower dry weight, and were also less
viable than eggs originating from well fed
fish (P. Solemdal, personal communication).
Egg Viability in Wild versus Captive Fish

Varying egg quality may be characterized
by variations in chromosome aberrations
and biochemical composition (Kjersvik et
al. 1990). Only morphological characteristics provide as yet a possible direct measure,
which seems to be valid for many species

with pelagic fish eggs. For cod, studies of
egg quality have had two approaches: 1) to
provide optimal egg quality for fry production in aquaculture; and 2) to investigate
stock-recruitment relationships and effects
of pollution.
The reported studies provide a basis for
understanding the reproductive pattern in
cod and other fish, and for optimization of
broodstock handling and selection, as well
as of fry production. It is now possible to
draw conclusions that the spawning result
of cod during a spawning season is dependent on fish age, nutritional status, and batch
number of spawned eggs.
There are not necessarily differences in
survival potential of eggs from wild or captive spawners, but the number of eggs produced is clearly in favor of the well fed captive fish. It is also important not to stress

the fish during the spawning season, as this
seems to cause spawning activity to be out
of phase with ovulation, with poor viability
of the eggs as a result. Vitellogenesis and egg
maturation is continuous during the spawning season for a batch-spawning fish like
cod, and these processes are susceptible to
environmental influences. Cod will spawn
naturally in captivity, but some species, like
turbot, must be stripped. Stripping of eggs
may cause varying egg quality in several
ways, such as ovempening or disturbances
in the ovulation process. There is still little
practical knowledge on how to determine
the ovulation cycle in fish. A good method


28

KJQRSVIK

for prediction of egg maturation and ovulation would therefore be of great value, especially in the case where stripping is necessary.
Similar methods and combination of results from the field and the laboratory can
still provide more data on what will determine the egg’s chances of becoming a fish
fry. Different egg groups spawned both in
nature and in captivity have varying degrees
of cellular abnormalities, and further studies should provide a more detailed knowledge of how single spawning females contribute to the observed rates. Crucial aspects
in this research will be to elucidate factors
of importance for varying egg quality, how
the age of female spawners influences egg
viability, and if the use of cell malformations in egg groups can serve as quantitative

predictions for subsequent larval viability.
Acknowledgments
Thanks are due to the Norwegian Fisheries Research Council, for providing financial support for my part of the studies mentioned in this paper. I am also grateful to
Dr. Per Solemdal and Dr. Olav Sigurd Kjesbu for valuable comments on the manuscript.
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