Types and frequency of malformations and mortality in eggs of Arcto-Norwegian cod: A field study
Original version
This report is not to be cited without prior reference to the authorsAbstract
Embryological studies on living cod eggs in NUNC-trays
showed that spontaneous malformations during
embryo development are lethal. They are called natural
egg mortality in contrast to external lethal factors,
as predation, extreme physical conditions and
pollution.
The natural mortality in cod eggs, both coastal and
Arcto-Norwegian, is significantly reduced from first
to second spawning, from laboratory experiments.
The same embryological method was used during
field studies on natural egg mortality on planktonic
cod eggs in the spawning area in Lofoten, Northern
Norway, in the March - April years 2000 and 2001.
The types malformations which was found in a fild
also corresponding to certain morphogenetic movements
in embryodevelopment and equel discover in
the experimental work. Most part of lehal malformations
(more than 50 %) is identified after hatching
only. The pattern of types malformations differ
between spawning grounds and to be consistant
during two years. The frequencies of malformations
to the end spawning period significantly reduced of
29,7 to 9,8 % for Vestfjord.
To demonstrate the higher natural egg mortality in
eggs from first time spawners, two strategies were
followed:
1. Comparing natural egg mortality in egg samples
from 2000 and 2001, collected during same period
and from the same areas.
2.On the basis of the acoustic survey of the spawning
population of Arcto-Norwegian cod in Lofoten,
areas of high frequency of first time spawners were
compared.
Luckily, in the year 2001 the proportion of first time
2
spawners was specially high. A comparison between
the years 2000 and 2001 demonstrated an increase
in natural mortality from 23 to 31 %.
In Vesterålen, the area of highest frequency of first
time spawners, natural egg mortality increased from
31 to 46 %. Because of two systematic errors the
numbers are minimum values.