Jökull - 01.12.1969, Side 148

Fig. 6.
Herring migra-
tions in August
to September
1968 ancl
temperature
at 100 m.
turn is 1964 in spite o£ the fact that the sea
temperature off North-Iceland was relatively
high.
During the first few years under discussion
the temperature in the tongue of cold water
northeast off Iceland does not appear to have
a retarding effect on the westward migration
of the Norwegian herring ancl in 1964 the di-
stance of the southern boundary of the cold
water is closely correlated with the distribu-
tion of the herring northeast of Iceland. As
from 1965, however, the proportion of polar
water in the tongue of cold water has greatly
increased (Malmberg 1967, 1969) and at the
same time the herring no longer migrated
westwards across the cold current and as far
as the migrations along the southern edge of
the cold tongue are concerned these did not
reach further west than the areas east and
southeast of Iceland. Insteacl, the main migra-
tion headed north and northeast towards the
area south and southeast off Jan Mayen. Un-
til 1967 a second invasion of the Norwegian
herring usually arrived at the southern border
of the cold tongue east of Iceland sometime
in July and remained in the area east of Ice-
land throughout the summer feeding period.
144 JÖKULL 19. ÁR
This was e.g. very clear in 1964 and 1956
(.Jakobsson 1968). Since 1967 these summer
concentrations off east Iceland have not been
located except in very small quantities ancl it
appears that in 1967 and 1968 almost the en-
tire adult stock travelled all the way to the
Spitsbergen-Bear Island region during the sum-
mer feeding period. Thus the herring prob-
ably react to a tlireshold level of temperature
within the cold East Icelandic current. Since
1965 the increased proportion of polar water
within the Arctic water (see also Malmberg
1969) certainly brought the environmental
condition in tliis area below the endurable
threshold for the feeding migrations of the
Norwegian herring. Thus it appears from the
discussion above that the drastic changes in
the migrational pattern of the Norwegian
herring which took place during the periocl
1964—1967 was primarily due to changes in
temperature and changed water masses especi-
ally the greatly increased proportion of polar
water within the Arctic water in the area east
and northeast off Iceland. In addition one
must bear in mincl that complicated changes
in “stock units” within the Norwegian herring
may also have affected the migrational changes