Jökull - 01.12.1969, Síða 139

logical processes of the fish are therefore to a
large degree affected by environmental changes
in temperature. Thus it is well known that
the growth rate of herring as that o£ other
fishes is temperature dependant. The influence
of environmental changes on such vital physio-
logical process as spawning was clearly shown
when a prespawning herring shoal was kept
in captivity in a Norwegian fiord (Dragesund
1960) where the sea temperature was sorae-
what lower than on the natural spawning
grouncls further off shore. Thus the spawning
process was delayed about three rnonths or
until the sea temperature in the fiord had
reached the same level as on the spawning
grounds.
The influence of changes in sea temperature
on the herring physiology will, however, not
be particularly dealt with in this paper since
its purpose is to discuss the possible connection
between the drastic recent changes that have
taken place in the migration pattern of the
herring stocks on one hand and the changes
in sea temperature on the other.
THE HERRING STOCKS
The Icelandic herring fisheries are based on
three different spawning populations or herr-
ing stocks i.e. Icelandic spring and sumrner
spawners as well as Norwegian spring spawners.
Thus the herring stocks are named after their
spawning area and spawning season.
The spawning area of the Icelandic herring
stocks are off the south and southwest coast
of Iceland and the height of the spawning
season is in March ancl July for the spring
and summer spawners respectively. At the end
of the spawning season the Icelandic herring
stocks migrate in varying proportions to the
north and east coast of Iceland and often
spend their feeding periods there. These feed-
ing migrations of the Icelandic herring especi-
ally the spring spawners to the north coast
were very pronounced during the period 1955—
1962 wlien this herring stock contributed niore
that 50% o£ the catches. Since 1962 the pro-
portion of the Icelandic herring in the north
and east coast summer catches has decreased
whereas the proportion of the Norwegian
spring spawners has increased correspondingly.
The Norwegian herring has a spawning sea-
son in March off the west coast of Norway.
In the spring this liistorically largest stock of
the Atlanto-Scandian herring migrates in a
northwesterly direction and spends its feeding
period usually in the mixed waters of Atlantic
ancl Arctic origin in the western part of the
Norwegian sea and off Nortli- ancl East-Iceland.
During the spring and early summer one finds
that the feeding migration of the herring is
greatly affected by the distribution of C. fin-
marehicus especially the adult stages of tliis
copepod sp. (Ostvedt 1965). This feeding migra-
tion sometimes reaches as far west as the west-
ern north coast of Iceland ancl as far north
as Jan Mayen and Spitsbergen. The habitat
of the Norwegian herring during the spring,
summer and autumn is therefore to be found
within a range of rather low sea temperature
in the boundary areas of mixed Atlantic and
Arctic water masses as already mentioned.
HERRING MIGRATIONS 1961
When we begin this account of lierring
migration in relation to temperature in the
year 1961 the state of the Norwegian herring
stock is such that 11 year old fish i.e. the very
strong yearclass from 1950 is still predominant
in the age distribution. It was, liowever, known
that a strong yearclass from 1959 was maturing
and would recruit to the spawning stock in the
coming few years. As regards the Icelandic
herring stocks, 5 years old herring of the 1956
yearclass predominated in the age distribu-
tion.
The annual investigations carried out by Ice-
landic and Norwegian research vessels showed
that there were during the first week of June
two separate herring migrations approaching
the fishing grounds off the nortli coast of Ice-
land, i.e. one (Icelandic herring) from the west
and the other (Norwegian herring) from the
east. Both migrations were in the mixed waters
of the “polar front” where the temperature at
20 m depth was 3—4° C (Jakobsson and Ösl-
vedt 1961). By the middle of June the herring
had invaded the western and eastern parts off
the north coast fishing grounds and remained
in these areas for the succeeding 4—6 weeks.
JÖKULL 19. ÁR 135