Jökull - 01.12.1969, Blaðsíða 138

On Herring Migrations in Relation to Changes
in Sea Temperature
JAKOB JAKOBSSON,
MARINE RESEARCH INSTITUTE, REYKJAVÍK, ICELAND
INTRODUCTION
The Icelandic herring fisheries have for many
years been subjected to large fluctuations,
which have had severe influence on the na-
tional economy. Therefore great attention has
been given to all research concerning the na-
ture of these great fluctuations in the Icelandic
lierring landings. In this connection the irn-
portant international research program carried
out by Icelandic, Norwegian and Soviet scient-
ists should be notecl.
Such and other research work lias shown
that the causes of the íluctuations in landings
are so compiicated that it is usually not pos-
sible to pinpoint one single reason as the real
cause of the trouble. On the other hand it is
probably nearer the truth that the causes of the
fluctuations in the Icelandic herring fishery
are due to many factors some of which are
more or less independent, but other inter-
dependent. As an example of the former it is
sufficient to draw attention to such factors as
development of fishing gear and iluctuations
in the size of the herring stocks. The compli-
cated influence of the food chain in the sea is
on the other hand mentioned here as an ex-
aniple of interdependent factors influencing
the herring catches. Fig. 1 clearly shows the
large fluctuations in the Icelandic herring land-
ings during the period 1904—1967. It is, how-
ever, not within tlie scope of this paper to
analyse and discuss the causes of these fluctua-
tions generally but to consider the possible
influence of one single factor within the whole
complex of causes, i.e. variations in sea tem-
perature especially during the last few years.
Variations in sea temperature are bound to
affect the herring biologically in several ways.
In this connection it is sufficient to remind
the reader that the bloocl temperature of the
herring as that of other fishes is directly de-
pendant on the sea temperature. The physio-
134 JÖKULL 19. ÁR