Fróðskaparrit - 01.01.2002, Blaðsíða 154
152
NEWS AND PROGRESS 2001
tion and the zooplankton biomass, and it is
hypothesised that the observed variations in
timing of the production development dur-
ing spring and the calculated new, primary
productivity is most likely due to variable
grazing. It is hypothesised, moreover, that
this effect occurs mainly during the pre-
bloom period, when grazing by overwin-
tered C. finmarchicus may prolong the lag
phase.
First-feeding cod larvae on the Faroe
Shelf predate mainly on copepod eggs, of
which C. fmmarchicus eggs are considered
to be the main food source. In large part,
these larvae occur prior to the phytoplank-
ton spring bloom. Seasonal copepod pro-
duction on the central shelf region depends
largely on the development of phytoplank-
ton production. However, on the western
and north-western shelf and slope regions,
C. finmarchicus shows significant egg pro-
duction prior to the spring bloom. This is
roughly the same area that is considered to
be the key advective area of C. finmarchi-
cus onto the Shelf in most years. The
spawning grounds of Faroe Plateau cod are
in the same region also, but are restricted to
the area just inside the tidal front that sur-
rounds the Shelf water. Hence during early
spring, cod eggs and larvae are advected
along the same route as the offspring of
spawning C. finmarchicus. During spring
and summer, as cod and haddock larvae and
pelagic juveniles grow, they progressively
consume larger prey. First consuming
copepod nauplii and small copepodites,
followed by larger copepods, they finally
add decapod larvae and occasionally even
fish larvae to their diet.
Variability in the growth rates of cod and
haddock and seabird recruitment co-fluctu-
ated quite well. Long-term data also
showed that periods with large fish year-
classes generally had higher growth rates
than periods with the small year-classes, in-
dicating that food production for fish and
sea birds may exceed the increased preda-
tion pressure during productive periods. It
is hypothesised that variability in fish re-
cruitment during the studied period is
largely affected by variable plankton pro-
duction and abundance, while variable sea
bird recruitmenl and fish growth rates are
largely affected by variable production of
sandeel.