Fróðskaparrit - 31.12.2000, Blaðsíða 96

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REPRODUCTION OF FAROE PLATEAU COD:
SPAWNING GROUNDS, EGG ADVECTION AND LARVAL FEEDING
eggs are reported to decrease in size as the
spawning season advances (Kjesbu et al.
1992; 1996; Chambers and Waiwood,
1996; Bleil and Oeberst, 1998; Marteins-
dóttir and Steinarsson, 1998; Trippel,
1998). The eggs in 1995 were spawned
during the peak spawning period, while the
eggs and larvae sampled in 1999 were
spawned quite late in the spawning period.
We may thus speculate whether or not the
difference in larval lengths these two years
may have been due to the combined effect
of these two parameters (female size and
spawning stage).
A large number of studies show size-de-
pendent food selectivity by cod larvae,
where first-feeding larvae select small-
sized prey (mainly copepod eggs and nau-
plii), gradually switching over to larger
prey as they grow (e.g. Thorisson, 1989;
Kane, 1984; Otterá, 1993; McLaren and
Avendano, 1995). The present study pro-
vides similar results. A great number of
first-feeding larvae had phytoplankton in
their guts in April. This is a common íind-
ing in small larvae (Nordeng and Bratland,
1971; Ellertsen et al., 1980). Among the
zooplankton prey group, copepod eggs
(presumably Calanus fmmarchicus eggs)
were by far the most important food item
for first-feeding cod larvae. This finding
was also seen in the above-mentioned stud-
ies, but in those studies the number of eggs
was markedly lower in proportion to nau-
plii than in the present study (Table 4). As
the larvae grew, they switched over to larg-
er prey. In May 1995, cod larvae selected
mainly small and medium-sized cope-
podites and copepod nauplii (Table 5).
This increase in the size of selected prey
items continues as the larvae and pelagic
juveniles on the Faroe Shelf grow during
summer (Gaard and Reinert, 2000).
Since C. finmarchicus eggs seem to be a
key food source for the first-feeding cod
larvae during early spring, knowledge
about the egg production of C. finmarchi-
cus during the early spring is important in
order to understand feeding conditions for
first-feeding cod larvae on the Faroe Shelf.
The fecundity of copepods is generally
considered to be food limited (Diel and
Tande, 1992; Kiørboe et al„ 1990; Kiørboe
and Nielsen, 1994; Hirche, 1996; Niehoff
etal., 1999). On the Faroe Shelf, the spring
bloom develops during May in most years
(Gaard, 1996; Gaard et al., 1998; Gaard,
1999). However, the lag phase on the shelf
may be quite long. During this pre-bloom
period (in April), the phytoplankton bio-
mass is clearly higher on the shelf than off-
shore in most years (Gaard, 1994; 2000).
During spring, copepod production on the
central shelf, for the most part, increases si-
multaneously with the increase in phyto-
plankton (Gaard, 1999; 2000). However,
overwintered C. finmarchicus that are ad-
vected onto the shelf slope start to repro-
duce on the western and north-western
shelf and slope regions during the pre-
bloom period in most years. Offspring
from this reproduction appear to be advect-
ed clockwise in an easterly direction in the
northern shelf region, and, subsequently,
are dispersed throughout the central shelf
area. This happens in most years prior to
the increase in copepod reproduction in the
central shelf region (Gaard. 1999; 2000).