Fróðskaparrit - 31.12.2000, Síða 95

NØRINGIN HIA TOSKINUM A LANDGRUNNINUM:
GÝTINGARØKI, EGGSPJAÐING OG FØÐIN HJA LARVUNUM
99
plete circle around the archipelago prior to
the samplings.
Despite the fact that the samplings were
carried out almost two weeks earlier in
1995 than in 1999, most larvae were
markedly larger in 1995 than in 1999. The
use of different sampling gears in each of
the years (WP2 net in 1995 and Bongo net
in 1999) may have caused some differences
in selectivity of the cod larvae. The mesh
sizes of the two nets were the same (200
pm), but the Bongo net was towed approx-
imately four times faster than the WP2 net,
which would result in a higher selectivity of
the largest larvae in the Bongo net. Since
cod larvae collected with the WP2 net
(1995) were larger than larvae collected
with the Bongo net (1999), the size differ-
ence of the cod larvae that were observed
between the two years must be assumed to
have been real and not due to the different
nets used.
The maturity-stage distribution of the
spawning cod indicates that peak spawning
during these two years occurred at about
the same time (in the western area), or pos-
sibly a little later in 1995 than in 1999 (in
the northern area). The water temperature
was almost the same in both years (or even
a little higher in 1999 than in 1995), and the
gut content was not much different as well,
with the same frequency of empty gut in
both years (Table 4). Different growth
rates, therefore, can hardly explain the size
difference between the years. One could
ask whether mortality of the larvae was
higher in 1999 than in 1995, but this does
not explain the lack of the smallest larvae in
1999 compared to 1995, especially when
considering the fact that yolk sac larvae and
young larval stages (Fossum, 1986) were
commonly found in both years. One could
speculate also as to whether or not the size
of the cod eggs might have been different
these two years. If so, then hatching larvae
collected in 1999 would have been smaller
than those collected in 1995.
Despite a large number of studies, there
is still a general lack of information (and
some contradictions too) about the factors
that may affect cod egg size. Available in-
formation, however, indicates that egg size
may be affected by a combination of sever-
al factors, including female size (or age),
condition, spawning stage, and sea temper-
ature. Based on the available information,
it is not possible to determine which factors
may have affected the observed size differ-
ence these two years, although some fac-
tors are more likely than others. Pepin et al.
(1997) reported an increaseđ length at
hatch with increasing temperature. How-
ever, since the in situ temperature was
about 0.5°C lower in the year with the larg-
er larvae (1995,) this cannot explain the dif-
ference in larval lengths between the years.
Other studies demonstrated that large cod
females generally produce larger eggs than
small females (Kjesbu et al., 1996;
Marteinsdóttir and Steinarsson, 1998). The
spawning cod in 1995 was about six cm
larger than in 1999 (Fig. 3; Table 2). This
may have affected egg sizes, but this factor
alone does not seem to be enough to ex-
plain the larval size difference between the
two years. Another possible explanation is
that the sampling in 1999 was carried out
almost two weeks later than in 1995. Cod