Fróðskaparrit - 01.01.2004, Page 279

KYNSKYKNU-BÚNING OG -GÝTING HJÁ KRÆKLINGI (MYTILUS EDULIS) í FØROYUM 277
ton was not measured at the study site in
the present sludy but investigations from
the Faroe shelf show that the phytoplank-
ton concentrations are highest in sum-
nter (June-July) and that phytoplankton is
sparse after September (Gaard, 2000). The
main spawning in the Faroe Islands was
observed in early summer. Therefore both
adults and larvae had probably access to
abundant food during sunrmer.
A gametogenic cycle similar to the one
in the Faroe Islands with spawning during
summer and developing gonads during win-
ter has been observed in a number of popu-
lations in Western Europe e.g. in Ireland
(Snooden and Roberts, 1997), Wales (Lowe
et al, 1982), Germany (Sprung, 1983),
Portugal (Reis-Henriques and Coimbra,
1990), the Baltic (Kautsky, 1982), western
Norway (Barkati, 1990) and the Wadden
Sea (Pieters et al, 1979). On the east coast
of the United States, Brousseau (1983) ob-
served a population with a similar pattern
of gametogenesis in Long Island sound. In
the Faroe Islands nutrient reserves, which
have accumulated in the mantle tissue dur-
ing the summer, probably have functioned
as an energy source for gametogenesis dur-
ing autumn and winter when food supply
was limiting, a process common in bivalves
(Gabbot, 1983).
Another pattern of gametogenic cycle is
found when carbohydrate stores are used
primarily for non-reproductive metabolic
requirements during the winter and the
gonads are synthesized in late winter and
early spring in conjunction with the onset
of phytoplankton spring growth followed
by spawning in summer. This has been
observed in populations in the north-west
Allantic as in Newfoundland (Thompson,
1984), off the east coast (Newell et al.,
1982) and the west coast of the United
States (Emmet et al, 1987).
Both types of gametogenic patterns occur
in Mytilus edulis in Iceland. The initiation
of gonad development has been observed
in January in west Iceland at a sea tempera-
ture of 0-1.8°C while on the east coast the
gonads starts developing in October at a
temperature of 7°C. This is thought to be
related to the fact that in autumn food sup-
ply measured as chlorophyll-a and winter
temperatures are higher in the east than in
the west. Spawning starts in June or July
(10-11°C) and continues through Novem-
ber at both sites (Thorarinsdóttir, 1996;
Thorarinsdóttir and Gunnarsson, 2003).
The gametogenic cycle in the mussels
in Faroe Islands is more similar to the one
observed on the east coast of Iceland than
on the west coast. However, the onset and
duration of spawning is different at the two
sites and can probably be related to the dif-
ference in sea temperatures observed. The
sea temperature in the Faroe Islands does
not fluctuate as much as in Iceland. The
maximum. sea temperature is observed in
August/September in Faroese waters with
mónthly averages at 9.4-10.7°C and in Au-
gust in Icelandic waters at 9.8-11.0°C. At
both sites the minimum sea temperature is
observed in March which is considerably
lower in Icelandic (0-2°C) than in Faroese
waters (4-5°C). The spring temperature is
higher in Faroe Islands and probably there-
fore the gametogenesis starts earlier and is
followed by earlier spawning.