Fróðskaparrit - 31.12.2000, Blaðsíða 84

88
REPRODUCTION OF FAROE PLATEAU COD:
SPAWNING GROUNDS, EGG ADVECTION AND LARVAL FEEDING
Introduction
The water on the Faroe Shelf (Fig. 1) is rel-
atively well separated from the offshore en-
vironment by a persistent tidal front that
surrounds all the islands. The tidal front is
usually situated between the 100 and 150 m
isobath (Gaard et al, 1998; Gaard and
Hansen, 2000). Due to extremely strong
tidal currents, the water column over the
shallow parts of the shelf is well mixed
from the surface to the seabed. In addition
to the tidal currents, a residual current
flows anticyclonically on the shelf
(Hansen, 1992; Simonsen, 1999; Hansen
and Larsen, 1999). The renewal of the
shelf water is highly variable and is, for ex-
ample, affected by prolonged wind stress.
The average flushing time of the central
shelf water is estimated to be about three
months (Gaard and Hansen, 2000).
The shelf water maintains its own neritic
plankton ecosystem, which, with regard to
the phytoplankton (Gaard, 1996; Gaard et
ai, 1998) and zooplankton (Gaard, 1999)
is, in most years, quite different from the
surrounding oceanic environment. Zoo-
plankton on the shelf is basically dominat-
ed by neritic species, of which Acartia lon-
giremis and Temora longicornis are usually
the most abundant copepod species during
summer. During spring, barnacle larvae
may also be quite abundant. The shelf
ecosystem, however, is also affected by
oceanic zooplankton. This influence is
mainly by the copepod Calanus finmarchi-
cus, which is advected onto the shelf in in-
terannually, highly variable abundances
(Gaard, 1999). This species is rarely found
on the shelf during winter, but is advected
7°W
Fig. 1. Bottom topography and main flow field around
the Faroes. The dotted line indicates a typical position
ofthe tidalfront.
Mynd 1. Botnskapið og høvuðsrákið rundan um Før-
oyar. Brotastrikan vísir, hvar sjóvarfalsfronturin vanliga
kann vera.
onto the shelf during spring and summer
(Gaard, 1999; Gaard and Hansen, 2000)
and reproduces on the shelf (Gaard, 2000).
C. finmarchicus overwinters in diapause,
mainly in the cold waters of the Norwegian
Sea at depths below about 500 m (Heath et
al., 2000a). During that time, large quanti-
ties of C. fmmarchicus are transported by
the deep overflow through the Faroe-Shet-
land Channel (Heath and Jónasdóttir, 1999)
and further through the Faroe Bank Chan-
nel (Gaard and Hansen, 2000). During
spring, it migrates towards the surface
(Heath, 1999), and advection onto the
Faroe Shelf is considered largely to origi-
nate from the Faroe Bank Channel (Gaard
and Hansen, 2000). During early spring