Fróðskaparrit - 01.01.2005, Page 124

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MIGRATION OF COD (GADUS MORHUA):
TAGGING EXPERIMENTS AT THE FAROES 1952-1965
et al., 1986). Large cod (35-100 cm)
north of Scotland moved somewhat more
than cod in this study (Easey, 1987). The
reader is referred to Robichaud and Rose
(2004) for a comprehensive comparison of
all cod stocks in the Atlantic ocean.
Migration distance depended on tagging
location (Figure 14) and several factors
may have been involved. The bottom to-
pography sets the limits what is possible,
and the topography is different between ar-
eas, which again may influence the abun-
dance of food or hiding places. Stomach
investigations at the time of these tagging
experiments indicate that there were dif-
ferences in foođ organisms between areas
(Rae, 1967) and stomach analyses during
the period 1997-2004 show that both spe-
cies composition and abundance may vary
considerably between areas (unpubl.). The
deciding factor (or factors) is, however,
not known.
The movement of cod to deeper waters
is likely related to fish size and abundance
of suitable food organisms. Results from
the summer groundfish survey on Faroe
Plateau (ICES, 2004) show that the mean
size of cod increases with increasing depth
(unpubl.) and stomach analyses show - on
a broad scale - that prey size increases
with increasing depth (unpubl.). Thus the
relation between predator and prey size
(Floeter and Temming, 2003) seems to be
an important factor that is influencing the
distribution of cod on Faroe Plateau.
The swimming capacity of cod is lim-
ited (Soofiani and Priede, 1985) and may
affect migration. In order to minimise
swimming, cod may select the shortest
way to deeper waters, which normally
can be achieved by moving a few nautical
miles farther from land. This is one way of
keeping the populations of cod east (Fig-
ure 7-9) and west of Faroe Islands (Figure
9-11) separated. The Faroe Plateau is, on
the other hand, a small area compared to
other areas (e.g. Icelandic waters) suggest-
ing that other factors than energy minimi-
sation could control the migration pattern
to deeper waters.
It is interesting that there always were
some cod that moved to new and distant
areas. The size of these fish was quite
similar to local cod, but we have indica-
tions, that diet played a role. On 21 No-
vember 1952, 75 cod were tagged south
of Vágoy (SVA), that were characterised
as “sand-eel preying cod that were caught
off bottom”. Seventeen of them were re-
captured, and the average distance to the
tagging locality was 22 nautical miles.
The next day 31 cod were tagged on the
same locality and they were “not sand-eel
preying cod and caught close to bottom”-
Eight of them were recaptured, and the av-
erage distance was only one nautical rnile.
The difference was statistically significant
(Mann-Whitney U-test, p<0.05). Faroese
fishermen have told the first author that
sand-eel preying cod move much more
than cod eating benthic food. Migrants to
new areas may play an important role by
enhancing local populations of cod that for
some reason, for example high fishing in-
tensity, are on a low level.
Migration distance differed between
years and one reason may be a variable
fishing pattern (i.e. the observed differ-