Fróðskaparrit - 01.01.2002, Blaðsíða 114

1 12 MYXIDIUM GADl (GEORGEVITCH, 1916) AND MYXIDIUM OVIFORME (PARISI, 1912)
1N THE GALL BLADDERS OF COD (GADUS MORHUA L.)
examined cod were infected with Myxidi-
um gadi. In other areas around New Found-
land the prevalence was 23%-48%. Other
parasites have been found suitable as bio-
logical indicators of migration and stock
delimitation of cod in the New Foundland
area (Khan and Tuck, 1995).
In the New England area the copepod
Lernaeocera branchialis was investigated
for its potential as a biological indicator of
cod stocks (Sherman and Wise, 1961).
Comparisons between the parasite fauna
of cod in the Barents Sea and Balsfjord in
North Norway showed that many parasites
were potential biological tags (Hem-
mingsen et ai, 1991). Especially Myxidium
sp. was thought to have a large potential as
a biological indicator because of the long
duration of the infection. Myxidium sp. was
later found to comprise two species Myxid-
ium bergense and Myxidium oviforme (Lar-
sen et al., 1997). The duration of the infec-
tions of the digeneans in the digestive tract
of cod is less than a year, and this single
fact makes the digeneans less suitable as
biological indicator. Nevertheless Hem-
mingsen et al. (1991) find Hemiuris levin-
seni suited because of the high prevalence
in the fjords. Køie (1984) also argues that
digeneans might be useful biological indi-
cators because of their high intermediate
host specificity. This narrows down the
area where the cod can get infected. Thus
migration can be determined more precise-
iy-
The larvae of the nematodes Anisakis
simplex, Psedoterranova decipiens and
Phocascaris sp. are l'ound useful as biolog-
ical indicators because of a long duration of
the infection. Adult stages of nematodes
such as Cucculanus cirratus and Hys-
terothylacium aduncum are not found use-
ful because of the short duration of the in-
fection (Hemmingsen et al., 1991).
Further north in the West Murman part of
the Barents Sea in the Kislaya inlet and the
Ura inlet Karasev et al. (1996) examined
165 cod specimens and found 36 parasite
species. In these studies Echinorhyncus
gadi and Anisakis simplex were found the
most suitable biological indicators. In
Greenland Boje (1987) did use the parasites
Hemiuris levinseni and Hysterothylacium
aduncum to separate the cod into fjord and
ocean stocks. In the area around Bornholm,
Buchmann (1986) finds the prevalence of
the metacercarian stage of Cryptocotyle
lingua to be very low 0-3.5%. Two different
cod stocks in the Baltic Sea are separated at
the Bornholm area, and Buchmann (1986)
explains the low infection frequency as a
result of intermingling between the highly
infected cod to the west, and the non in-
fected cod to the east of Bornholm.
The use of parasites as biological indica-
tors, and the criteria that have to be set
when choosing parasites for fish stock in-
vestigations are best reviewed in Kabata
(1963), MacKenzie (1987), Lester (1990),
Moser (1991), Williams et al. (1992) and
MacKenzie and Abaunza (1998).
The parasite fauna ofthe Cod
ofthe Faroe Plateau and the Faroe Bank
Several studies have been made on the par-
asite fauna of the Faroe Plateau and the
Faroe Bank cod. Only very few of these in-
vestigations were intented for fínding para-