Fróðskaparrit

						66

SEASONAL CHANGES IN THE INFECTION OF YOUNG SAITHE

chus gadi give an impression which is

much more varied (Fig.6c,d). These para-

sites were mainly found in the gut, which is

a very unstable environment.

Polyanski (1966) in an extensive para-

sitological research of fishes from the Bar-

ents Sea reported 45.5 % of 1-year old

saithe and 75.0 % of 2-year old saithe in-

fected in/on the liver with Anisakis sp. The

same figures for Contracaecum sp. were 3-

4 times higher than in the present study, but

he did not find P. decipiens. Polyanski

(1966) examined seasonal variations of in-

fection for cod and haddock, but not for the

saithe.

Scott (1985) investigated the helmints of

the alimentary tract of Canadian pollock

(=saithe, Pollachius virens L.). His results

do not clearly distinguish between the years

examined, but the pooled prevalence of in-

fection of A. simplex show a rapid increase

in the first 3 length groups (approximately

corresponding to ages 0-3 years) up to 100

%; then a fall for the following length

groups (presumably 4-6 years old) down to

77 %. Neither H. aduncum nor P. decipiens

were found, but McClelland et al. (1990)

reported the prevalence of infection of

Canadian pollock, length groups 31-71+

cm, to be 6-11 % for P. decipiens , and 89-

100 % for A. simplex. Comparing these fig-

ures with the present study it seems that the

main trends in the infections are similar

with age, but there are some distinct dis-

similarities in the parasite species found on

each side of the Atlantic Ocean. More sim-

ilarity is found when comparing with Ice-

landic saithe seems to be more similar;

Hauksson (1992) reported prevalences of

infection to be 72 % with A. simplex, 44 %

with Contracaecum and 44 % with Hys-

terothylacium, but no P. decipiens in 26-39

cm saithe (most likely 1-2 years old). The

relatively high prevalence of infection up to

60 % with P. decipiens in young Faroese

saithe (Fig.4 b) thus seems exceptional.

Wootten (1978) investigated five species

of offshore, small gadoid fishes (cod, whit-

ing, haddock, Norway pout and poor cod,

all primarily 0-2 years old) and found three

types of nematodes: Hysterothylacium

aduncum (then called Thynnascaris adun-

cum), Anisakis sp. and Contracaecum sp.

He did not find Pseudoterranova decipiens

(then Phocanema decipiens), because this

nematode requires seals as final host and

seals are more commonly found in inshore

waters. Interestingly Wootten (1978) recor-

ded more Hysterothylacium than Anisakis

in all five fish species, both in mean inten-

sity and prevalence of infection. He, too

found a peak for Hysterothylacium in Octo-

ber (compared to June/July). However,

Wootten (1978) did not collect nematodes

from the gut lumen and the present study is

not taking into account stomach nema-

todes, so the figures are not completely

comparable. Also it is difficult to recover

all Anisakis of the musculature in a dissec-

tion, while a digestion technique is more re-

liable in discovering the hidden nematodes

and other multicellular parasites.

An examination of the diet of the saithe

would give valuable information in the

search to obtain a more detailed knowledge

in the correlation of the parasite infection to

the composition of the food. Such work

could throw more light on the other intrigu-