SEALWORM (PSEUDOTERRANOVA-DECIPIENS) INFECTION IN COMMON SEALS (PHOCA-VITULINA) AND POTENTIAL INTERMEDIATE FISH HOSTS FROM THE OUTER OSLOFJORD

Infection with Pseudoterranova decipiens in 45 common seals (Phoca vit ulina) from the outer Oslofjord was investigated, Seals were collected before and during the phocine distemper virus epizootic in 1988 when the seal population in the area was reduced from about 350 to 100. In 1989, cod (Gadus morhua) and other fish species were sampled for compa rison with earlier investigations on prevalence and abundance of P. de cipiens infection in fishes from the outer Oslofjord. Seventy-two per cent of cod were infected with P. decipiens larvae in shallow waters b etween the seal skerries; the corresponding abundance was 2.2. At othe r fishing sites at increasing distances from the seal colony, abundanc e dropped to 0.05-0.20. Otoliths recovered from stomachs and seats ind icated that gadids (Micromesistius poutassu, Trisopterus esmaukii and G. morhua) made up more than 80% of the seal diet, Bullrout (Myxocepha lus scorpius) was also heavily infected with P. decipiens (prevalence 93% and abundance 8.4), but was not found in the seal diet. However, u ninfected cod that enter shallow water from the surrounding deep water s became infected when they feed on bullrout, Recriutment to mature st ock of P. decipiens occurs when highly infected cod are eaten by seals . The number of reproducing P. decipiens is very low in common seals. Only 2 seals (i.e. less than 5% of the sample) were simultaneously inf ected with mature worms of both sexes, The dramatic reduction of the s eal stock in the outer Oslofjord by the epizootic did not seem to affe ct the abundance of P. decipiens in its intermediate hosts.