Discussion

The Baltic Sea is a semi-enclosed brackish water system carrying local and stationary populations of Atlantic cod Gadus morhua and grey seal Halichoerus grypus. During the latest years the grey seal population has increased markedly from a very low level during the 1980es and is at present counting around 50,000 individuals in the eastern Baltic. The lack of natural predators and enemies (e.g. killer whale and man) of the protected grey seals may suggest that further population increases can be foreseen. Local fishermen and the industry are seriously affected by three seal associated problems. First of all, predation of grey seals on fish stocks is estimated to reach an annual loss of 90,000 metric tonnes. Secondly, seals attack fishing gear and ingest fish caught in nets and hooks often leaving only the head intact when fishermen recover their equipment. Thirdly, nematodes carried by grey seals use cod as intermediate/transport host and recent investigations have documented significant increases of infection levels in Baltic cod - in the primary spawning area in the southern Baltic &hypen; of two species of zoonotic nematodes, third stage larvae of Pseudoterranova decipiens (in musculature) and Contracaecum osculatum (in liver). The prevalence of the latter species - which parasitizes the liver of cod - is at present 100 %. The mean infection intensity may reach 84 worms per fish with maximum values of more than 300 worm larvae per cod liver - even in smaller cod between 35 and 45 cm.
For comparison it can be mentioned that a survey of cod from the same area conducted during the 1980es showed 14-16% infection prevalence and intensities around 1-2 larvae per fish in this size class of cod. Due to the extreme infection intensities it has been hypothesized that the worms have effect on the survival of larger cod which has shown an otherwise unexplainable decline during the latest 9 years. Thus, spawning and survival of cod fry and juveniles in the eastern Baltic has been successful during the last 10 years but it never resulted in corresponding increases of larger size classes of cod. Direct parasite-induced host mortality and increased seal predation on the most heavily C. osculatum infected cod may explain this scenario.