Introduced bullheads Cottus gobio and infection with plerocercoids of Schistocephalus cotti in the Utsjoki, an Arctic river in Finland

Abstract

Bullheads Cottus gobio, first found in the River Utsjoki, an Arctic river in Finland, in 1979, were sampled at six sites in August 1991 (two of these sites were also sampled previously in June and July 1991). Mean total length (LT) of C. gobio was 45 mm (n= 1080 fish). Overall, there were slightly more females than males, with males larger than females (mean LT males 52·1 and females 47·6 mm). Prevalence of infection with plerocercoids of Schistocephalus cotti varied from 20·0 to 43·0% over the six sites in August. Juvenile C. gobio had a prevalence of infection of 3·5% whereas in mature fish prevalence was consistently higher in females than in males (38·8 v. 20·1%). There was one plerocercoid per infected juvenile C. gobio. In mature fish the mean number varied between 1·3 and 1·8: 49% had one worm per fish, 38% two or three worms and 13% had four to eight plerocercoids. In juvenile C. gobio, S. cotti were randomly distributed (overdispersion index 1·10), whereas occurrence was slightly aggregated in the LT groups 40–59 and ≥60 mm (overdispersion indices both 1·90). Mean mass of infected fish was higher in all LT groups compared to uninfected C. gobio. After removal of plerocercoids, mean mass of fish tissue remaining was lower compared with uninfected fish for the two LT groups 40–59 and ≥ 60 mm, but higher in C. gobio <40 mm LT. Prevalence and mean intensity of infection over the 3 months did not differ either for the two sites individually or the pooled data. Mean mass of both uninfected and infected fish increased over this period, and were both highest in August as was the mean worm burden. There was, however, no difference in Fulton’s condition factor at any time. Mean mass of plerocercoids in C. gobio of ≥40 mm LT increased significantly comparing June and July with August. Comparison of backcalculated LT of infected and non‐infected bullheads of different age‐groups did not show any significant differences in growth at any age. In each parasite infrapopulation as the number of worms increased so their mean mass decreased significantly but at the same time the total mass of worms in the fish host remained more or less constant in relation to length. Aggregation of parasites in the fish population was low, as the majority were infected by one S. cotti plerocercoid. Gonad development was suppressed in infected C. gobio.