Infections of Diphyllobothrium dendriticum, D. ditremum (Cestoda), and Cystidicola farionis (Nematoda) in a north Norwegian population of Arctic charr (Salvelinus alpinus) during winter RUNE KNUDSEN AND ANDERS KLEMETSEN The Norwegian College of Fishery Science, University of TromsQ, N-9037 TromsQ, Norway Received October 14, 1993 Accepted August 17, 1994 KNUDSEN, R., and KLEMETSEN, A. 1994. Infections of Diphyllobothrium dendriticum, D. ditremum (Cestoda), and Cysti- dicola farionis (Nematoda) in a north Norwegian population of Arctic charr (Salvelinus alpinus) during winter. Can. J. Zool. 72: 1922-1930. Infections of the parasites Diphyllobothrium dendriticum, Diphyllobothrium ditremum, and Cystidicola farionis in Arctic charr (Salvelinus alpinus) from Takvatn, northern Norway, were followed throughout a 9-month overwintering period. The relative density of all three parasite species increased with the age of the host in response to ontogenetic shifts in habitat and food of Arctic charr in the lake. Our results indicate that some recruitment of the three parasite species, particularly D. ditre- mum, takes place during the winter. However, winter parasite recruitment was lower than in the ice-free season. In spite of high parasite infection in some individuals, no strong indications of parasite-induced host winter mortality were found for any of the three species. Low overwintering mortality of heavily parasitized charr may be connected with a stock-reduction program in the lake that has improved the conditions of the fish, and the residual stock is probably less affected by parasites. KNUDSEN, R., et KLEMETSEN, A. 1994. Infections of Diphyllobothrium dendriticum, D. ditremum (Cestoda) , and Cysti- dicola farionis (Nematoda) in a north Norwegian population of Arctic charr (Salvelinus alpinus) during winter. Can. J. Zool. 72 : 1922- 1930. Les infections causees chez 1'Omble chevalier (Salvelinus alpinus) par les parasites Diphyllobothrium dendriticum, D. ditre- mum et Cystidicola farionis ont ete suivies i Takvatn, dans le nord de la Norvkge, au cours des 9 mois d'hiver. La densite relative des trois espkces de parasites augmente en fonction de l'Age de l'h6te en reaction i des changements ontogeniques dans le choix d'habitat et le choix de nourriture des ombles dans le lac. Nos resultats indiquent qu'il se produit un certain recrutement des trois espkces de parasites au cours de l'hiver, particulikrement dans le cas de D. ditremum. Cependant, le recrutement en hiver est faible comparativement i celui qui se produit au cours des mois sans glace. Bien que les infections se soient averees graves chez certains individus, les parasites ne semblent pas entrainer de mortalite chez les h6tes en hiver. Cette faible mortalite des ombles fortement parasites en hiver est peut-&re le resultat d'un programme de reduction des stocks qui a ameliore la condition physique des poissons et fait en sorte que le stock actuellement present est moins affecte par les parasites. [Traduit par la Redaction] Introduction with Diphyllobothrium spp. by eating copepods containing Winter studies of macroparasites of freshwater fishes in ice- bound lakes are rare, both in Scandinavia and elsewhere. The only studies on Arctic charr are from Sweden (Henricson 1977, 1978) and northern Norway (Kristoffersen 1993). These studies dealt with the infection dynamics of the plerocercoids of Diphyllobothrium spp. throughout the year, and both found indications that heavily infected fish disappeared during the winter. Parasites may diminish the fish host's chance of sur- vival directly or in combination with various stress factors (Sindermann 1987). Many investigations have indicated that plerocercoids of the genus Diphyllobothrium may cause the death of fish hosts (Chubb 1980; Halvorsen and Andersen 1984), particularly during the winter (Henricson 1977, 1978; Curtis 1984; Kristoffersen 1993). There are also indications that the nematode Cystidicola farionis may cause the death of Arctic charr during winter (Giaever et al. 1991). Such parasite-induced winter mortality of the fish is believed to be important in regulating some host-parasite systems (Lemly and Esch 1984; B6rube and Curtis 1986; Fischer and Kelso 1988). Parasite infections in Arctic charr (Salvelinus alpinus) are closely associated with food and habitat choices (Henricson and Nyman 1976; Halvorsen and Andersen 1984; Curtis 1984; Berub6 and Curtis 1986; Frandsen et al. 1989). Cystidicola farionis is transmitted to charr from amphipods with infective L3 larvae (Smith and Lankester 1979). Fish become infected procercoids (Chubb 1980) or small fish infected with pler& cercoids (Halvorsen and Wissler 1973; Curtis 1984). Previous Scandinavian studies on the winter feeding ecology of Arctic charr have shown that both copepods and amphipods are com- mon food items (Nilsson 1955; Klemetsen and Grotnes 1975; Hindar and Jonsson 1982). In northern Sweden, Henricson (1978) found indications of D. dendriticum recruitment late in the autumn and in the spring under ice cover, when Arctic charr increased feeding on copepods. In this study we pre- dicted that charr feeding on amphipods and copepods during the winter experience recruitment of all three parasite species at the time of year when the habitat is ice-bound. Earlier work in 1980 and 1985 on charr parasites in Tak- vatn, our study site in northern Norway, indicated the occur- rence of parasite-induced host mortality during winter (Giaever et al. 1991 ; Kristoffersen 1993). However, because the lake had been subjected to an intensive stock depletion experiment, we wished to test whether these patterns of mor- tality had changed. This was achieved by applying indirect methods based on the work of Anderson and Gordon (1982). Materials and methods Takvatn (69"07'N, 19'05'E; 214 m asl) is an oligotrophic lake situated in northern Norway. The surface area is 14.2 km2 and the lake has two main basins with a maximum depth of 70-80 m. Fish species in the lake are Arctic charr, three-spined stickleback Printed in Canada i IrnprirnC au Canada Can. J. Zool. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/04/13 For personal use only.