The distribution of Eubothrium crassum (Cestoda: Pseudophyllidea) in brown trout, Salmo trutta, in allopatric and sympatric populations

Sticklebacks in particular have been suspected to have a role in transmission of E. crassum to its final host, the brown trout. In this study, prevalence and mean abundance of E. crassum in trout have been examined in allopatric and sympatric populations to address the suspected role of sticklebacks in the life cycle of E. crassum. 1106 trout from 22 lakes in the northern parts of Norway were examined for E. crassum infections. The lakes represented four different compositions of fish communities: 1) allopatric trout, 2) trout in sympatry with charr, 3) trout in sympatry with charr and sticklebacks and, 4) trout in sympatry with sticklebacks. 605 of sampled trout, from 13 lakes were examined regarding piscivory. All four fish communities were represented. 19.6 % (217) of examined trout were infected with E. crassum. The prevalence and mean abundance of E. crassum in trout were significantly higher in sympatric populations. In the allopatric populations none of the trout were infected with E. crassum. Trout in sympatry with charr had low prevalence (< 7 %) and mean abundance (< 0.1) of E. crassum in some of the lakes, but the majority of lakes had no infected trout. The most significant prevalence and mean abundance were recorded in lakes where sticklebacks were present. In lakes with a fish community of trout, charr and stickleback, the prevalence and mean abundance of E. crassum varied among the lakes (prevalence: 0 - 90 %, mean abundance (0 - 13.8) but were overall significantly higher than the fish communities without sticklebacks. The lakes with a fish community of trout and sticklebacks had significantly higher prevalence (57 - 84 %) and mean abundance (4.1 - 39.4) than any of the other fish communities. Trout was found to be more frequently piscivorous in sympatric populations with sticklebacks. Intensity of E. crassum in trout was found to have a strong, positive correlation with degree of piscivory. Accordingly, length of trout had a slight, positive correlation with intensity of E. crassum. Sticklebacks were documented to have a crucial role in transmission of E. crassum to trout. From the data in this study, copepods seems to be of low importance in transmission of E. crassum to trout.