Plerocercoids of the cestode Diphyllobothrium ditremum in brown trout Salmo trutta: substantial increase in infection after establishment of European minnow Phoxinus phoxinus.

This study focuses on plerocercoids of the cestode Diphyllobothrium ditremum in brown trout Salmo trutta from the subalpine lake Øvre Heimdalsvatn in south-central Norway. Salmo trutta was the only fish species in this lake until European minnow Phoxinus phoxinus was registered in 1969. The P. phoxinus population increased substantially in the following years. In contrast with the 1969-1972 period, when plerocercoids of D. ditremum were practically absent in S. trutta, there was a high prevalence and intensity of infection in the 2013 S. trutta samples. Because the life cycle of D. ditremum involves two larval stages, in copepods and salmonids and mature worms in piscivorous birds, such as mergansers and loons, a change in feeding ecology of S. trutta or changes in population densities of copepods, fish or birds might have influenced the infection pattern. No relationships between D. ditremum infection and muscle-tissue δ15 N signature or Hg concentration were found, indicating that infection is not a result of piscivory or cannibalism. Furthermore, consumption of copepods by S. trutta during summer and autumn was low. On the other hand, the number of piscivorous birds has increased, probably due to the presence of P. phoxinus as a new and numerous prey. An increased number of final D. ditremum hosts may have produced a higher output of cestode eggs, resulting in more infected copepods that in turn are consumed by S. trutta. Indirectly, P. phoxinus may therefore have caused the observed increased infection in S. trutta and thereby imposed further negative effects on S. trutta in high mountain areas.

A comparison of AFLPs and microsatellites to identify the population structure of brown trout (Salmo trutta L.) populations from Hardangervidda, Norway.

A total of 495 fish from 11 Hardangervidda lakes were genotyped in order to compare amplified fragment length polymorphisms (AFLP) and microsatellites in terms of their capacity to infer population genetic structure. The 11 microsatellites used in this study gave a greater polymorphism information content and greater gene diversity, with an average of 14.8 alleles per locus, than the six AFLP primer combinations used. However, the AFLPs resulted in 178 polymorphic loci and a 3.1 times larger marker index (effective multiplex ratio multiplied with the gene diversity). Comparable population structuring, for example in terms of distinguishing fish from the different river systems, was obtained with both marker systems. An AFLP and microsatellite multilocus Bayesian assignment test with the structure program divided the fish into six groups largely concurrent with main branches on a population neighbour-joining tree. Yet, the admixture status of individuals is mostly contradictory in the AFLP and the microsatellite analyses. The results are discussed concerning migration between lake populations.