For some years now the parasitic swim bladder nematode Anguillicola crassus of the European eel Anguilla anguilla L., has been reported from several European countries. The enhre life history of this parasite has recently been elucidated in our laboratory. Young larvae leave the swim bladder of the host via the pneumatic duct and reach the water through the digestive tract. They are ingested by small copepods (Cyclopoida), which act as intermediate hosts. Larvae remain in the hemocoel until the copepods are eaten by the final host, the European eel. Larvae penetrate through the intestinal wall and reach the swim bladder where they develop into adults. When infected copepods are eaten by other small fish, such as carp Cyprinus carpio L. or ide Leuciscus idus L., larvae do not reach the adult stage. However, when larger eels feed on such facultative reservoir hosts, they too become infected.

The host specificity and population dynamics of Anguillicola crassus in a number of paratenic hosts were investigated. Various freshwater flsh species were sampled monthly (March 1990 to March 1991) from the Kolenhaven (Albertcanal, Genk, Belgium) and examined for L3-larvae of Anguillicola crassus. Sixteen species were found to be infected: all the physoclist species examined (Gymnocephalus cernua, Lepomis gibbosus, Ictalurus nebulosus, Stizostedion lucioperca, Gasterosteus aculeatus, Oreochromis niloticus and Perca fluviatilis) and those physostome species of which a sufficient number could be examined to detect the infection (Gobio gobio, Leuciscus cephalus, Chondrostoma nasus, Leuciscus leuciscus, Alburnus alburnus, Leuciscus idus, Scardinius erytrophthalmus, Rutilus rutilus and Tinca tinca). There were large differences in prevalence among the fish species examined but generally the prevalence was higher in physoclist fishes and was highest in G. cernua ( 9 6 % ) . In 4 species there was a significant positive correlation between fish length and parasite abundance. The percentage of grown larvae varied among fish species, being lowest in G. cernua and highest in P. fluviatilis. No clear seasonal incidence cycle was observed.

Mork, K. A., Gilbey, J., Hansen, L. P., Jensen, A. J., Jacobsen, J. A., Holm, M., Holst, J. C., Ó Maoiléidigh, N., Vikebø, F., McGinnity, P., Melle, W., Thomas, K., Verspoor, E., and Wennevik, V. 2012. Modelling the migration of post-smolt Atlantic salmon (Salmo salar) in the Northeast Atlantic. – ICES Journal of Marine Science, 69: 1616–1624. The migration of post-smolt Atlantic salmon (Salmo salar) during their first 4 months at sea in the Northeast Atlantic was simulated using an individual-based model that combined a particle-tracking scheme with growth and behaviour routines. The migration was decomposed into both passive pelagic drift with the surface currents, provided by an ocean model, and active horizontal swimming behaviour. The active swimming direction was aligned with the surface current. Swimming speed was a function of body length and calculated from recaptured tagged salmon. Releases of particles in the model were made to the west of Ireland and to the southwest of Norway. The modelled post-smolt distributions were compared with the observed distributions, and a sensitivity analysis using different swimming speeds was performed. The strength and direction of the flow can transport the post-smolts towards areas with favourable feeding conditions. However, in some areas, the direction of migration was sensitive to interannual changes in the windforcing, leading the post-smolts to areas with a different environment and prey. Inclusion in the swimming behaviour of a preference for water with higher temperature and salinity displaced the northward migration more offshore, away from coastal areas.

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Jensen, A. J., Ó Maoiléidigh, N., Thomas, K., Einarsson, S. M., Haugland, M., Erkinaro, J., Fiske, P., Friedland, K. D., Gudmundsdottir, A. K., Haantie, J., Holm, M., Holst, J. C., Jacobsen, J. A., Jensås, J. G., Kuusela, J., Melle, W., Mork, K. A., Wennevik, V., and Østborg, G. M. 2012. Age and fine-scale marine growth of Atlantic salmon post-smolts in the Northeast Atlantic. – ICES Journal of Marine Science, 69: 1668–1677. Surface trawls were conducted over a large area of the Northeast Atlantic in 2002, 2003, 2008, and 2009 to collect samples of Atlantic salmon (Salmo salar) post-smolts during their marine feeding migration (n = 2242). The dominant smolt age of wild post-smolts was 2 years, followed by 1- and 3-year-old fish, and a few 4-year-old fish. The average rate of circulus formation in the marine zone of scales was estimated to be 6.3 d circulus−1. Both the age structure and the number of marine circuli in the scales suggest that the majority of the post-smolts originated in rivers in southern Europe. Applying intercirculi distances in scales as a proxy variable of growth rate suggests that putative marine growth rates varied among years, with the fastest growth rates in 2002 and the slowest growth rates in 2008. Further, the first marine intercirculi distances were narrowest in 1-year-old smolts, successively increasing with smolt age, indicating that growth rates during the first period at sea were lowest for salmon of southernmost origin. Growth indices are linked to prevailing environmental and biological conditions.