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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.

We released 598 Atlantic salmon (Salmo salar) hatchery smolts in a river in Iceland in 2005 and 2006 tagged with data storage tags recording temperature and (depth) continuously. Five salmon returned in 2006 and two in 2007, all spending 1 year at sea. The complete temperature and depth profiles of the whole ocean migration were measured. The salmon stayed close to the surface most of the time and showed diurnal behavior, staying at slightly deeper waters during day. The salmon were in temperatures from 6 to 15 °C, with warmer temperatures in the summer. We compared the fish-recorded temperature with sea surface temperature from an available National Oceanic and Atmospheric Administration database to locate the fish at different times. Utilizing the diurnal behavior of the salmon, we estimated solar noon each day during winter. Sea surface temperatures and diurnal activity were used to estimate daily locations using established Hidden Markov Model for fish geolocation. The salmon in the study stayed southwest of Iceland in the Irminger Sea during the first summer months, but in the fall they moved towards the Faroes Islands and then back to the Irminger Sea where they stayed, until returning to the river. The salmon also took shorter and deeper dives (>100 m) during the latter part of their ocean migration.

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The grilse to salmon ratio of each smolt class of Atlantic salmon (Salmo salar) during the last three decades were examined using data from catch records and from direct counter data in several rivers in western and northern Iceland. The counter data show that the catch data in Icelandic rivers reflects the number of spawners entering the rivers. Both long- and short-term changes in the grilse to salmon ratio were seen. Climatic changes can be responsible for long term changes in sea age composition of Icelandic salmon stocks. Short-term changes in the ratio can be explained by changes in marine conditions that influence the survival of salmon during their second year at sea.