Smolt Emigration and Survival to Adulthood in Two Icelandic Stocks of Atlantic Salmon

Atlantic salmon Salmo salar smolts were sampled at weirs from two Icelandic rivers (Ellidaar and Vesturdalsa) for a 15-year period to test four hypotheses: (1) longer smolts (i.e., at the high end of the length-frequency distribution) will have higher survival to adulthood than shorter smolts (i.e., in the middle and lower parts of the distribution); (2) plumper (i.e., higher condition factor) smolts will have higher survival to adulthood than leaner (slim or average) smolts; (3) middle-age smolts (i.e., in the middle part of the freshwater age distribution) will have higher survival rates than fish at the extremes of the age distribution; and (4) fish in the middle portion of the smolt run will have higher smolt-to-adult survival than smolts emigrating early and late. Among the three length-groups, significant differences were found in smolt-to-adult survival in each river. In pairwise comparisons, long smolts had significantly higher survival rates than middle-sized and short smolts for both rivers. Among the three condition groups, significant differences were found in smolt-to-adult survival for the Ellidaar but not for the Vesturdalsa. Ages of smolts in the Ellidaar ranged from 1 to 5 years, and more than 98% of smolts were age 2-4. Smolts in the Vesturdalsa ranged in age from 2 to 7 years, and more than 99% of smolts were age 3-5. Among the dominant three age-groups for each river, significant differences were found in smolt-to-adult survival. Among the three migration timing groups, significant differences were found in smolt-to-adult survival in both rivers. In pairwise comparisons for the Ellidaar, late-migrating smolts had significantly higher survival rates than middle and early smolts. In the Vesturdalsa, late-migrating smolts had significantly higher survival than early and middle smolts, which did not differ from each other. These results are placed in the context of optimization of life history strategies and ideas about the process of smoltification in Atlantic salmon life history.