| Summary: | The size of the eggs in fish long been regarded as an important parameter in comparative ichthyology, as it has been linked to fitness related traits such as the survival and competitive ability of the offspring. However, very few studies exist were variation in egg-size within a population has been investigated over long time. In this thesis, I have analyzed the size of the eggs in 1126 returning female Baltic salmon (Salmo salar L.) caught in a breeding fishery in river Ume, Northern Sweden between 1974 to 2016. My main goal was to investigate how condition factor or egg number affected the egg-size in salmon, and whether any trend in egg-size existed over time. Independently of the body size and yearly effect, mean egg size of the hatchery (6.01899 mm) and wild (6.004838 mm) reared females didn’t have any statistical significant difference (p value>0.05). Larger females produced significantly bigger (diameter) and more eggs. in diameter. When the sample was fragmented with 7 length classes (70-75, 75.1-80, 80.1-85, 85.1-90, 90.1-95, 95.1-100 and 100.1-105 cm), females showed increasing egg size trend over the years. But the females didn’t tend to increase producing more eggs over time which can be the evolutionary price they paid to achieve larger egg size. Condition factor of the fishes didn’t increase significantly over the years. But larger fish length groups with better condition factor laid larger and more eggs. Several hypothesizes were discussed to explain the observed egg size variation, but no clear cut explanation could be found to the somewhat contradictory results except for a clue to have an indirect relationship with the sprat population in the Baltic which is a major food for salmon.
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