Hur påverkar ålder och fetthalt laxsmoltens utvandring till havet?

Sammanfattning: Abstract Previous studies have shown that wild smolt have 2.5-4.5 times higher survival rate than smolt grown in culture and that starved reared smolt behave more like wild born smolt. The purpose of this project was to study whether there are differences in migration patterns and survival during migration to the coast between wild and reared smolt and between different types of reared smolt. In 2009, 25 smolts in three different groups were tagged with acoustic transmitters: Wild Born, conventionally reared 2-year and 1 –year old smolt. For the 2010 trial there was also added a group of reared 2 –year old smolt that were starved before release. Receivers were placed in the river and in a line outside the estuary. The receivers recorded when the tagged fish passed and which fish it was. Based on information from the receivers the survival rate and migration speed could be calculated for each group and be compared to the smolts amount of body fat. In this study the condition factor (weight/length3 '100000) was used as an indicator of the smolts body fat levels. The study was conducted in river Dalälven. The results provide some indications that the wild born smolt in accordance with previous studies had a slightly higher (not significant) survival rate than the other groups. No major signs of differences in survival were seen between the groups of the conventionally reared and the starved reared smolt. The annual smolt seemed to have a slightly higher survival rate than these two groups. Unlike the results in earlier studies the wild born smolt did not migrate significantly faster than any of the other groups. No significant difference was observed between the conventionally reared and the starved smolt. The group that migrated fastest was the 1 –year old smolt. It was the group of 1 –year old smolt that was most similar to the wild born smolt in regard to the condition factor. But the wild born smolt were still considerably leaner. There was a significant difference in the condition factor between the starved smolt and those grown conventionally. But the difference was probably too small to show any differences in migration behavior when they still were much larger than the wild born smolt. There also tended to be a negative correlation between low condition factor and higher survival. In this study the starved smolt behaved more like the conventionally reared than the wild hatched. They were not starved for a sufficient amount of time to reach as low levels of body fat as the wild hatched smolt. It would require studies in which smolt were starved for longer period to evaluate whether the condition factor is crucial for if reared smolt in river Dalälven possibly can behave as wild smolt.