Martin Kesler will defend his PhD thesis titled "Biological characteristics and restoration of Atlantic salmon Salmo salar populations in the Rivers of Northern Estonia" on the 4 September in the Faculty of Science and Technology in the University of Tartu (Vanemuise 46-301).
Supervisors: Dr. Markus Vetemaa and Prof. Toomas Saat, University of Tartu
Opponent: Dr. Atso Romakkaniemi, Natural Resource Institute (Finland)
The abundance of Atlantic salmon has declined throughout its range. Originally salmon populations existed in eleven North Estonian rivers and by the end of 1990s there remained only four. To recover the abundance of salmon a great emphasis has been directed to hatchery rearing and releases of juveniles to wild. First study of this thesis compares the sea entry timing of the hatchery reared and wild smolts. Majority of the 2-year old smolts migrated to the sea within a week after the release. Reared 1-year smolts have slower migration rate and their sea entry time coincided more with the wild smolts. During the freshwater period the survival rate of 2-year old smolts was 44 % and 19 % for the 1-year old. Some released 1-year old fish stayed in the river for a year and migrated to the sea a year later and their sea entry coincided with wild smolts. Original salmon population in R. Purtse went extinct due to the pollution from the oil shale industry. The water quality in the river started to improve in the 1990s and the first spawning salmon were identified in 2005.
In 2006 young-of-the-year parr were found from the river and this clearly proves that the salmon is able to naturally reproduce in R. Purtse. This demonstrates that salmon is quickly able to repopulate suitable habitats on their own. Third paper shows that part of salmon parr migrate downstream to the sea also during the autumn. Parr are not physiologically adapted to live in the saline environment; however the low salinity of the Baltic Sea is not a physiological barrier. The final paper the immune response of salmon parr. The parr had a negative correlation with the immune response and size. This result is somewhat contradictory to the general perception that the larger juveniles have better survival and the hatcheries should produce juveniles that are similar in size and behavior to wild fish.