Atlantic salmon (Salmo salar L.) stocking in the Simojoki river as a management practice
1University of Oulu, Faculty of Science, Department of Biology
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|Academic dissertation to be presented, with the assent of the Faculty of Science of the University of Oulu, for public defence in Raahensali (Auditorium L10), Linnanmaa, on November 24th, 2006, at 12 noon
Professor Hans Lundqvist
Doctor Timo Marjomäki
Long-term monitoring of the wild salmon (Salmo salar L.) stock of the Simojoki river and the stocked hatchery-reared salmon parr and smolts has provided a considerable amount of information on the development of the stock and factors affecting it. Data on the relationships between wild and reared salmon were collected by tagging and trapping both smolts and adult salmon having either a wild or reared background. The tag recapture rate of wild smolts was about twice as high as that of smolts stocked as two-year-olds and slightly greater than for smolts stocked as parr. When survival was measured in relation to the smolt size, the difference between the wild and reared smolts was even greater, and it seemed to be emphasized in years with a low survival rate.
The difference observed between the wild and reared salmon in the smolt phase generally disappeared in the adult phase. When adult salmon returned to the river to spawn the difference in the timing of the ascent depended more on the age or sex of the salmon, and less on their origin. Similarly, the survival of adult salmon in the river before or after spawning and later after returning to the sea depended on the sex and age of the fish. The origin of fish affected their behaviour, the reared salmon wandering more than wild adults before settling down into spawning areas. When the yield of wild and reared smolts as returning adults was compared, the wild smolts gave the best results, although the survival from smolt to adult was low in all smolt groups, probably due to the high fishing pressure in the sea. The smolts stocked as parr and those stocked as two-year-old fish were similar in this respect. The former group gave better results if the yield was measured as the number of returning multi-sea-winter adults, while the latter group gave better results if one-sea-winter grilse were also included in the yield.
The low yield of adult salmon from stocking and the generally low survival of smolt groups irrespective of their origin emphasises the importance of fishing regulations as a tool in the maintenance or enhancement of naturally reproducing salmon stocks. However, despite the low profitability of stocking, it probably safeguarded the existence of the wild Simojoki salmon stock during its critical phase in the early 1990s. At that time the fishing regulations were not strict enough to prevent the alarming decrease in the salmon stock, and the adult spawners produced by stocking of young salmon may have had a relatively higher value than their number suggests.
Acta Universitatis Ouluensis. A, Scientiae rerum naturalium
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