One-year-old and 1-summer-old salmon parr have been stocked in the Simojoki, a northern Baltic river, to protect the natural salmon stock from extinction. This enhancement practice was studied during a 5-year stocking period, when an average of 14.3 1-summer-old parr and 5.4 1-year-old parr was needed to produce one sea running smolt. The mean yield from stocking was 70 smolts per 1000 stocked ... [Show full abstract] 1-summer-old parr and 186 smolts per 1000 stocked 1-year-old parr. The mean cost of smolts produced by stocking 1-year-old and 1-summer-old parr was 1.2 and 2.6 times the cost of producing one 50 g smolt in the hatchery, respectively. There was a negative relationship between density of wild parr and length of smolts stocked as 1-summer-old parr in the river, but no relationship between wild parr density and length of smolts stocked as 1-year-old parr. This indicated that 1-summer-old parr were more susceptible than the older parr to competition with wild parr. Considering production costs and smolt yield, stocking of 1-year-old parr appeared to be more profitable than that of 1-summer-old parr in the enhancement of the endangered salmon stock.