The marine survival and growth of wild and hatchery-reared Atlantic salmon

Published source details Jonsson N., Jonsson B. & Hansen L.P. (2003) The marine survival and growth of wild and hatchery-reared Atlantic salmon. Journal of Applied Ecology, 40, 900-911.
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Summary

Catches of Atlantic salmon Salmo salar have decreased dramatically in recent decades over its entire natural range in the North Atlantic. In order to try and redress declines, captive-bred smolts were often released as a management intervention for stock enhancement. However, there are questions about their survival and performance relative to wild fish. This paper reports on the survival and sea growth of River Imsa (Noway) salmon released from 1981 to 1999 as 1- and 2-year-old hatchery and wild smolts.

Atlantic salmon Salmo salar spawn in the River Imsa, south-western Norway, and the young (parr) use the river as a nursery before smolting, mostly (78%) at 2 years of age (78%). Most (c. 80%) mature sexually after one winter at sea (one-sea-winter fish or grilse). The rest mature as multi-sea-winter fish (i.e. mostly two winters at sea).

From 1981 to 1999, 20,655 wild smolts migrated from the river to the sea. During the same period, 133,875 1- and 2-year-old hatchery-reared smolts (brood stock taken from wild Imsa parents) were released in May when the majority of wild smolts leave the river.

Survival was significantly higher for wild (8.9% recapture rate) than hatchery salmon (fish released as 1- and 2-year-old smolts, recapture rate of 3.3% and 2.9%, respectively). Hatchery salmon released as 2-year-old smolts, which had lower survival, were captured more in coastal than freshwaters, grew more slowly and attained maturity younger than released 1-year-old smolts.

The survival rate of hatchery fish released as 2-year-old smolts, but not 1-year-olds and wild smolts, decreased during the 1980s and 1990s. Growth rates at sea, adult size and the proportion of multi-sea-winter fish of all three groups also decreased over time.

Catches in coastal relative to freshwaters were higher for two- than one-sea-winter fish. Mean specific growth rate at sea was similar for wild and hatchery salmon released as 1-year-old smolts, and higher than in hatchery fish released as 2-year-olds.

The proportion of two-sea-winter salmon correlated positively with the specific growth rate in the first year at sea. Total capture of wild adult salmon in rivers and Norwegian home waters each year correlated positively with the specific growth rate in the first year at sea. The same correlation held for hatchery fish released as 2- but not 1-year-old smolts.

Conclusions Survival was significantly higher for wild than hatchery salmon (fish released as 1- and 2-year-old smolts). Release of 1-year-old smolts was financially more profitable than those of 2-year-olds.

Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://blackwellpublishing.com/submit.asp?ref=0021-8901