ESCAPED FARMED ATLANTIC SALMON REPLACE THE ORIGINAL SALMON STOCK IN THE RIVER VOSSO, WESTERN NORWAY

Eggs and alevins were collected from 36 redds in the River Vosso in la te March 1996, The redds had been made by 20 individual female Atlanti c salmon and 12 female brown trout. Species-specific allozyme variatio n was used to distinguish trout from salmon and egg size and pigment a nalyses were used to distinguish farmed from wild salmon females. Nine (45%) of the 20 female salmon spawners in the sample were of confirme d farmed origin, because their offspring contained synthetic astaxanth in which is an additive to commercial fish feed. Most of the remaining female salmon were also likely to have been farmed escapees because o nly about half of the actual farmed spawners can be identified by thei r astaxanthin content due to intake of carotenoids from natural food s ources. The estimated peak spawning for both confirmed and putative fa rmed females was 32 d earlier than peak spawning of wild females. Egg survival was high and similar to previous estimates for wild and farme d salmon in the River Vosso. Based on the astaxanthin content in sampl ed eggs and the time of peak spawning it is concluded that most Atlant ic salmon fry that hatched in the River Vosso in 1996 were produced by escaped farmed females. The frequency of redds made by farmed females was in accordance with their estimated representation (81%) in the po pulation of spawners during autumn 1995. (C) 1997 International Counci l for the Exploration of the Sea.