Escaped farmed Atlantic salmon replace the original salmon stock in the River Vosso, western Norway

Abstract

Eggs and alevins were collected from 36 redds in the River Vosso in late March 1996. The redds had been made by 20 individual female Atlantic salmon and 12 female brown trout. Species-specific allozyme variation was used to distinguish trout from salmon and egg size and pigment analyses were used to distinguish farmed from wild salmon females. Nine (45%) of the 20 female salmon spawners in the sample were of confirmed farmed origin, because their offspring contained synthetic astaxanthin which is an additive to commercial fish feed. Most of the remaining female salmon were also likely to have been farmed escapees because only about half of the actual farmed spawners can be identified by their astaxanthin content due to intake of carotenoids from natural food sources. The estimated peak spawning for both confirmed and putative farmed females was 32 d earlier than peak spawning of wild females. Egg survival was high and similar to previous estimates for wild and farmed salmon in the River Vosso. Based on the astaxanthin content in sampled eggs and the time of peak spawning it is concluded that most Atlantic 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 population of spawners during autumn 1995.