Escape- and diet-history among farmed Atlantic Salmon caught in Norwegian rivers and coast in the period 2011-2021
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Every year thousands of farmed Atlantic salmon (Salmo salar) escape from net pens in Norway, and introgression has been documented in two-thirds of the more than 200 wild populations investigated thus far. Farmed salmon display reduced spawning success compared to their wild conspecifics, but how long the escapees have been on the run is likely of importance for whether they succeed on the spawning grounds and thus, contribute to introgression. Due to greater experience in the wild, salmon escaping early in the production cycle are presumed to be better competitors and thus more likely to contribute to spawning than fish that have escaped late in the production cycle. However, knowledge about time-since-escape of farmed salmon approaching the spawning grounds, and the variation in escape history in time and space, is limited. Shortly after escaping, farmed escapees have a fatty acid (FA) profile which reflects the aquaculture feed. Thus, the terrestrial FA 18:2n-6 can be used as a marker for the aquaculture diet and separate the escapees based on time-since-escape. Using FA profiling, I show that in conjunction with the overall decline in escapees the proportion of early escapees has changed from 2011-2021, with a decrease in the Southern populations and an increase in the proportion of early escapees in the Northern populations. Moreover, the early escapees have the same FA profiles and thus, diet as wild salmon, and therefore most likely use the same feeding grounds as their wild conspecifics. Early escapees were also larger than both wild salmon and recent escapees. Moreover, six6 was associated with inter-individual dietary specialization, while no such link was detected for vgll3 or genetic sex. My findings indicate that the aquaculture management strategies have likely had an impact on the ratio of early escapees through the standardization of mesh width relative to fish size and suggests that it is worth examining the use of out-of-season smolt as a management strategy to reduce the proportion of farmed escapees which survive long term.
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