Farm salmon escape event: levels of farm/wild hybridisation

Results of a genetic survey to examine levels of farm/wild hybrid salmon in rivers local to a large-scale farm escape in in south west Scotland and north east England. This occurred in 2020 when MOWI’s Carradale North fish farm shifted position after its seabed anchors became dislodged during Storm Ellen.


Family structure

The majority of families in both years (88.8% and 86.8% in 2020 and 2021, respectively) consisted of single fish (i.e. there were no full siblings) or family groups of two full sib members (8.6% and 9.2% in 2020 and 2021, respectively) (Table 2, Figure 2). In 4% of cases, in both years, full-sib family sizes ranged from 4 up to a maximum of 24 individuals. Overall, in most cases, family groups were small, suggesting the sampling had captured the diversity present at a site. As such all data were retained for analysis.

Table 2: Full-sib family size across the two years of screening. Values represent the proportion (%) of families of a particular size observed across the two years.
Family size Year
2020 2021
1 88.8 86.8
2 8.6 9.2
3 1.4 1.5
4 0.3 0.8
5 0.3 0.3
6 0.1 0.3
7 0.3 0.3
8 0.1 0.1
9 0.0 0.3
10 0.0 0.2
11 0.1 0.1
12 0.1 0.1
13 0.1 0.1
24 0.0 0.1
Figure 2 Full-sib family sizes per site in 2022 and 2021.
The distribution of different family sizes across the two years of sampling. Family sizes are shown to be mostly single-sib groups, with a small number of 2 and 3 sib groups.

There was no significant difference in family composition between the two years of sample collection (Paired t-test df = 13, p-value = 0.215).

Contact

Email: David.Morris@gov.scot

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