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Publication - Research and analysis

Farm salmon escape event: levels of farm/wild hybridisation

Published
15 December 2022
Directorate
Marine Directorate
Part of
Environment and climate change, Farming and rural, Marine and fisheries
ISBN
9781805253334

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.

View supporting documents Supporting documents

Levels of introgression as measured in simulated first generation (F1) hybrids

Figure 3 Introgression status, as measured by P(wild), for 500 in silico generated F1 hybrid fish. Vertical lines represent mean (solid), plus/minus one standard deviation (dashed), and plus/minus two standard deviations (dotted).
The distribution is normally distributed around the 0.5 P(wild) value.

Individual P(wild) values for the 500 in silico generated F1 hybrid fish can be seen to have a distribution with a mean of 0.503 and standard deviation (SD) of 0.047 (Figure 3). Identification of F1 fish in the Scottish wild collected samples was thus undertaken using the mean ± two SD of the simulated F1 fish, which resulted in confidence bounds which spanned from 0.409 to 0.597.

Levels of introgression of wild fish

Introgression in Scotland

Examination of the P(wild) values across both years in Scotland (Figure 4) identified a single F1 fish. This was a fish from 2020 from the river Clyde that had a P(wild) value of 0.5. No F1 fish were detected in 2021.

Figure 4 Introgression status, as measured by P(wild), for wild caught fish in Scotland in 2020 and 2021. Vertical solid lines represent mean (thick) and ± 2 x SD (thin) of simulated F1 fish. Vertical dashed line represents the hybrid cut-off used in Gilbey et al. (2021) to identify farm/wild hybrid fish.
A single F1 fish from 2020 is identified in the Clyde region.

Twelve and fifteen fish in 2020 and 2021, respectively, had P(wild) values below the cut-off used by Gilbey et al. (2021) to identify farm/wild hybrid fish (Table 3). These proportions of hybrid fish are similar to those previously found in the areas under investigation here (Gilbey et al., 2021).

Table 3: All wild caught fish in Scotland with P(wild) values below the cut-off used by Gilbey et al. (2021) for the identification of hybrid fish.
Area River Site Latitude Longitude P(wild)
2020
Argyll Glen Rosa Water CarGenArgyllAdhoc_001 55.59401 -5.18084 0.692
Argyll Lusragan Burn CarGen_Argyll_059 56.43569 -5.39851 0.609
Argyll River Creran CarGenArgyllAdhoc_007 56.56396 -5.23203 0.690
Argyll River Creran CarGenArgyllAdhoc_007 56.56396 -5.23203 0.726
Argyll River Creran CarGenArgyllAdhoc_006 56.58804 -5.19216 0.744
Argyll River Fyne CarGenArgyllAdhoc_005 56.28921 -4.89669 0.616
Argyll River Shira CarGen_Argyll_076 56.29638 -5.00610 0.638
Clyde River Kelvin CarGen_Adhoc_Clyde_008 55.90655 -4.31539 0.526
Clyde River Kelvin CarGen_Clyde_172 55.93929 -4.33814 0.676
Clyde River Kelvin CarGen_Clyde_170 55.98858 -4.24113 0.745
Clyde River Leven CarGen_Clyde_161 56.02686 -4.67677 0.721
Galloway River Bladnoch CarGen_Adhoc_Galloway_001 54.8554 -4.53795 0.714
2021
Galloway River Bladnoch CarGen_Adhoc_Galloway_005 54.92281 -4.60209 0.719
Argyll River Eachaig CarGen_Argyll_047 56.14867 -5.05116 0.715
Argyll River Eachaig CarGen_Argyll_047 56.14867 -5.05116 0.744
Argyll River Fyne CarGenArgyllAdhoc_005 56.28921 -4.89669 0.679
Argyll River Ruel CarGenArgyllAdhoc_009 55.99864 -5.21408 0.724
Ayrshire Water Of Girvan CarGen_Ayrshire_111 55.31707 -4.57661 0.736
Carradale Machrie Water CarGen_Carradale_028 55.56547 -5.26877 0.747
Clyde River Clyde CarGen_Clyde_165 55.67757 -4.02746 0.621
Clyde River Clyde CarGen_Clyde_165 55.67757 -4.02746 0.707
Clyde River Clyde CarGen_Clyde_196 55.95109 -4.33743 0.713
Clyde River Clyde CarGen_Clyde_165 55.67757 -4.02746 0.723
Clyde River Clyde CarGen_Adhoc_Clyde_010 55.73413 -3.98817 0.731
Clyde River Kelvin CarGen_Clyde_191 55.96901 -4.19301 0.682
Galloway River Bladnoch CarGen_Adhoc_Galloway_005 54.92281 -4.60209 0.708
Galloway River Bladnoch CarGen_Adhoc_Galloway_004 54.95613 -4.72664 0.744

Introgression in England

The same F1 identification confidence bounds were used for the fish from English rivers as were used for the Scottish sample. However, it must be remembered that all wild reference samples used here were based on individuals collected from Scotland. These represented the most relevant available reference fish and had been chosen such that the influence of any wild phylogeographic structure across the whole country was minimised. Thus, even though there were no wild English fish in the reference samples, there is no reason to expect that the results would be influenced to a significant degree.

Figure 5 Introgression status, as measured by P(wild), for wild fry and parr caught in England in 2021. Vertical solid lines represent mean (thick) and ± 2 x SD (thin) of simulated F1 fish. Vertical dashed line represents the hybrid cut-off used in Gilbey et al. (2021) to identify farm/wild hybrid fish.
A single F1 fry is identified.

There was a single fish whose P(Wild) fell just below the lower bound used to identify F1 fish. The fish was from the Aglionby Crosby site on the river Eden (Lat. 54.923169, Long. -2.8732639) and had a P(wild) value of 0.369. This was also the only fish with a P(wild) value below the cut-off used by Gilbey et al. (2021) to identify farm/wild hybrid fish.

Contact

Email: David.Morris@gov.scot

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