EVIDENCE OF GENETIC INTERACTION BETWEEN ESCAPED FARMED SALMON AND WILD ATLANTIC SALMON (SALMO-SALAR L) IN A NORTHERN IRISH RIVER

As a result of an escape of farmed Atlantic salmon (Salmo salar L. ) from sea cages in Glenarm Bay in Northern Ireland in 1990, presumed escaped salmon were observed in the nearby Glenarm River. An electrophoretic study was undertaken to determine whether any genetic impact on the wild salmon population could be detected. Samples of presumed escapees ascending the river were taken, together with baseline samples of the wild population. Electrophoretic variation at seven polymorphic loci was used to assess genetic differences between the wild and escaped salmon. Heterogeneity of allelic frequencies was significant al three of the foci tested (sAAT-3*; sMDH-B1,2*; mMEP-2*) and summed over all loci. Two variant alleles were detected in the presumed escapees (sMDH-B1,2*85; sSOD* 120) which were absent in the wild baseline sample. Follow-up samples of 0+ salmon were taken from the Glenarm River in summer 1991, to test for genetic changes in the wild population after possible spawning by escapees. These samples indicated that at four of the seven loci examined (sAAT-3*; IDDH-1*; sMDH-B1,2*; mMEP-2*), allele frequencies in the wild population had shifted in the direction of those in the farmed strain, contributing to lower overall heterogeneity of allelic frequencies between the two types. This was particularly evident in juvenile salmon from the lower part of the river. Gametic disequilibrium was detected in the follow-up samples, supporting the view that these results indicate interbreeding between the farmed strain and the wild population.