Allozyme evidence for reproductively isolated sympatric populations of brown trout Salmo trutta L. in Lough Melvin, Ireland

Electrophoretic studies of five polymorphic enzyme loci (G‐3‐PDH‐2, LDH‐I, LDH‐5, PGI‐2, PGI‐3) in brown trout from Lough Melvin in northwestern Ireland have demonstrated that the morphotypes known by the vernacular names of ‘ferox’, ‘gillaroo’ and ‘sonaghen’, are not merely ecophenotypes but represent genetically distinct and reproductively isolated populations. The results suggest that the long life and higher growth potential of ferox trout of this lake, and possibly others, has a genetic basis. These separate demes of brown trout are probably the result of multiple invasions in post‐glacial times of allopatrically derived stocks. Lough Melvin's isolated position and absence of pike, Esox lucius, and large cyprinids have probably contributed to its pristine condition. As such it is one of the few remaining examples of what may once have been a widespread situation in Britain and Ireland.