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A genetic study of the mixed trout populations of the Lough Ennell catchment

A genetic study of the mixed trout populations of the Lough Ennell catchment

Technical Report for Inland Fisheries Ireland

After losing breeding habitat due to arterial drainage of inflowing streams in the 1950s, Lough Ennell’s wild brown trout population subsequently crashed following serious eutrophication of the lake in the early 1970s.  During this period, very large numbers of hatchery trout from the Roscrea fish farm were released into the lake and its inflowing streams to enhance the trout fishery. 
In 2011, an OPW-funded collaborative project by IFI and the UCD School of Biology & Environmental Science released a report comparing the genetic structure of the wild brown trout population in the Lough Ennell catchment with a sample from the Roscrea fish farm.  The project primarily aimed to investigate whether the fish stocking programme changed the genetic diversity of the catchment’s wild brown trout population.  The report’s findings indicate the following conclusions:  

  • The brown trout population of Lough Ennell and its stream subcatchments clustered into three main groups that were genetically distinct from the Roscrea fish farm sample. 
  • There was evidence of admixture and migration of trout between subcatchments, with the River Dysart having the largest effective population size and providing most migrants.
  • The stocked trout from the Roscrea fish farm made no genetic contribution to the wild brown trout populations of the lake fishery or of the inflowing streams.
  • Management efforts should concentrate on rehabilitating spawning and nursery areas in the catchment, rather than fish stocking.