Identifying and conserving sympatric diversity in trout of the genus Salmo, with particular reference to Lough Melvin, Ireland

Salmonid (Salmonidae) sympatric diversity is the co-occurrence, in a lake or river, of two or more reproductively isolated populations/subpopulations, or phenotypes resulting from phenotypic plasticity. Sympatric populations can arise through allopatric and/or sympatric evolution. Subsequently, allopatric lineages can occur in sympatry due to independent colonisation and/or through anthropogenic introduction. Sympatric divergence is often driven by feeding opportunities, with populations segregating as planktivorous, benthivorous and piscivorous ecotypes ("trophic polymorphism"), and further segregation occurring by feeding depth and body size. Subpopulations evolve by natal homing where a water has two or more discrete spawning areas, often resulting in phenotypically and ecologically cryptic sympatry. Most known sympatric populations/phenotypes in trout of the genus Salmo (Eurasian trout aka brown trout) involve sympatric piscivorous (ferox) and lifetime invertivorous trout. Segregation on the benthic-limnetic axis has been poorly studied in Eurasian trout compared with other salmonids but is likely commoner than currently described. While three sympatric populations/species of Eurasian trout are recognised from Lake Ohrid (Albania/North Macedonia), limited ecological information is available and there are only two lakes with three or four sympatric populations with described benthic, limnetic and piscivorous trophic segregation: Lough Melvin (Ireland) and Loch Laidon (Scotland), the latter having the only identified case of a sympatric profundal benthic feeding populations, possibly due to the absence of Arctic charr (Salvelinus alpinus) in the lake. Many thousands of waters are yet to be examined. Some sympatric populations are extinct, and others are vulnerable with conservation action being urgently required. This should ideally be based on populations/conservation units, but the lack of recognition of intraspecific units in most legislations in the native Eurasian trout range necessitates a pragmatic approach, with species classification, where appropriate, based on integrative taxonomy. Some sympatric populations clearly merit species status and should be formally classified as such if a valid previous name is not available.