Unique sympatric quartet of limnetic, benthic, profundal and piscivorous brown trout populations resolved by 3D sampling and focused molecular marker selection

The full extent of sympatric intraspecific population diversity (SIPD) i.e. structuring into multiple genetically distinct Mendelian populations, remains uncertain for most lacustrine fish species, particularly in northern lakes. However, increasing application of molecular genetics is advancing understanding and has shown both that many known intraspecific sympatric lacustrine orphological and behavioural polymorphisms represent SIPD, and resolved the existence of phenotypically cryptic structuring.
Uncertainty remains as only a few northern lakes have been comprehensively surveyed and existing studies focus on known phenotypic polymorphisms or exploit ad hoc field sampling, marker selection and data analysis methods. Such unfocused approaches constrain resolving power and increase the likelihood of failing to detect SIPD when present.
Brown trout (Salmo trutta L.) in a previously unstudied Scottish lake were collected by 3D stratified random netting. They were screened with both an arbitrary set of molecular markers and markers pre‐selected for their ability to resolve regional allopatric brown trout population structuring and data analysed by both Bayesian and non‐Bayesian approaches.
Depth clines for mitochondrial and microsatellite markers variation were observed and found to arise from different, but overlapping, depth distributions of four genetically distinct piscivorous, limnetic, shallow benthic, and profundal benthic populations. This is the highest number of ecologically and phenotypically distinct sympatric brown populations identified in any lake to date, and includes the first reported profundal benthic form.
The detection of the SIPD in Loch Laidon depended critically on the random 3D sampling and using markers preselected for their power to differentiate regional allopatric populations of trout, as well as aided by using both Bayesian and non‐Bayesian analytical approaches.
The findings support the view that the extent of ecologically and evolutionarily significant SIPD is probably underestimated in brown trout and other fish species, and probably is a significant component of the biodiversity in many northern hemisphere lakes.