The genomic and adaptive consequences of extreme, human-mediated population decline

The consequences of severe population decline in wild organisms are poorly understood. Survival of species experiencing such declines may be hampered by a limited ability to adapt to changing environments, reduced fertility and greater susceptibility to disease - all stemming from low genetic diversity. When applied to systems that have experienced population bottlenecks as a result of human activities, genomic tools allow us to examine the genomic consequences in terms of inbreeding, genetic load, disease resistance and adaptability. Our aim is to understand the consequences of extreme, rapid population decline using an example of drastic human intervention in which the fish stocks of entire lakes were eradicated and repopulated.

The introduction of the salmonid ectoparasite Gyrodactylus salaris to Norway resulted in the collapse of wild Atlantic salmon populations. Consequently, authorities applied Rotenone - a broad-spectrum biocide - to entire river catchments to kill all fish hosts. Once treated, waterways are restocked with uninfected captive-bred juveniles from local fish stocks. In 2012 the Fusta catchment (four linearly-connected sub-arctic lakes) underwent a rotenone program. Prior to treatment, fish communities included inter-lake migratory brown trout. Genetic assessment revealed significant population structure and a high proportion of anadromous trout caught at sea could be assigned to Fusta populations. Furthermore, these analyses detected loci under selection suggesting local adaptation across the system. Brown trout and the anadromous form, in particular, have experienced substantial declines in recent years and such treatments may threaten the genetic health of brown trout.

This system represents a unique opportunity to understand (i) the impact of extreme bottlenecks on the genome of a species, (ii) how loss of genetic diversity influences immunity to parasites and (iii) the processes by which rapid adaptation occurs in genetically depauperate populations.