Anadromy in brown trout (Salmo trutta): A review of the relative roles of genes and environmental factors and the implications for management

While many brown trout (Salmo trutta) populations spend their entire life cycle in freshwater, especially as river-lake migrants, others show facultative anadromy. That is, some trout migrate to sea while other individuals of the same population remain resident within their natal river. Sea trout can give rise to river resident offspring and vice versa, although there is a strong tendency to track the parental life history. Anadromy delivers better feeding and thus larger size, which, especially for females, results in higher fecundity and enhanced mate choice. River residence, more prevalent in males, can give higher survival and avoids the energy expenditure required by anadromy. Overall, the costs and benefits of anadromy versus residency, measured in terms of survival and reproduction, are finely balanced and small changes can lead to evolutionary changes in life history. The decision to be anadromous or resident is a quantitative threshold trait, which is controlled by multiple genes and environmental factors. The binary nature of the trait is postulated to be the result of the environmentally determined physiological condition (e.g. energy status) relative to a genetically determined threshold, although this may not be the only mechanism involved. Anadromy ensues when an individual’s condition fails to meet the threshold level, which varies between sexes and among individuals and populations. Environmental factors and genetic architecture may also directly influence life history, e.g., by altering gene expression. A strong genetic influence on the anadromy decision means that facultative anadromy can be altered by natural selection driven by changes such as differential exploitation, stocking, partial barriers to migration, and changes in climate, and freshwater and marine productivity, together with parasite, pathogen and predator abundance resulting in reduced marine survival and growth. Further studies of the factors determining life history choice, together with multiple population estimates of heritability and differential reproductive success (fitness), are required to understand fully the impact of natural and anthropogenic environmental changes on sea trout dynamics.