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

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Ferguson, Andy
Reed, Thomas E.
McGinnity, Philip
Prodöhl, Paolo A.
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Troubador Publishing Ltd.
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While many brown trout (Salmo trutta) populations spend their entire life cycle in freshwater, especially as river-lake migrants or river residents, others show facultative anadromy. That is, some trout migrate to sea while other individuals of the same population remain within their natal river. Sea trout can give rise to 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 results in higher fecundity in females, enhanced mate choice, and other reproductive benefits. River residence, more prevalent in males as anadromy conveys fewer benefits, 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 to the cost-benefit equation can lead to evolutionary changes in life history. The decision to be anadromous or resident is a quantitative threshold trait, controlled by multiple genes and environmental factors. The dichotomous nature of the trait is postulated to be the result of the environmentally influenced physiological condition (e.g. energy status) relative to a genetically determined threshold. 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 with farm-reared brown trout, 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.
Sea trout , River resident trout , Quantitative threshold trait , Physiological condition , Sexual maturation , Parental effects
Ferguson, A., Reed, T. E, McGinnity, P. and Prodohl, P. (2016) 'Anadromy in brown trout (Salmo trutta): A review of the relative roles of genes and environmental factors and the implications for management and conservation', in Harris, G. (ed.) Sea Trout: from Science to Management, Proceedings of the 2nd International Sea Trout Symposium, Dundalk, Ireland, 20-22 October, pp. 1-40. isbn: 9781788035354
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