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Large and long‐lived piscivorous brown trout, Salmo trutta, colloquially known as ferox trout, have been described from a number of oligotrophic lakes in Britain and Ireland. The "ferox" life history strategy is associated with accelerated growth following an ontogenetic switch to piscivory and extended longevity (up to 23 years in the UK). Thus, ferox trout often reach much larger sizes and older ages than sympatric lacustrine invertebrate‐feeding trout. Conventional models suggest that S. trutta adopting this life history strategy grow slowly before a size threshold is reached, after which, this gape‐limited predator undergoes a diet switch to a highly nutritional prey source (fish) resulting in a measurable growth acceleration. This conventional model of ferox trout growth was tested by comparing growth trajectories and age structures of ferox trout and sympatric invertebrate‐feeding trout in multiple lake systems in Scotland. In two of the three lakes examined, fish displaying alternative life history strategies, but living in sympatry, exhibited distinctly different growth trajectories. In the third lake, a similar pattern of growth was observed between trophic groups. Piscivorous trout were significantly older than sympatric invertebrate‐feeding trout at all sites, but ultimate body size was greater in only two of three sites. This study demonstrates that there are multiple ontogenetic growth pathways to achieving piscivory in S. trutta and that the adoption of a piscivorous diet may be a factor contributing to the extension of lifespan. [ABSTRACT FROM AUTHOR]

Dominance hierarchies, where they exist, affect individual food acquisition ability and fitness, both of which have the potential to influence life history pathways. Juvenile salmonids exhibit clear dominance hierarchies in early life. As one of the drivers for the adoption of alternative life histories in salmonids is the relative rate of resource acquisition, there is potential for juvenile behaviour to influence the subsequent life history strategy of the individual. Lacustrine brown trout, Salmo trutta, exhibit a multitude of life histories which includes among others the piscivorous (ferox) life history where individuals grow to large size and have delayed maturity and benthivorous and pelagic life histories where individuals grow to much smaller sizes, however mature earlier. Using a number of observable characteristics of dominance, this study compared differences in behaviour between size-matched pairs of progeny, reared under common garden conditions which are derived from alternative, co-existing life history strategy parents. We found that first-generation progeny of ferox trout were more aggressive, acquired more food, had lighter skin pigmentation and held more desirable positions than the progeny of benthivorous brown trout in an experimental stream system. Ferox trout progeny were dominant over benthivorous brown trout progeny in 90% of trials in dyadic contests. Given such clear differences in dominance, this study indicates that parentally acquired dominance-related differences, passed through either, or both, of genetic and nongenetic (e.g. maternal effects) means, are likely a contributing factor to the continued maintenance of distinct life history strategies of brown trout. [ABSTRACT FROM AUTHOR]

Identifying the genetic basis underlying phenotypic divergence and reproductive isolation is a longstanding problem in evolutionary biology. Genetic signals of adaptation and reproductive isolation are often confounded by a wide range of factors, such as variation in demographic history or genomic features. Brown trout (Salmo trutta) in the Loch Maree catchment, Scotland, exhibit reproductively isolated divergent life history morphs, including a rare piscivorous (ferox) life history form displaying larger body size, greater longevity and delayed maturation compared to sympatric benthivorous brown trout. Using a dataset of 16,066 SNPs, we analyzed the evolutionary history and genetic architecture underlying this divergence. We found that ferox trout and benthivorous brown trout most likely evolved after recent secondary contact of two distinct glacial lineages, and identified 33 genomic outlier windows across the genome, of which several have most likely formed through selection. We further identified twelve candidate genes and biological pathways related to growth, development and immune response potentially underpinning the observed phenotypic differences. The identification of clear genomic signals divergent between life history phenotypes and potentially linked to reproductive isolation, through size assortative mating, as well as the identification of the underlying demographic history, highlights the power of genomic studies of young species pairs for understanding the factors shaping genetic differentiation. [ABSTRACT FROM AUTHOR]