The general drivers in partial migration relative to environmental norms are not fully understood, yet. The present study investigates the juvenile freshwater growth stage in partially anadromous brown trout Salmo trutta L. in an Irish lake system over a long- time series of 14 years (1977 – 2016). A scale growth analysis was conducted to assess juvenile growth between sea trout and brown trout. The effect of environmental parameters was tested on juvenile growth in brown trout only. Lastly, the periodicity of scale growth measurements was examined by analysing brown trout recaptures. Sea trout exhibited significantly more circuli than the brown trout population, indicating faster growth in early years. Temperature had diverse effects on growth in juvenile brown trout. The circuli formation rate in the brown trout recaptures was not related to fish length increment, demanding caution when interpreting scale growth measurements. It is argued that the partially anadromous brown trout population follows a dichotomous population, exhibiting two different life histories: anadromy and residency. Temperature shows seasonal patterns in juvenile brown trout growth. It is assumed that a more complex set of intrinsic and extrinsic factors mediate juvenile growth in the partially anadromous brown trout population.
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