This thesis investigated life history characteristics in anadromous brown trout (Salmo trutta L.) populations inhabiting small coastal streams in the Orkney Islands, Scotland. The main findings were as follows. 1) A total of 36 separate brown trout populations were identified with evidence of anadromy detected in 23. 2) Significant variation in freshwater growth rate occurred even between closely neighbouring populations. 3) Mature resident trout were predominantly male. Their incidence and size between populations was directly related to stream size. 4) Smolt size also varied directly with stream size although age reflected growth rates in each population. Smolts were smaller and younger on average compared to other Scottish populations. 5) Smolts represented both the fastest and slowest growing members in each population studied. Resident mature males exhibited an intermediate growth rate. 6) Some trout de-smolted one year but survived and re-smolted the next year. 7) B-growth in smolts occurred in freshwater and resulted in a significant growth increase between the end of winter and seawater entry. A strong inverse relationship was apparent between individual size at the end of winter and B-growth expression. Information was presented to rekindle the discussion on the presence of a threshold size for seaward migration among anadromous trout.