By Peter Landergren
Fisheries Research, Volume 67, Issue 3, May 2004, Pages 283-294, ISSN 0165-7836, DOI: 10.1016/j.fishres.2003.10.005.

The factors influencing the early migration of young-of-the-year sea trout, Salmo trutta, in small coastal streams on the Baltic Sea Island of Gotland, Sweden, were studied in a series of laboratory stream section experiments. In addition, downstream parr migration was monitored in a natural stream for 2 years. High initial fry density and fluctuating or decreasing water depth influenced the number of non-migrating parr in artificial stream sections. Furthermore, size influenced the ability of parr to establish and hold territories in the artificial stream troughs. Thus smaller parr left the stream sections in higher proportion than did larger conspecifics. However, length and dry mass of down-migrators increased with time, indicating a dynamic process with successive elimination of territory holders as the parr grow and expand their territorial boundaries. Thus, the largest individuals remained in the system at the end of the experiments. A study in a natural stream confirmed the laboratory result, with larger individuals remaining in the stream whilst smaller conspecifics migrated downstream, some entering the coastal zone. Early out-migration of parr seems to be generated by the competition for food and space following emergence, and be promoted by variable hydrological conditions.

Keywords: Brown trout; Emergence; Smolt; Stream; Competition

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