Precocious maturation among wild sea trout, Salmo trutta (L.), was studied in two small streams in south-western Sweden. The proportion of parr males varied between 17.9 and 57.0%. Parr males and immature individuals of the same age were compared with respect to length distribution, migratory tendency and seawater tolerance. Precocious males were, on average, longer than immature fish. Marking and recapture showed a reduced or inhibited migration tendency of parr males. The seawater tolerance of these was also consistently lower than for juveniles. The seasonal pattern in osmoregulatory ability was similar for both categories, with a peak during the smolt run.
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