From 1993 to 1995, more than 2000 of 0 + brown trout (Salmo trutta L.) were captured in a small brook in October and released after clipping the left pelvic fin. More than 1700 of them were also tagged with a small Passive Integrated Transponder (PIT) tag (II mm long) implanted in the body cavity. Tagged juveniles were recaptured in May in the brook but also one year later in the river Oir, and continuously in a trap during spring downstream migration in the river Oir.The combined marking experiment (PIT tag and fin-clipping) carried out during the two first years showed an average transponder loss rate of 3.38% after seven months for 0+ juveniles tagged in October. Pelvic fin regeneration occurred at a average low rate of 2.37% during the same period. Futhermore, PIT tag showed no significant effect on growth, survival in 0+ juveniles (55 to 127 mm fork length), tagged in October 1995 and recaptured in May 1996.PIT tagging has been used to analyse brown trout movement inside the river. Recapture rate in the brook from tagged 0+ juveniles after seven months decreased while size increased. The medium sized juveniles migrated in the main system Oir and the largest ones out of the main river.Results showed (I) PIT tagging is a reliable technique to analyse the movement and population dynamics of brown trout by taking into account the individual life history, (2) recruitment of juvenile trout in the river depends on fish size.
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