Using PIT technology to study the fate of hatchery-reared YOY northern pike released into shallow vegetated areas

This study uses passive integrated transponder (PIT) technology to analyse the fate of 192 PIT-tagged hatchery-reared young-of-the-year northern pike Esox lucius (mean fork length FL 51.0 mm ± 5.3 S.D.) released into an experimental area located in the Brière Marsh (France), together with 72 untagged individuals for control. Survival and emigration were studied by trapping and using a portable PIT detector from release (20 May) to complete drying out of the grassland (15 June 2005). Only 19.3% and 6.9% of the PIT-tagged and control fish, respectively, successfully emigrated from the experimental area before drying out. Small individuals (FL <50 mm at tagging) suffered a higher mortality rate, and size-dependent effects of cannibalism rather than PIT-tagging procedure on survival were suspected. No evidence of tag loss was found for surviving fish, and individual growth conformed to values reported in literature. The portable PIT antenna detected 71.4% of the PIT-tagged pike in the flooded grassland. More than 30% of tagged fish disappeared from the experimental area throughout the study period, and predation by birds was suspected. Results support the view that more attention should be paid to limiting the detrimental effects of mortality on the stocking success of northern pike.