Stocking efficiency and the effects of diet preconditioning on the post-release adaptation of hatchery-reared juveniles of Atlantic salmon (Salmo salar L.) in an Atlantic temperate stream

摘要

This study explores the stocking efficiency of Atlantic salmon juveniles in a 4th order stream located at the southern limit of its Eastern Atlantic distribution. Moreover, with the idea of implementing new protocols and methods to improve "traditional" hatchery practices and rear a more "wild" like fish, the study examines the response of this species to natural diet preconditioning prior to release. The field study indicated that most juveniles disappeared from the stocked reaches within two monthsof their release, resulting in a recapture rate after a year of 0.0-1.1 %. There were no differences in stocked juvenile's density along the stream during the first months following stocking. The preconditioning experiments showed that juveniles of Atlantic salmon suffered a significant decrease in their condition factor when diet was changed from artificial pellets to live preys, either if the change occurred before (i.e., in a manipulative experiment) or after their release in the stream. Although weight, length, condition factor and specific growth rates reached by juveniles at the end of the rearing period were higher in individuals fed on pellets than in those fed on macroinvertebrates, differences disappeared only a week after their release in the stream. These results may have implications regarding the stocking success when releasing hatchery-rearedAtlantic salmons in suboptimal environments. In particular, the study indicates that stocked salmon adaptation in the selected stream may not be improved by short conditioning periods to natural diet prior to their release, while we discuss potential explanations for the observed results.