Effects of broodstock origin, rearing environment and release method on post-stocking performance of Atlantic salmon : Enriched rearing promotes post-stocking performance of Atlantic salmon

An increasing number of programs are releasing animals to nature to conserve threatened populations or to reintroduce animal populations that are extinct from the wild. Crucial to the success of these programs is the survival of the animal. Animals reared in captivity show often maladaptive behaviour and low survival in the wild. Genetic domestication and adaptation to the rearing environment are influencing the development of animals. It has been shown that enriching the rearing environment by e.g. adding structure is promoting animal behaviour and survival. Many studies are now investigating the effects of genetic domestication and the effects of rearing environment on behavioural development of animals and survival in the wild. However, few studies have addressed the effects of genetic domestication and enriched rearing on behaviour and survival simultaneously. The main aim of this thesis was therefore to investigate the effects of broodstock origin (wild vs. captive) and rearing environment (enriched vs. standard) of endangered Atlantic salmon (Salmo salar) on important life skills (e.g. foraging and antipredation), migration and survival after release to the wild. \n\nSurvival after release is not only thought to be influenced by the quality of fish, but partly due to the release methods. Handling and transportation to a release site represent major stressors for animals and can impair important survival traits. Methods that aim at decreasing stress before release to the wild have been developed and are used in many animal taxa with promising results. The second part of this thesis was therefore designed to investigate the effects of stocking procedures on stress and if so-called soft release methods (acclimatization after transport to decrease stress levels) could benefit post-release performance. \n\nThe results showed that enriched rearing improved foraging capacity and decreased maladaptive behaviour after release to semi-natural environments of salmon parr. Enriched rearing also promoted migration and survival after release to the wild of salmon smolts. The effects of genetic domestication were less clear. Acclimatization after transport (soft release) proved important for lowering stress before release, but no direct evidence on survival was found. \n\nThese results show clearly that conventional rearing does not produce fish that are prepared for a life in the wild and indicates that environmental enrichment can improve life skills and survival of fish significantly.