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The brain monoamines serotonin (5-hydroxytryptamine; 5-HT) and dopamine (DA) both play an integrative role in behavioural and neuroendocrine responses to challenges, and comparative models suggest common mechanisms for dietary modulation of transmission by these signal substances in vertebrates. Previous studies in teleosts demonstrate that 7 d of dietary administration with L-tryptophan (Trp), the direct precursor of 5-HT, suppresses the endocrine stress response. The present study investigated how long the suppressive effects of a Trp-enriched feed regimen, at doses corresponding to two, three or four times the Trp levels in commercial feed, last in juvenile Atlantic cod (Gadus morhua) when the fish are reintroduced to a diet with standard amino acid composition. We also wanted to determine whether Trp supplementation induced changes in brain monoaminergic neurochemistry in those forebrain structures innervated by DAand 5-HTergic neurons, by measuring regional activity of DA and 5-HT in the lateral pallial regions (Dl) of the telencephalon and nucleus lateralis tuberis (NLT) of the hypothalamus. Dietary Trp resulted in a dose-dependent suppression in plasma cortisol among fish exposed to confinement stress on the first day following experimental diet; however, such an effect was not observed at 2 or 6 d after Trp treatment. Feeding the fish with moderate Trp doses also evoked a general increase in DA and 5-HT-ergic activity, suggesting that these neural circuits within the NLT and Dl may be indirectly involved in regulating the acute stress response.Key words: L-Tryptophan: Atlantic cod: Stress response: Nucleus lateralis tuberis: Lateral pallial telencephalonThe monoamines serotonin (5-hydroxytryptamine; 5-HT) and dopamine (DA) are important neurotransmitters/modulators involved in integration of the behavioural and neuroendocrine responses to stress (1 -4) . Comparative models suggest common mechanisms for dietary modulation of release and transmission by these signal substances in vertebrates. For instance, it has been demonstrated that dietary supplementation with the essential amino acid L-tryptophan (Trp), the immediate precursor of 5-HT, enhances brain serotonergic activity in teleosts (5 -8) and mammals, including man (9) . In general, the synthesis of central 5-HT depends largely on the plasma concentration ratio between Trp and other large neutral amino acids (LNAA), since they all enter the brain through the same carrier transporter located on the blood -brain barrier (10,11) . Once inside the neuron, Trp is transformed by tryptophan-5-hydroxylase (EC 1.14.16.4) into 5-hydroxytryptophan, which * Corresponding author: Dean Basic, email dean.basic@nvh.no Abbreviations: 1xTrp, control feed containing 0·47 % L-tryptophan; 2xTrp, experimental feed containing 0·82 % L-tryptophan; 3xTrp, experimental feed containing 1·14 % L-tryptophan; 4xTrp, experimental feed containing 1·65 % L-tryptophan; 5-HIAA, 5-hydroxyindoleacetic acid; 5-HT, 5-hydroxytryptamine (serotonin); DA, dopamine; Dl, lateral pall...

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Escaped farmed Atlantic salmon (Salmo salar) is considered a threat to wild salmon. In order to take action to reduce the impact of escapees, methods to distinguish escapees from wild fish and to trace them back to their origin are in demand. This paper gives an overview of available methods and discusses the impact on fish welfare, both in the short‐ and long‐term. Adipose fin clipping, freeze branding, different external and internal tags, as well as natural and chemical marks are considered. All marking procedures that involve handling of fish have an impact on fish welfare. Spraying with pigments and most externally attached tags significantly reduce the welfare, both on short‐term and long‐term perspectives. Although the use of natural or chemical marks, like exposure to stable isotopes via egg immersion or vaccination, involves no or no extra handling, subsequent analysis may require killing the fish after catch. Large‐scale marking with implanted or external tags could represent higher risks of human errors and reduced fish welfare, as opposed to small‐scale marking. In general, the knowledge about effects of marking on fish welfare is limited for most of the available methods, in particular regarding other effects than mortality and growth reductions.