This paper reviews results of experimental and field studies of downstream migration of young fish carried out in the Institute of Ecology and Evolution (Russian Academy of Sciences) as a part of the research programme on fish behaviour and ecology. Studies of fish migrations have been dominated by descriptions of migratory routes and proximate control of orientation. Behaviour and ecological factors influencing variability of downstream migrations of fish larvae and fry are still poorly understood. Life cycles of most of freshwater fishes can be roughly considered as a sequence of ''residential'' and ''migratory'' phases. Migrating, or just moving between microhabitats, fish are especially vulnerable to various threats including impacts of water abstraction systems. Abiotic (water flow velocity, rheogradients, turbulence, visual habitat heterogeneity) and biotic (foraging conditions, predators, competitors and parasites) factors can cause changes in fish spatial distribution as well as shift from residential to migratory behaviour. From the viewpoint of fish protection, these changes are important not only in the vicinity of water abstraction sites, but also in more distant parts of the aquatic system that should be taken into account in long-term assessments of impacts of abstraction on fish populations in regulated rivers. Besides macro-and mesoscale impacts that affect fish migrations and spatial distribution, micro-scale habitat heterogeneity (hydrodynamic and visual) are important as factors triggering and controlling various aspects of young fish behaviour. Interactions of water abstraction zones with fish populations have to be analysed at ecological (meso-scale spatial overlapping and longterm dynamics) and behavioural (effects of microscale structures and events) levels.

The lipid fraction of larvae of the black soldier fly Hermetia illucens was shown to contain lauric acid (38.43 wt %) and its esters, azelaic and sebacic acids, and azelaic acid dibutyl ester. The dominant compound in the group of identified glycerides was lauric acid monoglyceride (0.70 wt %). Glycerides were also represented by triglycerides and diglycerides of lauric acid. Sterols were represented primarily by phytosterols (over 75%), the major of which was alpha-sitosterol (45%). The identified lipid complex composition is apparently determined by the biological characteristics of the fly Hermetia illucens and ensures antibacterial defence of larvae and stability of lipids at changing ambient temperature.

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The dynamics of fatty acid composition have been studied in differently aged salmon juveniles (0+, 1+, 2+ parrs and 3+ smolts) inhabiting two biotopes of the Varzuga River (the White Sea basin) differing in ecological conditions. Differences in the levels of linoleic (18:2n 6), linolenic (18:3n 3), and oleic (18:1n 9) acids have been revealed. It has also been shown that 3+ smolts have an increased content of long chain fatty acids-arachidonic (20:4n 6), eicosapentaenoic (20:5n 3), and docosahexaenoic (22:6n 3)-characteristic of the composition of marine type lipids.

Downstream migration (DSM) of young fish has traditionally been considered a passive process. A review of studies of the patterns and mechanisms of DSM showed that it met the main criterion for migration: that animals move between well-defined habitats according to a predictable schedule. During DSM, fish often leave the habitat in which they reside (inshore zone) and enter a migratory habitat (main channel). DSM is manifested at a seasonal or diel scale and in species-specific spatial distribution patterns. According to our "dispersal with low dilution" hypothesis, the supposed adaptive value of DSM is due not only to dispersal but also to maintenance of aggregations and synchronization of migratory activity. Temporal and spatial peaks of migration promote efficient intraspecific interactions, such as shoaling, and the integrity of populations. We conclude that migratory behaviour is most successful in habitats with optimum complexity and that the influence of behaviour on DSM is most important at key periods and sites (twilight windows and interfaces between habitats).Résumé : L'avalaison de jeunes poissons a traditionnellement été considérée comme étant un processus passif. Un examen des études des motifs et mécanismes d'avalaison montre qu'elle répond au principal critère d'une migration, soit que les animaux se déplacent entre des habitats bien définis selon un cadre temporel prévisible. Durant l'avalaison, les poissons quittent souvent l'habitat où ils résident (zone côtière) et entrent dans un habitat migratoire (chenal principal). L'avalaison se manifeste à une échelle saisonnière ou journalière et selon des motifs de répartition spatiale propres à chaque espèce. Selon notre hypothèse de « dispersion avec peu de dilution », la valeur adaptative supposée de l'avalaison résulte non seulement de la dispersion, mais également du maintien de groupements et de la synchronisation de l'activité migratoire. Les pointes de migration temporelles et spatiales favorisent des interactions intraspécifiques efficientes, comme le regroupement en bancs, et l'intégrité des populations. Nous concluons que le comportement migratoire a le plus de succès dans des habitats de complexité optimale et que l'influence du comportement sur l'avalaison est la plus importante dans des périodes et des sites clés (fenêtres crépusculaires et interfaces entre habitats). [Traduit par la Rédaction]