The colonization by both resident and migrating spawner populations of brown trout and the characteristics of resident and migrating juveniles derived from the two populations have been studied in a brook and its tributary over 4 years. Resident trout spawns mainly in the upstream part of the brook and migrating trout in the downstream part. There are density and growth variations for the two age classes (0+ and 1 +) ofjuveniles in autumn according to the year and the environment. In the brook, the population of0 + fish increases from downstream to upstream while the density of other age classes decreases. The migrating juvenile population of the brook changes annually and consists mainly of 1s (one summer) individualscorning from the upper part. These individuals migrate generally in autumn and winter while young trout produced in the middle and downstream parts of the brook migrate mainly in the spring. The emigration process of the O + population decreases markedly from upstream to downstream and appears to be independent of the autumn length and sex ratio. In the tributary, most trout are O+ years old, the population structure is different, and no migrating fish is observed. The results are discussed and a colonization strategy of the brown trout population in this brook is suggested.
Migration and spawning behaviour of eight Atlantic salmon adult females were analysed by radio-tracking in relation to the degree of sexual maturity in a spawning tributary of the R.Selune. Six of them were grilse and four of them were ripe. All the fish migrated upstream until reaching their spawning site at a distance o f 4 1 2 km from the trap. Thedaily migration rate up to this site was inversely correlated with the length of the female. Spawning occurred in all fish at the same time when the water temperature increased dramatically. Spawning lasted 1-10 days according to the fish. After spawning, females quickly moved downstream only small distances and then stayed in approximately the same location until death. This study concluded that ripeness did not influence behaviour on the spawning migration and describes certain characteristics of the reproductive phase in a spawning tributary.
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