Movements of hatchery reared Salmo trutta during the smolting period, under experimental conditions

In an experimental flume tank, two-year-old, 20 cm Salmo trutta from both a nadromous and freshwater resident stocks showed increased downstream moveme nt, mostly diurnally, as the water temperature rose, from early May to earl y June, most intensely from 23 May-9 June, after which movement ceased. Wat er velocity was 20 cm s(-1): at first the downstream speed of the fish was ca. 7 cm s(-1), suggesting that the fish were resisting displacement. At th e most intense movement, the fish headed downstream, at only ca. 4 cm s(-1) (0.2 body lengths s(-1)) Easter than the water. Maturing males moved signi ficantly less than did immature males and females. Individual fish that ate more, and that grew faster, moved less. The more that condition factors in creased between February and mid-May, the less the fish moved. Taken togeth er, these findings suggest that spring smelt emigration of trout, whether o r not they belong to anadromous or freshwater resident stock, is principall y by passive displacement. (C) 1998 Elsevier Science B.V. All rights reserv ed.