The Atlantic salmon (Salmo salar L.) is a migratory species

that has a remarkably variable and flexible life history. Adult salmon spawn in fresh water in the autumn and winter, burying their eggs in river bed gravels (Stabell, 1984); the eggs hatch and the fry emerge the following spring. Young fish, known as parr, spend between 1 and 5 years in the river before smolting and migrating to the sea. Oceanic migrations take salmon to feeding grounds where they remain and grow for 1–4 years before becoming sexually mature and returning to their natal streams to spawn. Some male parr mature precociously in fresh water and fertilize the eggs of adult females at spawning. There is evidence that the homing habit of salmon, on both broad geographical scales (between-river) and narrower (within-river) scales, has resulted in local adaptation of populations (see Taylor, 1991). Tracking and radiotelemetry studies on the River Dee in Aberdeenshire, Scotland, have shown that fish entering the river in the winter and spring spawn at high altitudes, whereas summerand autumn-running 2539 The Journal of Experimental Biology 203, 2539–2552 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2689