Large variations in the number of female adult salmon spawning in the Girnock Burn each year (range 28–127) produced smaller variations each year (range 2900–5600) in the number of juvenile migrants. The relative constancy of the parr migrations was achieved by changes in their age composition. There may be no advantage in allowing ova deposition to exceed a level around 200 000 (3.4 m−2) in the Girnock Burn, which would give rise to an average of about 4000 juvenile migrants (0.07 m−2) per season. Large numbers of ova (up to 12.5 m−2) did not decrease the number of juvenile