Life-history traits of Brown Trout vary with the size of small streams

1. Brown Trout Salmo trutta L. life-history traits were studied in 17 coastal streams with mean annual water discharges less than or equal to1 m(3) s(-1), and with anadromous and freshwater resident trout present in all streams. 2. Mean smolt length and age were positively correlated with mean annual water discharge and latitude. The variation (CV) in smolt length and age was negatively correlated with water discharge. Mean length and age at maturity were also positively correlated with mean annual water discharge. 3. Freshwater resident females were sparse (mean: 3.7%) but males were abundant in all the investigated populations (mean 48.9%). Mean body length at maturity of resident males increased with discharge in parallel to that of the anadromous males. Mean length at maturity increased with smolt length in anadromous males, but not females. Mean sea-age at maturity in females, but not males, was negatively correlated with smolt age. 4. It is concluded that several life-history traits of Brown Trout are strongly influenced by habitat variables associated with the size of small nursery streams.