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 coast al 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 strea ms. 2. Mean smolt length and age were positively correlated with mean annual wa ter 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 abun dant 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 i n anadromous males, but not females. Mean sea-age at maturity in females, b ut not males, was negatively correlated with smolt age. 4. It is concluded that several life-history traits of Brown Trout are stro ngly influenced by habitat variables associated with the size of small nurs ery streams.