EFFECT OF SMOLT AGE, SEX AND ENVIRONMENTAL-CONDITIONS ON SEA AGE AT 1ST MATURITY OF ANADROMOUS BROWN TROUT, SALMO-TRUTTA, IN NORWAY

Anadromous brown trout (Salmo trutta) were sampled at spawning in six Norwegian rivers, located between 58 and 64-degrees-N. Sea age at firs t maturity was significantly influenced by the growth rate at sea, but not by the growth rate in the river. Between rivers, sea age at matur ity declined with increasing smolt age. Within rivers, brown trout fro m upper stretches smoltified at an older age and matured at an older s ea age than brown trout from the lower stretches. Males matured at a y ounger sea age than females. Smolt age, sex and environmental conditio ns accounted for about 30% of the overall variation in sea age at firs t maturity.