BIOLOGICAL CHARACTERISTICS OF SEA-TROUT ( SALMO-TRUTTA L) IN NORTHERNSPAIN, IN 2 RIVERS OF ASTURIAS

We studied size, age structure, feeding and reproductive parameters in order to make a preliminary characterization of sea-trouts from river s of Asturias (northern Spain). Samples were obtained from sport fishe ry (June and July, cheafly) in the rivers Cares and Narcea and from el ectrofishing in the spawning grounds of the Narcea basin during the re productive season (November to January).Samples from the summer fishin g season showed similar age structure for sea trout in both rivers. Ab out 85% of the individuals remained two years in freshwater prior to s eaward migration, and more than 95% of them were in sea age classes 0 and 1+. Relative frequency of 0+ individuals (finnocks) was slightly higher in the river Cares (68%) than in the Narcea (51%). The number o f females exceeds that of males in all age classes. Age structure of s ea trout caught in the spawning grounds didn't differ from that of the fishing season, also showing a high proportion of finnocks (32% among mature fishes). Maturation rate of 'finnocks' was noticeably high (81 % of females and 100% of males), and all individuals in older sea age classes were mature. Egg number (571 to 2086 eggs per mature female) a nd relative clutch weight were positively correlated with size and sea age of the females. Sea trout feeds actively in freshwater during sum mer, as 81% of stomachs were full in the fishing samples. The main pre ys were Ephemeroptera, Diptera and Trichoptera, but the diet of sea tr outs also included a number of terrestrial preys, mainly Arthropods.