VARIATION IN AGE, SIZE AND REPEAT SPAWNING OF ADULT ATLANTIC SALMON IN RELATION TO RIVER DISCHARGE

(1) Mean body length and mean sea-age at maturity of Atlantic salmon, Salmo salar, increased with increasing mean annual water discharge (1-300 m3 s-1) of the spawning river, r2 = 0.53 and r2 = 0.41, respectively. Rivers between 40 and 300 m3s-1, however, showed no such significant correlation. (2) Mean body length and mean sea-age at maturity also increased with passable length (4-126 km) of the spawning river, r2 = 0.30 and r2 = 0.24, respectively. However, passable river length did not add significantly to regression models having mean annual water discharge as the independent variable. (3) Among rivers, body length at sexual maturity for one-(r2 = 0.31) and two-sea-winter (r2 = 0.34) fish increased with mean sea-age at maturity of the population. (4) Among post-spawners, some spawned annually, others biennially. The proportion of biennial spawners increased significantly with sea-age at maturity (r2 = 0.69). In rivers with mean sea-age at maturity between 1.0 and 1.4 years, 0-40% of the fish spawned biennially. In rivers where sea-age at maturity was between 2.0 and 2.3 years, 80-100% spawned biennially. (5) The proportion of biennial spawners depended on fish size (r2 = 0.97). The majority of the smallest adults (< 60 cm) spawned annually, the largest (> 90 cm) biennially. (6) Recapture rates of tagged post-spawners decreased with increasing body length (r2= 0.45) suggesting that small adults had higher sea survival than large ones.