Smolt development and subsequent sexual maturity in previously mature male Atlantic salmon ( Salmo salar)

Abstract

Individually tagged, age 0 +, upper modal and lower modal group (UM, LM), sexually mature male and immature male and female Atlantic salmon parr were observed for 1 + smolt development during winter-spring and survival, growth and subsequent maturation during 4.5 months (15 May to 30 September) in seawater. Both previously mature and immature UM fish became competent smolts as judged by development of high salinity tolerance (96 h in 37.5‰ salinity), elevated gill Na + K + - ATPase activity and decreasing condition factor (CF); LM fish did not show these smolt characteristics. Mortality in seawater was 15.4 and 9.1% in LM previously mature and immature groups and 0 and 4% in UM previously mature and immature groups, respectively. Previously mature and immature LM fish, though smaller (ca. 13 cm) than UM fish (ca. 20 cm) when entering seawater, had instantaneous growth rates as high as or higher than UM fish. CF of both LM and UM, previously mature and immature, fish rose during the period in seawater. CF of rematuring fish and those that matured initially in seawater rose more steeply than in immature fish. There was a higher incidence of rematuration and initial maturation in seawater in the LM than in the UM groups. After 4.5 months in seawater, plasma osmolalities were in the normal range (300–350 mOsmol·kg −1) in all groups and gill Na + K + - ATPase activity was uniformly higher in the immature than in the mature fish in both LM and UM. These findings are discussed in relation to the suggestion in earlier studies that sexual maturation and smolting in male Atlantic salmon are in developmental conflict.