Growth performance and sexual maturation in diploid and triploid Atlantic salmon ( Salmo salar L.) in seawater tanks exposed to continuous light or simulated natural photoperiod

Abstract

Triploid Atlantic salmon are considered to be functionally sterile and the use of these in intensive fish farming would potentially reduce interactions between escapees and wild stocks. However, variable performance under commercial production conditions in seawater has been reported. The experiment described here is part of a larger comprehensive study on triploid Atlantic salmon, and investigates seasonal growth performance in diploid and triploid salmon under simulated natural light, and an accelerated production regime using continuous light during winter and spring. Four full-sib families of diploid and triploid Atlantic salmon postsmolts were held in six 20-m 3 circular indoor tanks in the January–October period after their transfer to seawater in July. Two tanks were given continuous light (LL) from January to June and subsequently put on simulated natural photoperiod (SNP), while four tanks were kept on SNP throughout. Variation in feed intake was monitored by waste feed collection, and fish were sampled monthly for weight and length. A depression in feed consumption was noted in the LL groups during the first 6–8 weeks after onset of light. As expected, the LL groups outperformed their SNP counterparts, but displayed an unexpected equal incidence of sexual maturation. Triploids displayed distinctly enhanced feed intake during their second sea autumn, which resulted in their being heavier and significantly longer than diploid fish in October. This increased feed intake in triploids was advanced in the LL treated fish. Only males showed signs of becoming ‘sexually mature’ (increased GSI) as grilse (after 1.5 years in seawater), and equal proportions of the diploid and triploid fish were characterised as sexually mature. Thus, lack of ‘sexual maturation’ could not explain the improved triploid growth. There were no distinct differences in mortality or occurrence of deformities between ploidy types. Both triploids and diploids responded to the accelerated light regime with an advanced seasonal development of growth and condition factor, long-term enhanced growth and an indicated improved feed conversion. In general, all sibling groups responded to the ploidy and light treatments. Growth differences between sibling groups within treatments were mainly ascribed to different proportions of sexual maturation. This study indicated that triploid Atlantic salmon performed equally, or better, compared to diploid conspecifics reared in indoor seawater tanks.