Photoperiod regulation of plasma growth hormone levels during induced smoltification of underyearling Atlantic salmon

Earlier studies have established that increased daylength increases plasma growth hormone (GH) levels during spring smoltification of yearling Atlanti c salmon. Recently, the Atlantic salmon aquaculture industry has started th e production of underyearling ("summer") smolts. This involves fast juvenil e growth on continuous light (24L), the transfer of juveniles over 8 cm in length to short day (12L) for 6 weeks in the summer, followed by transfer t o 24L for another 6 weeks before transfer to seawater in late October. Thre e groups were studied in fresh water from July to the following May in orde r to elucidate the GH response to this photoperiod manipulation: one group was kept on 24L throughout (long-day group), while the other two groups wer e exposed to short day from July 15th. Of these, one was brought back onto long day on September Ist (winter group) while the other was kept on short day (short-day group). Plasma GH levels of the long-day group were around 1 .6 ng/ml throughout the study. The short-day transfer suppressed GH levels to 0.7 ng/ml within 2 weeks (short-day and winter groups). The long-day tra nsfer (winter group) increased GH levels to 11 ng/ml within 3 weeks, and th is elevation of GH levels was sustained for about 3 months, before declinin g to pretreatment levels. The study demonstrates that underyearling Atlanti c salmon react to increased daylength in a way similar to traditional yearl ing smolts. It also demonstrates for the first time that decreased daylengt h can suppress plasma GH levels in fish. It is concluded that winter photop eriod manipulation causes an out-of-season initiation and completion of the parr-smelt transformation of underyearling Atlantic salmon and that growth hormone plays a major role in this process. (C) 2000 Academic Press.