This study investigated how light quality control during the seedling stage affects the long-term growth of cultivated Sargassum horneri and aimed to identify suitable light conditions. Seedlings were reared under four light-emitting diode (LED) treatments (White, Red–Blue mixture, Red, Blue) for approximately six weeks, then transferred to an outdoor tank under natural sunlight for approximately five weeks. Under LEDs, White (broad spectrum) and the Red–Blue mixture tuned to the species’ absorption spectrum supported superior growth; however, after transfer to sunlight, the Blue-preconditioned seedlings showed a rapid surge in growth, and across the entire period, relative growth rates were highest in the Red–Blue and Blue treatments. The daily stem elongation observed during the Blue-preconditioned surge reached approximately 1.5 times the value reported in a previous study of the same population. The Red–Blue mixture produced the smallest interindividual variability, thus suggesting superior uniformity, whereas the Blue treatment is practically promising. These results indicate that controlling light quality during seedling production can considerably enhance the growth potential of S. horneri and that the effects of prior light treatment are likely to become evident after exposure to sunlight. Field-scale trials and physiological studies are recommended to validate its applicability and clarify the underlying mechanisms.

　To clarify the spawning period of pond smelt Hypomesus nipponensis inhabiting Lake Biwa, the maturity of fish in the lake, plus the number of eggs deposited in two inflowing rivers and lakeshores, were investigated over two seasons. Laid eggs in the rivers were first observed in late December, peaking from mid- to late January, and then gradually decreasing, with eggs being confirmed until early March. The GSI (gonadosomatic index) of sampled females was highest from December to January. The peak spawning period of pond smelt in Lake Biwa is currently found to be from mid-to late January, suggesting that the spawning period become some one to two months earlier than around 1995. Because changes in spawning timing may greatly affect recruitment fluctuations through shifts in hatching timing, this study provides important information for understanding future fluctuations in stock abundance.