Life cycle and growth rate of the chaetognath Parasagitta elegans in the northern North Pacific Ocean

Breeding periods and growth rate of the typical boreal epipelagic chaetognath Parasagitta elegans (Verrill) were investigated in the northwestern North Pacific off eastern Hokkaido. Seasonally, P. elegans were most abundant in the epipelagic layers in early June to October, while they were always poor in abundance during December to April. It is suggested that the decrease in abundance in early December was partly due to vertical migration into water deeper than 150 m in winter. The main breeding period of the chaetognath was observed in late May to early June. Some cohorts produced by the spawning populations were able to be traced at least until early December. Thus, the mean growth rate of the cohorts was estimated to be 0.03 to 0.29 mm d super(-1), averaging approximately 0.1 mm d super(-1), which corresponds to 3 mm month super(-1). This value is lower than that (5 to 6 mm month super(-1)) reported from the Gulf of Alaska, eastern North Pacific. Although the generation length of P. elegans is still uncertain in the present area, it is probably longer than that in the northeastern Pacific Ocean, being 1 year or possibly 2 years. This suggests that reproductive and high growth periods in P. elegans are seasonally more strictly defined in the western parts of the North Pacific Ocean than in the eastern, due to the larger magnitude of seasonal fluctuations in oceanographic conditions in the former than in the latter.