Growth and life cycle of an euphausiid crustacean (Euphausia pacifica Hansen) in Toyama bey, southern Japan sea
1993
Iguchi, N. (Japan Sea National Fisheries Research Inst., Niigata (Japan)) | Ikeda, T. | Imamura, A.
The growth and life cycle of a euphausiid Euphausia pacifica were investigated in Toyama Bay based on vertical haul (0-500m) samples collected with Norpac nets during 2-4 week intervals over one full year (February 1990 through January 1991). MTD nets were also towed to evaluate its vertical distribution pattern in June, September, and December 1986. The results of Norpac nets indicated that the occurrence of eggs and larvae were from mid-February through late June. After a rapid growth (3.1mm/month) of the larvae and early postlarvae, the model size (-10mm) of postlarvae remained unchanged from summer to winter, indicating no growth of E. pacifica during this season. The results of MTD nets revealed a diurnal migration pattern of E. pacifica in the order of 300m in June and December. However, in September, the magnitude of night ascent was reduced because of a higher water temperature barrier (20 degrees C) developed in the euphotic zone. This limited night ascent should have reduced feeding of this species in the euphotic zone and considered to be the major cause for its growth stagnation in Toyama Bay. After this long growth stagnation from summer to winter, E. pacifica grew rapidly once again through the spring phytoplankton bloom of the next year and spawned. In Toyama Bay E. pacifica was considered to have only one generation per year. The minimum maturity size and the maximum size were 11mm and 22mm, respectively, for males and were 12mm and 23mm, respectively, for females. The estimated life span was less than 21 months. The present results are compared with those for other regions of the North Pacific
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