Observation of the ovarian eggs and spawning habits in Euphausia superba DANA

The development of ovarian eggs of a total of 197 individuals of Euphausia superba DANA was observed. One of three material sources was from the stomachs of minke whales caught by the CHIYO MARU fleet during the 1973/74 season, while others were collected by net during two cruises of the R. V. KAIYO MARU in 1979/80 and 1980/81. Histologically, the oogenetic development of ovarian eggs in female E. superba (23-57mm) was classified into the following nine stages : I. oogonia stage, II. early chromatin nucleolus stage, III. late chromatin nucleolus stage, IV. early peripheral nucleolus stage, V. middle peripheral nucleolus stage, VI. late peripheral nucleolus stage, VII. early vitellogenic stage, VIII. late vitellogenic stage, and IX. final maturity stage. In females smaller than 30mm (T.L.) from November to February, ovarian eggs were composed largely of the stages I to III, whereas those in animals larger than 31mm collected in early to mid January showed a more developed stage composition. Near the end of January a few post-spawning krill larger than 35mm began to occur and their relative abundance in the population increased toward early February. Changes in the composition of ovarian eggs by the developmental stages during late January to February suggested that some females of 35-40mm may spawn within the current breeding season whereas the remaining non-spawning females of similar body size may spawn first in the subsequent breeding season. In the ovary of post-spawning females some oocytes of stages II to III were observed, which were considered to become mature by the beginning of the subsequent breeding season. These results strongly suggest that E. superba is likely to spawn separately, twice or more in different breeding seasons throughout its life-span. The number of eggs released by 26 individuals of 44-54mm in the shipboard rearing experiments ranged from 627 to 3115 (x=1417,s=117) per female. This figure was considerably smaller than the known apparent fecundity of this animal