| Summary: | Abundance and population structure of 3 Paraeuchaeta copepods (P. elongata, P. birostrata, and P. rubra) were investigated over one year, based on monthly samplings down to 2000 m depth in the Oyashio region, western subarctic Pacific. Annual mean abundance of P. elongata, P. birostrata, and P. rubra were 597, 516, and 86indiv. m^-2, respectively. In terms of biomass, these figures equate to 210, 308, and 143 mg DW m^-2, respectively. All copepodid stages occurred throughout the year, except for C1-C3 of P. rubra which are distributed below 2000 m depth and were thereby not sampled. In all 3 species, no clear sequence in stage-development was observed from the population structure data, suggesting more or less constant reproduction modes throughout the year. Adult sex ratios (female: male) were biased to females (71% in P. elongata, 67% in P. birostrata, and 92% in P. rubra), reflecting the fact that males are short-lived. No significant correlation was found between the sex ratios and the incidence of spermatophore-attached and egg-carrying females in these Paraeuchaeta copepods. Possibly undescribed epizoic ciliates on the Paraeuchaeta spp. were noted, and discussed in the light of their possible effect on the copepods. Occurrences of carcasses of various copepodid stages of the Paraeuchaeta spp. were also noted. The carcasses outnumbered the living specimens sometimes, and possible causes of the high abundance of Paraeuchaeta carcasses in the Oyashio region are discussed.
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