Population dynamics of the euphausiids Euphausia pacifica and Thysanoessa inspinata in the Oyashio region during the 2007 spring phytoplankton bloom

The abundance and population structure of Euphausia pacifica and Thysanoessa inspinata were investigated during the period 9 March–29 April 2007 at a single station in the Oyashio region. Water temperature and chlorophyll a concentration varied from 1.1 to 6.1 °C and from 0.02 to 6.3 mg m−3, respectively, primarily reflecting the effects of varying mixtures of the water masses prevailing in this region. The abundance of euphausiids, collected by oblique hauls with Bongo nets (0–200 m) at night, fluctuated from 41 to 1040 individuals m−2 for E. pacifica and from 50 to 186 individuals m−2 for T. inspinata. Correlation analyses revealed that the variations in the abundance of E. pacifica were related to water temperature and chlorophyll a, while abundance of T. inspinata only varied with temperature. Adult males and females (with and without spermatophores) were the major components of both euphausiid stocks (>90% of the total). Modal size (=total length or TL) analysis revealed that one group with large sizes (14–18 mm TL for E. pacifica, 16–18 mm TL for T. inspinata) occurred throughout the study period, often accompanied by one discrete group of smaller individuals (<11 mm TL for both euphausiids), suggesting the distribution of the same euphausiid populations across the heterogeneous water masses. Adult males were significantly smaller than adult females in T. inspinata, but this was not the case in E. pacifica. Tracing the progressive increase in the mean sizes of the larger size groups, growth rates were estimated to be 0.082 mm day−1 for E. pacifica and 0.022 mm day−1 for T. inspinata. The difference in the growth rates of the two euphausiids is discussed in light of species-specific differences in the allocation of energy to reproduction and somatic growth during the present study period.