2008 Volume 72 Issue 3 Pages 165-173
Isada krill (Euphausia pacifica) is a target species of krill fishery in the North Pacific as well as an important component as a prey organism in marine ecosystem. Objective information of their distribution has been anticipated for the current and future ecosystem-based fisheries management. We thus aimed to quantitatively analyze the distribution characteristics (structures and environments) of isada krill near southern Kuril Islands in the North Pacific Ocean. In 2005, acoustic data at two different frequencies (38 and 120 kHz) and specimens from MOCNESS trawls were obtained as well as temperature and salinity at multiple sampling stations. Using the difference of backscattering at each frequency, backscatterance due to krill was extracted from echograms and numbers of krill in an individual water column over the transect was calculated. Temperature and salinity data were interpolated over transect using Kriging, and compared with the calculated krill numbers at specific locations. In exploratory analyses with vertical profiles along transect, different features of krill distribution and marine environment between north and south of 4°C isotherm, located at 43.27°N, were observed. Also, temperature and salinity where krill were concentrated were significantly different between the north and south of this observed isotherm (p<0.05). Using semivariograms, structural differences of krill distribution between the north and south became obvious. In northern area of the 4°C isotherm, the krill distribution was more disperse and extended to the north-south direction than it was in the southern area where aggregations with variable densities were observed. Specimens in the northern area were mostly of furcilia stages. On the other hand, in the southern area, mainly found were adults. It is likely that life stages of the krill determine their habitat in a given oceanic structure in the area.
