Abundance, biomass and body size of the hydromedusa Aglantha digitale in the western and eastern subarctic Pacific during the summers of 2003^|^ndash;2006

Abstract

The aim of this study is to evaluate regional and interannual changes in abundance, biomass and body size of the hy- dromedusa Aglantha digitale in the subarctic Pacific. A. digitale was sampled by 0-150 m vertical tows using a 100 μm mesh-size NORPAC net at stations from 41°30'N to 49°30'N along the 165°E line (western subarctic Pacific) and from 39°00'N to 53°30'N along the 165°W line (eastern subarctic Pacific) during the summers of 2003-2006. The mean numerical abundances during the course of the study in the west and east were 32.9 (range, 0-368) and 169.2 (0-768) ind. m -2 , respectively, and those of biomass were 26.3 (0-264) and 69.1 (0-418) mg DM m -2 , respectively. The abundance and biomass showed no significant north-south or year-to-year differences within each transect but they were significantly greater in the east in some years. The body size was char- acterized with the dominance of small-sized individuals in the east. The higher abundance and biomass, and smaller body size in the east were probably due to sampling during or just after their reproduction in the east.