Community structure of demersal fish over the continental shelf off the southeastern coast of Hokkaido Island: The effect of eurythermy in dominant species

Abstract

The seasonal dynamics of a boreal demersal fish community and the factor governing its structure have been analyzed, using a total of 169 Danish seine samples collected off the southeastern coast of Hokkaido Island where is an important nursery for Japan Pacific population of walleye pollock Gadus chalcogrammus. The sea floor of the area was covered by cold (<2 °C) Coastal Oyashio Water during spring which was replaced by Oyashio Water and Modified Soya Current Water by autumn. Antlered sculpin Enophrys dicerus was the dominant species accounting for 23.2% of total biomass followed by plain sculpin Myoxocephalus jaok (19.8%), Kamchatka flounder Atheresthes evermanni (8.2%) and walleye pollock (7.4%). Based on species composition and biomass, the samples were classified into the four faunal groups: i) upper-shelf assemblage in which antlered and plain sculpins in combined accounted for 61.4%, ii) mid-shelf assemblage dominated by antlered sculpin (27.0%) and various fish taxa iii) lower shelf assemblage dominated by gadids (32.8%) and iv) less abundant lower-shelf assemblage dominated by antlered sculpin (33.3%) and other cottoid species (30.4%). Depth was the most important factor determining the structure of assemblages accounting for 66.2% of the variance explained by environmental variables, outweighing the effects of season (19.4%) and temperature (14.4%). This result was ascribed to the eurythermy of the dominant species which persisted at relatively narrow depth ranges under the seasonally fluctuating thermal environments.