NAKATANI Toshikuni Hokkaido University, Faculty of Fisheries, Instructor, 水産学部, 助手 (80188979)
MAEDA Tatsuaki Hokkaido University, Faculty of Fisheries, Professor, 水産学部, 教授 (00001595)
|Budget Amount *help
¥2,400,000 (Direct Cost: ¥2,400,000)
Fiscal Year 1990: ¥200,000 (Direct Cost: ¥200,000)
Fiscal Year 1989: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1988: ¥1,400,000 (Direct Cost: ¥1,400,000)
The annual life period of adult Hippoglossoides dubius in Funka Bay and its offshore waters at a depth less than 200 m was divided into the following three: spawning (December-May), active feeding (May-June), and inactive feeding (July-November). They were concentrated in the deep area (more than 80 m depth) of the bay, except for the spawning season. Bottom water temperature in the deepest area of the bay was low (<8^ﾟC) throughout all seasons, because the cold water (Winter Funka Bay Water) originated from the Kuroshio was distributed during the warm months. The existence of cold water near the bottom is essential for H. dubius which the optimal temperature seems 2-7^ﾟC.
H dubius fed chiefly on megalobenthos, but changed to fish or plankton diet with relative availability of prey species, suggesting that they are euryphagous species. Direct interspecific competition for food between H. dubius and other main fishes apparently was absent. Therefore, their food-use patterns, either in qu
antity or kind, were unrelated to interspecific competitive activity. In addition, H. dubius partitioned the food resource of the environment within species by the dietary change with increase of body size. Such intrainterspecific relationships are connected to the fact that H. dubius are predominated in the demersal fish community.
H. dubius eggs were sampled from December to April and were abundant for the period January to March. Distributions of eggs devided into five developmental stages indicated that the main spawning ground was formed in the coastal waters from inner part to southern part of the bay. Eggs were trasported by current anticlockwise and mostly hatched from southern part of the bay to the bay mouth. A part of eggs found in the bay were of those brought from eastern area off the bay, and some of the eggs originated from the bay flowed to the outside of the bay. The density of copepoda nuplii, which are the primary food of H. Dubius larvae, increased from February to March with inflow of the Oyashio current. This fact suggests that the Funka Bay during the main spawning season provides the suitable food condition for H. Dubius larvae at the first feeding stage. In this study area, however, there is a great annual fluctuation in abundance of H. dubius. To clarify this cause, further investigation concerning the relation between survival rate of larvae and food density and damage by predator are necessary. Less