Distribution and abundance of the Ishikari Bay herring (Clupea pallasii) natural spawning beds and the seaweed beds on the northwestern coast of Hokkaido [Japan]
2010
Akaike, S., Hokkaido. Hakodate Fisheries Experiment Station (Japan) | Takahashi, K. | Hoshikawa, H. | Takiya, A. | Tsuda, F. | Goda, H. | Nakajima, K. | Kawai, T.
Distribution and abundance of natural spawning beds of Ishikari Bay herring (Clupea pallasii) were investigated along the northwestern coast of Hokkaido, Japan during 1996-2007. Geographic areas of seaweed beds were determined based on interpretation of aerial photographs using multispectoral images and color photographs taken during 1999-2004. Spawning beds were found at 1-7 sites per district after 1998. Their geographic distribution changed temporally from northern to southwestern districts and showed a wider range of depths shallower than 12 m depth. The maximum number eggs was 381 x 108 at Atsuta district in 2004. This phenomenon was attributed to a large population of the 200l-year-class. The seaweed bed areas per district ranged 169 x 104 - 254 x 104 square m except Wakkanai. More than 80% of seaweed beds were shallower than 5 m depth. The percentage of seaweed bed area used as spawning beds were 4-9% at Rumoi district and 2% at other districts. The present seaweed beds areas are sufficient for herring spawning beds sustaining the present resource level. For enlarging the resource level, however, seaweed bed areas might be insufficient for spawning beds. Therefore, it will be necessary to preserve the present seaweed beds, restore degradated areas, and afforest a wider range of depth zones.
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