| Budget Amount *help |
¥2,600,000 (Direct Cost: ¥2,600,000)
Fiscal Year 1996: ¥1,200,000 (Direct Cost: ¥1,200,000)
Fiscal Year 1995: ¥1,400,000 (Direct Cost: ¥1,400,000)
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| Research Abstract |
In 1970s, Japanese anchovy population, particularly its spring spawners started to decline corresponding to the rapid increase of Japanese sardine population in the Enshu-nada Sea. The absence of predatory plankton and competitive pelagic fish such as anchovy during the main spawning period of the sardine (February-March) was apparently advantageous to recruitment success of the sardine. In late 1980s, sardine started to decline, being replaced by anchovy. It was noteworthy that during this phase the spawning period of the sardine was delayd a few months.
One of the possible mechanisms behind the replacement could be the changes in the reproductive environment of these two species due to variations in the flow pattern of the Kuroshio. It has been, in fact, reported that anchovy preferred non meandering Kuroshio patterns, while large Kuroshio meander seemed favorable to sardine. We have investigated the influences of the Kuroshio meander on the reproductive environment, focusing on the c
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hanges in current fields, temperature and food availability due to the Kuroshio meander.
The large meander of the Kuroshio off the central Pacific coast possibly enhances the inshore transport of fish eggs and larvae and also contributes to temperature increase in the shelf regions, where coastal spawning grounds and nurseries are located. This could favor the sardine recruitment because sardines spawn more offshore and are more dependent on the spawning temperature (15-17ﾟC) compared to anchovies. It shoud be noted that sardines could spawn during February-March only the years, when large Kuroshio meanders were observed. On the other hand, it is revealed that anchovy's spawning is more closely related to lower transparency, which may be indicative of coastal water prevalence and therefore higher food availability, frequently corresponding to non-meandering Kuroshio patterns.
It has been also revealed from intensive field surveys that anchovy eggs and larvae hatched in the coastal water of the Enshu-nada Sea were possibly entrained into cyclonic eddies associated with the Kuroshio front, which could provide favorable conditions for feeding and larval survival due to the enhanced biological productivity. Less
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