Study on the mechanisms behind species replacement between sardine and anchovy populations in the Enshu-nada Sea

20世纪70年代，随着恩树那达海日本沙丁鱼种群数量的快速增长，日本鳀鱼种群数量，特别是其春季产卵量开始下降。在沙丁鱼的主要产卵期（2 - 3月），没有捕食性浮游生物和竞争性远洋鱼类，如凤尾鱼，显然有利于沙丁鱼的招募成功。在20世纪80年代末，沙丁鱼开始减少，被凤尾鱼取代。值得注意的是，在这一阶段，沙丁鱼的产卵期推迟了几个月。其中一个可能的机制是，由于黑潮水流模式的变化，这两个物种的生殖环境发生了变化。事实上，据报道，凤尾鱼更喜欢无曲流的黑潮模式，而大黑潮曲流似乎对沙丁鱼有利。本文研究了黑潮潮对鱼类生殖环境的影响，重点探讨了黑潮潮对鱼类生殖环境的影响。黑潮在太平洋中部沿海的巨大蜿蜒可能会加强鱼卵和幼鱼的近岸运输，也会导致陆架地区的温度升高，而陆架地区是沿海产卵地和苗圃的所在地。这可能有利于沙丁鱼的招募，因为与凤尾鱼相比，沙丁鱼在近海产卵更多，更依赖于产卵温度（15-17ºC）。值得注意的是，沙丁鱼只有在观察到大黑潮曲流的年份，才能在2月至3月产卵。另一方面，研究表明，凤尾鱼的产卵与较低的透明度密切相关，这可能表明沿海水域普遍存在，因此食物可得性较高，通常对应于非蜿蜒的黑潮模式。大量野外调查还发现，在恩树那达海沿岸海域孵化的凤尾鱼卵和幼鱼可能被卷入与黑潮锋相关的气旋漩涡中，生物生产力的提高为其提供了有利的摄食条件和幼鱼的生存条件。少

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 … More 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