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

20世纪70年代，随着日本仁州海沙丁鱼数量的迅速增加，日本凤尾鱼的数量，特别是春季产卵的数量开始下降。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中上层鱼类(如凤尾鱼)，显然有利于沙丁鱼的补充成功。20世纪80年代末，沙丁鱼开始衰落，取而代之的是凤尾鱼。值得注意的是，在这一阶段，沙丁鱼的产卵期推迟了几个月。这种更替背后的可能机制之一可能是由于黑潮水流模式的变化而改变了这两个物种的繁殖环境。事实上，据报道，凤尾鱼更喜欢非曲折的黑潮模式，而大的黑潮曲流似乎有利于沙丁鱼。我们研究了黑潮弯曲对繁殖环境的影响，重点是c…。黑潮在太平洋中部海岸附近的大范围蜿蜒，可能加强了鱼卵和仔鱼的近岸运输，也导致了沿海产卵场和苗圃所在陆架地区的温度上升。这可能有利于沙丁鱼的补充，因为与凤尾鱼相比，沙丁鱼在近海产卵更多，而且更依赖产卵温度(15-17ﾟC)。应该指出的是，沙丁鱼只能在2-3月期间产卵，这是观察到大黑潮曲折的年份。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，因此有更高的食物可获得性，通常对应于非曲折的黑潮模式。此外，密集的实地调查还显示，在内州-那田海沿岸水域孵化的凤尾鱼卵和仔鱼可能被卷入与黑潮锋面有关的气旋性涡流中，这可能由于生物生产力的提高而为摄食和幼体生存提供有利条件。较少

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