The Yamato Rise(YR) is the largest seamount in the central part of the East Sea. The YR and its ambient sea are historically known as one of major fishing ground in the East Sea fisheries, and also known main migration route for Japanese common squid, Todarodes pacificus. The sea around YR is forming Sub Polar Front(SPF) every year by joining between warm water and cold water mass, and vertical structure of water column in this region are intensively influenced by alteration of formation and disappearance of SPF.
The formation and location SPF, and vertical structures of water temperature around YR were illustrated using SST gradient derived from both satellite images and in-situ observation data, estimating by numerical modeling. Squid samples around YR were collected monthly Korean squid jigging vessels in November 2015, and from July to October 2016. The composition of stomach contents was analyzed and compared using both microscopic observations and Next Generation Sequencing(NGS) by molecular genetic method.
Spatial and temporal variabilities of SPF were varied with changes in marine environment especially sea surface water temperature(SST) around YR. SPF was generating from October to February, and degenerating from March to May, but there were no identifiable SPF between June and September throughout East Sea. From the stomach content analyses using both microscopic and NGS methods, Japanese Common Squid mainly consumed fishes and cephalopods during study period. The compositions of food items were varied monthly, with fish being abundant in August and September in the diets of the species, whereas cephalopods consuming more in the other months. During summer when SPF was absent, the upper layer which squid was captured from surface to 150m was influenced by warm water mass. Consequently the prey items of Japanese common squid were dominated warm water species such as Japanese anchovy(Engraulis japonicus) and Spotted sardine(Sardinops sagax). On the other hand, in the phase of SPF when the water temperature from the surface to the depth of 150 m became colder by vertical mixing, the foods of Japanese Common Squid was mainly constituted cold water species including Japanese firefly squid (Watasenia scintillans) and Japanese sandfish(Arctoscopus japonicus). Strong cannibalisms were also evident in July and October when the samples were transported by live status.
In conclusion, the present study yields important insight into the diets and variations of prey resources Japanese common squid by environmental changes in the waters around of YR. This study attempted the first combining analyses of diet choice and SPF formation, and gives reliable interpretations of generalized feeding behavior of the species dependently abundant prey resources in their ambient environments which was driven SPF. Investigation of dietary habits of marine animals in relation to environmental changes is important for providing baseline data to improve trophic and ecosystem modeling in marine ecosystems of the East Sea.