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In this study, we investigated the diving behavior of sei whales relative to the vertical distributionoftheirpotentialpreyinthewesternNorthPacificduringthesummerof2013.Acoustic time-depthtransmitterswereattachedtotwoseiwhalesfor10.2and32.0h,respectively.Thevertical distributionanddensity(expressedasthevolumebackscatteringstrength,SV)oftheirpotentialprey were recorded by an echosounder. Diving behavior was classified into two shapes: U-shaped and V-shaped.Forbothindividuals,U-shapeddivingwasassociatedwithhigherSVvaluesthanV-shaped divingandthefrequencyofU-shapeddivingincreasedfromlateafternoonuntilsunset.Duringthe daytime,densescatteringlayers(presumablyzooplankton)weredistributedatapproximately40mand theythenmigratedtowardthesurfacearoundsunset.Thedivingdepthofthewhalesfollowedthediel migrationof thescatteringlayersandthedivingwas concentratedintheselayerswhenthedensity becamehigh.Theresultsofthisstudyindicatethatseiwhaleschangetheirdivingdepthandshapesin responsetothedielverticalmigrationoftheirpotentialprey.

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The plain area in the east part of Tokyo, Musashino Terrace and Shitamachi Lowland, is underlain by Quaternary thick sediments. These sediments are divided stratigraphically into two groups, the lower is the Kitatama Formation and the upper the Tokyo Formation. The general strike of the Formations tends NNW-SSE with a very low dip to the northeast. Upper layers of the Tokyo Formation are mainly composed of silt, sand and gravel, forming the aquifer system of artesian groundwater. According to the distribution of Tritium ratio in the groundwater, which of Musashino Terrace has mainly supplied from the surface water of the Tama River.Withdrawal groundwater from these aquifers has caused the lowering of groundwater level and the land subsidence all over the plain. The lowering of groundwater level and the land subsidence, however, which have continued half a century, rapidly taken favorable turns during recent about ten years due to restrictions of groundwater withdrawal.Recovering of groundwater level has not only brought about the favorable turn of the land subsidence, but also decreasing the spout of oxygen deficient air through water wells and the cracks of basements at the time of air pressure change or pnuematic work in the underground.It also has advanced the secondary storage of natural water soluble gas, which has reserved originally in the deep layers, the Kazusa Group, into gravel layers beneath the Yurakucho Formation.

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