Diurnal Tidal Current On The Eastern Shelf Of Hidaka Bay: Can Juvenile Walleye Pollock, Theragra Chalcogramma, Move Southeastward With The Diurnal Tidal Current?

摘要

It has been reported that juvenile walleye pollock (age 0) moves southeastward along the eastern shelf of Hidaka Bay, Japan, in the early summer, when the mean flow direction on this shelf is opposite to its movement, i.e., northwestward flow. A hypothesis was previously proposed that the juvenile can move against the mean flow by combining an active diurnal vertical migration with background diurnal tidal currents. A strong vertical shear of the diurnal tidal current is essential for this hypothesis, but the presence has not been clarified. In order to describe characteristics of the diurnal tidal current on the eastern shelf of Hidaka Bay and to argue the validity of this hypothesis, we conducted a mooring current measurement from December 2002 to July 2003 and one-shot/25.8-hour ADCP surveys in the early summer (June) of 2003. It is shown that the O_1 current is by far the largest of the four major tidal constituents, followed by the K_1 current. This magnitude relation is inconsistent with that of the tidal height amplitude, M_2 > K_1 > O_1 > S_2. The spatial structure of the O_1 current from the ADCP survey is compared with that from free coastal-trapped wave (CTW) dynamics. As a result, it is indicated that the O_1 tidal height and current are mainly governed by a basin-scale external gravity wave and a first-mode baroclinic CTW with about 100-km wavelength, respectively. The O_1 current related to the CTW exhibits a nearly barotropic structure on the shelf without a strong vertical shear, independent of the seasonal stratification. Consequently, it is concluded that the proposed hypothesis must be quantitatively invalid.