Yoshitaka Morikawa

Bifurcation Current along the Southwest Coast of the Kii Peninsula

Along the southwest coast of the Kii Peninsula, a bifurcation current is regularly observed. By using ADCP data taken on board the R/V Wakayama of the Wakayama Prefectural Fisheries Experimental Station, characteristics of this bifurcation current are analyzed. The occurrence frequency of the bifurcation current reaches about 70% in the period from 1988 to 1996. The bifurcation point appears to be changeable and occurs almost evenly between Cape Ichie and Cape Shionomisaki. The current divergence in the alongshore direction was also investigated. Positive divergence values dominated in the whole analyzed area, and an onshore current appears to be dominant along the southwest coast of the Kii Peninsula, except in 1990 when the Kuroshio flowed in a large meandering path.

Current Nature of the Kuroshio in the Vicinity of the Kii Peninsula

The Kuroshio flows very close to Cape Shionomisaki when it takes a straight path. The detailed observations of the Kuroshio were made both on board the R/V Seisui-maru of Mie University and on board the R/V Wakayama of the Wakayama Prefectural Fisheries Experimental Station on June 11–14, 1996. It was confirmed that the current zone of the Kuroshio touches the coast and bottom slope just off Cape Shionomiaki, and that the coastal water to the east of the cape was completely separated from that to the west. The relatively high sea level difference between Kushimoto and Uragami could be caused by this separation of the coastal waters when the Kuroshio takes a straight path. This flow is rather curious, as the geostrophic flow, which has a barotropic nature and touches the bottom, would be constrained to follow bottom contours due to the vorticity conservation law. The reason why the Kuroshio leaves the bottom slope to the east of Cape Shionomisaki is attributed to the high curvature of the bottom contours there: if the current were to follow the contours, the centrifugal term in the equation of motion would become large and comparablee to the Coriolis (or pressure gradient) term, and the geostrophic balance would be destroyed. This creates a current-shadow zone just to the east of the cape. As the reason why the current zone of the Kuroshio intrudes into the coastal region to the west of the cape, it is suggested that the Kii Bifurcation Current off the southwest coast of the Kii Peninsula, which is usually found when the Kuroshio takes the straight path, has the effect of drawing the Kuroshio water into the coastal region. The sea level difference between Kushimoto and Uragami is often used to monitor the flow pattern of the Kuroshio near the Kii Peninsula. It should be noted that Uragami is located in the current shadow zone, while Kushimoto lies in the region where the offshore Kuroshio water intrudes into the coastal region. The resulting large sea level difference indicates that the Kuroshio is flowing along the straight path.

On Structure and Temporal Variation of the Bifurcation Current off the Kii Peninsula

The Kii Bifurcation Current is often found along the southwest coast of the Kii Peninsula, and its frequency of occurrence reaches about 70% in the period from 1988 to 1996 (Takeuchi et al., 1998a). In order to clarify the structure and short-period variability of the Kii Bifurcation Current, detailed observations were made four times on board the R/V Seisui-maru of Mie University on October 29–31, 1996, on June 24–26, 1997, October 14–16, 1997, and December 3–4, 1997. The measured horizontal structure of the Kii Bifurcation Current indicates that the eastern portion of the Current (eastward flow near Cape Shionomisaki) consists of a part of the current zone of the Kuroshio. It is shown that the current structure, including the Kii Bifurcation Current in the vicinity of Cape Shionomisaki, is stable when the Kuroshio is flowing in a stationary straight path, but that the current structure is considerably changed when small-scale eddies pass by the cape. Such short-period variation can be monitored by using the daily variation of the sea level difference between Kushimoto and Uragami. In particular, in the case of October 29–31, 1996, when an eminent small-scale eddy passed by Cape Shionomisaki, and when the Kuroshio axis tentatively moved southwards about 50 km apart from the coast, the Kii Bifurcation Current seems to have disappeared.

Visual contrast threshold of striped beak-perch Oplegnathus fasciatus

Underwater visibility is lowered with increasing suspended materials in coastal waters. Low contrast threshold and high visual acuity enable a fish to locate precisely its prey and find the presence of a predator. The striped beak-perch Oplegnathus fasciatus is widely found in the coastal waters of Japan and often used in the laboratory experiments to examine the visual ability of fish. In spite of intensive studies on the visual acuity of this fish, no study on the contrast threshold has been conducted. In the present study, the contrast threshold by behavioral response of the fish to prey was examined. The principle of the experimental equipment followed Nakamura. The equipment was installed in a darkroom. As shown in Figure 1, a rectangular parallelepiped water tank was divided into a waiting tank of the length 60 cm, and a response tank of the length 140 cm. A gate to restrict the movement of fish was installed at the border between the waiting tank and the response tank. Opening and closing of the gate was carried out by remote control. The water tank was filled with filtered sea water (beam attenuation coefficient: ca. 0.5 m-1 at 486 nm) to a depth of 30 cm. Filtered sea water was supplied to the end of the waiting tank at a rate of 6–8 L/min. The sea water was discharged from the back of the response tank so that a weak flow (less than 1 cm/ s) from the waiting tank to the response tank could be maintained. By this method, the behavior of fish is not influenced by the smell of a target. The temperature of filtered sea water was 21–24°C during the experiment. The irradiance of the water surface of the tank was kept uniform at ca. 60 lx. A white piece of mantle tissue of frozen Japanese common squid was used as the target and its shape was a circle (diameter 5 mm; thickness 4–5 mm). The size of the target did not influence the visual recognition. In order to generate various contrasts for the target, black to white background plates with six grades of lightness were placed behind the target. The apparent contrast C(r), for the target against the background, at the distance r can be expressed by the following equation.