Alternating dominance of postlarval sardine and anchovy caught by coastal fishery in relation to the Kuroshio meander in the Enshu-nada Sea

In the mid 1970s, the fishery catch of postlarval Japanese anchovy (Engraul is japonica) in a shelf region of the Enshu-nada Sea, off the central Pacif ic coast of Japan, started to decline corresponding to a rapid increase of postlarval sardine (Sardinops melanostictus). In late 1980s, sardine starte d to decline, and it was replaced by anchovy in the 1990s. This alternating dominance of postlarval sardine and anchovy corresponded to the alternatio n in egg abundance of these two species in the spawning habitat of this sea . It was also noteworthy that during the period of sardine decline, sardine spawning occurred in April-May, a delay of two months compared with spawni ng in the late 1970s. The implication of oceanographic changes in the spawning habitat for the al ternating dominance of sardine and anchovy eggs was explored using time-ser ies data obtained in 1975-1998, focusing on the effect of the Kuroshio mean der. Large meanders of the Kuroshio may have enhanced the onshore intrusion of the warm water into the shelf region and contributed to an increase in temperature in the spawning habitat. This might favour sardine, because its egg abundance in the shelf region was more dependent on the temperature in early spring than was that of anchovy. In addition, enhanced onshore intru sion could contribute to transport of sardine larvae from upstream spawning grounds of the Kuroshio region. On the other hand, anchovy egg abundance w as more closely related to lower transparency at the shelf edge, which may indicate the prevalence and prolonged residence of the coastal water, and t herefore higher food availability, frequently accompanying non-meandering K uroshio. The expansion/shrinkage of the spawning habitat of sardine and anchovy in t he shelf region, apparently responding to the change in the Kuroshio, possi bly makes the alternation in dominance of postlarval sardine and anchovy mo st prominent in the Enshu-nada Sea, in combination with changes in the abun dance of spawning adults, which occurred almost simultaneously in the overa ll Kuroshio region. The implication of this rather regional feature for the alternating dominance of sardine and anchovy populations on a larger spati al scale is also discussed.