LIFE HISTORY AND POPULATION FLUCTUATION IN THE LABRID FISH, LABROIDES DIMIDIATUS, NEAR THE NORTHERN LIMIT OF ITS RANGE

Abstract

The ecology of the Indo-Pacific tropicallabrid fish, Labroides dimidiatus, was studied at the shallow rocky reefs in Shirahama, near the northern limit of its distribution. The population increases in late summer and autumn when juveniles are recruited, but considerably decreases in winter, when water temperatures decline as low as 10°C. The fish are socially and sexually active only in the warmer period, from April to November or December, and spawning occurs restrictedly within 4-5 months from late May to early September. .Juveniles settle out of the plankton, probably within one or two months after spawing. The rapid growth of juveniles in summer and autumn makes up for the low growth in the next winter, and subsequently the one-year old fish are matured, usually as females, in the next breeding season. Though protogynous sex-change usually occurs shortly before 3-years old, even the one-year old fish can become a male when there are no older or larger individuals in its vicinity. The adult population declines also in late summer and early autumn, coincidentally with the beginning of the juvenile recruitment. The life history pattern of L. dimidiatus in Shirahama is compared with those in southern localities, for which information is available, and the modifications in the life history pattern are discussed in relation to the fluctuation of population.