Timing of parturition, planktonic duration, and settlement patterns of the black rockfish, Sebastes inermis

The microstructure of otoliths was used to examine the relationship among timing of parturition, duration of planktonic stage, and temporal patterns of settlement in young-of-the-year (YOY) Sebastes inermis collected during the settlement seasons of 1998-2001 in a seagrass bed at the northwestern coast of Japan. Parturition occurred in winter, predominantly around the new and full moons, and parturition dates were distributed mainly from early January to late February. In the 4 years YOY S. inermis settled into distinctive groups at early spring, synchronized mainly to a semi-lunar cycle at new and full moon phases; except for 1998, in which settlement occurred around the first and third quarter moons. In all years settlers extruded earlier in the parturition season encompassed the first settlement groups that appeared in the bed and vice versa. Planktonic period ranged from 53 to 113 days, and its mean values varied significantly among settlement groups and years. Mean size at settlement showed less variability among settled groups and between years (range: 18-26 mm total length), which suggested an ideal size for settling and active migration. On the basis of our findings we discuss the extent to which the 'tidal control', 'settlement linkage', and 'competent size' hypotheses explain the temporal pattern of settlement.