Growth of larval and juvenile Diaphus theta (Pisces : Myctophidae) in the transitional waters of the western North Pacific

Diaphus theta is one of the most common myctophid fish species in the subar ctic and transitional waters of the North Pacific. The growth of larval and juvenile D. theta was investigated using sagittal otolith increment analys is of specimens caught in transitional waters of the western North Pacific. Samples taken over a 24-h period demonstrated that otoliths; exhibited dai ly growth cycles, allowing accurate determination of age. Calcification of the incremental zone of otoliths took place only at night, suggesting that the formation cycle of the increment of juvenile D. theta was different fro m that of shallow-water fishes and would be related to their diel vertical migration. The relationships between standard length (SL) and daily growth increment (D) were expressed as linear equations: SL 2.65 + 0.141 D (r(2) = 0.942) for larvae of 5.1-9.6 mm SL and SL = 3.54 + 0.129 D (r(2) 0.933) fo r juveniles of 13.7-27.6 mm SL. The growth rates were 0.14 mm d(-1) in larv ae and 0.13 mm d(-1) in juveniles, this is slow compared with tropical or s ubtropical myctophid species. in which growth occurs at about twice these r ates. The larval period, including the metamorphic stage, was long compared with species at lower latitudes and was estimated to be 71 days. The slow growth rate and long period of larval stage of D. theta would be the life h istory pattern of high-latitudinal species adapted to a low-temperature hab itat.