The diel vertical migration patterns of adult myctophid fishes were determined in the transitional waters of the western North Pacific off Japan, using day–night sampling from 20 to 700 m depths with a commercial otter trawl in the summer of 1995. A total of 12 species belonging to 9 genera were collected. Four patterns were recognized in the diel vertical migration of 11 of the 12 species. (1) Migrants showing clear day–night habitat separation with peak abundance above 200 m at night: Symbolophorus californiensis, Tarletonbeania taylori, Notoscopelus japonicus, Diaphus theta, Ceratoscopelus warmingi, and Diaphus gigas. (2) Semi‐migrants, in which part of the population often remains in the daytime habitat at night. The distribution depths of migratory and nonmigratory individuals do not overlap: Stenobrachius leucopsarus. (3) Passive‐migrants, in which there is no separation of day–night habitats, but the upper limit of daytime distribution depth shifts to a shallower layer at night, probably as the fish follow migratory prey: Lampanyctus jordani. (4) Nonmigrants: Stenobrachius nannochir, Lampanyctus regalis (> 140 mm SL), and Protomyctophum thompsoni. The day–night habitat temperature ranges are also given for the 11 species. No remarkable east–west differences were seen in the vertical migration patterns compared with previous knowledge of eight of these species in the eastern Pacific. The diel migration patterns are newly described for three other species endemic to the western Pacific. The standing stock of myctophids in the study area was conservatively estimated at 18.5 ± 4.7 g m−2 (avg. ± SD).

The feeding habits of 3 dominant myctophid fishes, Diaphus theta (vertical migrant), Stenobrachius leucopsarus (semi-migrant), and S. nannochir (non-migrant), were studied in relation to their diel vertical migration patterns using time-series sampling during the summers of 1994 to 1996 in the subarctic and transitional waters of the western North Pacific. D. theta and S. leucopsarus fed mainly on euphausiids (mainly Euphausia pacifica), copepods (mainly Metridia pacifica and the Neocalanus plumchrus/flemingeri group), and amphipods (mainly the Themisto japonica /pacifica group). The prey of S. nannochir was mainly copepods, but the species were different from those preyed on by D. theta and S. leucopsarus. D. theta fed on euphausiids, with 1 feeding peak during the daytime and another at night, but fed on amphipods only at night and on copepods throughout the entire diel cycle. The vertical migratory component of S. leucopsarus showed a significant diel feeding periodicity, feeding on euphausiids, copepods, and amphipods from dusk to midnight, while the non-migratory component did not feed actively at nighttime, and had a high frequency of empty stomachs. The non-migratory S. nannochir showed no diel feeding periodicity. The average ratio of stomach content weight/body weight was 2.2% during the day and 2.0% at night in D. theta, 0.8% during the daytime and 1.1% at night in the migratory S. leucopsarus, 0.6% at night in the nonmigratory S. leucopsarus, and 0.07 to 0.13% (avg. 0.11%) in S. nannochir. KEY WORDS: Myctophid fish · Diet composition · Diel feeding periodicity · Daily ration · Diel vertical migrationResale or republication not permitted without written consent of the publisher

To estimate the age and growth of dominant lanternfishes in the Kuroshio-Oyashio Transition Zone, we examined the sagittal otolith microstructure of Symbolophorus californiensis (n = 30) and Ceratoscopelus warmingii (n = 93) collected from the western North Pacific during 1997-2003. Age of S. californiensis ranged from 81 to 541 days corresponding to postmetamorphosis stage (juveniles and adults), and the von Bertalanffy model was fitted: L t = 128[1 − exp{−0.003(t − 1.52)}], where L is the standard length (mm) and t is age in days. Age of C. warmingii ranged from 6 to 416 days, and growth before metamorphosis was linear (L t = 0.346t + 1.51), and the von Bertalanffy model was fitted to the postmetamorphosis stage: L t = 80.8 [1 − exp{−0.00769(t − 34.4)}]. Growth of these two lanternfishes was faster than that of other lanternfishes in previous studies but considerably slower than that of Japanese sardine (Sardinops melanostictus) and anchovy (Engraulis japonicus) distributed in the KuroshioOyashio Transition Zone. Temperature and prey availability are discussed in relation to this difference in growth rate.

Diaphus theta is one of the most common myctophid fish species in the subarctic and transitional waters of the North Pacific. The growth of larval and juvenile D. theta was investigated using sagittal otolith increment analysis of specimens caught in transitional waters of the western North Pacific. Samples taken over a 24-h period demonstrated that otoliths exhibited daily 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 from that of shallowwater 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 ϭ 2.65 ϩ 0.141D (r 2 ϭ 0.942) for larvae of 5.1-9.6 mm SL and SL ϭ 3.54 ϩ 0.129D (r 2 ϭ 0.933) for juveniles of 13.7-27.6 mm SL. The growth rates were 0.14 mm d Ϫ1 in larvae and 0.13 mm d Ϫ1 in juveniles; this is slow compared with tropical or subtropical myctophid species, in which growth occurs at about twice these rates. 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 history pattern of high-latitudinal species adapted to a low-temperature habitat.

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