Feeding habits of juvenile surface-migratory myctophid fishes (family Myctophidae) in the Kuroshio region of the western North Pacific

We examined the diets and nighttime feeding chronologies of the most abundant juvenile surface-migratory myctophid fishes in the Kuroshio waters of the western North Pacific: Myctophum asperum, M. nitidulum, Symbolophorus evermanni and Centrobranchus brevirostris. Samples were collected from water 0 to 1 m deep at night from January to March during 10 yr between 1961 and 1992. The 4 species exhibited resource partitioning by having different main prey items at night in the top 1 m layer. M. asperum fed mainly on appendicularians, M nitidulum on copepods and amphipods, S. evermanni on euphausiids and C. brevirostris on pteropods. They also had species-specific peak feeding hours, demonstrated by the different hours during which each species had the greatest wet weight of stomach contents (percentage of body weight): 24:00 to 01:00 h (5.1 %) and 02:00 to 03:00 h (4.0 %) for M asperum; 03:00 to 04:00 h (7.3 %) for M nitidulum; 04:00 to 05:00 h (3.9 %) for S. evermanni; and 20:00 to 21:00 h, 22:00 to 23:00 h and 04:00 to 05:00 h (4.6, 5.3 and 3.8 %, respectively) for C. brevirostris. These results suggest that myctophids that migrate to the surface reduce trophic competition by specializing in different food organisms. We observed 2 and 3 feeding peaks for species that feed on gelatinous plankton (M. asperum and C. brevirostris, respectively), compared with 1 peak for crustacean feeders (S. evermanni and M nitidulum). We also estimated the impact on zooplankton of nocturnal feeding by these 4 species.