Evidence of niche segregation between mesopelagic fishes (Myctophidae and Gonostomatidae) and larval and juvenile Japanese anchovy from their gut contents and the diurnal vertical migration of prey zooplankton

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Food competition with mesopelagic fishes has been assumed to affect survival during early life stages of pelagic fish in Kuroshio-Oyashio Transition Zone (KOTZ) of the western North Pacific. The gut content of dominant larval and juvenile fishes (Japanese anchovy Engraulis japonicus and 8 mesopelagic species) was analyzed from samples caught by a frame trawl (MOHT, 5 m2 opening, 1.59 mm square mesh size) in KOTZ during 2001 and 2002. Feeding habits were compared by the volumetric ratio and body width frequency of items found in the gut, and both of them were considerably different between anchovy and mesopelagic fishes. Larval and juvenile anchovy fed mainly on copepods smaller than 0.4 mm in body width, while prey found in the gut content of mesopelagic fishes showed greater size varablility. Day/night differences in vertical distribution of prey copepods were also analyzed. Small copepods (< 0.4 mm in body width) had similar vertical distribution patterns throughout the day, while large copepods (> 0.4 mm) had considerably different patterns. The density of large copepods was 20-fold higher during nighttime than during daytime in the surface 30 m. It was suggested that mesopelagic fishes migrate and forage in the surface layer during nighttime, and possibly feed selectively on large mesopelagic zooplankton concurrently migrating into the surface layer. On the other hand, anchovy feed during daytime, but the low daytime density of large zooplankton in the surface layer does not allow selective feeding.