Assemblages of vertical migratory mesopelagic fish in the transitional region of the western North Pacific

Abstract

Assemblages of vertical migratory mesopelagic fish are described for the nighttime upper 20 m layer of the transitional and adjacent waters in the western North Pacific. Twenty‐three mesopelagic fish species belonging to six families and 16 genera were collected during June 1996. Family Myctophidae was most speciose, represented by 17 species, followed by Sternoptychidae (two spp.) and Bathylagidae (one spp.). Six assemblages are recognized based on their species composition: Subarctic, Northern Transition, Southern Transition, Subtropical, Slope Water and Seamount Assemblages. These assemblages are also characterized by size composition and biomass of the catches. The distributions of the Subarctic, Northern Transition, Southern Transition and Subtropical Assemblages are approximately defined by the Kuroshio and Oyashio fronts and the Subarctic Boundary. Slope Water and Seamount Assemblages are closely related to the slope water around the shelf edge off Japan and waters around the Shatsky Rise and the Emperor Seamount chain, respectively. The importance of the Southern Transition Assemblage area between the Kuroshio front and Subarctic Boundary as spawning and nursery grounds of many subarctic, transitional and subtropical fishes is elucidated, based on the geographical distribution patterns of their larvae and juveniles.

Feeding habits of neon flying squid Ommastrephes bartramii in the transitional region of the central North Pacific

“…) at night in the 0 to 20 m layer near the Shatsky Rise area (Sassa et al 2002), which would support the flying squid population in the TZ in summer.…”

“…Its biomass is very large in summer, and it occurs exclusively in the areas between 30 and 40°N and 160 and 180°E, which include the TZ of our study area (Boehlert et al 1994, Sassa et al 2002. Its abundance probably explains why adult neon flying squid mainly depended on this sternoptychid species in the TZ in July.…”

“…Maurolicus imperatorius, which is endemic to the insular slope and seamount waters, is one of the dominant micronektonic fish species in the TZ around the Emperor seamounts and the Shatsky Rise area (Parin & Kobyliansky 1996, Sassa et al 2002. Its biomass is very large in summer, and it occurs exclusively in the areas between 30 and 40°N and 160 and 180°E, which include the TZ of our study area (Boehlert et al 1994, Sassa et al 2002.…”

Larval feeding habits of Diaphus theta, Protomyctophum thompsoni, and Tarletonbeania taylori (Pisces: Myctophidae) in the transition region of the western North Pacific

“…Given the maximum body depth of larvae (Moser & Ahlstrom 1996), net extrusion would occur for larvae < 4.3 mm in notochord length, even when using a 0.33 mm mesh net. There is no significant difference between day and night catchability of small myctophid larvae < 5 mm in body length (Sassa et al 2002a(Sassa et al , 2004a. On the other hand, larger larvae, especially those >10 mm in standard length, may be able to avoid nets visually during daytime sampling, although the details of how they do this are unclear.…”

“…Several subtropical species, such as Diaphus garmani and Myctophum asperum, and 1 transitional water species, Notoscopelus japonicus, spawn in the Kuroshio region, and their larvae are transported into the transition region by the Kuroshio Current (Sassa et al 2002b(Sassa et al , 2004a, which usually occurs in the southern part of this region, south of the Oyashio Front (Sassa et al 2004b). On the other hand, several subarctic species, including D. theta, Protomyctophum thompsoni, and Tarletonbeania taylori, which were investigated in this study, undergo a southward spawning migration from their feeding grounds in the subarctic waters to the transition region, and their larvae are found in the warm epipelagic layer of the transition region (Sassa 2001, Sassa et al 2002a, 2004b, Moku et al 2003. Larvae of subtropical and subarctic species sometimes mix in the southern part of the transition region owing to the complicated physical oceanographic structures there, such as warm water tongues or streamers, although the spawning grounds of subtropical and subarctic species are quite different.…”