Distribution and feeding of Myctophum orientale juveniles (Teleostei: Myctophidae) on the onshore side of the Kuroshio off Japan

Highlights

• M. orientale occurred in the surface layer on the onshore side of the Kuroshio axis.

• They preyed on appendicularians, copepods, and euphausiid calyptopis and furcilia stages.

• The nighttime ration was estimated to be 10.5–13.8% of body weight.

• Predatory impact on production rate of the mesozooplankton was estimated to be 6.0–15.5%.

• M. orientale is a competitor for prey with the small pelagic fish in the Kuroshio region.

Abstract

We examined the distribution and feeding habits of the diel vertical migratory myctophid Myctophum orientale in the Kuroshio region of the western North Pacific in late winter. All 1985 specimens collected were in the juvenile stage, with a mean standard length (SL) of 29.9 mm. They occurred on the onshore side of the Kuroshio axis along the slope region off Japan, showing a typical pseudoceanic distribution. Juveniles <35 mm SL depended heavily on appendicularians, while juveniles ≥35 mm SL preyed on various species of copepods (predominantly Candacia bipinnata) and euphausiid calyptopis and furcilia stages. Based on estimated biomass (2.85 mg m^-3) and nighttime ration (10.5–13.8% of body weight d^-1), the predatory impacts of M. orientale on the biomass and production rate of the mesozooplankton were calculated to be 0.3–0.8% and 6.0–15.5%, respectively, in the surface layer. In the study area, M. orientale co-occurs with larvae and juveniles of Japanese sardine and mackerels, both of which also prey on appendicularians. Predatory impacts of M. orientale <35 mm SL on the biomass and production rate of appendicularians were estimated to be 2.5–7.4% and 4.7–14.1%, respectively. This suggests that M. orientale is a competitor for prey with the small pelagic fish on the onshore side of the Kuroshio axis during late winter.

Introduction

Myctophid fish are one of the most abundant and widespread mesopelagic fish groups, distributing throughout the oceans from subarctic and Antarctic to tropical waters (Gjøsæter and Kawaguchi, 1980; Brodeur and Yamamura, 2005; Olivar et al., 2016). Worldwide, 249 species in 32 genera are reported in this family (Priede, 2017). Of these, approximately 20 species occur associated with submerged bottom features such as islands, seamounts, or continental slope regions, which have been called ‘pseudoceanic’, ‘mesopelagic-boundary’, or ‘slope water’ species in the literature (Hulley and Lutjeharms, 1989; Reid et al., 1991; Brodeur and Yamamura, 2005). High biomasses of pseudoceanic myctophids have been reported over the continental shelf and slope in various parts of the world (Gjøsæter and Kawaguchi, 1980; Hulley and Prosch, 1987; Pakhomov and Yamamura, 2010; Ohshimo et al., 2012), which correspond to areas of enhanced plankton productivity (Longhurst, 2006). The pseudoceanic myctophids are at an important mid-trophic level that transfers energy from the mesozooplankton to upper trophic levels, linking the epipelagic production and mesopelagic and benthopelagic environments through diel vertical migration (DVM) (Young and Blaber, 1986; Williams et al., 2001; Yamamura and Inada, 2001; Bulman et al., 2002).

Myctophum orientale (Gilbert, 1913) is an endemic pseudoceanic myctophid occurring on the slope region of the western North Pacific (Kawaguchi and Aioi, 1972). They are categorized as an intermediate-sized species, reaching approximately 78 mm standard length (SL). This species is one of the so-called surface migratory myctophids that migrate at night from the mesopelagic layer up to the surface to be commonly collected at the surface by nighttime neuston net sampling or dip-netting from the side of a ship (Kawaguchi et al., 1972; Watanabe et al., 2002; Brodeur and Yamamura, 2005; Olivar et al., 2016). The surface migratory myctophids are generally included in the subfamily Myctophinae with species of the genus Myctophum often being dominant (Kawaguchi and Aioi, 1972; Priede, 2017). The lower limit of the nighttime depth range where Myctophum spp. occur abundantly has been estimated to be approximately <10 m from the surface (Gartner et al., 1987; Clarke, 1973; Watanabe et al., 2002). Myctophum orientale is one of the main components of the neuston fish assemblages at night over the slope region off Japan (Kawaguchi, 1977; Watanabe and Kawaguchi, 2003) where primary spawning grounds for various commercially important small pelagic fish, such as Japanese sardine (Sardinops melanostictus), Japanese anchovy (Engraulis japonicus), chub mackerel (Scomber japonicus), spotted mackerel (S. australasicus), and Pacific saury (Cololabis saira) are formed during winter (Oozeki et al., 2007; Sugisaki et al., 2010; Takasuka et al., 2014).

In the present study, we examined the distribution and feeding of M. orientale in the Kuroshio region off southern and central Japan during winter. Firstly, we described the distribution and biomass in relation to the bottom topography and the position of the Kuroshio axis. Then, we examined the dietary composition in relation to SL, feeding chronology, and nighttime ration. Finally, based on data for biomass over the slope region off Japan and nighttime ration, we estimated the food requirements of M. orientale and their predatory impact on the biomass and production of the mesozooplankton. Potential for competition for prey among M. orientale and small pelagic fish is also discussed.