Myctophid fish are a major component of the micronektonic fish community in oceanic ecosystems. Diaphus theta Eigenmann & Eigenmann, Stenobrachius leucopsarus (Eigenmann & Eigenmannn), and S. nannochir (Gilbert) are mainly distributed throughout the subarctic and transition waters of the North Pacific Ocean, the Bering Sea, and the Sea of Okhotsk, and they are the dominant myctophid species in these areas (Pearcy, Reference Pearcy1964; Pearcy et al., Reference Pearcy, Krygier, Mesecar and Ramsey1977; Willis et al., Reference Willis, Pearcy and Parin1988; Balanov & Il'inskii Reference Balanov and Il'inskii1992; Beamish et al., Reference Beamish, Leask, Ivanov, Balanov, Orlov and Sinclair1999; Watanabe et al., Reference Watanabe, Moku, Kawaguchi, Ishimaru and Ohno1999). Diaphus theta is a vertically migrant species with distinct day–night habitat separation; S. leucopsarus is a semi-migrant species in which part of the population remains in the daytime habitat at night; and S. nannochir is a non-migrant species (Pearcy et al., Reference Pearcy, Krygier, Mesecar and Ramsey1977; Frost & McCrone, Reference Frost and McCrone1979; Willis & Pearcy, Reference Willis and Pearcy1982; Watanabe et al., Reference Watanabe, Moku, Kawaguchi, Ishimaru and Ohno1999). These species mainly feed on crustacean zooplankton, i.e. copepods such as Neocalanus spp. and Metridia spp., and euphausiids such as Euphausia pacifica and Thysanoessa spp., which are key species in the subarctic Pacific ecosystem (Tyler & Pearcy, Reference Tyler and Pearcy1975; Pearcy et al., Reference Pearcy, Lorz and Peterson1979; Balanov, Reference Balanov1994; Balanov et al., Reference Balanov, Gorbatenko and Efimkin1995; Moku et al., Reference Moku, Kawaguchi, Watanabe and Ohno2000). Myctophids also play an important role in transporting organic matter from the surface to the deep layer of the ocean. Therefore, ecological studies of myctophids are required to understand energy flow from lower to higher trophic levels, carbon and nitrogen cycles, and the vertical transport of organic materials from the epipelagic to the mesopelagic layers.

Carbon and nitrogen cycling are commonly used as indicators of energy flow. However, there is insufficient information concerning the chemical composition of myctophids in the subarctic and transition waters of the North Pacific (Butler & Pearcy, Reference Butler and Pearcy1972; Childress & Nigaard, Reference Childress and Nygaard1973; Neighbors & Nafpaktitis, Reference Neighbors and Nafpaktitis1982; Bailey & Robison, Reference Bailey and Robison1986). Most previous studies have attempted to quantitatively clarify the role of these fish in the subarctic Pacific and adjacent seas, but these results were based on one or very few individuals of limited body size (Butler & Pearcy, Reference Butler and Pearcy1972; Childress & Nigaard, Reference Childress and Nygaard1973; Bailey & Robison, Reference Bailey and Robison1986; Lindsay, Reference Lindsay2003). We analysed the relationship between standard length (SL) and wet weight, dry weight, water content, carbon content, and nitrogen content in juvenile and adult specimens of D. theta, S. leucopsarus and S. nannochir. We also discuss the results in relation to the life-history traits of these species.

Samples were collected in the subarctic and transition waters of the western North Pacific. Sampling was performed from onboard a number of vessels: the commercial trawler ‘Marusada-Maru’ under charter by the Japan Marine Fishery Resources Research Center; the research vessels ‘Hakuho-Maru’ and ‘Tansei-Maru’ of the Ocean Research Institute, University of Tokyo; and the fisheries research vessel ‘Shunyo-Maru’ of the National Research Institute of Far Seas Fisheries. Samples were collected from July 1995 to April 1998 using an otter trawl and a 3-m Isaacs–Kidd midwater trawl (Table 1). Samples were frozen onboard at −20°C or −80°C. After thawing in the laboratory, the samples were measured to the nearest 0.1 mm in SL and weighed to the nearest 0.01 g. In total, 30 individuals were obtained for both Diaphus theta, ranging from 15.7 to 94.2 mm SL, and Stenobrachius leucopsarus, ranging from 25.1 to 92.5 mm SL. Only 24 individuals were obtained for S. nannochir, ranging from 31.4 to 107.2 mm SL. These specimens were analysed for growth-associated differences in chemical composition. Each fish was placed on a pre-weighed aluminium sample boat and dried in an oven at 60°C for 2–3 days until it reached a constant weight. The dried fish was then ground to a fine powder in a glass tissue grinder. Samples were kept in a desiccator until analysis. Three samples of each homogenized fish were placed into ultra-clean tin capsules. The carbon and nitrogen contents were analysed using a Fisons NA-1500 elemental analyser (Fisons Instruments, Italy). Values from three samples were used to calculate the mean carbon and nitrogen contents of each fish.

Table 1. Sampling data of Diaphus theta (Dt), Stenobrachius leucopsarus (Sl) and S. nannochir (Sn) in the western North Pacific.

For each species, the relationships between SL (mm) and wet weight (WW in g), dry weight (DW in g), carbon content (C in g), and nitrogen content (N in g) were expressed as follows. These equations are written on a log₁₀ basis because the double logarithmic relationship gave the best fit.

Diaphus theta:

Stenobrachius leucopsarus:

Stenobrachius nannochir:

In D. theta, the water and nitrogen contents were lowest for the middle size-class (40–80 mm SL; ANOVA, P < 0.05), and the carbon content was highest for middle size-class (ANOVA, P < 0.05; Table 2). We observed similar trends in S. leucopsarus for water (ANOVA, P < 0.05) and carbon contents, although a significant difference in carbon content was only observed between the smallest (<40 mm SL) and middle (ANOVA, P < 0.05) size-classes (Table 2). The differences in nitrogen content were significant between the smallest size-class and all other classes (ANOVA, P < 0.05). In S. nannochir, the water and nitrogen contents (% dry weight) showed a tendency to decrease with growth; in contrast, carbon content tended to increase with growth (Table 2).

Table 2. Chemical composition (mean±standard deviation) of the three myctophid fish.

SL, standard length.

Our carbon and nitrogen content results by size-class allow us to more accurately estimate the quantity of carbon and nitrogen bases in the fish biomass, the flow of energy from mesozooplankton to animals at higher trophic levels through myctophid fish, and vertical fluxes from epipelagic to deep-sea systems in the subarctic and transition waters of the North Pacific. Of these three myctophid species, S. leucopsarus is a key species in the subarctic Pacific ecosystem (e.g. Beamish et al., Reference Beamish, Leask, Ivanov, Balanov, Orlov and Sinclair1999). The biomass estimates of S. leucopsarus in the western North Pacific, including the western Bering Sea and the Sea of Okhotsk, and in the eastern North Pacific were ~13000 and 300 thousand tons wet weight, respectively (Beamish et al., Reference Beamish, Leask, Ivanov, Balanov, Orlov and Sinclair1999). Using the mean carbon and nitrogen content percentages quantified in the present study, the estimates of carbon and nitrogen bases contained in the biomass of this species are 2930 and 213 thousand tons, respectively. If the size distributions of all populations are determined, more detailed estimates of biomass carbon and nitrogen could be possible because our dataset includes a greater variety of fish sizes, especially smaller classes, compared to previous studies.

The carbon contents (% wet weight) for D. theta, S. leucopsarus, and S. nannochir ranged from 15 to 24%, which is two to five times higher than values reported for tropical and subtropical myctophid species (Childress & Nygaard, Reference Childress and Nygaard1973; Childress et al., Reference Childress, Price, Favuzzi and Cowles1990). This may be attributable to the elevated lipid contents observed in subarctic myctophids compared to tropical and subtropical myctophids (Seo et al., Reference Seo, Endo, Fujimoto, Watanabe and Kawaguchi1996). In addition, our data for the carbon and nitrogen contents in D. theta and S. leucopsarus are consistent with those reported by Childress & Nygaard (Reference Childress and Nygaard1973), although our analysis included a much wider range of body sizes.

In general, mesopelagic fish have the highest water content and the lowest lipid and caloric contents among deeper-living species, suggesting a relationship between chemical composition and food availability (Bailey & Robison, Reference Bailey and Robison1986). While the depth layer used by D. theta during the daytime is the shallowest among the three myctophid species examined in this study, and D. theta undergoes a diel vertical migration, S. nannochir is the deepest-living and non-migratory species (Watanabe et al., Reference Watanabe, Moku, Kawaguchi, Ishimaru and Ohno1999). However, except for nitrogen content, there were no clear trends in the water and carbon contents among the three species. This suggests that the biomass of zooplankton prey for S. nannnochir in the deep layer is relatively high. In the subarctic Pacific, the biomass of dominant copepods, such as three Neocalanus spp., especially N. cristatus, which is the main prey of S. nannochir (Moku et al., Reference Moku, Kawaguchi, Watanabe and Ohno2000), is very high, maintaining the mesopelagic ecosystem through their seasonal ontogenetic vertical migration from the epipelagic to the mesopelagic layer (e.g. Kobari & Ikeda, Reference Kobari and Ikeda1999, Reference Kobari and Ikeda2001; Tsuda et al., Reference Tsuda, Saito and Kasai1999, Reference Tsuda, Saito and Kasai2004).

Because males and females were not analysed separately, our data cannot be used to discuss sex-dependent relationships between reproduction and growth. However, it is expected that egg production in females would be influenced by growth and the subsequent changes in tissue chemical composition. Changes in water, carbon, and nitrogen contents may be related to reproduction in these three species. In both D. theta and S. leucopsarus, females attain maturity at lengths of 55 mm SL and 60–65 mm SL, respectively (Smoker & Pearcy, Reference Smoker and Pearcy1970; Moku, Reference Moku2000). Accordingly, we observed changes in the chemical contents of these two species in the 40–80 mm SL size-class compared to the <40 mm SL size-class. Individuals in the >80 mm SL size-class exhibited higher water contents and lower carbon contents than did individuals in the 40–80 mm SL size- class, suggesting that female reproduction may have decreased or ceased altogether. In S. nannochir, the carbon content and the water and nitrogen contents continued to increase and decrease, respectively, as body size increased. However, S. nannochir attains maturity at a larger body size (90 mm SL; Moku, Reference Moku2000) than do the other two species; thus, these individuals were likely not yet mature. In some myctophid species, including S. leucopsarus and D. theta, lipid contents increased with increasing fish size (Butler & Pearcy, Reference Butler and Pearcy1972; Neighbors & Nafpaktitis, Reference Neighbors and Nafpaktitis1982). Although these studies did not examine changes in water content with size, lipid content would be related to the water, carbon and nitrogen contents of the three myctophids in this study. In the present study, the samples were collected from spring to autumn. If there were strong relationships between chemical composition and reproduction in these three species, the chemical compositions and lipid accumulation would be strongly related to spawning seasons and it would be necessary to analyse the relationships in detail based on seasonal sampling of the myctophids.