49 



Abstract. -Metabolic activity 

 (=oxygen consumption), chemical com- 

 position (water, ash, carbon [C], hydro- 

 gen [H], nitrogen [N]), and caloric con- 

 tent were determined for various devel- 

 opmental stages of the mesopelagic fish 

 Maurolicus muelleri from the Sea of 

 Japan. Oxygen consumption rates were 

 estimated from enzyme activity in- 

 volved in the respiratory electron trans- 

 port system (ETS). Body composition 

 varied with development of the fish, 

 ranging from 68.3 to 78.3% of wet 

 weight (WW) for water, 10.7 to 25.7% 

 of dry weight (DW) for ash, 34.1 to 

 54.4% of DW for C, 5.1 to 8.4% of DW 

 for H, 7.2 to 12.2% of DW for N, and 

 1.02 to 1.98 Kcal/g WW or 3.51 to 6.33 

 Kcal/g DW for caloric content. Condi- 

 tion factor index (CFI) ranged from 5.2 

 to 9.5 (mg WWxl0 3 /(mm TL) 3 ). Caloric 

 content, C, H, and CFI increased with 

 development, but the reverse was the 

 case for water, ash, and N. Age-to-age 

 and lifetime energy budgets were es- 

 tablished, i.e. metabolic data from ETS 

 assay and caloric data from body com- 

 position analyses, combined with natu- 

 ral growth data. The results indicated 

 that higher age-specific net growth ef- 

 ficiency was associated with younger 

 ages, decreasing toward the end of life, 

 with a lifetime average of 17%. Esti- 

 mated age-specific daily ration followed 

 the same pattern, with a lifetime aver- 

 age of 2.9%/day. From the gonad index 

 and caloric contents of gonads, energy 

 invested in reproduction was calculated 

 as 1.3% of the lifetime sum of assimi- 

 lated energy for females and as 0.6% 

 for males. Because the number of itera- 

 tive spawnings during the lifetime of 

 M. muelleri is unknown, this energy 

 partition to reproduction is a conserva- 

 tive estimate. From the comparison of 

 the present results with those for other 

 mesopelagic fishes off southern Califor- 

 nia, it is suggested that M. muelleri is 

 a more efficient mediator of energy flow 

 and matter cycling in the pelagic eco- 

 system because of its smaller size and 

 shorter life span. 



Metabolism, body composition, 

 and energy budget of the 

 mesopelagic fish Maurolicus muelleri 

 in the Sea of Japan 



Tsutomu Ikeda 



Japan Sea National Fisheries Research Institute 

 1 Suido-cho, Niigata 951. Japan 

 Present address: Nansei National Fisheries Research Institute 



Ohno-cho, Saeki-gun, Hiroshima 739-04, Japan 



Manuscript accepted 8 September 1995. 

 Fishery Bulletin 94:49-58 ( 1996). 



Maurolicus muelleri (Gmelin) is a 

 small, short-lived sternoptychid fish 

 distributed in the mesopelagic zone 

 of tropical, subtropical, subarctic, 

 and subantarctic waters of the Pa- 

 cific and Atlantic Oceans (Kaw- 

 aguchi and Mauchline, 1987). Gj0- 

 saeter (1981) and Kawaguchi and 

 Mauchline ( 1987) have reviewed the 

 biology and ecology of M. muelleri and 

 have pointed out the geographical 

 variability in the life mode of this fish. 



Maurolicus muelleri is the only 

 micronektonic fish in the Sea of Ja- 

 pan, yet its stock size has been esti- 

 mated as 3.3 x 10 6 metric tons 

 (Okiyama, 1981). It is reported to 

 mature at age 1 yr in the Sea of Ja- 

 pan, and to grow to 60 mm SL 

 (Yuuki, 1982, 1984; Ikeda, 1994). 

 The estimated life span of the fish 

 is 20-22 months. Spawning is 

 aseasonal with two peaks in spring 

 and autumn or only one peak in 

 autumn (Yuuki, 1982; Ikeda, 1994). 

 Possible iterative spawnings have 

 been suggested (Okiyama, 1971; 

 Yuuki, 1982), and migrations up to 

 a 50-m depth at night and down to 

 below a 150-m depth during the day 

 have been reported (Hamano et al., 

 1992). Copepods are numerically 

 the most important dietary compo- 

 nent of this species, with euphausi- 

 ids second (Ikeda et al., 1994). 



In order to evaluate the quanti- 

 tative roles of M. muelleri in the 

 pelagic ecosystem of the Sea of Ja- 



pan, information about partitioning 

 of materials or energy for various 

 processes is required. However, 

 little information about material 

 and energy budget is available for 

 this fish because of the lack of ap- 

 propriate physiological data. A ma- 

 jor problem inherent in an experi- 

 mental study of M. muelleri is the 

 difficulty in maintaining them un- 

 der laboratory conditions. This 

 problem is not limited only to M. 

 muelleri but is also seen in other 

 mesopelagic fishes (cf. Robinson, 

 1973). Along with this difficulty, di- 

 rect measurement of the respira- 

 tory oxygen consumption rate of me- 

 sopelagic fishes is currently limited 

 to those living for a short period of 

 time in the laboratory after capture 

 (Torres et al., 1979; Donnelly and 

 Torres, 1988). Since "metabolism," 

 estimated from respiratory oxygen 

 consumption, is of central impor- 

 tance in estimating the energy bud- 

 get of fishes (Winberg, 1956), the 

 lack of adequate techniques for 

 maintaining mesopelagic fishes 

 makes it difficult to calculate energy 

 used for metabolism. 



Because of this difficulty in mea- 

 suring respiratory oxygen consump- 

 tion directly, Ikeda (1989) used the 

 enzyme activity of the respiratory 

 electron transport system (ETS) as 

 an indirect measure of routine me- 

 tabolism of myctophid fishes in the 

 field. The energy-producing process 



