Metabolism, body composition, and energy budget of the mesopelagic fish Maurolicus muelleri in the Sea of Japan

Metabolic activity (= oxygen consumption), chemical composition (water, ash, carbon [C], hydrogen [H], nitrogen [M]), and caloric content were determined for various developmental stages of the mesopelagic fish Maurolicus muelleri from the Sea of Japan. Oxygen consumption rates were estimated from enzyme activity involved in the respiratory electron transport 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. Condition factor index (CFI) ranged from 5.2 to 9.5 (mg WW x 10(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 established, i.e. metabolic data from ETS assay and caloric data from body composition analyses, combined with natural growth data. The results indicated that higher age-specific net growth efficiency was associated with younger ages, decreasing toward the end of life, with a lifetime average of 17%. Estimated age-specific daily ration followed the same pattern, with a lifetime average 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 assimilated energy for females and as 0.6% for males. Because the number of iterative spawnings during the lifetime of M. muelleri is unknown, this energy partition to reproduction is a conservative estimate. From the comparison of the present results with those for other mesopelagic fishes off southern California, it is suggested that M. muelleri is a more efficient mediator of energy flow and matter cycling in the pelagic ecosystem because of its smaller size and shorter life span.