Otolith-derived field metabolic rates of myctophids (Family Myctophidae) from the Scotia Sea (Southern Ocean)

Myctophids (Family Myctophidae, commonly known as the lanternfishes) are critical components of open ocean food webs and an important part of the ocean biological carbon pump, as many species actively transport carbon to the deep ocean through their diel vertical migrations. Estimating the magnitude of myctophids' contribution to the biological carbon pump requires knowledge of their metabolic rate. Unfortunately, data on myctophid metabolic rates are sparse, as they rarely survive being captured and placed in a respirometer. Because of this limitation, many studies estimate myctophid metabolic rates indirectly from body mass and temperature scaling relationships, often extrapolating regressions from global data sets to regional scales. To test the validity of these estimates, we employed a newly developed proxy for mass-specific field metabolic rate (C-resp: the proportion of metabolically derived carbon in the otolith) based on the stable carbon isotope composition (delta C-13) of otolith aragonite. We recovered estimates of C-resp for individuals of 6 species of myctophids from the Scotia Sea, giving a range in C-resp values from 0.123-0.248. We found that ecological and physiological differences among species are better predictors of variation in C-resp values than body mass and temperature. We compared our results to estimates of metabolic rates derived from scaling relationships and from measurements of electron transport system activity. When considering myctophids as a whole, we found that estimates of oxygen consumption from different methods are broadly similar; however, there are considerable discrepancies at the species level. Our study highlights the usefulness of metabolic proxies where respirometry is currently unavailable, and provides valuable information on field metabolic rates of myctophids.