Thermal, trophic and metabolic life histories of inaccessible fishes revealed from stable‐isotope analyses: a case study using orange roughy Hoplostethus atlanticus

A time‐resolved record of inhabited water depth, metabolic rate and trophic behaviour of the orange roughy Hoplostethus atlanticus was recovered from combined stable‐isotope analyses of otolith and muscle tissue. The results demonstrate that H. atlanticus from the north‐east Atlantic Ocean have a complex life history with three distinct depth‐stratified life stages. Early juvenile H. atlanticus occupy relatively shallow habitats, juvenile H. atlanticus show a deep‐demersal phase, rising at sexual maturity, and adult H. atlanticus exploit increasingly deep habitats with increasing age. At all sampled sizes, H. atlanticus muscle tissues have an isotopic composition suggesting a benthic rather than benthopelagic or pelagic diet. Isotopic measures of relative metabolic rate provide an insight into energy partitioning throughout ontogeny. Hoplostethus atlanticus have relatively low metabolic rates compared to coexisting deep‐water benthic fishes, consistent with their unusually high longevity. Surprisingly, lifetime fastest growth rates are achieved during juvenile stages when otolith isotopes imply deep‐water residency and relatively low metabolic rates. Fast growth may be sustained during a period of high efficiency associated with reduced metabolic costs of prey capture or predator evasion. The stable‐isotope approach can be applied to any teleost and provides a rapid, cost‐effective technique for studying deep‐water fish communities.