Mesopelagic fishes (0-1,000 m depth) are major components of the oceans usually presenting global distribution, vertical migratory behavior, and a diverse array of morphological adaptations (Priede, 2017). These species are crucial for several ecosystem processes, such as carbon sequestration, nutrient regeneration, fisheries production, and waste absorption (Mengerink et al., 2014; Cavan et al., 2019; Eduardo et al., 2020). However, despite their importance, mesopelagic species remain mostly understudied and increasingly threatened by anthropogenic impacts (e.g. seabed mining, plastic pollution, ocean warming, and deoxygenation) (Steinberg et al., 2012; Mengerink et al., 2014; Levin & Bris, 2015). Given the difficulty of sampling in the deep-sea, even basic biological knowledge is lacking for many species, hampering the sustainable management of mesopelagic species as well as the comprehension of their numerous functions in the ecosystem (e.g. transport of carbon between oceanic layers). One of the key basic knowledge concerns the length-weight relationships (LWR), which is widely applied in the management of fish populations (Froese & Binohlan, 2000; Froese, 2006; Froese et al., 2011). For example, LWR is an effective approach for assessing fish biomass based on the conveniently obtained length data (Froese, 2006). Additionally, this parameter may be used for
