Brought to the surface from obscurity: The distribution and biology of Coelorhinchus gilberti (Macrouridae, Gadiformes, Teleostei) off the Emperor Seamounts (Northwestern Pacific)

Coelorinchus gilberti is a member of the grenadier family (Macrouridae) found off the Pacific coast of Japan (from Shikoku Island to Hokkaido Island), the Kyushu-Palau submarine ridge, and the Emperor Seamounts. This species is a common bycatch in the longline fishery for the skilfish Erilepis zonifer (Anoplopomatidae) and some rockfishes (Sebastidae). However, there is no published information on its distribution and biology. Based on observations on commercial longlines between 2014 and 2018, the catch distribution, size and sex, size of otoliths, age and growth, sex ratio and sexual maturity of C. gilberti in the waters of the Emperor Seamounts (NW Pacific) are presented. This species has been caught from Nintoku Seamount in the north to Kammu Seamount in the south and is most commonly found off the Jingu, Ojin, and Kammu seamounts. The species has been recorded from 215 to 1840 m, which significantly extends its known bathymetric range. The catches included individuals from 34 to 93 cm (mean 53.2 cm), weight 140–5240 g (mean 688.2 g) and age from 24 to 48 years (mean 37.9 years). The females were significantly longer, heavier and older than the males and generally dominated the catches (the proportion of males <40%). The growth of C. gilberti is best described by a von Bertalanffy growth function (VBGF), and the parameters of the von Bertalanffy growth equation were L∞ = 61.6, k = 0.09, t₀ = −1.4. Spawning individuals were recorded in catches only in April, which is the end of spawning. Further research with year-round trawl samples is needed to obtain more detailed information on the life cycle of C. gilberti.