Deep-sea fishes in a sauna: Viperfishes dominate a submarine caldera

Barbeled dragonfishes (family Stomiidae) are considered rare, solitary deep-sea predators. We document the fish fauna of the Kurose Hole, a submarine caldera within Japanese waters. The area has been surveyed on three occasions: twice in 2000 during the RV 'Natsushima' cruise and once in 2020 during the RV 'Kaimei' cruise. Within the span of twenty years, the base of the caldera warmed from 11.1 degrees C to 17.8 degrees C at 790 m. No drag-onfishes were observed during the 2000 'Natsushima' expedition while over 1500 viperfishes (genus Chauliodus) were observed during a five and a half hour period in the 2020 'Kaimei' expedition. Viperfishes have never been seen at such high densities, accounting for 61.4% of fish observations with 6.7 viperfish per 100 m3 within the benthic boundary layer. All captured dragonfishes were juveniles, suggesting that this population arose from larvae becoming trapped within the caldera as they developed.