A unique ecological phenomenon quantified by hydroacoustics: the deep-sea eel spawning aggregation at Patience Seamount

Patience Seamount, within Australia's Huon Marine Park, hosts persistent deep-sea fish aggregations dominated by basketwork eels (Diastobranchus capensis) and orange roughy (Hoplostethus atlanticus). Using integrated hydroacoustic surveys, deep towed camera imagery and physical sampling from 2023 to 2025, we quantified the spatial and temporal dynamics of these assemblages and produced species-specific biomass estimates. Acoustic backscatter attribution followed a two-phase approach: (i) camera-derived species distributions to inform spatial apportioning of backscatter and (ii) conducting biomass-constrained sensitivity analyses applying conservative bounds on orange roughy biomass (200-850 t) informed by historical assessments. We found a large, seasonally variable basketwork eel aggregation that increased markedly from March to May, when biomass reached 3547-3557 t (approx. 2.8 million individuals), likely corresponding to spawning. Distinct habitat preferences-with basketwork eels on deeper, flatter terrain and orange roughy on steep summit slopes-improved spatial separation of backscatter in this mixed-species environment. Without biomass constraints, orange roughy estimates became implausible relative to regional stock assessments, underscoring the need for species discrimination in deep-sea acoustic analyses. Patience Seamount's protection and recovery from historical fishing provide a rare opportunity for long-term study of deep-sea ecological processes. Our methods offer a scalable, non-invasive framework for monitoring deep-sea aggregations in marine protected areas.