Autonomous visual monitoring of pantropical fish larvae recruitment using acoustic enrichment and mesoscale artificial reef structures

Acoustic enrichment (AE), the playback of ambient sound from healthy reefs, shows promise in attracting both fish and coral larvae to degraded or artificial reefs. We explored the effect of a fully autonomous AE system (3.8 kW-hr capacity) in attracting fish at various life stages in Kāne'ohe Bay (O‘ahu, Hawai‘i) over three consecutive spawning events (summer 2023, 2024, and 2025). Control and treatment sites at 5-m depth were separated by 42-m built using artificial structures on a sandy seabed. Each site was equipped with low-power cameras, underwater speakers, and monitoring hydrophones. The speaker at the treatment site broadcasted local healthy reef sounds every night for 3 weeks. The autonomous cameras operated from dawn to dusk and captured five sequential burst images every 5 min. Manual and semi-automated image analysis enabled fish larvae counts, which peaked around the new moon. The treatment and control site were switched for each deployment. The treatment site attracted up to 13-times more larvae than the silent control site, but both sites had similar numbers of mature fish. These results demonstrate autonomous camera systems can be used to study fish larval presence, and they provide further evidence that AE can enhance larval fish recruitment.