Reef riffs: biological soundscapes & fish communities of reefs in the Wadden Sea
This thesis investigates how underwater soundscapes can support the understanding and monitoring of marine ecosystems, with a focus on reef habitats and restoration in the Wadden Sea. Soundscapes integrate biological activity, habitat characteristics, and human pressures, offering a powerful lens for studying ecosystem health. Advances in passive acoustic monitoring now allow these soundscapes to be recorded at large scales, providing a non-invasive and cost-effective tool to study fish populations, reef-associated communities, and habitat use. By linking ecological knowledge of reef systems with acoustic approaches, this work explores how sound can function both as an ecological signal and as a method for assessing reef community activity and restoration outcomes in a heavily modified coastal environment.
The research shows that natural shellfish reefs generate richer and more diverse biological sounds than surrounding soft-sediment areas, providing a baseline for the region. Harbor seal vocalizations were also recorded, revealing distinct call types and seasonal patterns consistent with breeding activity, offering the first description of seal acoustic behavior in the Wadden Sea. Experimental reef structures demonstrated that habitat complexity strongly influences community composition: both small reef cages and larger wood-based reefs rapidly supported higher abundance, diversity, and biomass of fish and prawns compared to controls, with large, complex, and biodegradable substrates attracting the greatest responses. Across all studies, passive acoustic monitoring consistently detected elevated biological activity, particularly from fish and seals, that traditional nets and traps only partially revealed. Together, these findings highlight the ecological value of reef structures and demonstrate the potential of soundscapes as a sensitive, scalable tool for monitoring ecosystem restoration.