Identification of mussel clusters in multibeam echosounder backscatter data

Hydroacoustic remote sensing represents a non-invasive, repeatable approach to monitoring benthic communities, supporting sustainable management, conservation efforts, and the detection of environmental change. Mussel clusters on a sandy seafloor in the Oder Bank area (10 to 15 m water depth, southwestern Baltic Sea) were detected in high-frequency backscatter data recorded with a Norbit STX multibeam echosounder in 2019. The blue mussel ( Mytilus edulis ) complexes appear as narrow bands in backscatter mosaics, showing average backscatter intensity increases between 0.2 and 1.0 dB, with localized peaks of up to 2 dB compared to the surrounding sand. Mussel coverage, verified by underwater video sledge observations, reaches up to 50% in isolated patches but typically remains below 15%. The acoustic response of the mussel clusters shows a weak to moderate yet significant correlation with mussel cover, independent of frequency (tested at 200, 400, and 700 kHz). This response is observed at incidence angles greater than 40°. A persistent shell hash layer found at 8 cm depth in sediment cores was not detected acoustically at any frequency. Comparison with data from 2024 suggests that these mussel clusters are ephemeral. Due to the shallowness of the Oder Bank they are influenced by natural processes such as wind and current-driven circulation. This causes the Mytilus bands to roll back and forth on the sand, but can also lead to the dissolution and dispersal of the clusters. The mussel complexes cannot be reliably captured using point-based sampling and short video transects. • Mytilus edulis clusters on sand are detectable in multibeam backscatter data. • Change in mussel abundance explains changes in backscatter intensity. • The clusters are short-lived features and not detected in repeated surveys.