Recent distribution and population structure of the ocean quahog, Arctica islandica (Linnaeus, 1767), in the German waters of the Baltic Sea – Ecological insights and relevance for conservation

Arctica islandica, a long-lived bivalve species, reaches its southern distribution limit in the Baltic Sea, where it plays a key role in benthic ecosystem functioning. In this region, the species is listed as vulnerable, and its main associated biotope holds critically endangered status. Despite its ecological importance, current knowledge of its distribution and population structure in this marginal environment has remained limited.

This study presents the most comprehensive assessment to date of A. islandica in the German waters of the Baltic Sea, integrating species distribution modelling (SDM) with empirical data on population structure. Using Random Forest algorithms and benthic survey data collected between 2015 and 2024, we mapped spatial patterns of abundance and biomass and analysed size-frequency distributions to assess demographic trends.

Arctica islandica was primarily found in fine sediments below the halocline in the central Kiel Bay and the Bay of Mecklenburg, and was absent from shallower coastal zones (<10 m). The recorded maximum shell length of 82.5 mm was smaller than in Atlantic populations, likely due to the brackish conditions. Size-frequency data indicated recent recruitment events alongside high early-life mortality. While the Baltic Sea population appears broadly stable, regional shifts were evident – for example, increased stock biomass in the Arkona Basin but also a population decline in Lübeck Bay.

In the Baltic Sea, A. islandica lives near its physiological tolerance limits, making it particularly susceptible to anthropogenic impacts and climate-driven environmental change. Population hotspots within the Marine Protected Area “Fehmarn Belt” were identified as conservation priorities, supporting both species protection and ecosystem functioning. More broadly, this study demonstrates the utility of SDM in supporting evidence-based marine spatial planning and targeted environmental management.