A unique historical baseline uncovers harvesting impacts on intertidal molluscs at Inhaca Island, southern Mozambique

The effects of ocean exploitation are especially pronounced in coastal, more easily accessible, regions. However, long-term ecological shifts often go unnoticed due to the absence of historical reference points, a phenomenon known as the shifting baseline syndrome. These two aspects are particularly pronounced in the Global South where coastal communities still heavily rely on marine resources for subsistence, but data is rarely available to quantify change, a major shortcoming in a context of demographic growth that is increasing extraction and worsening impacts. Intertidal mollusc harvesting is a vital source of protein and income for coastal communities in Mozambique, but its impacts have hardly been quantified so far. We here focus on Inhaca Island in southern Mozambique, where demographic growth has intensified since the late 1980s and molluscs are harvested primarily for consumption, while other species may be collected for ornamental purposes. The island hosts a marine biological station almost a century old with a collection of local biodiversity with the oldest samples from the 1940s. Leveraging on this unique baseline, we quantified changes in body sizes of intertidal molluscs over time, as a proxy of over-exploitation. We targeted species used for consumption or potentially for the souvenir trade. Edible species showed a significant decline in median and maximum size. In contrast, species potentially collected as souvenirs showed no consistent trend. These findings suggest that the sustainable exploitation of intertidal marine resources urgently requires management to guarantee their long-term availability and highlight the importance of historical collections in reconstructing baselines. Such collections, despite being built in colonial times, are a precious heritage that contemporary societies should preserve.