Benign, and currently sustainable, forms of shellfish-harvesting in the Bijagós Archipélago, Guinea-Bissau

Submitted by editor on 17 April 2025.

by Theunis Piersma

It has been an enormous privilege to contribute to ecological studies on the intertidal flats in the Bijagós Archipélago; better still, to experience a tropical tidal world where people have made livings for long times whilst leaving the ecosystems that they relied on pretty much intact. In the study now published in Wildlife Biology, we set out to verify our impression that the ecological effects of shellfish harvesting, especially the effects on the co-consuming migratory shorebirds, are small or absent (Coelho et al. 2024). The story revolves around two bivalve species, both with the capacity to grow to lengths of several centimeters. The first is a razor clam called Tagelus adansonii and locally known as ‘lingron’, the second is the West African bloody cockle Senilia senilis, locally known as ‘combé’. On mudflats close to the villages both bivalve species are harvested by hand by women and children. As the thick-shelled cockles live closer to the surface than the thin-shelled razor clams, the harvesting of lingron will be a harder task. Because the two shellfish species play part in ceremonies that maintain societal organization, their role for humans is bigger than ‘just’ being important food items.

Using questionnaires, we systematically interviewed local shellfish harvesters and questioned them on their assessment of stocks, on stock renewal and on harvest expectations. The island-specific differences between local management regimes then enabled us to show that on mudflats with formal protection, the condition of the shellfish stocks were considered better than in areas with traditional management based on cultural beliefs. There is no evidence for direct competition between shorebirds and humans, as the Bijagós harvesters selected the larger individual shellfish, the birds selecting the smaller ones. Indirect impacts caused by sediment disturbance or depletion were either very small or absent. Shellfishing was highest in December, with strong declines towards northern spring. This would have reduced any form of disturbance during the period when shorebirds need to double their intake rates to fuel up before their long northward flights to the (mostly) tundra breeding areas.

The evidence that the manual harvesting of the two shellfish species on intertidal flats in the Bijagós Archipélago is indeed a benign human activity stands in stark contrast to what I have experienced at home, in the Wadden Sea (actually used by exactly the same migratory shorebirds), and in comparable temperate intertidal systems in Asia (Piersma et al. 2001, Kraan et al. 2007, Compton et al. 2016, Peng et al. 2021). Here, invasive forms of fishery, for example the use of mechanical harvesting techniques (or dredging; in the Wadden Sea) in addition to the seeding of mudflats by the commercially most profitable shellfish types (including the use of pesticides to control unwanted invertebrate predators; in coastal China) have affected intertidal food webs to great extents, most often with strongly negative impacts on the migratory shorebirds.

The cultural care for lingron and combé and for the mudflats that the bivalves live in, makes the Bijagós people an inspiration for the rest of the world.

Overlooking the mudflats in the northwest of Bubaque island, Bijagós, with some large baoban trees marking the shoreline.

An early morning scne at the edge of the mudflats in the east of Bubaque Island.

First author Ana Coelho looking into a gully in the mangroves that mark the transition between open mud- and sandflats and the higher land of the Bijagós islands in many places.

References:

Coelho, A.P., Ramos, C., Regala de Barros, A., Piersma, T. & Alves, J.A. (2024) Human–wildlife interactions on the tidal flats of the Bijagós Archipelago: does shellfishing affect migratory shorebirds? Wildlife Biology 2024, e01134. https://doi/10.1002/wlb3.01134

Compton, T.J., Bodnar, W., Koolhaas, A., Dekinga, A., Holthuijsen, S., ten Horn, J., McSweeney, N., van Gils, J.A. & Piersma, T. (2016) Burrowing behavior of a deposit feeding bivalve predicts change in intertidal ecosystem change. Frontiers in Ecology and Evolution 4, 19. doi: 10.3389/fevo.2016.00019.

Kraan, C., Piersma, T., Dekinga, A., Koolhaas, A. & van der Meer, J. (2007) Dredging for edible cockles (Cerastoderma edule) on intertidal flats: short-term consequences of fisher patch-choice decisions for target and non-target benthic fauna. ICES Journal of Marine Science, 64, 1735-1742.

Peng, H.-B., Chan, Y.-C., Compton, T.J., Cheng, X.-F., Melville, D.S, Zhang, S.-D., Zhang, Z., Lei, G., Ma, Z. & Piersma, T. (2021) Mollusc aquaculture homogenizes intertidal soft-sediment communities along the 18,400 km long coastline of China. Diversity and Distributions 27, 1553–1567.

Piersma, T., Koolhaas, A., Dekinga, A., Beukema, J.J., Dekker, R., & Essink, K. (2001). Long-term indirect effects of mechanical cockle-dredging on intertidal bivalve stocks in the Wadden Sea. Journal of Applied Ecology, 38, 976-990.

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