Trade-offs between shellfish aquaculture and benthic biodiversity: A modelling approach for sustainable management

Abstract

This paper presents an ecosystem modelling approach aimed at improving shellfish aquaculture management by explicitly considering natural benthic biodiversity. This methodology uses a combination of benthic field survey data, sediment and bathymetric mapping, physiological models and dynamic ecosystem modelling. The Wild species Integration for Shellfish Ecoaquaculture (WISE) approach helps to understand the baseline food requirements for maintaining natural benthic biodiversity of suspension-feeding organisms, thus informing managers on potential upper thresholds for shellfish aquaculture. WISE was tested in four coastal systems in Europe and China, including bays, estuaries and sea loughs with widely differing aquaculture activities. In the European systems, where the aquaculture industry is developing, species diversity and abundance are much higher and suspension-feeding wild species play an important role in the consumption of food resources. Densities of wild individuals were estimated to be 13 ind m−2 in Sanggou Bay (total: 2×109), 33 ind m−2 in Xiangshan Gang (total: 122×109), 95 ind m−2 in Carlingford Lough (total: 4.62×109) and 175 ind m−2 in Loch Creran (total: 2.62×109). Total clearance rates by wild populations were calculated as 5% of the total volume d−1 in Sanggou Bay (75×106 m3 d−1), 11% d−1 in Xiangshan Gang (434×106 m3 d−1), 40% d−1 in Loch Creran (93 – 99×106 m3 d−1) and 45% d−1 in Carlingford Lough (170–250×106 m3 d−1). In relative terms, wild populations play a more important role than cultivated shellfish in clearing suspended particles from the European systems due to the much lower aquaculture activity. 56% and 76% of total primary production in Loch Creran and Carlingford Lough, respectively, are consumed annually by wild organisms, while less than 50% is consumed in Chinese systems (45% in Sanggou Bay and 2.9% in Xiangshan Gang). Integration of the WISE approach within broader ecological modelling illustrates some of the trade-offs between commercial aquaculture and the conservation of biodiversity, showing that rates of and capacities for shellfish culture are reduced when both wild and cultured suspension-feeding species are considered in relation to the available seston. When food resources are partitioned between wild and cultivated species, there is a decrease in individual length and weight (9 to 22% reduction in shell length and 24 to 52% reduction in total fresh weight for the Pacific oyster; reductions of 6% in length and 20% in weight for blue mussel; reductions of 4% in length and 13% in individual weight for bivalves in Xiangshan Gang), resulting in a lower aquaculture production (e.g. for Pacific oyster, a reduction of 12.5% in Carlingford Lough, 34% in Loch Creran and of 9% for bivalves in Xiangshan Gang).