Spatial variation in cockle growth (Cerastoderma edule) in the Dutch Wadden Sea
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Sea-level rise is expected to affect agricultural land in low-lying coastal areas by salinization of the groundwater as the result of seawater intrusion. One of the potential solutions to the decreasing yield on salt affect agricultural land is the change in land-use from traditional agriculture to inland marine aquaculture, e.g. sustainable cultivation of marine shellfish. Exploration of presence and ambient growth rates of shellfish in surrounding coastal seas might aid in finding suitable locations for sustainable inland shellfish cultivation. At present, cockles (Cerastoderma edule) are a national export product of the Netherlands and are manually harvested in the Dutch Wadden Sea. However, the cultivation of cockles is being considered for low-lying polders in this area. To find suitable locations for these activities, we calculated the growth rates of cockles from annual SIBES (Synoptic Intertidal Benthic Sampling Program) field surveys that started in 2008, covering all tidal flats of the Wadden Sea. Data on shell length and age was used to fit Von Bertalanffy growth functions. To better understand the potential causes of the observed variation in growth, the relationship between cockle growth and environmental conditions was modelled by means of a generalized additive model. Cockle growth was related to sampling year, distance to gullies and median grain size. Model outcomes were used to map the growth conditions for cockles on the tidal flats of the Wadden Sea. Cockle growth is predicted to be high near tidal inlets (in particular those near the islands of Texel, Vlieland, Terschelling and Borkum) and low at the tidal divides. This map and its underlying information on growth conditions provides a baseline to identify and modify suitable inland locations for sustainable cockle cultivations along the coastlines of the Wadden Sea.