The carrying capacity for Mediterranean bivalve suspension feeders: evidence from analysis of food availability and hydrodynamics and their integration into a local model

Abstract

In order to assess the carrying capacity of two Mediterranean areas, the Incze et al. model and its modification were applied. Our measures were carried out in the Gulf of Gaeta (Central MED), where mussels (Mytilus galloprovincialis) are intensively cultivated (production of approximately 200 t per year) and the Gulf of Castellammare (Southern MED), where bivalve culture is not widely practised. Velocities of water current and in field filtration rates were measured in each area. Total suspended matter (TSM), suspended chlorophyll-a (CHLa), lipid, protein and carbohydrate concentrations in the particulate were measured seasonally and used as tools to evaluate the trophic status of the two areas. The two areas differed strongly in current velocities (in the Gulf of Castellammare were higher than in the Gulf of Gaeta); vice versa, cholorophyll-a concentration was higher in Gaeta and calculated average clearance rates were lower in the Gulf of Gaeta (2.2 ± 1.2 l h−1; and in Castellammare, 3.2 ± 1.2 l h−1). With these values measured in field, in the Gulf of Gaeta, the average potential mollusc biomass that can be cultivated using the original Incze et al. model was 403 t, while the same value using the modified model was 160 t in Castellammare average potential mollusc biomass; in the original model, it was 2034 t and about 200 t calculated with the modified model. Our results lead us to hypothesise that in the Gulf of Castellammare both the hydrodynamics and the total gross available food may not represent limiting factors for the expansion of bivalve culture. Although the quantity of available food is lower than in the Gulf of Gaeta, the higher current velocities compensate for the lower quantity of TSM and the overall outcome is that in Castellammare it would potentially be possible to obtain greater biomass than in Gaeta. In the Gulf of Gaeta, if TSM does not represent a limiting factor, the low current velocities measured seasonally in the area may represent the real limiting factor for bivalve culture. In the Gulf of Gaeta, we believe that the existing trophic and hydrodynamic conditions do not permit further development of bivalve cultivation and that the maximum carrying capacity of this area may already have been achieved.