Interactions of cage aquaculture in Nile Delta lakes : Insights from field data and models

In this paper, we present a cost-effective approach to the assessment of the impact of fish cage culture in transitional water bodies characterized by limited exchanges with the sea. The approach, previously applied to marine coastal areas, is based on two steps: (1) the application of a Lagrangian model, driven by purposely collected data concerning the local hydrodynamic, for an ‘‘a priori’’ simulation of the dispersion of uneaten feed and fecal material from a cage; (2) the collection of a set of field data concerning both water column and sediment indicators, sensitive to the release of organic matter from a cage, along a transect: the latter is determined on the basis of the results provided by the dispersion model. The methodology was tested at a seabass/seabream farm located in a coastal lake on the Nile Delta, namely Lake Maryut. Model results indicated that the area impacted by the cage is relatively small, about 5 m from the edge of the cage: this finding was later corroborated by field observations. As far as the water column is concerned, Dissolved Inorganic Nitrogen was found to be higher near the farm. The most sensitive sediment indicators were found to be surface sediment TOC, and benthic macrofauna abundances, which were, respectively, higher and lower in the proximity of the farm. These findings suggest that a cost-effective monitoring programme of the environmental impact of intensive aquaculture could be implemented in Egyptian coastal Lakes, thus providing a science-based support to the implementation of the ecosystem approach to aquaculture in these important ecosystems.