The paper deals with some base trophic factors in relation to two herbivores, namely oysters and mullets, which derive their energy directly from the first trophic level of an estuarine ecosystem. The analysis of the gut contents of both organisms has shown that they feed mostly on micro-organisms, especially planktonic and benthic diatoms, and a significant percentage of detritus materials, including inorganic particles, is always present. Their dietary components are very much related to seasonal abundance of food production of the estuary. Current yields of oysters and mullets are far too low and food source is not a limiting factor for their production on large scale. Tidal flows, particularly in relation to salinity, have profound effect on culture grounds. Mortality is largely due to predation.