Cascading effects of the flathead grey mullet Mugil cephalus in freshwatereutrophic microcosmos

Flathead grey mullet (Mugil cephalus L.) stocked in fish ponds were long co nsidered to feed primarily on detritus. However, recent research has found that they obtain much of their food from plankton and that they have a detr imental effect on pond zooplankton and large phytoplankton, whilst enhancin g small phytoplankton. It has been suggested that flathead grey mullet may also increase the internal phosphorus loading of the ecosystem, which would also increase phytoplankton density. To test whether zooplankton removal o r nutrient release from the sediment is the better explanation for phytopla nkton enhancement in the presence of flathead grey mullet, the ecosystems o f fish-less tanks, tanks with a 60 mu m mesh filter and tanks stocked at a fish density of 243 g m(-3) were compared. In the presence of flathead grey mullets, cladocerans, ostracods and chironomid larvae became scarcer than in the control tanks, while there were more small phytoplankton and mud sna ils. The green algae Cladophora sp. did not occur at all. The presence of a mechanical filter also reduced cladoceran, ostracod and chironomid densiti es and increased phtyoplankton and mud snail density. However, the situatio n observed in filter tanks was intermediate between that observed in the fi sh tanks and the control tanks, due to the lower filtering efficiency of th e mechanical filter. The organic matter content of the sediment decreased t hroughout the experiment in the control and filter tanks, but remained stab le in fish tanks. Phosphorus and nitrogen concentrations were not affected by any treatment. These results showed that flathead grey mullet enhanced p hytoplankton development due to the removal of large cladocerans and not as a consequence of nutrient release. Furthermore, the flathead grey mullet s trongly modified the benthic community, probably due to direct predation.