THE EFFECT OF STRIPED GREY MULLET (MUGIL CEPHALUS L) ON FRESH-WATER ECOSYSTEMS

Striped grey mullet, stocked in fish ponds, were long considered to fe ed primarily on detritus. However, recent research has indicated that they obtain much of their food from plankton. If this were indeed the case, this species would have a detrimental effect on pond zooplankton and large phytoplankton, whilst enhancing small phytoplankton. To tes t this hypothesis, the ecosystems of fishless tanks and tanks stocked at a fish density of 0.188 kg/m(3) were compared. In the presence of s triped grey mullets, the rotifer density was low, cladocerans were com pletely absent and small phytoplankton were very dense. Water quality was also affected; very little organic matter accumulated in the sedim ent while chironomid midges failed to colonize it. These results show that striped grey mullet obtain food both from the benthos and plankto n and that they dramatically affect plankton and benthos composition. This species efficiently exploits certain other resources that are min or components in the diet of other pond cultured fish species, i.e., f ine detritus and small zooplankton. This might explain the enhancement of total fish yields in ponds stocked with striped grey mullet. Howev er, the experiment also shows that striped grey mullet have a detrimen tal effect on large zooplankton and chironomid midges, which constitut e a significant proportion of the diet of common carp and tilapia, two species usually raised together with striped grey mullet. The reducti on in these food sources might explain why the performance of common c arp and tilapia is affected by the presence of striped grey mullet, de spite the overall increase in fish yield.