Controlling aquatic weeds in a Saudi drainage canal using grass carp (Ctenophatyngodon idella Val.)

Aquatic weeds have a negative effect on agriculture by blocking water drainage system. This is especially true in hot climate countries, like Saudi Arabia, where aquatic weeds grew quickly. For this reason, a field experiment was conducted as follows: grass carp (Ctenopharyngodon idella Val.) fingerlings (average weight 3.3±0.3 g) were randomly stocked in 12 sections (50 m L x 5 m W x 1.5 m D) of a drainage canal at a rate of 0, 1, 3 and 5 fish m ^-2 in triplicate. Starting from upstream, the drainage canal was divided into 12 sections using 13 metal frames covered from both sides with 0.5-inch plastic mesh screens. The canal was infested with 850 km^-2 of Phragmites australis Cav. (5.1±0.6 kg m^-2), 100 km ^-2 of Ceratophyllum demersum L. (2.39±0.22 kg m^-2) and filamentous algae Cladophora globulina Kützing (0.35 kg section ^-1). Fish grew on aquatic weeds for a period of one year. The results indicated that the grass carp completely eliminated filamentous algae within 5 months. The number of Phragmites australis m^-2 was significantly reduced in all treatment groups as compared to the control group. Sections containing 3 and 5 fish m^-2 were yielding similar results and significantly higher than those containing 1 fish m^-2. Average daily growth rates were 3.47, 5 and 6.25 g day^-1 for treatments with 1,3 and 5 fish m^-2, respectively. Removal of aquatic macrophytes by grass carp affected water quality parameters by increasing levels of dissolved oxygen, carbon dioxide, turbidity, chlorophyll a (phytoplankton), conductivity and total dissolved solids in the down-stream water samples, as compared to that of p-stream.