Biological Control of Aquatic Weeds by Grass Carp in Kelvin River (Scotland)

Biological control of aquatic weeds has raised much interest because of the sudden and often short-lasting effects of mechanical and chemical controls. The use of grass carp (Ctenopharyngodon idella) is mainly for the production of food and control of aquatic weeds. The aims of this study were to assess the degree and duration of weed control achieved at two stocking densities (150 and 250 kg ha^-1 ) and to determine the change of fish size and their survival in Kelvin River. Analysis of variance showed that both stocking densities of grass carp caused a significant (p < 0.001) reduction of aquatic weeds. Results showed that free floating and submerged weeds by their soft tissues are often regarded as amongst the palatable species than emergent and floating-leaved species. Experimental results showed that about 60% fish survival with substatial weed reduction by the end of first year after releasing. Results showed that the average weight of grass carp in both 150 and 250 kg ha^-1 at l, 2, and 3 years after releasing were 930, 1680, 2350, and 840, 1550, and 2200 grams respectively. Experimental results showed that weeds control by grass carp is temperature dependent and, to be effective, requires water temperature in excess of 16ᵒC for at least three months of the year.