Abstract:

The water quality and aquatic vegetation were investigated in Lake Xiaosihai from 2006 to 2008, and water environmental characteristics at different fisheries developmental stages from 1987 to 2008 were also compared based upon previous literatures. The results showed that: (1) The SD ranged from 44 cm to 120 cm from 2006 to 2008, and was highest in 2008 and lowest in 2007 in the same season. (2) The concentrations of TP and Chl.a were higher in 2007 than those in 2006 and 2008 in the same season, and the concentration of TN was lower in 2008 than those in 2007 and 2006. (3) Apparent seasonal variations of SD, TP, TN and Chl.a were found in 2007. SD was highest in winter and lowest in summer. The concentrations of TP and TN were highest in spring, but lowest in winter and autumn, respectively. The concentration of Chl.a was highest in summer and lowest in winter. The lake was at eutrophic level in spring, summer and autumn and was at mesotrophic level in winter. (4) The TN and TP of 1992 showed an obvious increase compared with 1987, while those of 2002 clearly decreased compared with 1992 and reached the historically low levels. From 2002 to 2008, the TN rose up and then fell down gradually, and the TP rose up slowly first, then increased sharply, and fell sharply in the end. (5) The trophic states from 1987 to 2008 underwent five stages: mesotrophiceutrophicmesotrophiceutrophicmesotrophic, and the submersed vegetation underwent four stages: disappearancerecoveryluxuriancedegeneration. Such changes were mainly influenced by fisheries forms in different stages. Irrational fisheries forms such as overstocking of herbivorous fish and Chinese mitten crab (Eriocheir sinensis), extensive use of chemical fertilizers and overfeeding of exogenous baits resulted in rapid disappearance of submersed vegetation and water eutrophication. It was suggested that reasonable fisheries forms would not have negative effects on water environment. Aiming at resolving the ubiquitous problem that fishery development was in contradiction with environmental protection, some suggestions of sustainable development of fishery were put forward. First of all, putting the fertilizers and exogenous baits into lakes should be strictly prohibited. Secondly, strictly control the stocking of herbivorous fishes in order to reconstruct and restore the aquatic vegetation. The nutrients in water and sediment were assimilated through aquatic vegetation for suppressing the reproduction of phytoplankton, and consequently reduceing nitrogen and phosphorus contents and purifying water quality. Finally, it was suggested that the species of the stocked fish should be collocated reasonably, and the protection and reproduction of piscivorous fishes should be strengthened, so as to make full use of the rich resources of small fish and shrimp, and improve the quality of the fish products and the economic benefits