Lake fishery is an important component of Chinese freshwater fisheries. The fish is at the top or higher trophic levels of the food chain in aquatic ecosystems. Stocking artificially in lakes (reservoirs) would lead to the increase of fish biomass and overfishing would lead to a sharp reduction of fish biomass. All these would have a deep impact on the lake ecosystem through food web trophic relation. Lake Tianmu fishery take both the protection of the water environment and fisheries production into account, mainly implementing the non-classical biological manipulation by stocking silver carp and bighead carp to filter the phytoplankton to prevent and control cyanobacterial bloom, which aims to improve water quality. Ecological monitoring of the fish resources is the foundation of fisheries regulation. Hydroacoustic method provides an efficient means of achieving this goal with such superiorities: quick and efficient, large surveying area, no damage to fish resource, providing continuous data, positioning the fish spatial distribution in natural state.
Combining fisheries catching statistics and hydroacoustic method, an ecological monitoring of fish resources' (fish in aggregating cage was exclusive of fish resources after the fishery driving) catching and stocking in Lake Tianmu was carried out in three fishing stages, before the fishing driving(December 2011), after the fishing driving (January 2012) and after the fishing catching and stocking(March 2012). And then fish population structure, size component, fish density, fish aggregation, fish biomass and its distribution were gained with developing GIS model, which could provide scientific basis to the implementation of aquatic environment protection oriented fishery and fisheries production in Lake Tianmu. The cyprinids are the main fish populations in Lake Tianmu with the silver carp and bighead carp contributing weight ratio of 98.07% in the fisheries production of 2011 and mantissa ratio of 68.72 in single network bamboo sampling statistics. Fish resources are affected significantly by the types and size of stocking fish. The fish average target strength(TS) of the three fishing stages was (-47.84±4.79) dB, (-48.58±4.98) dB and (-47.24±5.10) dB respectively with significantly(P < 0.05)difference and the ratio of fish TS in -45— -40 dB went up to 24.40% after the catching and stocking. The fish density(FPCM)of the three fishing stages was (0.0124±0.0292) ind/m³, (0.0062±0.0227) ind/m³ and (0.0098±0.0185) ind/m³ respectively. The fishing driving decreased the fish density significantly(P < 0.05), while the fish density after fishing catching and stocking was significantly(P < 0.05)lower than before due to the arise of the water depth. Furthermore, classic fish aggregations were detected in the middle and bottom water body in winter and agglomeration improved with decreasing temperature. Assessment of the biomass were gained by developing GIS model with 610000 ind before the fishing driving, 380000 ind after the fishing driving and 670000 ind after the fishing catching and stocking. In addition, higher fish biomass was distributed in the middle and down reaches of the lake. Finally, discussions were performed on the efficacy of the fishing driving and the accuracy of the assessment of fish resources by hydroacoustic survey method in Lake Tianmu.