This study was carried out to unravel the mechanism of reductions in production performances in high stocking density geese flocks during summer months in “geese-fish” production system.Experiment 1 observed the water bacterial growth,lipopolysaccharde concentrations in water and geese blood,and geese reproductive performances from summer to winter,in two flocks with varying on water stocking densities.Results showed that counts of total bacteria,Escherichia coli and Salmonella in water,as well as water and geese plasma LPS concentrations,exhibited a tendency decreasing from the highest levels in summer,to intermediate levels in autumn,and to the lowest values in winter.Such seasonal decreases in bacteria and LPS concentrations were associated with similar seasonal decreases in embryo mortality during incubation.In addition,embryos dead or showing development retardation by day 25 of incubation contained copious LPS in allantoic fluid,in contrast to the negligible amount in normal developing embryos.Raising on water stocking density elevated bacteria counts,LPS concentrations in water and geese plasma,and decreased egg fertility but increased embryo mortality during incubation.In experiment 2,exogenous LPS treatment to the geese depressed egg laying,reduced egg hatchability,caused sickness behavior in the goslings hatched.In experiment 3,exogenous LPS directly administered to day 8 and 18 embryos during incubation dose dependently increased mortality and decreased hatchability,and caused sickness behavior in the goslings hatched.It is concluded that the raising on water geese stocking density stimulates pathogenic bacteria growth in water,which via LPS contamination impaires embryo development in incubation and therefore reduces geese reproductive performance and gosling quality during the hot summer months.