At present,many scientists pay more attention to the incidental catch of top predator of food chain in longline fisheries,such as sharks,sea turtles and sea birds.Given the long life span and relatively low reproductive capacity of many shark species,reducing the incidental catch of sharks during commercial fishing operations is thus critical in the conservation of shark species.In 1999,FAO promulgated the International Plan of Action for the Conservation and Management of Sharks.In order to protect the marine environment and conserve the shark species,many countries and regions declared a law prohibiting commercial shark fishing in its national waters.Shark's habitat environment is complex with respect to both spatial and temporal variations.Mechanisms of their migration are not fully understood even though many studies suggested that they might be related to the behavior and dynamics of zooplankton.The effective fishing gears and methods to mitigate the incidental catch rate of Carcharhinus falciformis and the effects of 16/0 and 18/0 circle hooks to the incidental catch rate of C.falciformis should be studied further.Based on these studies,the robust results could be obtained and could be used to reduce the incidental catch rate of C.falciformis effectively.On the other hand,there were many studies about the biological characteristics of C.falciformis,but the studies about the impacts of marine environment on the incidental catch rate of C.falciformis were rare.The aim of this study is to reduce the incidental catch rate of C.falciformis in longline fisheries.In this study,the mean nominal catch rates of C.falciformis and environmental variables were obtained from two longlining surveys in waters near Gilbert Islands from 4 October 2009 through 25 December 2009 and from 20 November 2010 through 20 January 2011.Data included:hook depth data,temperature,salinity,dissolved oxygen concentration and chlorophyll concentration vertical profile data,operating parameters,catch statistics.Stepwise regression was used to develop the hook depth calculation model.Wilcoxon-test was used to test if there were significant differences among C.falciformis incidental catch rates of four kinds of fishing gear or hook types.Statistics and clustering analysis were used to analyze environmental effects on the C.falciformis catch rate.Results showed that:(1)C.falciformis incidental catch rate can be significantly reduced by using the experimental gear or 18/0 circle hook;(2)the depth,temperature,salinity,chlorophyll concentration and dissolved oxygen(DO)range with the high incidental catch rate for C.falciformis was 40.0-79.9 m,24.0-24.9 ℃ and 29.0-29.9 ℃,35.40-35.99,0.120-0.199 μg/L,and 4.50-4.99 mg/L,respectively.This study suggests that:(1)the numbers of 18/0 circle hook or hook deeper than 120 m should be increased;(2)the numbers of hook should be reduced in the higher incidental catch rate ranges of hook depth,temperature,salinity,dissolved oxygen concentration and chlorophyll concentration.