Reducing Bycatch of Silky Shark (Carcharhinus falciformis) in Pelagic Longlines Fishing in Waters Near Gilbert Islands through Better Understanding of Environmental Factors and Fishing Gear Parameters

The aim of this study is to reduce the incidental catch rate of Carcharhinus 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. 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 rate 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-a concentration and dissolved oxygen range with the high incidental catch rate for C. falciformis was 40.0～79.9 m, 24.0～24.9 ºC and 29.0～29.9 ºC, 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 decreased in the higher incidental catch rate ranges of hook depth, temperature, salinity, dissolved oxygen concentration and chlorophyll-a concentration.